Code sequence generator in a CDMA modem

Description

BACKGROUND OF THE INVENTION

Providing quality telecommunication services to user groups which are clasified as remote, such as rural telephone systems and telephone systems in underdeveloped countries, has proved to be a challenge over recent years. The past needs created by these services have been partially satisfied by wireless radio services, such as fixed or mobile frequency division multiplex (FDM), frequency division multiple access (FDMA), time division multiplex (TDM), time division multiple access (TDMA) systems, combination frequency and time division systems (FD/TDMA), and other land mobile radio systems.Often, these remote services are faced with more potential users than can be supported simultaneously by their frequency or spectral bandwidth capacity.

Recognizing these limitations, recent advances in wireless communications have used spread spectrum modulation techniques to provide simultaneous communication by multiple users. Spread spectrum modulation refers to modulating a information signal with a spreading code signal; the spreading code signal being generated by a code generator where the period Tc of the spreading code is substantially less than the period of the information data bit or symbol signal. The code may modulate the carrier frequency upon which the information has been sent, called frequency-hopped spreading, or may directly modulate the signal by multiplying the spreading code with the information data signal, called direct-sequence spreading (DS). Spread-spectrum modulation produces a signal with bandwidth substantially greater than that required to transmit the information signal, and synchronous reception and despreading of the signal at the receiver demodulator recovers the original information. The synchronous demodulator uses a reference signal to synchronize the despreading circuits to the input spread-spectrum modulated signal in order to recover the carrier and information signals. The reference signal can be a spreading code which is not modulated by an information signal. Such use of a synchronous spread-spectrum modulation and demodulation for wireless communication is described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling, which is incorporated herein by reference.

Spread-spectrum modulation in wireless networks offers many advantages because multiple users may use the same frequency band with minimal interference to each user's receiver. Spread-spectrum modulation also reduces effects from other sources of interference. In addition, synchronous spread-spectrum modulation and demodulation techniques may be expanded by providing multiple message channels for a user, each spread with a different spreading code, while still transmitting only a single reference signal to the user. Such use of multiple message channels modulated by a family of spreading codes synchronized to a pilot spreading codes for wireless communication is described in U.S. Pat. No. 5,166,951 entitled HIGH CAPACITY SPREAD-SPECTRUM CHANNEL by Donald L. Schilling, which is incorporated herein by reference.

One area in which spread-spectrum techniques are used is in the field of mobile cellular communications to provide personal communication services (PCS). Such systems desirably support large numbers of users, control Doppler shift and fade, and provide high speed digital data signals with low bit error rates. These systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence synchronized to the family of codes. Each user is assigned one of the spreading codes as a spreading function. Related problems of such a system are: supporting a large number of users with the orthogonal codes, handling reduced power available to remote units, and handling multipath fading effects. Solutions to such problems include using phased-array antennas to generate multiple steerable beams, using very long orthogonal or quasi-orthogonal code sequences which are reused by cyclic shifting of the code synchronized to a central reference, and diversity combining of multipath signals. Such problems associated with spread spectrum communications, and methods to increase capacity of a multiple access, spread-spectrum system are described in U.S. Pat. No. 4,901,307 entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS by Gilhousen et al. which is incorporated herein by reference.

SUMMARY OF THE INVENTION

An exemplary system which includes a modem according to the present invention provides local-loop telephone service using radio link between one or more base stations and multiple remote subscriber units. In the exemplary embodiment, the radio link is described for a base station communicating with a fixed subscriber unit (FSU), but the system is equally applicable to systems including multiple base stations with radio links to both FSUs and Mobile Subscriber Units (MSUs). Consequently, the remote subscriber units are referred to herein as Subscriber Units (SUs). Referring to

FIG. 1

, in the exemplary system, the Base Station (BS)

101

provides call connection to the local exchange (LE)

103

or other and telephone network switching interface, and includes the Radio Carrier Station RCS (

104

). One or more RCSs

104

,

105

,

110

connect to the Radio Distribution Unit (RDU)

102

through the links

131

,

132

,

137

,

138

,

139

, and the RDU

102

in turn interfaces with the LE

103

by transmitting and receiving call set-up, control, and information signals through telco links

141

,

142

,

150

. The SUs

116

,

119

communicate with the RCS

104

through radio links

161

,

162

,

163

,

164

,

165

. Both the RCS and the SUs include CDMA modems which establish and maintain the radio links. Alternatively, another embodiment of the invention may include several SUs and a “master” SU which functions in much the same was as the RCS to allow communication among the SUs. Such embodiment may or may not have connection to a local telephone network.

Although the described embodiment uses different spread-spectrum bandwidths centered around a carrier for the transmit and receive spread-spectrum channels, the present method is readily extended to systems using multiple spread-spectrum bandwidths for the transmit channels and multiple spread-spectrum bandwidths for the receive channels. Alternatively, because spread-spectrum communication systems have the inherent feature that one user's transmission appears as noise to another user's despreading receiver, an embodiment can employ the same spread-spectrum channel for both the transmit and receive path channels. In other words, Uplink and Downlink transmissions can occupy the same frequency band.

The spread binary symbol information is transmitted over the radio links

161

to

165

using Quadrature Phase Shift Keying (QPSK) modulation with Nyquist Pulse Shaping in the present embodiment, although other modulation techniques may be used, including, but not limited to, Offset QPSK (OQPSK) and Minimum Shift Keying (MSK).

The RCS and the SUs each contain CDMA modems according to the present invention for transmission and reception of telecommunication signals including information signals and connection control signals. A CDMA modem which includes an embodiment of the present invention includes a modem transmitter having: a code generator which provides an associated pilot code signal and which generates a plurality of message code signals; a spreading circuit which combines each of the information signals, with a respective one of the message code signals to generate a spread-spectrum processed message signal; and a global pilot code generator that provides a global pilot code signal to which the message code signals are synchronized.

The exemplary CDMA modem also includes a modem receiver having associated pilot code acquisition and tracking logic. The associated pilot code acquisition logic includes an associated pilot code generator and a group of associated pilot code correlators for correlating code-phase delayed versions of the associated pilot signal with a receive CDM signal to produce a despread associated pilot signal. The code phase of the associated pilot signal is changed responsive to an acquisition signal value until a detector indicates the presence of the despread associated pilot code signal by changing the value of the acquisition signal. The associated pilot code signal is synchronized to the global pilot signal. The associated pilot code tracking logic adjusts the associated pilot code signal in phase responsive to the acquisition signal so that the signal power level of the despread associated pilot code signal is maximized. Finally, the CDMA modem receiver includes a group of message signal acquisition circuits. Each message signal acquisition circuit includes a plurality of receive message signal correlators which correlate the local received message code signal with the CDM signal to produce a respective despread received message signal.

To generate large families of nearly mutually orthogonal codes used by the CDMA modems, the exemplary modem includes a code sequence generator. The code sequences are assigned to a respective logical channel of the spread-spectrum communication system, whch includes In-phase (I) and Quadrature (Q) transmission over RF communication channels. One set of sequences is used as pilot sequences that are transmitted without being modulated by a data signal. The code sequence generator circuit includes a long code sequence generator including a linear feedback shift register, a memory which provides a short, even code sequence, and a plurality of cyclic shift, feedforward sections, each of which provides a respective code sequence in the family of code sequences. The code sequence generator further includes a group of code sequence combiners for combining each Generated code sequence with the short, even code sequence to produce a group, or family, of long code sequences having relatively low mutual correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a block diagram of a code division multiple access communication system according to the present invention.

FIG. 2

a

is a block diagram of a 36 stage linear shift register suitable for use with long spreading-code of the code generator of the present invention.

FIG. 2

b

is a block diagram of circuitry which illustrates the feed-forward operation of the code generator.

FIG. 2

c

is a block diagram of an exemplary code generator of the present invention including the circuit for generating spreading-code sequences from the long spreading-code and the short spreading-codes.

FIG. 2

d

is an alternate embodiment of the code generator circuit including delays to compensate for electrical circuit delays.

FIG. 3

a

is a graph of the constellation points of the pilot spreading-code QPSK signal.

FIG. 3

b

is a graph of the constellation points of the message channel QPSK signal.

FIG. 3

c

is a block diagram of exemplary circuitry which implements the method of tracking the received spreading-code phase of the present invention.

FIG. 4

is a block diagram of the tracking circuit that tracks the median of the received multipath signal components.

FIG. 5

a

is a block diagram of the tracking circuit that tracks the centroid of the received multipath signal components.

FIG. 5

b

is a block digram of the Adaptive Vector Correlator.

FIG. 6

is a block diagram of exemplary circuitry which implements the acquisition decision method of the correct spreading-code phase of the received pilot code of the present invention.

FIG. 7

is a block diagram of an exemplary pilot rake filter which includes the tracking circuit and digital phase locked loop for despreading the pilot spreading-code, and generator of the derotation factors of the present invention.

FIG. 8

a

is a block diagram of an exemplary adaptive vector correlator and matched filter for despreading and combining the multipath components of the present invention.

FIG. 8

b

is a block diagram of an alternative implementation of the adaptive vector correlator and adaptive matched filter for despreading and combining the multipath components of the present invention.

FIG. 8

c

is a block diagram of an alternative embodiment of the adaptive vector correlator and adaptive matched filter for despreading and combining the multipath components of the present invention.

FIG. 8

d

is a block diagram of the Adaptive Matched Filter of one embodiment of the present invention.

FIG. 9

is a block diagram of the elements of an exemplary radio carrier station (RCS) of the present invention.

FIG. 10

is a block diagram of the elements of an exemplary modem interface unit (MIU) of the RCS shown in FIG.

9

.

FIG. 11

is a high level block diagram showing the transmit, receive, control, and code generation circuitry of the CDMA modem.

FIG. 12

is a block diagram of the transmit section of the CDMA modem.

FIG. 13

is a block diagram of an exemplary modem input signal receiver.

FIG. 14

is a block diagram of an exemplary convolutional encoder as used in the present invention.

FIG. 15

is a block diagram of the receive section of the CDMA modem.

FIG. 16

is a block diagram of an exemplary adaptive matched filter as used in the CDMA modem receive section.

FIG. 17

is a block diagram of an exemplary pilot rake as used in the CDMA modem receive section.

FIG. 18

is a block diagram of an exemplary auxiliary pilot rake as used in the CDMA modem receive section.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

The entire disclosure of U.S. patent application Ser. No. 08/669,769, filed Jun. 27, 1996 now U.S. Pat. No. 5,796,776, is expressly incorporated by reference herein.

Referring to

FIG. 1

, the radio links

161

to

165

incorporate Broadband Code Division Multiple Access (B-CDMA™) as the mode of transmission in both the Uplink and Downlink directions. CDMA (also known as Spread Spectrum) communication techniques used in multiple access systems are well-known, and are described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATION SYSTEM AND METHOD by Donald T Schilling which is incorporated herein by reference. The described exemplary system uses the Direct Sequence (DS) spreading technique. In each modem, one or more CDMA modulators performs the spread-spectrum spreading code sequence generationIn addition, the modems generate, for example, a pseudonoise (PN) spreading sequence; and pefrorm complex DS modulation to produce quadrature phase shift keying (QPSK) signals for the In-phase (I) and Quadrature (Q) channels. Pilot signals are generated and transmitted with the modulated signals. The pilot signals of the present embodiment are spreading codes which are not modulated by data. The pilot signals are used for system synchronization, carrier phase recovery, and for estimating the impulse response of the radio channel. Each SU includes a single pilot generator and at least one CDMA modulator and demodulator, called a CDMA modem. Each RCS

104

,

105

,

110

has a single pilot generator plus sufficient CDMA modulators and demodulators for all the logical channels in use by all SUs.

The CDMA demodulator despreads the signal, with appropriate processing to combat or exploit multipath propagation effects. Parameters concerning the received power level are used to generate the Automatic Power Control (APC) information which, in turn, is transmitted to the other end (i.e. from the SU to the RCS or from the RCS to the SU). The APC information is used to control transmit power of the automatic forward power control (AFPC) and automatic reverse power control (ARPC) links. In addition, each RCS

104

,

105

and

110

may perform Maintenance Power Control (MPC), in a manner similar to APC, to adjust the initial transmit power of each SU

111

,

112

,

115

,

117

and

118

.

Diversity combining at the radio antennas of the RCS

104

,

105

and

110

is not necessary because CDMA has inherent frequency diversity due to the spread bandwidth. Receivers may include Adaptive Matched Filters (AMFs) (not shown in FIG.

1

), however, which combine the multipath signals. In the exemplary embodiment, AMFs perform Maximal Ratio Combining.

Logical Communication Channels

A ‘channel’ of the prior art is usually regarded as a communications path which is part of an interface and which can be distinguished from other paths of that interface without regard to its content. In the case of CDMA, however, separate communications paths are distinguished only by their content. The term ‘logical channel’ is used to distinguish the separate data streams, which are logically equivalent to channels in the conventional sense. All logical channels and sub-channels of the present invention are mapped to a common 64 kilo-symbols per second (ksym/s) QPSK stream. Some channels are synchronized to associated pilot codes which are generated in the same way and perform much the same function as the system Global Pilot Code. The system pilot signals are not, however, considered logical channels.

Several logical communication channels are used over the RF communication link between the RCS and SU. Each logical communication channel has either a fixed, pre-determined spreading code or a dynamically assigned spreading code. For both predetermined and assigned codes, the code phase is in synchronism with the Pilot Code. Logical communication channels are divided into two groups: the Global Channel (GC) group includes those channels which are either transmitted from the base station RCS to all the remote SUs or from any SU to the RCS of the base station regardless of the SU's identity, These channels contain for all users and include the channels used by SUs to gain access to message communication channels. Channels in the Assigned Channels (AC) group are those channels which are dedicated to communication between the RCS and a particular SU.

The Global Channels (GC) group provides for 1) Broadcast logical channels, which provide point to multipoint services for broadcasting messages to all SUs and paging messages to SUs; and 2) Access Control logical channels which provide point-to-point services on global channels for SUs to access the system and obtain assigned channels.

An Assigned Channel (AC) group contains the logical channels that control a single telecommunication connection between the RCS and a SU. The functions developed when an AC group is formed consists of a pair of power control logical message channels for each of the Uplink and Downlink connections, and depending on the type of connection, one or more pairs of traffic channels. The Bearer Control function performs the required forward error control, bearer rate modification, and encryption functions. The logical channels which constitute the BC and AC groups are summarized below in Table 1.

TABLE 1

Logical Channels and sub-channels of the B-CDMA Air Interface

Di-

Brief

rection

De-

(forward

Channel

scrip-

or

Bit

Max

Power

name

Abbr.

tion

reverse)

rate

BER

level

Pilot

Global Channels

Fast

FBCH

Broad-

F

16

le-4

Fixed

GLPT

Broad-

casts

kbit/

cast

fast-

s

Channel

chang-

ing

system

infor-

mation

Slow

SBCH

Broad-

F

16

le-7

Fixed

GLPT

Broad-

casts

kbit/

cast

paging

s

Channel

mes-

sages to

FSUs

and

slow-

chang-

ing

system

infor-

mation

Access

AXCH

For

R

32

le-7

Control-

LAXP

Chan-

(i)

initial

kbit/

led

T(i)

nels

access

s

by APC

attempts

by

FSUs

Control

CTCH

For

F

32

le-7

Fixed

GLPT

Chan-

(i)

granting

kbit/

nels

access

s

Assigned Channels

16

TRCH/

General

F/R

16

le-4

Control-

F-

kbit/s

16

POTS

kbit/

led

GLPT

POTS

use

s

by APC

R-

ASPT

32

TRCH/

General

F/R

32

le-4

Control-

F-

kbit/s

32

POTS

kbit/

led

GLPT

POTS

use

s

by APC

R-

ASPT

64

TRCH/

POTS

F/R

64

le-4

Control-

F-

kbit/s

64N

use for

kbit/

led

GLPT

POTS

in-band

s

by APC

R-

mod-

ASPT

ems/fax

Traffic

TRCH/

ISDN B

F/R

64

le-7

Control-

F-

channel

64L

channel

kbit/

led

GLPT

@ 64

or LL

s

by APC

R-

kbit/s-

ASPT

low

BER

D

TRCH/

ISDN D

F/R

16

le-7

Control-

F-

channel

16L

channel

kbit/

led

GLPT

s

by APC

R-

ASPT

Order

OW

assign-

F/R

32

le-7

Control-

F-

wire

ed

kbit/

led

GLPT

channel

signal-

s

by APC

R-

ing

ASPT

channel

APC

APC

carries

F/R

64

2e-1

Control-

F-

channel

APC

kbit/

led

GLPT

com-

s

by APC

R-

mands

ASPT

The APC data is sent at 64 kbit/sec. The APC logical channel is not FEC coded to avoid delay and is transmitted at a low power level to minimize capacity used for APC. Alternatively, the APC and order wire (OW) data may be separately modulated using complex spreading code sequences, or they may be time division multilplexed with a 16 kbit/s traffic channel.

The Spreading Codes

The CDMA code generators used to encode the logical channels of the present invention employ Linear Shift Registers (LSRs) with feedback logic which is a method well known in the art. The code generators of the present embodiment of the invention generate 64 synchronous unique sequences. Each RF communication channel uses a pair of these sequences for complex spreading (in-phase and quadrature) of the logical channels, so the generator gives 32 complex spreading sequences. The sequences are generated by a single seed which is initially loaded into a shift register circuit.

The Generation of Spreading Code Sequences and Seed Selection

The spreading code period of the present invention is defined as an integer multiple of the symbol duration, and the beginning of the code period is also the beginning of the symbol. The relation between bandwidths and the symbol lengths chosen for the exemplary embodiment of the present invention is:

BW (MHZ)

L(chips/symbol)

7

91

10

130

10.5

133

14

182

15

195

The spreading code length is also a multiple of 64 and of 96 for ISDN frame support. The spreading code is a sequence of symbols, called chips or chip values. The general methods of generating pseudorandom sequences using Galois Field mathematics is known to those skilled in the art; hoever, the inventor has derived a unique set, or family, of code sequences for the present invention. First, the length of the linear feedback shift register to generate a code sequence is chosen, and the initial value of the register is called a “seed”. Second, the constraint is imposed that no code sequence generated by a code seed can be a cyclic shift of another code sequence generated by the same code seed. Finally, no code sequence generated from one seed can be a cyclic shift of a code sequence generated by another seed.

The inventor has determined that the spreading code length of chip values of the present invention is:

128×233415=29877120 (1)

The spreading codes are generated by combining a linear sequence of period 233415 and a nonlinear sequence of period 128

The nonlinear sequence of length 128 is implemented as a fixed sequence loaded into a shift register with a feed-back connection. The fixed sequence can be generated by an m-sequence of length 127 padded with an extra logic 0, 1, or random value as is well known in the art.

The linear sequence of length L=233415 is generated using a linear feedback shift register (LFSR) circuit with 36 stages. The feedback connections correspond to a irreducible polynomial h(n) of degree 36. The polynomial h(x) chosen by the inventor for the exemplary embodiment of the present invention is

h

(

x

)=

x

36

+x

35

+x

30

+x

28

+x

26

+x

25

+x

22

+x

20

+x

19

+x

17

+x

16

+x

15

+x

14

+x

12

+x

11

+x

9

+x

8

+x

4

+x

3

+x

2

+x

1

(2)

A group of “seed” values for a LFSR representing the polynomial h(x) of equation (2) which generates code sequences that are nearly orthogonal with each other is determined. The first requirement of the seed values is that the seed values do not generate two code sequences which are simply cyclic shifts of each other.

The present invention includes a method to increase the number of available seeds for use in a CDMA communication system by recognizing that certain cyclic shifts of the previously determined code sequences may be used simultaneously. The round trip delay for the cell sizes and bandwidths of the present invention are less than 3000 chips. In one embodiment of the present invention, sufficiently separated cyclic shifts of a sequence can be used within the same cell without causing ambiguity for a receiver attempting to determine the code sequence. This method enlarges the set of sequences available for use.

By implementing the tests previously described, a total of 3879 primary seeds were determined by the inventor through numerical computation. These seeds are given mathematically as

d

n

modulo h(x) (3)

where 3879 values of n are listed in the appendix, with d=(00, . . . 00111).

When all primary seeds are known, all secondary seeds of the present invention are derived from the primary seeds by shifting them multiples of 4095 chips modulo h(x). Once a family of seed values is determined, these values are stored in memory and assigned to logical channels as necessary. Once assigned, the initial seed value is simply loaded into LFSR to produce the required spreading-code sequence associated with the seed value.

Epoch and Sub-epoch Structures

The long complex spreading codes used for the system of the current invention have a number of chips after which the code repeats. The repetition period of the spreading sequence is called an epoch. To map the logical channels to CDMA spreading codes, the present invention uses an Epoch and Sub-epoch structure. The code period for the CDMA spreading code to modulate logical channels is 29877120 chips/code period which is the same number of chips for all bandwidths. The code period is the epoch of the present invention, and the Table 2 defines the epoch duration for the supported chip rates. In addition, two sub-epochs are defined over the spreading code epoch and are 233415 chips and 128 chips long.

The 233415 chip sub-epoch is referred to as a long sub-epoch, and is used for synchronizing events on the RF communication interface such as encryption key switching and changing from global to assigned codes. The 128 chip short epoch is defined for use as an additional timing reference. The highest symbol rate used with a single CDMA code is 64 ksym/sec. There is always an integer number of chips in a symbol duration for the supported symbol rates 64, 32, 16, and 8 ksym/s.

TABLE 2

Bandwidths, Chip Rates, and Epochs

number of

128 chip

233415 chip

Chip Rate,

chips in a

sub-epoch

sub-epoch

Epoch

Bandwidth

Complex

64 kbit/sec

duration*

duration*

duration

(MHz)

(Mchip/sec)

symbol

(μs)

(ms)

(sec)

7

5.824

91

21.978

40.078

5.130

10

8.320

130

15.385

28.055

3.591

10.5

8.512

133

15.038

27.422

3.510

14

11.648

182

10.989

20.039

2.565

15

12.480

195

10.256

18.703

2.394

*numbers in these columns are rounded to 5 digits.

Cyclic sequences of the prior art are generated using linear feedback shift register (LFSR) circuits. This method, however, does not generate sequences of even length. One embodiment of the spreading code sequence generator using the code seeds generated previously is shown in

FIG. 2

a,

FIG. 2

b,

and

FIG. 2

c.

The exemplary system uses a 36 stage LFSR

201

to generate a sequence of period N′=233415=3

3

×5×7×13×19, which is C

o

in

FIG. 2

a.

In the

FIGS. 2

a

,

2

b,

and

2

c

the symbol ⊕ represents a binary addition (EXCLUSIVE-OR). A sequence generator designed as above generates the in-phase and quadrature parts of a set of complex sequences. The tap connections and initial state of the 36 stage LFSR determine the sequence generated by this circuit. The tap coefficients of the 36 stage LFSR are determined such that the resulting sequences have the period 233415. Note that the tap connections shown in

FIG. 2

a

correspond to the polynomial given in equation (2). Each resulting sequence is then overlaid by binary addition with the 128 length sequence C* to obtain the epoch period 29877120.

FIG. 2

b

shows a Feed Forward (FF) circuit

202

which is used in the code generator. The signal X[n−1] is output of the chip delay

211

, and the input of the chip delay

211

is X[n]. The code chip C[n] is formed by the logical adder

212

from the input X[n] and X[n−1].

FIG. 2

c

shows the complete spreading-code generator. From the LFSR

201

, output signals go through a chain of up to 63 single stage FFs

203

cascaded as shown. The output of each FF is overlaid with the short, even code sequence C* which has a period of 128=2

7

. the short code sequence C* is stored in code memory

222

and exhibits spectral characteristics of a pseudorandom sequence to obtain the epoch N=29877120 when combined with the sequences provided by the FFs

203

. This sequence of 128 is determined by using an m-sequence (PN sequence) of length 127=2

7

−1 and adding a bit-value, such as logic 0, to the sequence to increase the length to 128 chips. The even code sequence C* is input to the even code shift register

221

, which is a cyclic register, that continually outputs the sequence. The short sequence is then combined with the long sequence using an EXCLUSIVE-OR operation

213

,

214

,

220

.

As shown in

FIG. 2

c,

up to

63

spreading-code sequences C

o

through C

63

are generated by tapping the output signals of FFs

203

and logically adding the short sequence C* in a binary adders

213

,

214

, and

220

, for example. One skilled in the art would realize that the implementation of the FF

203

create a cumulative delay effect for the code sequences produced at each FF stage in the chain. This delay is due to the nonzero electrical delay in the electronic components of the implementation. The timing problems associated with the delay can be mitigated by inserting additional delay elements into the FF chain. An exemplary FF chain with additional delay elements is shown in

FIG. 2

d.

The code-generators in the exemplary system are configured to generate either global codes, or assigned codes. Global codes are CDMA codes that can be received or transmitted by all users of the system. Assigned codes are CDMA codes that are allocated for a particular connection. When a family of sequences is generated from the same generator as described, only the seed of the 36 stage LFSR is specified. Sequences for all the global codes, are generated using the same LFSR circuit. Therefore, once an SU has synchronized to the Global pilot signal from an RCS and knows the seed for the LFSR circuit for the Global Channel codes, it can generate not only the pilot sequence but also all other global codes used by the RCS.

The signal that is upconverted to RF is generated as follows. The spreading sequences produced by the above shift register circuits are converted to an antipodal sequence (0 maps into +1, 1 maps into −1). The Logical channels are initially converted to QPSK signals, which are mapped as constellation points as is well known in the art. The In-phase and Quadrature channels of each QPSK signal form the real and imaginary parts of the complex data value. Similarly, two spreading codes are used to form complex spreading chip values. The complex data and complex spreading code are multiplied to produce a spread-spectrum data signal. Similarly, for despreading, the received complex data is correlated with the conjugate of the complex spreading code to recover the data signal.

Short Codes

Short codes are used for the initial ramp-up process when an SU accesses an RCS. The period of the short codes is equal to the symbol duration and the start of each period is aligned with a symbol boundary. Both the SUs and the RCS derive the real and imaginary parts of the short codes from the last eight feed-forward sections of the sequence generator to produce the global codes for that cell. Details on the implementation of the initial ramp-up process may be found in a U.S. patent application entitled “A METHOD OF CONTROLLING INITIAL POWER RAMP-UP IN CDMA SYSTEMS BY USING SHORT CODES”, filed on even date herewith which is incorporated herein by reference.

The signals represented by these short codes are known as Short Access Channel pilots (SAXPTs)

Mapping of Logical Channels to Spreading Codes

The exact relationship between the spreading-code sequences and the CDMA logical channels and pilot signals is documented in Table 3a and Table 3b. Those signal names ending in ‘-CH’ correspond to logical channels. Those signal names ending in ‘-PT’ correspond to pilot signals, which are described in detail below.

TABLE 3a

Spreading code sequences and global CDMA codes

Logical Channel

Sequence

Quadrature

or Pilot Signal

Direction

C

0

I

FBCH

Forward (F)

C

1

Q

FBCH

F

C

2

⊕C*

I

GLPT

F

C

3

⊕C*

Q

GLPT

F

C

4

⊕C*

I

SBCH

F

C

5

⊕C*

Q

SBCH

F

C

6

⊕C*

I

CTCH (0)

F

C

7

⊕C*

Q

CTCH (0)

F

C

8

⊕C*

I

APCH (1)

F

C

9

⊕C*

Q

APCH (1)

F

C

10

⊕C*

I

CTCH (1)

F

C

11

⊕C*

Q

CTCH (1)

F

C

12

⊕C*

I

APCH (1)

F

C

13

⊕C*

Q

APCH (1)

F

C

14

⊕C*

I

CTCH (2)

F

C

15

⊕C*

Q

CTCH (2)

F

C

16

⊕C*

I

APCH (2)

F

C

17

⊕C*

Q

APCH (2)

F

C

18

⊕C*

I

CTCH (3)

F

C

19

⊕C*

Q

CTCH (3)

F

C

20

⊕C*

I

APCH (3)

F

C

21

⊕C*

Q

APCH (3)

F

C

22

⊕C*

I

reserved

—

C

23

⊕C*

Q

reserved

—

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

C

40

⊕C*

I

reserved

—

C

41

⊕C*

Q

reserved

—

C

42

⊕C*

I

AXCH(3)

Reverse (R)

C

43

⊕C*

Q

AXCH(3)

R

C

44

⊕C*

I

LAXPT(3)

R

SAXPT(3) seed

C

45

⊕C*

Q

LAXPT(3)

R

SAXPT(3) seed

C

46

⊕C*

I

AXCH(2)

R

C

47

⊕C*

Q

AXCH(2)

R

C

48

⊕C*

I

LAXPT(2)

R

SAXPT(2) seed

C

49

⊕C*

Q

LAXPT(2)

R

SAXPT(2) seed

C

50

⊕C*

I

AXCH(1)

R

C

51

⊕C*

Q

AXCH(1)

R

C

52

⊕C*

I

LAXPT(1)

R

SAXPT(1) seed

C

53

⊕C*

Q

LAXPT(1)

R

SAXPT(1) seed

C

54

⊕C*

I

AXCH(0)

R

C

55

⊕C*

Q

AXCH(0)

R

C

56

⊕C*

I

LAXPT(0)

R

SAXPT(0) seed

C

57

⊕C*

Q

LAXPT(0)

R

SAXPT(0) seed

C58⊕C*

I

IDLE

—

C59⊕C*

Q

IDLE

—

C60⊕C*

I

AUX

R

C61⊕C*

Q

AUX

R

C62⊕C*

I

reserved

—

C63⊕C*

Q

reserved

—

TABLE 3b

Spreading code sequences and assigned CDMA codes.

Logical Channel

Sequence

Quadrature

or Pilot Signal

Direction

C

0

⊕C*

I

ASPT

Reverse (R)

C

1

⊕C*

Q

ASPT

R

C

2

⊕C*

I

APCH

R

C

3

⊕C*

Q

APCH

R

C

4

⊕C*

I

OWCH

R

C

5

⊕C*

Q

OWCH

R

C

6

⊕C*

I

TRCH(0)

R

C

7

⊕C*

Q

TRCH(0)

R

C

8

⊕C*

I

TRCH(1)

R

C

9

⊕C*

Q

TRCH(1)

R

C

10

⊕C*

I

TRCH(2)

R

C

11

⊕C*

Q

TRCH(2)

R

C

12

⊕C*

I

TRCH(3)

R

C

13

⊕C*

Q

TRCH(3)

R

C

14

⊕C*

I

reserved

—

C

15

⊕C*

Q

reserved

—

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

C

44

⊕C*

I

reserved

—

C

45

⊕C*

Q

reserved

—

C

46

⊕C*

I

TRCH(3)

Forward (F)

C

47

⊕C*

Q

TRCH(3)

F

C

48

⊕C*

I

TRCH(2)

F

C

49

⊕C*

Q

TRCH(2)

F

C

50

⊕C*

I

TRCH(1)

F

C

51

⊕C*

Q

TRCH(1)

F

C

52

⊕C*

I

TRCH(0)

F

C

53

⊕C*

Q

TRCH(0)

F

C

54

⊕C*

I

OWCH

F

C

55

⊕C*

Q

OWCH

F

C

56

⊕C*

I

APCH

F

C

57

⊕C*

Q

APCH

F

C

58

⊕C*

I

IDLE

—

C

59

⊕C*

Q

IDLE

—

C

60

⊕C*

I

reserved

—

C

61

⊕C*

Q

reserved

—

C

62

⊕C*

I

reserved

—

C

63

⊕C*

Q

reserved

—

Pilot Signals

As described above, the pilot signals are used for synchronization, carrier phase recovery, and for estimating the impulse response of the radio channel. The RCS

104

transmits a forward link pilot carrier reference as a complex pilot code sequence to provide a time and phase reference for all SUs

111

,

112

,

115

,

117

and

118

in its service area. The power level of the Global Pilot (GLPT) signal is set to provide adequate coverage over the whole RCS service area, which area depends on the cell size. With only one pilot signal in the forward link, the reduction in system capacity due to the pilot energy is negligible.

Each of the SUs

111

,

112

,

115

,

117

and

118

transmits a pilot carrier reference as a quadrature modulated (complex-valued) pilot spreading-code sequence to provide time and phase reference to the RCS for the reverse link. The pilot signal transmitted by the SU of one embodiment of the invention is 6 dB lower than the power of the 32 kbit/s POTS (plain old telephone service) traffic channel. The reverse pilot channel is subject to APC. The reverse link pilot associated with a particular connection is called the Assigned Pilot (ASPT). In addition, there are pilot signals associated with access channels, and these are called the Long Access Channel Pilots (LAXPTs). Short access channel pilots (SAXPTs) are also associated with the access channels and used for spreading-code acquisition and initial power ramp-up.

All pilot signals are formed from complex codes, as defined below:

GLPT

(forward)={

C

2

⊕C

*)+

j

.(

C

3

⊕C

*)}·{(±1)+

j

.(0)}{Complex Code}·{Carrier }

The complex pilot signals are de-spread by multiplication with conjugate spreading codes: {(C

2

⊕C*)−j.(C

3

⊕C*)}. By contrast, traffic channels are of the form:

TRCH

n

(forward/reverse)={(

C

k

⊕C

*)+

j

.(

C

1

⊕C

*)}·{ (±1)+

j

(±1)}{Complex Codes}·{Data Symbol}

which thus form a constellation set at π/4 radians with respect to the pilot signal constellations.

The GLPT constellation is shown in

FIG. 3

a,

and the TRCH

n

traffic channel constellation is shown in

FIG. 3

b.

Logical Channel Assignment of the FBCH, SBCH, and Traffic Channels

The fast broadcast channel (FBCH) is a global forward link channel used to broadcast dynamic information about the availability of services and access channels (AXCHs). The messages are sent continuously, and each message lasts approximately I ms. The FBCH message is 16 bits long, repeated continuously, and epoch aligned. The FBCH is formatted as defined in Table 4.

TABLE 4

FBCH format

Bit

Definition

0

Traffic Light 0

1

Traffic Light 1

2

Traffic Light 2

3

Traffic Light 3

4-7

service indicator bits

8

Traffic Light 0

9

Traffic Light 1

10

Traffic Light 2

11

Traffic Light 3

12-15

service indicator bits

For the FBCH, bit 0 is transmitted first. A traffic light corresponds to an Access Channel (AXCH) and indicates whether the particular access channel is currently in use (red) or not in use (green). A logic ‘1’ indicates that the traffic light is green, and a logic ‘0’ indicates the traffic light is red. The values of the traffic light bits may change from octet to octet, and each 16 bit message contains distinct service indicator bits which describe which types of service are available for the AXCHs.

One embodiment of the present invention uses service indicator bits as follows to indicate the availability of services or AXCHs. The service indicator bits {4,5,6,7,12,13,14,15} are interpreted as an unsigned binary number, with bit 4 as the MSB and bit 15 as the LSB. Each service type increment has an associated nominal measure of the capacity required, and the FBCH continuously broadcasts the available capacity. This is scaled to have a maximum value equivalent to the largest single service increment possible. When an SU requires a new service or an increase in the number of bearers), it compares the capacity required to that indicated by the FBCH, and then considers itself blocked if the capacity is not available. The FBCH and the traffic channels are aligned to the epoch.

Slow Broadcast Information frames contain system or other general information that is available to all SUs, and Paging Information frames contain information about call requests for particular SUs. Slow Broadcast Information frames and Paging Information frames are multiplexed together on a single logical channel which forms the Slow Broadcast Channel (SBCH). As previously defined, the code epoch is a sequence of 29 877 20 chips having an epoch duration which is a function of the chip rate defined in Table 5 below. In order to facilitate power saving, the channel is divided into N “Sleep” Cycles, and each Cycle is subdivided into M Slots, which are 19 ms long, except for 10.5 Mhz bandwidth which has slots of 18 ms.

TABLE 5

SBCH Channel Format Outline

Spreading

Epoch

Cycles/

Cycle

Slots/

Slot

Bandwidth

Code Rate

Length

Epoch

Length

Cycle

Length

(MHz)

(MHz)

(ms)

N

(ms)

M

(ms)

7.0

5.824

5130

5

1026

54

19

10.0

8.320

3591

3

1197

63

19

10.5

8.512

3510

3

1170

65

18

14.0

11.648

2565

3

855

45

19

15.0

12.480

2394

2

1197

63

19

Sleep Cycle Slot #1 is always used for slow broadcast information. Slots #2 to #M−1 are used for paging groups unless extended slow broadcast information is inserted. The pattern of cycles and slots in one embodiment of the present invention run continuously at 16 kbit/s.

Within each Sleep Cycle the SU may power-up the receiver and re-acquire pilot code to achieve carrier lock to a sufficient precision for satisfactory demodulation and Viterbi decoding. This settling time may be up to 3 Slots in duration. For example, an SU assigned to Slot #7 may power up the Receiver at the start of Slot #4. Having monitored its Slot the SU either recognizes its Paging Address and initiates an access request, or fails to recognize its Paging Address in which case it reverts to the Sleep mode.

Spreading code Tracking and AMF Detection in Multipath Channels

Spreading code Tracking

Three CDMA spreading-code tracking methods in multipath fading environments are described which track the code phase of a received multipath spread-spectrum signal. The first method uses the prior art tracking circuit which simply tracks the spreading code phase of the detector having the highest output signal value, the second method uses a tracking circuit that tracks the median value of the code phase of the group of multipath signals, and the third method of the present invention, is the centroid tracking circuit which tracks the code-phase of an optimized, least mean squared weighted average of the multipath signal components. The following describes the algorithms by which the spreading code phase of the received CDMA signal is tracked.

A tracking circuit has operating characteristics that reveal the relationship between the time error and the control voltage that drives a Voltage Controlled Oscillator (VCO) of a spreading-code phase tracking circuit. When there is a positive timing error, the exemplary tracking circuit generates a negative control voltage to offset the timing error. When there is a negative timing error, the exemplary tracking circuit generates a positive control voltage to offset the timing error. When the tracking circuit generates a zero value, this value corresponds to the perfect time alignment called the ‘lock-point’.

FIG. 3

c

shows the basic tracking circuit. Received signal r(t) is applied to matched filter

301

, which correlates r(t) with a local code-sequence c(t) generated by Code Generator

303

. The output signal of the matched filter x(t) is sampled at the sampler

302

to produce samples x[nT] and x[nT+T/2]. The samples x[nT] and x[nT+T/2] are used by a tracking circuit

304

to determine if the phase of the spreading-code c(t) of the code generator

303

is correct. The tracking circuit

304

produces an error signal e(t) as an input to the code generator

303

. The code generator

303

uses this signal e(t) as an input signal to adjust the code-phase it generates.

In a CDMA system, the signal transmitted by the reference user is written in the low-pass representation as

\begin{matrix} s (t) = \sum_{k = - \infty}^{\infty} c_{k} P_{Tc} (t - {kT}_{c}) & (4) \end{matrix}

where c

k

represents the spreading code coefficients, P

Tc

(t) represents the spreading code chip waveform, and T

c

is the chip duration. Assuming that the reference user is not transmitting data, only the spreading code modulates the carrier. Referring to

FIG. 3

, the received signal is

\begin{matrix} r (t) = \sum_{i = 1}^{M} a_{i} s (t - τ_{i}) & (5) \end{matrix}

Here, a

i

is due to fading effect of the multipath channel on the i-th path and τ

i

is the random time delay associated with the same path. The receiver passes the received signal through a matched filter, which is implemented as a correlation receiver and is described below. This operation is done in two steps: first the signal is passed through a chip matched filter and sampled to recover the spreading code chip values, then this chip sequence is correlated with the locally generated code sequence.

FIG. 3

c

shows the chip matched filter

301

, matched to the chip waveform P

Tc

(t), and the sampler

302

. The signal x(t) at the output terminal of the chip matched filter is

\begin{matrix} x (t) = \sum_{i = k}^{M} \sum_{k = - \infty}^{\infty} a_{i} c_{k} g (t - τ_{i} - {kT}_{c}) & (6) \end{matrix}

where

g

(

t

)=

P

Tc

(

t

)*

h

R

(

t

) (7)

Here, h

R

(t) is the impulse response of the chip matched filter and ‘*’ denotes convolution. The order of the summations, can be rewritten as:

\begin{matrix} x (t) = \sum_{k = - \infty}^{\infty} c_{k} f (t - {kT}_{c}) & (8) \end{matrix}

where

\begin{matrix} f (t) = \sum_{i = 1}^{M} a_{i} g (t - τ_{i}) & (9) \end{matrix}

In the multipath channel described above, the sampler samples the output signal of the matched filter to produce x(nT) at the maximum power level points of g(t). In practice, however, the waveform g(t) is often severely distorted because of the effect of the multipath signal reception, and a perfect time alignment of the signals is not available.

When the multipath in the channel is negligible and a perfect estimate of the timing is available, i.e., a

1

=1, τ

1

=0, and a

1

=0, i=2, . . . , M, the received signal is r(t)=s(t). Then, with this ideal channel model, the output of the chip matched filter becomes

\begin{matrix} x (t) = \sum_{k = - \infty}^{\infty} c_{k} g (t - {kT}_{c}) & (10) \end{matrix}

When there is multipath fading, however, the received spreading code chip value waveform is distorted, and has a number of local maxima that can change from one sampling interval to another depending on the channel characteristics.

For multipath fading channels with quickly changing channel characteristics, it is not practical to try to locate the maximum of the waveformf(t) in every chip period interval. Instead, a time reference can be obtained from the characteristics off(t) that may not change as quickly. Three tracking methods are described based on different characteristics of f(t).

Prior Art Spreading-Code Tracking Method:

Prior art tracking methods include a code tracking circuit in which the receiver attempts to determine where the maximum matched filter output value of the chip waveform occurs and sample the signal at that point. In multipath fading channels, however, the receiver despreading code waveform can have a number of local maxima, especially in a mobile environment. If f(t) represents the received signal waveform of the spreading code chip convolved with the channel impulse response, the shape off(t) and where its maximum occurs can change rather quickly making it impractical to track the maximum off(t).

Define τ to be the time estimate that the tracking circuit calculates during a particular sampling interval. Also, define the following error function

\begin{matrix} \begin{matrix} ε = {\int \underset{{t : &LeftBracketingBar; τ - t &RightBracketingBar; > δ}}{f (t) ⅆ t}, & &LeftBracketingBar; τ - t &RightBracketingBar; > δ \\ ε = 0 & &LeftBracketingBar; τ - t &RightBracketingBar; < δ \end{matrix} & (11) \end{matrix}

The tracking circuits of the prior art calculate a value of the input signal that minimizes the error ε. One can write

\begin{matrix} \min ε = 1 - \max_{τ} \int_{τ - δ}^{τ + δ} f (t) ⅆ t & (12) \end{matrix}

Assuming f(τ) has a smooth shape in the values given, the value of τ for which f(τ) is maximum minimizes the error ε, SO the tracking circuit tracks the maximum point of f(t).

Median Weighted Value Tracking Method:

The Median Weighted Tracking Method of one embodiment of the present invention, minimizes the absolute weighted error, defined as

\begin{matrix} ε = \int_{- \infty}^{\infty} &LeftBracketingBar; t - τ &RightBracketingBar; f (t) ⅆ t & (13) \end{matrix}

This tracking method calculates the ‘median’ signal value off(t) by collecting information from all paths, where f(τ) is as in equation (9). In a multipath fading environment, the waveformf(t) can have multiple local maxima, but only one median.

To minimize ε, the derivative of equation (13) is taken with respect to τ and equated it to zero, which gives

\begin{matrix} \int_{- \infty}^{τ} f (t) ⅆ t = \int_{τ}^{\infty} f (t) ⅆ t & (14) \end{matrix}

The value of τ that satisfies (

14

) is called the ‘median’ off(t). Therefore, the Median Tracking Method of the present embodiment tracks the median off(t).

FIG. 4

shows an implementation of the tracking circuit based on minimizing the absolute weighted error defined above. The signal x(t) and its one-half chip offset version x(t+T/2) are sampled by the analog-to-digital A/D converter

401

at a rate 1/T. The following equation determines the operating characteristic of the circuit in FIG.

4

:

\begin{matrix} ε (τ) = \sum_{n = 1}^{2 L} &LeftBracketingBar; f (τ - nT / 2) &RightBracketingBar; - &LeftBracketingBar; f (τ + nT / 2) &RightBracketingBar; & (15) \end{matrix}

Tracking the median of a group of multipath signals keeps the received energy of the multipath signal components equal on the early and late sides of the median point of the correct locally generated spreading-code phase c

n

. The tracking circuit consists of an A/D converter

401

which samples an input signal x(t) to form the half chip offset samples. The half chip offset samples are alternatively grouped into even samples called an early set of samples x(nT+τ) and odd samples called a late set of samples x(nT+(T/2)+τ). The first correlation bank adaptive matched filter

402

multiplies each early sample by the spreading-code phases c(n+1), c(n+2), . . . , c(n+L), where L is small compared to the code length and approximately equal to number of chips of delay between the earliest and latest multipath signal. The output of each correlator is applied to a respective first sum-and-dump bank

404

. The magnitudes of the output values of the L sum-and-dumps are calculated in the calculator

406

and then summed in a summer

408

to give an output value proportional to the signal energy in the early multipath signals. Similarly, a second correlation bank adaptive matched filter

403

operates on the late samples, using code phases c(n−1), c(n−2), . . . , c(n−L), and each output signal is applied to a respective sum-and-dump in an integrator

405

. The magnitudes of the L sum-and-dump outputs are calculated in calculator

407

and then summed in summer

409

to give a value for the late multipath signal energy. Finally, the subtractor

410

calculates the difference and produces error signal ε(t) of the early and late signal energy values.

The tracking circuit adjusts, by means of error signal ε(τ), the locally generated code phases c(t) to cause the difference between the early and late values to tend toward 0.

Centroid Tracking Method

Another spreading-code tracking circuit of one embodiment of the present invention is called the squared weighted tracking (or centroid) circuit. Defining τ to denote the time estimate that the tracking circuit calculates, based on some characteristic off(t), the centroid tracking circuit minimizes the squared weighted error defined as

\begin{matrix} ε = \int_{- \infty}^{\infty} {&LeftBracketingBar; t - τ &RightBracketingBar;}^{2} f (t) ⅆ t & (16) \end{matrix}

This function inside the integral has a quadratic form, which has a unique minimum. The value of τ that minimizes ε can be found by taking the derivative of the above equation with respect to τ and equating to zero, which gives

\begin{matrix} \int_{- \infty}^{\infty} (- 2 t + 2 τ) f (t) ⅆ t = 0 & (17) \end{matrix}

Therefore, the value of τ that satisfies

\begin{matrix} τ - \frac{1}{β} \int_{- \infty}^{\infty} tf (t) ⅆ t = 0 & (18) \end{matrix}

is the timing estimate that the tracking circuit calculates, and β is a constant value.

Based on these observations, a realization of the tracking circuit minimizing the squared weighted error is shown in FIG.

5

. The following equation determines the error signal ε(τ) of the centroid tracking circuit:

\begin{matrix} ε (τ) = \sum_{n = 1}^{2 L} n [&LeftBracketingBar; f (τ - nT / 2) &RightBracketingBar; - &LeftBracketingBar; f (τ + nT / 2) &RightBracketingBar;] = 0 & (19) \end{matrix}

The value that satisfies ε(τ)=0 is the optimized estimate of the timing.

The early and late multipath signal energy on each side of the centroid point are equal. The centroid tracking circuit shown in

FIG. 5

consists of an A/D converter

501

which samples an input signal x(t), as described above with reference to

FIG. 4

to form half chip offset samples. The half chip offset samples are alternatively grouped as an early set of samples x(nT+τ) and a late set of samples x(nT+(T/2)+τ). The first correlation bank adaptive matched filter

502

multiplies each early sample and each late sample by the positive spreading-code phases c(n+1), c( n+2), . . . , c(n+L), where L is small compared to the code length and is approximately equal to number of chips of delay between the earliest and latest multipath signal. The output signal of each correlator is applied to a respective one of L sum-and-dump circuits of the first sum and dump bank

504

. The magnitude value of the output signal produced by each sum-and-dump circuit of the sum and dump bank

504

is calculated by the respective calculator in the calulator bank

506

and applied to a corresponding weighting amplifier of the first weighting bank

508

. The output signal of each weighting amplifier represents the weighted signal energy in a multipath component signal.

The weighted early multipath signal energy values are summed in sample adder

510

to give an output value that is proportional to the signal energy in the group of multipath signals corresponding to positive code phases which are the early multipath signals. Similarly, a second correlation bank adaptive matched filter

503

operates on the early and late samples, using the negative spreading-code phases c(n−1), c(n−2), . . . , c(n−L), each output signal is provided to a respective sum-and-dump circuit of discrete integrator

505

. The magnitude value of the L sum-and-dump output signals are calculated by the respective calculator of calculator bank

507

and then weighted in weighting bank

509

. The weighted late multipath signal energy values are summed in sample adder

511

to give an energy value for the group of multipath signals corresponding to the negative code phases which are the late multipath signals. Finally, the subtracter

512

calculates the difference of the early and late signal energy values to produce error sample value ε(τ).

The tracking circuit of

FIG. 5

produces error signal ε(τ) which is used to adjust the locally generated code phase c(nT) to keep the weighted average energy in the early and late multipath signal groups equal. The embodiment shown uses weighting, values that increase as the distance from the centroid increases. The signal energy in the earliest and latest multipath signals is probably less than the multipath signal values near the centroid. Consequently, the difference calculated by the subtracter

512

is more sensitive to variations in delay of the earliest and latest multipath signals.

Quadratic Detector for Tracking

In another exemplary tracking method, the tracking circuit adjusts sampling phase to be “optimal” and robust to multipath. If f(t) represent the received signal waveform as in equation (9) above. The particular method of optimizing starts with a delay locked loop with an error signal ε(τ) that drives the loop. The function ε(τ) desirably has only one zero at τ=τ

0

where τ

0

is optimal. The optimal form for ε(τ) has the canonical form:

\begin{matrix} ε (τ) = \int_{- \infty}^{\infty} w (t, τ) {&LeftBracketingBar; f (t) &RightBracketingBar;}^{2} ⅆ t & (20) \end{matrix}

where w(t, τ) is a weighting function relating f(t) to the error ε(τ), and the following holds

\begin{matrix} ε (τ + τ_{0}) = \int_{- \infty}^{\infty} w (t, τ + τ_{0}) {&LeftBracketingBar; f (t) &RightBracketingBar;}^{2} ⅆ t & (21) \end{matrix}

It follows from equation (21) that w(t, τ) is equivalent to w(t−τ). Considering the slope M of the error signal in the neighborhood of a lock point τ

0

:

\begin{matrix} {M = \frac{ⅆ ε (τ)}{ⅆ τ} &RightBracketingBar;}_{τ_{0}} = - \int_{- \infty}^{\infty} w^{'} (t - τ_{0}) g (t) ⅆ t & (22) \end{matrix}

where w′(t, τ) is the derivative of w(t, τ) with respect to τ, and g(t) is the average of |f(t)|

2

.

The error ε(τ) has a deterministic part and a noise part. Let z denote the noise component in ε(τ), then |z|

2

is the average noise power in the error function ε(τ). Consequently, the optimal tracking circuit maximizes the ratio:

\begin{matrix} F = \frac{M^{2}}{{&LeftBracketingBar; z &RightBracketingBar;}^{2}} & (23) \end{matrix}

The implementation of the Quadratic Detector is now described. The discrete error value e of an error signal ε(τ) is generated by performing the operation

e=y

T

By

(24)

where the vector y represents the received signal components yi, i=0, 1, . . . L−1, as shown in

FIG. 5

b.

The matrix B is an L by L matrix and the elements are determined by calculating values such that the ratio F of equation 23 is maximized.

Determining the Minimum Value of L needed:

The value of L in the previous section determines the minimum number of correlators and sum-and-dump elements. L is chosen as small as possible without compromising the functionality of the tracking circuit.

The multipath characteristic of the channel is such that the received chip waveformf(t) is spread over QT

c

seconds, or the multipath components occupy a time period of Q chips duration. The value of L chosen is L=Q. Q is found by measuring the particular RF channel transmission characteristics to determine the earliest and latest multipath component signal propagation delay. QT

c

is the difference between the earliest and latest multipath component arrival time at a receiver.

The Quadratic Detector described above may be used to implement the centroid tracking system described above with reference to

FIG. 5

a.

For this implementation, the vector y is the output signal of the sum and dump circuits

504

: y={f(τ−LT), f(τ−LT+T/2), f(τ−(L−1)T), . . . f(τ), f(τ+T/2), f(τ+T), . . . f(τ+LT)} and the matrix B is set forth in table 6.

TABLE 6

B matrix for quadratic form of Centroid Tracking System

L

0

0

0

0

0

0

0

0

0

0

0

L-1/2

0

0

0

0

0

0

0

0

0

0

0

L-1

0

0

0

0

0

0

0

0

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0

0

0

0

1/2

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

−1/2

0

0

0

0

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0

0

0

0

0

0

0

0

−L + 1

0

0

0

0

0

0

0

0

0

0

0

−L + 1/2

0

0

0

0

0

0

0

0

0

0

0

−L

Adaptive Vector Correlator

An embodiment of the present invention uses an adaptive vector correlator (AVC) to estimate the channel impulse response and to obtain a reference value for coherent combining of received multipath signal components. The described embodiment employs an array of correlators to estimate the complex channel response affecting each multipath component, then the receiver compensates for the channel response and coherently combines the received multipath signal components. This approach is referred to as maximal ratio combining.

Referring to

FIG. 6

, The input signal x(t) to the system is composed of interference noise of other message channels, multipath signals of message channels, thermal noise, and multipath signals of the pilot signal. The signal is provided to AVC

601

which includes a despreading means

602

, channel estimation means for estimating the channel response

604

, correction means for correcting a signal for effects of the channel response

603

, and adder

605

in the present invention. The AVC despreading means

602

is composed of multiple code correlators, with each correlator using a different phase of the pilot code c(t) provided by the pilot code generator

608

. The output of this despreading means corresponds to a noise power level if the phase of the local pilot code of the despreading means is not in phase with the input code signal, or it corresponds to a received pilot signal power level plus noise power level if the input pilot code and locally generated pilot code phases are the same. The output signals of the correlators of the despreading means corrected for the channel response by the correction means

603

and are applied to the adder

605

which collects all multipath pilot signal power. The channel response estimation means

604

receives the combined pilot signal and the output signals of the despreading means

602

, and provides a channel response estimate signal, w(t), to the correction means

603

of the AVC, and the estimate signal w(t) is also available to the adaptive matched filter (AMF) described subsequently. The output signal of the despreading means

602

is also provided to the acquisition decision means

606

which decides, based on a particular algorithm, such as a sequential probability ratio test (SPRT), if the present output levels of the despreading circuits correspond to synchronization of the locally generated code to the desired input code phase. If the detector finds no synchronization, then the acquisition decision means sends a control signal a(t) to the local pilot code generator

608

to offset its phase by one or more chip periods. When synchronization is found, the acquisition decision means informs the tracking circuit

607

, which achieves and maintains a close synchronization between the received and locally generated code sequences.

An exemplary implementation of the Pilot AVC used to despread the pilot spreading-code is shown in FIG.

7

. The described embodiment assumes that the input signal x(t) has been sampled with sampling period T to form x(nT+τ), and is composed of interference noise of other message channels, multipath signals of message channels, thermal noise, and multipath signals of the pilot code. The signal x(nT+τ) is applied to L correlators, where L is the number of code phases over which the uncertainty within the multipath signals exists. Each correlator

701

,

702

,

703

comprises a respective multiplier

704

,

705

,

706

, which multiples the input signal with a particular phase of the Pilot spreading code signal c((n+i)T), and a sum-and-dump circuit

708

,

709

,

710

. The output signal of each multiplier

704

,

705

,

706

is applied to a respective sum-and dump circuit

708

,

709

,

710

to perform discrete integration. Before summing the signal energy contained in the outputs of the correlators, the AVC compensates for the channel response and the carrier phase rotation of the different multipath signals. Each output signal of each sum-and-dump

708

,

709

,

710

is multiplied by a derotation phasor [complex conjugate of ep(nT)] obtained from the digital phase lock loop (DPLL)

721

. This phasor is applied to one input port of a respective multiplier

714

,

715

,

716

to account for the phase and frequency offset of the carrier signal. The Pilot Rake AMF calculates, complex weighting factors, wk, k=1, . . . , L, for each multipath signal by passing the output of each multiplier

714

,

715

,

716

through a low pass filter (LPF)

711

,

712

,

713

. Each despread multipath signal is multiplied by its corresponding weighting factor in a respective multiplier

717

,

718

,

719

. The output signals of the multipliers

717

,

718

,

719

are summed in a master adder

720

, and the output signal p(nT) of the accumulator

720

consists of the combined despread multipath pilot signals in noise. The output signal p(nT) is also applied to the DPLL

721

to produce the error signal ep(nT) for tracking of the carrier phase.

FIGS. 8

a

and

8

b

show alternate embodiments of the AVC which can be used for detection and multipath signal component combining. The message signal AVCs of

FIGS. 8

a

and

8

b

use the weighting factors produced by the Pilot AVC to correct the message data multipath signals. The spreading code signal, c(nT) is the spreading sequence used by a particular message channel and is synchronous with the pilot spreading code signal. The value L is the number of correlators in the AVC circuit

The circuit of

FIG. 8

a

calculates the decision variable Z which is given by

\begin{matrix} Z = w_{1} \sum_{i = 1}^{N} x (iT + τ) c (iT) + w_{2} \sum_{i = 1}^{N} x (iT + τ) c ((i + 1) T) + \dots + w_{L} \sum_{i = 1}^{L} x (iT + τ) + c ((i + L) T) & (25) \end{matrix}

where N is the number of chips in the correlation window. Equivalently, the decision statistic is given by

\begin{matrix} Z = x (T + τ) \sum_{i = 1}^{L} w_{1} c (iT) + x (2 T + τ) \sum_{i = 1}^{L} w_{2} c ((i + 1) T) + \dots + x (NT + τ) \sum_{i = 1}^{L} w_{N} c ((i + N) T) = \sum_{k = 1}^{N} x ({kT}_{—} τ) \sum_{i = 1}^{L} w_{k} c ((i + k - 1) T) & (26) \end{matrix}

The alternative implementation that results from equation (26) is shown in

FIG. 8

b.

Referring to

FIG. 8

a

, the input signal x(t) is sampled to form x(nT+τ), and is composed of interference noise of other message channels, multipath signals of message channels, thermal noise, and multipath signals of the pilot code. The signal x(nT+τ) is applied to L correlators, where L is the number of code phases over which the uncertainty within the multipath signals exists. Each correlator

801

,

802

,

803

comprises a multiplier

804

,

805

,

806

, which multiples the input signal by a particular phase of the message channel spreading code signal, and a respective sum-and-dump circuit

808

,

809

,

810

. The output of each multiplier

804

,

805

,

806

is applied to a respective sum-and dump circuit

808

,

809

,

810

which performs discrete integration. Before summing the signal energy contained in the output signals of the correlators, the AVC compensates for the different multipath signals. Each despread multipath signal and its corresponding weighting factor, which is obtained from the corresponding multipath weighting factor of the pilot AVC, are multiplied by multiplier

817

,

818

,

819

. The output signals of the multipliers

817

,

818

,

819

. The output signals of the multipliers

817

,

818

,

819

are summed in a master adder

820

, and the output signal z(nT) of the accumulator

820

consists of sampled levels of a despread message signal in noise.

The alternative embodiment of the invention includes a new implementation of the AVC despreading circuit for the message channels which performs the sum-and-dump for each multipath signal component simultaneously. The advantage of this circuit is that only one sum-and dump circuit and one adder is necessary. Referring to

FIG. 8

b,

the message code sequence generator

830

provides a message code sequence to shift register

831

of length L. The output signal of each register

832

,

833

,

834

,

835

of the shift register

831

corresponds to the message code sequence shifted in phase by one chip. The output value of each register

832

,

833

,

834

,

835

is multiplied in multipliers

836

,

837

,

838

,

839

with the corresponding weighting factor w

k

, k=1, . . . , L obtained from the Pilot AVC. The output signals of the L multipliers

836

,

837

,

838

,

839

are summed by the adding circuit

840

. The adding circuit output signal and the receiver input signal x(nT+τ) are then multiplied in the multiplier

841

and integrated by the sum-and-dump circuit

842

to produce message signal z(nT).

A third embodiment of the adaptive vector correlator is shown in

FIG. 8

c.

This embodiment uses the least mean square (LMS) statistic to implement the vector correlator and determines the derotation factors for each multipath component from the received multipath signal. The AVC of

FIG. 8

c

is similar to the exemplary implementation of the Pilot AVC used to despread the pilot spreading-code shown in FIG.

7

. The digtal phase locked loop

721

is replaced by a phase locked loop

850

having a voltage controlled oscillator

851

, loop filter

852

, limiter

853

, and imaginary component separator

854

. The difference between the corrected despread output signal dos and an ideal despread output is provided by adder

855

, and the difference signal is a despread error value ide which is further used by the derotation circuits to compensate for errors in the derotation factors.

In a multipath signal environment, the signal energy of a transmitted symbol is spread out over the multipath signal components. The advantage of multipath signal addition is that a substantial portion of signal energy is recovered in an output signal from the AVC. Consequently, a detection circuit has an input signal from the AVC with a higher signal-to-noise ratio (SNR), and so can detect the presence of a symbol with a lower bit-error ratio (BER). In addition, measuring the output of the AVC is a good indication of the transmit power of the transmitter, and a good measure of the system's interference noise.

Adaptive Matched Filter

One embodiment of the current invention includes an Adaptive Matched Filter (AMF) to optimally combine the multipath signal components in a received spread spectrum message signal. The AMF is a tapped delay line which holds shifted values of the sampled message signal and combines these after correcting for the channel response. The correction for the channel response is done using the channel response estimate calculated in the AVC which operates on the Pilot sequence signal. The output signal of the AMF is the combination of the multipath components which are summed to give a maximum value. This combination corrects for the distortion of multipath signal reception. The various message despreading circuits operate on this combined multipath component signal from the AMF.

FIG. 8

d

shows an exemplary embodiment of the AMF. The sampled signal from the A/D converter

870

is applied to th L-stage delay line

872

. Each stage of this delay line

872

holds the signal corresponding to a different multipath signal component. Correction for the channel response is applied to each delayed signal component by multiplying the component in the respective multiplier of multiplier bank

874

with the respective weighting factor w

1

, w

2

, . . . , w

L

from the AVC corresponding to the delayed signal component. All weighted signal components are summed in the adder

876

to give the combined multipath component signal y(t).

The combined multipath component signal y(t) does not include the correction due to phase and frequency offset of the carrier signal. The correction for the phase and frequency offset of the carrier signal is made to y(t) by multiplying y(t) with carrier phase and frequency correction (derotation phasor) in multiplier

878

. The phase and frequency correction is produced by the AVC as described previously.

FIG. 8

d

shows the correction before the despreading circuits

880

, but alternate embodiments of the invention can apply the correction after the despreading circuits.

The Radio Carrier Station (RCS)

The Radio Carrier Station (RCS) of the present invention acts as a central interface between the SU and the remote processing control network element, such as a Radio Distribution Unit (RDU). The interface to the RDU of the exemplary system follows the G.704 standard and an interface according to a modified version of DECT V5.1, but the present invention may support any interface that can exchange call control and traffic channels. The RCS receives information channels from the RDU including call control data, and traffic channel data such as, but not limited to, 32 kb/s ADPCM, 64 kb/s PCM, and ISDN, as well as system configuration and maintenance data. The RCS also terminates the CDMA radio interface bearer channels with SUs, which channels include both control data, and traffic channel data. In response to the call control data from either the RDU or a SU, the RCS allocates traffic channels to bearer channels on the RF communication link and establishes a communication connection between the SU and the telephone network through an RDU.

As shown in

FIG. 9

, the RCS receives call control and message information data into the MUXs

905

,

906

and

907

through interface lines

901

,

902

and

903

. Although E

1

format is shown, other similar telecommunication formats can be supported in the same manner as described below. Each MUX provides a connection to the Wireless Access Controller (WAC)

920

through the PCM highway

910

. While the exemplary system shown in

FIG. 1

uses an E

1

Interface, it is contemplated that other types of telephone lines which convey multiple calls may be used, for example, T1 lines or lines which interface to a Private Branch Exchange (PBX).

The Wireless Access Controller (WAC)

920

is the RCS system controller which manages call control functions and interconnection of data streams between the MUXs

905

,

906

,

907

and the Modem Interface Units (MIUs)

931

,

932

,

933

. The WAC

920

also controls and monitors other RCS elements such as the VDC

940

, RF

950

, and Power Amplifiers

960

.

A low speed bus

912

is connected to the WAC

920

for transferring control and status signals between the RF Transmitter/Receiver

950

, VDC

940

, RF

950

and Power Amplifier

960

. The control signals are sent from the WAC

920

to enable or disable the RF Transmitters/Receiver

950

or Power amplifier

960

, and the status signals are sent from the RF Transmitters/Receiver

950

or Power amplifier

960

to monitor the presence of a fault condition.

The exemplary RCS contains at least one MIU

931

, which is shown in FIG.

10

. The MIU of the exemplary embodiment includes six CDMA modems, but the invention is not limited to this number of modems. The MIU includes: a System PCM Highway

1201

connected to each of the CDMA Modems

1210

,

1211

,

1212

,

1215

through a PCM Interface

1220

; a Control Channel Bus

1221

connected to MIU controller

1230

and each of the CDMA Modems

1210

,

1211

,

1212

,

1213

; an MIU clock signal generator (CLK)

1231

; and a modem output combiner

1232

. The MIU provides the RCS with the following functions: the MIU controller receives CDMA Channel Assignment Instructions from the WAC and assigns a first modem to a user information signal which is applied to the line interface of the MUX and a second modem to receive the CDMA channel from the SU; the MIU also combines the CDMA Transmit Modem Data for each of the MIU CDMA modems; multiplexes I and Q transmit message data from the CDMA modems for transmission to the VDC; receives Analog I and Q receive message data from the VDC; distributes the I and Q data to the CDMA modems; transmits and receives digital AGC Data; distributes the AGC data to the CDMA modems; and sends MIU Board Status and Maintenance Information to the WAC

920

.

The MIU controller

1230

of the exemplary embodiment of the present invention contains one communication microprocessor

1240

, such as the MC68360 “QUICC” Processor, and includes a memory

1242

having a Flash Prom memory

1243

and a SRAM memory

1244

. Flash Prom

1243

is provided to contain the program code for the Microprocessors

1240

, and the memory

1243

is downloadable and reprogrammable to support new program versions. SRAM

1244

is provided to contain the temporary data space needed by the MC68360 Microprocessor

1240

when the MIU controller

1230

reads or writes data to memory

The MIU CLK circuit

1231

provides a timing signal to the MIU controller

1230

, and also provides a timing signal to the CDMA modems. The MIU CLK circuit

1231

receives and is synchronized to the system clock signal wo(t). The controller clock signal generator

1213

also receives and synchronizes to the spreading code clock signal pn(t) which is distributed to the CDMA modems

1210

,

1211

,

1212

,

1215

from the MUX.

The RCS of the present embodiment includes a System Modem

1210

contained on one MIU. The System Modem

1210

includes a Broadcast spreader (not shown) and a Pilot Generator (not shown). The Broadcast Modem provides the broadcast information used by the exemplary system, and the broadcast message data is transferred from the MIU controller

1230

to the System Modem

1210

. The System Modem also includes four additional modems (not shown) which are used to transmit the signals CT

1

through CT

4

and AX

1

through AX

4

. The System Modem

1210

provides unweighted I and Q Broadcast message data signals which are applied to the VDC. The VDC adds the Broadcast message data signal to the MIU CDMA Modem Transmit Data of all CDMA modems

1210

,

1211

,

1212

,

1215

, and the Global Pilot signal.

The Pilot Generator (PG)

1250

provides the Global Pilot signal which is used by the present invention, and the Global Pilot signal is provided to the CDMA modems

1210

,

1211

,

1212

,

1215

by the MIU controller

1230

. Other embodiments of the present invention, however, do not require the MIU controller to generate the Global Pilot signal, but include a Global Pilot signal generated by any form of CDMA Code Sequence generator. In the described embodiment of the invention, the unweighted I and Q Global Pilot signal is also sent to the VDC where it is assigned a weight, and added to the MIU CDMA Modem transmit data and Broadcast message data signal.

System timing in the exemplary RCS is derived from the El interface. There are four MUXs in an RCS, three of which (

905

,

906

and

907

) are shown in FIG.

9

. Two MUXs are located on each chassis. One of the two MUXs on each chassis is designated as the master, and one of the masters is designated as the system master. The MUX which is the system master derives a 2.048 Mhz PCM clock signal from the E

1

interface using a phase locked loop (not shown). In turn, the system master MUX divides the 2.048 Mhz PCM clock signal in frequency by 16 to derive a 128 KHz reference clock signal. The 128 KHz reference clock signal is distributed from the MUX that is the system master to all the other MUXs. In turn, each MUX multiplies the 128 KHz reference clock signal in frequency to synthesize the system clock signal which has a frequency that is twice the frequency of the PN-clock signal. The MUX also divides the 128 KHz clock signal in frequency by 16 to generate the 8 KHz frame synch signal which is distributed to the MIUs. The system clock signal for the exemplary embodiment has a frequency of 11.648 Mhz for a 7 MHz bandwidth CDMA channel. Each MUX also divides the system clock signal in frequency by 2 to obtain the PN-clock signal and further divides the PN-clock signal in frequency by 29 877 120 (the PN sequence length) to generate the PN-synch signal which indicates the epoch boundaries. The PN-synch signal from the system master MUX is also distributed to all MUXs to maintain phase alignment of the internally generated clock signals for each MUX. The PN-synch signal and the frame synch signal are aligned. The two MUXs that are designated as the master MUXs for each chasis then distribute both the system clock signal and the PN-clock signal to the MIUs and the VDC.

The PCM Highway Interface

1220

connects the System PCM Highway

911

to each CDMA Modem

1210

,

1211

,

1212

,

1215

. The WAC controller transmits Modem Control information, including traffic message control signals for each respective user information signal, to the MIU controller

1230

through the HSB

970

. Each CDMA Modem

1210

,

1211

,

1212

,

1215

receives a traffic message control signal, which includes signaling information, from the MIU. Traffic message control signals also include call control (CC) information and spreading code and despreading code sequence information.

The MIU also includes the Transmit Data Combiner

1232

which adds weighted CDMA modem transmit data including In-phase (I) and Quadrature (Q) modem transmit data from the CDMA modems

1210

,

1211

,

1212

,

1215

on the MIU. The I modem transmit data is added separately from the Q modem transmit data. The combined I and Q modem transmit data output signal of the Transmit Data Combiner

1232

is applied to the I and Q multiplexer

1233

that creates a single CDMA transmit message channel composed of the I and Q modem transmit data multiplexed into a digital data stream.

The Receiver Data Input Circuit (RDI)

1234

receives the Analog Differential I and Q Data from the Video Distribution Circuit (VDC)

940

shown in FIG.

9

and distributes Analog Differential I and Q Data to each of the CDMA Modems

1210

,

1211

,

1212

,

1215

of the MIU. The Automatic Gain Control Distribution Circuit (AGC)

1235

receives the AGC Data signal from the VDC and distributes the AGC Data to each of the CDMA Modems of the MIU. The TRL circuit

1233

receives the Traffic lights information and similarly distributes the Traffic light data to each of the Modems

1210

,

1211

,

1212

,

1215

.

The CDMA Modem

The CDMA modem provides for generation of CDMA spreading-code sequences, synchronization between transmitter and receiver. It also provides four full duplex channels (TR

0

, TR

1

, TR

2

, TR

3

) programmable to 64, 32, 16, and 8 ksym/sec. each, spreading and transmission at a specific power level. The CDMA modem measures the received signal strength to allow Automatic Power Control, it generates and transmits pilot signals, encodes and decodes using the signal for forward error correction (FEC). The moden in a subscriber unit (SU) also performs transmitter spreading-code pulse shaping using an FIR filter. The CDMA modem is also used by the SU and, in the following discussion, those features which are used only by the SU are distinctly pointed out. The operating frequencies of the CDMA modem are given in Table 7.

TABLE 7

Operating Frequencies

Bandwidth

Chip Rate

Symbol Rate

Gain

(MHz)

(MHz)

(KHz)

(Chips/Symbol)

7

5.824

64

91

10

8.320

64

130

10.5

8.512

64

133

14

11.648

64

182

15

12.480

64

195

Each CDMA modem

1210

,

1211

,

1212

,

1215

of

FIG. 10

, and as shown in

FIG. 11

, is composed of a transmit section

1301

and a receive section

1302

. Also included in the CDMA modem is a control center

1303

which receives control messages CNTRL from the external system. These messages are used, for example, to assign particular spreading codes, to activate the spreading or despreading, or to assign transmission rates. In addition, the CDMA modem has a code generator means

1304

used to generate the various spreading and despreading codes used by the CDMA modem. The transmit section

1301

transmits the input information and control signals m

i

(t), i=1,2, . . . I as spread-spectrum processed user information signals sc

j

(t), j=1,2, . . . J. The transmit section

1301

receives the global pilot code from the code generator

1304

which is controlled by the control means

1303

. The spread spectrum processed user information signals are ultimately added with other similarly processed signals and transmitted as CDMA channels over the CDMA RF forward message link, for example to the SUs. The receive section

1302

receives CDMA channels as r(t) and despreads and recovers the user information and control signals rc

k

(t), k=1,2,. . . K transmitted over the CDMA RF reverse message link, for example to the RCS from the SUs.

CDMA Modem Transmitter Section

Referring to

FIG. 12

, the code generator means

1304

includes Transmit Timing Control Logic

1401

and spreading-code PN-Generator

1402

, and the Transmit Section

1301

includes MODEM Input Signal Receiver (MISR)

1410

, Convolution Encoders

1411

,

1412

,

1413

,

1414

, Spreaders

1420

,

1421

,

1422

,

1423

,

1424

, and Combiner

1430

. The Transmit Section

1301

receives the message data channels MESSAGE, convolutionally encodes each message data channel in the respective convolutional encoder

1411

,

1412

,

1413

,

1414

, modulates the data with random spreading-code sequence in the respective spreader

1420

,

1421

,

1422

,

1423

,

1424

, and combines modulated data from all channels, including the pilot code received in the described embodiment from the code generator, in the combiner

1430

to generate I and Q components for RF transmission. The Transmitter Section

1301

of the present embodiment supports four (TR

0

, TR

1

, TR

2

, TR

3

) 64, 32, 16, 8 Kbps programmable channels. The message channel data is a time multiplexed signal received from the PCM highway

1201

through PCM interface

1220

and input to the MISR

1410

.

FIG. 13

illustrates the block diagram of the MISR

1410

. For the exemplary embodiment of the present invention, a counter is set by the 8 KHz frame synchronization signal MPCMSYNC and is incremented by 2.048 MHz MPCMCLK from the timing circuit

1401

. The counter output is compared by comparator

1502

against TRCFG values corresponding to slot time location for TR

0

, TR

1

, TR

2

, TR

3

message channel data; and the TRCFG values are received from the MIU Controller

1230

in MCTRL. The comparator sends a count signal to the registers

1505

,

1506

,

1507

,

1508

which clocks message channel data into buffers

1510

,

1511

,

1512

,

1513

using the TXPCNCLK timing signal derived from the system clock. The message data is provided from the signal MSGDAT from the PCM highway signal MESSAGE when enable signals TR

0

EN, TR

1

EN, TR

2

EN and TR

3

EN from Timing Control Logic

1401

are active. In further embodiments, MESSAGE may also include signals that enable registers depending upon an encryption rate or data rate. If the counter output is equal to one of the channel location addresses, the specified transmit message data in registers

1510

,

1511

,

1512

,

1513

are input to the convolutional encoders

1411

,

1412

,

1413

,

1414

shown in FIG.

12

.

The convolutional encoder enables the use of Forward error correction (FEC) techniques, which are well known in the art. FEC techniques depend on introducing redundancy in generation of data in encoded form. Encoded data is transmitted and the redundancy in the data enables the receiver decoder device to detect and correct errors. One exemplary system which uses a modem according to the present invention employs convolutional encoding. Additional data bits are added to the data in the encoding process and are the coding overhead. The coding overhead is expressed as the ratio of data bits transmitted to the tool bits (code data+redundant data) transmitted and is called the rate “R” of the code

Convolution codes are codes where each code bit is generated by the convolution of each new uncoded bit with a number of previous coded bits. The total number of bits used in the encoding process is referred to as the constraint length, “K”, of the code. In convolutional coding, data is clocked into a shift register of K bits length so that an incoming bit is clocked into the register, and it and the existing K−1 bits are convolutionally encoded to create a new symbol. The convolution process consists of creating a symbol consisting of a module-2 sum of a certain pattern of available bits, always including the first bit and the last bit in at least one of the symbols.

FIG. 14

shows the block diagram of K=7, R=½ convolution encoder suitable for use as the encoder

1411

shown in FIG.

12

. This circuit encodes the TR

0

Channel as used in one embodiment of the present invention. Seven-bit Register

1601

with stages Q

1

through Q

7

uses the signal TXPNCLK to clock in TR

0

data when the TR

0

EN signal is asserted. The output value of stages Q

1

, Q

2

, Q

3

, Q

4

, Q

6

, and Q

7

are each combined using EXCLUSIVE-OR Logic

1602

,

1603

to produce respective I and Q channel FEC data for the TR

0

channel FECTR

0

DI and FECTR

0

DQ.

Two output symbol streams FECTR

0

DI and FECTR

0

DQ are generated. The FECTR

0

DI symbol stream is generated by EXCLUSIVE OR Logic

1602

of shift register outputs corresponding to bits

6

,

5

,

4

,

3

, and

0

, (Octal

171

) and is designed as In phase component “I” of the transmit message channel data. The symbol stream FECTR

0

DQ is likewise generated by EXCLUSIVE-OR logic

1603

of shift register outputs from bits

6

,

4

3

,

1

and

0

, (Octal

133

) and is designated as Quadrature component “Q” of the transmit message channel data. Two symbols are transmitted to represent a single encoded bit creating the redundancy necessary to enable error correction to take place on the receiving end.

Referring to

FIG. 14

, the shift enable clock signal for the transmit message channel data is generated by the Control Timing Logic

1401

. The convolutionally encoded transmit message channel output data for each channel is applied to the respective spreader

1420

,

1421

,

1422

,

1423

,

1424

which multiplies the transmit message channel data by its preassigned spreading-code sequence from code generator

1402

. This spreading-code sequence is generated by control

1303

as previously described, and is called a random pseudonoise signature sequence (PN-code).

The output signal of each spreader

1420

,

1421

,

1422

,

1423

,

1424

is a spread transmit data channel. The operation of the spreader is as follows: the spreading of channel output (I+jQ) multiplied by a random sequence (PNI+jPNQ) yields the In-phase component I of the result being composed of (I xor PNI) and (-Q xor PNQ). Quadrature component Q of the result is (Q xor PNI) and (I xor PNQ). Since there is no channel data input to the pilot channel logic (I=1, Q values are prohibited), the spread output signal for pilot channels yields the respective sequences PNI for I component and PNQ for Q component.

The combiner

1430

receives the I and Q spread transmit data channels and combines the channels into an I modem transmit data (TXIDAT) and Q modem transmit data (TXQDAT) signals. The I-spread transmit data and the Q spread transmit data are added separately.

For an SU, the CDMA modem Transmit Section

1301

includes the FIR filters to receive the I and Q channels from the combiner to provide pulse shaping, close-in spectral control and x/sin (x) correction on the transmitted signal. Separate but identical FIR filters (not shown) receive the I and Q spread transmit data streams at the chipping rate, and the output signal of each of the filters is at twice the chipping rate. The FIR filters are 28 tap even symmetrical filters, which upsample (interpolate) by 2. The upsampling occurs before the filtering, so that 28 taps refers to 28 taps at twice the chipping rate, and the upsampling is accomplished by setting every other sample a zero. Exemplary coefficients are shown in Table 8.

TABLE 8

Coefficient Values

Coeff. No.:

0

1

2

3

4

5

6

7

8

9

10

11

12

13

Value:

3

−11

−34

−22

19

17

32

19

52

24

94

−31

277

468

Coeff. No.

14

15

16

17

18

19

20

21

22

24

25

26

27

Value

277

−31

−94

24

52

−19

−32

17

19

−22

−34

−11

3

CDMA Modem Receiver Section

Referring to

FIGS. 9 and 10

, the RF receiver

950

of the present embodiment accepts analog input I and Q CDMA channels, which are transmitted to the CDMA modems

1210

,

1211

,

1212

,

1215

through the MIUs

931

,

932

,

933

from the VDC

940

. These I and Q CMDA channel signals are sampled by the CDMA modem receive section

1302

(shown in

FIG. 11

) and converted to I and Q digital receive message signal using an Analog to Digital (A/D) converter

1730

of FIG.

15

. The sampling rate of the A/D converter of the exemplary embodiment of the present invention is equivalent to the despreading code rate. The I and Q digital receive message signals are then despread with correlators using six different complex despreading code sequences corresponding to the spreading code sequences of the four channels (TR

0

, TR

1

, TR

2

, TR

3

), APC information and the pilot code.

Time synchronization of the receiver to the received signal is separated into two phases; there is an initial acquisition phase and then a tracking phase after the signal timing has been acquired. The initial acquisition is done by sliding the locally generated pilot code sequence relative to the received signal and comparing the output signal of the pilot despreader to a threshold. The method used is called sequential search. Two thresholds (match and dismiss) are calculated from the auxiliary despreader. Once the signal is acquired, the search process is stopped and tracking begins. The tracking maintains the code generator

1304

(shown in

FIGS. 11 and 15

) used by the receiver in synchronization with the incoming signal. The tracking loop used is the Delay-Locked Loop (DLL) and is implemented in the acquisition & track

1701

and the IPM

1702

blocks of FIG.

15

.

In

FIG. 11

, the modem controller

1303

implements the Phase Lock Loop (PLL) as a software algorithm in SW PLL logic

1724

of

FIG. 15

that calculates the phase and frequency shift in the received signal relative to the transmitted signal. The calculated phase shifts are used to derotate the phase shifts in rotate and combine blocks

1718

,

1719

,

1720

,

1721

of the multipath data signals for combining to produce output signals corresponding to receive channels TR

0

′, TR

1

′, TR

2

′, TR

3

′. The data is then Viterbi decoded in Viterbi Decoders

1713

,

1714

,

1715

,

1716

to remove the convolutional encoding in each of the received message channels.

FIG. 15

indicates that the Code Generator

1304

provides the code sequences Pn

i

(t), I=1,2, . . . I used by the receive channel despreaders

1703

,

1704

,

1705

,

1706

,

1707

,

1708

,

1709

. The code sequences generated are timed in response to the SYNK signal of the system clock signal and are determined by the CCNTRL signal from the modem controller

1303

shown in FIG.

11

. Referring to

FIG. 15

, the CDMA modem receiver section

1302

includes Adaptive Matched Filter (AMF)

1710

, Channel despreaders

1703

,

1704

,

1705

,

1706

,

1707

,

1708

,

1709

, Pilot AVC

1711

, Auxiliary AVC

1712

, Viterbi decoders

1713

,

1714

,

1715

,

1716

, Modem output interface (MOI)

1717

, Rotate and Combine logic

1718

,

1719

,

1720

,

1721

, AMF Weight Generator

1722

, and Quantile Estimation logic

1723

.

In another embodiment of the invention, the CDMA modem receiver may also include a Bit error Integrator to measure the BER of the channel and idle code insertion logic between the Viterbi decoders

1713

,

1714

,

1715

,

1716

and the MOI

1717

to insert idle codes in the event of loss of the message data.

The Adaptive Matched Filter (AMF)

1710

resolves multipath interference introduced by the air channel The exemplary AMF

1717

uses an stage complex FIR filter as shown in FIG.

16

. The received I and Q digital message signals are received at the register

1820

from the A/D converter

1730

of FIG.

15

and are multiplied in multipliers

1801

,

1802

,

1803

,

1810

,

1811

by I and Q channel weights W

1

to W

11

received from AMF weight generator

1722

of FIG.

15

. In the exemplary embodiment, the A/D converter

1730

provides the I and Q digital receive message signal data as 2's complement 6 bits I and 6 bits Q which are clocked through an 11 stage shift register

1820

responsive to the receive spreading-code clock signal RXPNCLK. The signal RXPNCLK is generated by the timing section

1401

of code generation logic

1304

. Each stage of the shift register is tapped and complex multiplied in the multipliers

1801

,

1802

,

1803

,

1810

,

1811

by individual (6-bit I and 6-bit Q) weights to provide 11 tap-weighted products which are added in adder

1830

, and limited to 7-bit I and 7-bit Q values.

The CDMA modem receive section

1302

(shown in

FIG. 11

) provides independent channel despreaders

1703

,

1704

,

1705

,

1706

,

1707

,

1708

,

1709

(shown in

FIG. 15

) for despreading the message channels. The described embodiment despreads

7

message channels, each despreader accepting a 1-bit I b 1-bit Q spreading-code signal to perform a complex correlation of this code against a 8-bit I by 8-bit Q data input. The 7 despreaders correspond to the 7 channels: Traffic Channel

0

(TR

0

′), TR

1

′, TR

2

′, TR

3

′, AUX (a spare channel), Automatic Power Control (APC) and pilot (PLT).

The Pilot AVC

1711

shown in

FIG. 17

receives the I and Q Pilot Spreading-code sequence values PCI and PCQ into shift register

1920

responsive to the timing signal RXPNCLK, and includes

11

individual despreaders

1901

through

1911

each correlating the I and Q digital receive message signal data with a one chip delayed versions of the same pilot code sequence. Signals OE

1

, OE

2

, . . . OE

11

are used by the modem control

1303

to enable the despreading operation. The output signals of the despreaders are combined in combiner

1920

forming correlation signal DSPRDAT of the Pilot AVC

1711

, which is received by the ACQ & Track logic

1701

(shown in FIG.

15

), and ultimately by modem controller

1303

(shown in FIG.

11

). The ACQ & Track logic

1701

uses the correlation signal value to determine if the local receiver is synchronized with its remote transmitter.

The Auxiliary AVC

1712

also receives the I and Q digital receive message signal data and, in the described embodiment, includes four separate despreaders

2001

,

2002

,

2003

,

2004

as shown in FIG.

18

. Each despreader receives and correlates the I and Q digital receive message data with delayed versions of the same despreading-code sequence PARI and PARQ which are provided by code generator

1304

input to and contained in shift register

2020

. The output signals of the despreaders

2001

,

2002

,

2003

,

2004

are combined in combiner

2030

which provides noise correlation signal ARDSPRDAT. The auxiliary AVC despreading code sequence does not correspond to any transmit spreading-code sequence of the system. Signals OE

1

, OE

2

, . . . OE

4

are used by the modem control

1303

to enable the despreading operation. The Auxiliary AVC

1712

provides a noise correlation signal ARDSPRDAT from which quantile estimates are calculated by the Quantile estimator

1733

, and provides a noise level measurement to the ACQ & Track logic

1701

(shown in

FIG. 15

) and modem controller

1303

(shown in FIG.

11

).

Each despread channel output signal corresponding to the received message channels TR

0

′, TR

1

′, TR

2

′, and TR

3

′ is input to a corresponding Viterbi decoder

1713

,

1714

,

1715

,

1716

shown in

FIG. 15

which performs forward error correction on convolutionally encoded data. The Viterbi decoders of the exemplary embodiment have a constraint length of K=7 and a rate of R=½. The decoded despread message channel signals are transferred from the CDMA modem to the PCM Highway

1201

through the MOI

1717

. The operation of the MOI is very similar to the operation of the MISR of the transmit section

1301

(shown in FIG.

11

), except in reverse.

The CDMA modem receiver section

1302

implements several different algorithms during different phases of the acquisition, tracking and despreading of the receive CDMA message signal.

When the received signal is momentarily lost (or severely degraded) the idle code insertion algorithm inserts idle codes in place of the lost or degraded receive message data to prevent the user from hearing loud noise bursts on a voice call. The idle codes are sent to the MOI

1717

(shown in

FIG. 15

) in place of the decoded message channel output signal from the Viterbi decoders

1713

,

1714

,

1715

,

1716

. The idle code used for each traffic channel is programmed by the Modem Controller

1303

by writing the appropriate pattern IDLE to the MOI, which in the present embodiment is a 8 bit word for a 64 kbps stream, 4 bit word for a 32 kbps stream.

Modem Algorithms for Acquisition and Tracking of Received Pilot Signal

The acquisition and tracking algorithms are used by the receiver to determine the approximate code phase of a received signal, synchronize the local modem receiver despreaders to the incoming pilot signal, and track the phase of the locally generated pilot code sequence with the received pilot code sequence. Referring to

FIGS. 11 and 15

, the algorithms are performed by the Modem controller

1303

, which provides clock adjust signals to code generator

1304

. These adjust signals cause the code generator for the despreaders to adjust locally generated code sequences in response to measured output values of the Pilot Rake

1711

and Quantile values from quantile estimators

1723

B. Quantile values are noise statistics measured from the In-phase and Quadrature channels from the output values of the AUX Vector Correlator

1712

(shown in FIG.

15

). Synchronization of the receiver to the received signal is separated into two phases; an initial acquisition phase and a tracking phase. The initial acquisition phase is accomplished by clocking the locally generated pilot spreading-code sequence at a higher or lower rate than the received signal's spreading code rate, sliding the locally generated pilot spreading code sequence and performing sequential probability ratio test (SPRT) on the output of the Pilot Vector correlator

1711

. The tracking phase maintains the locally generated spreading-code pilot sequence in synchronization with the incoming pilot signal.

The SU cold acquisition algorithm is used by the SU CDMA modem when it is first powered up, and therefore has no knowledge of the correct pilot spreading code phase, or when an SU attempts to reacquire synchronization with the incoming pilot signal but has taken an excessive amount of time. The cold acquisition algorithm is divided into two sub-phases. The first subphase consists of a search over the length 233415 code used by the FBCCH. Once this sub-code phase is acquired, the pilot's 233415×128 length code is known to within an ambiguity of 128 possible phases. The second subphase is a search of these remaining 128 possible phases. In order not to lose synch with the FBCCH, the second phase of the search it is desirable to switch back and forth between tracking the FBCCH code and attempting acquisition of the pilot code.

The RCS acquisition of short access pilot (SAXPT) algorithm is used by an RCS CDMA modem to acquire the SAXPT pilot signal of an SU. The algorithm is a fast search algorithm because the SAXPT is a short code sequence of length N, where N=chips/symbol, and ranges from 45 to 195, depending on the system's bandwidth. The search cycles through all possible phases until acquisition is complete.

The RCS acquisition of the long access pilot (LAXPT) algorithm begins immediately after acquisition of SAXPT. The SU's code phase is known within a multiple of a symbol duration, so in the exemplary embodiment of the invention, there may be 7 to 66 phases to search within the round trip delay from the RCS. This bound is a result of the SU pilot signal being synchronized to the RCS Global pilot signal.

The re-acquisition algorithm begins when loss of code lock (LOL) occurs. A Z-search algorithm is used to speed the process on the assumption that the code phase has not drifted far from where it was the last time the system was locked. The RCS uses a maximum width of the Z-search windows bounded by the maximum round trip propagation delay.

The Pre-Track algorithm immediately follows the acquisition or re-acquisition algorithms and immediately precedes the tracking algorithm. Pre-track is a fixed duration period during which the receive data provided by the modem is not considered valid. The Pre-Track period allows other modem algorithms, such as those used by the ISW PLL

1724

, ACQ & Tracking, AMF Weight GEN

1722

, to prepare and adapt to the current channel. The Pre-track algorithm is two parts. The first part is the delay while the code tracking loop pulls in. The second part is the delay while the AMF tap weight calculations are performed by the AMF Weight Gen

1722

to produce settled weighting coefficients. Also in the second part of the Pre-Track period, the carrier tracking loop is allowed to pull in by the SW PLL

1724

, and the scalar quantile estimates are performed in the Quantile estimator

1723

A.

The Tracking process is entered after the Pre-Track period ends. This process is actually a repetitive cycle and is the only process phase during which receive data provided by the modem may be considered valid. The following operations are performed during this phase: AMF Tap Weight Update, Carrier Tracking, Code Tracking, Vector Quantile Update, Scalar Quantile Update, Code Lock Check, Derotation and Symbol Summing, and Power Control (forward and reverse).

If loss of lock (LOL) is detected, the modem receiver terminates the Track algorithm and automatically enters the reaquisition algorithm. In the SU, a LOL causes the transmitter to be shut down. In the RCS, LOL causes forward power control to be disabled with the transmit power held constant at the level immediately prior to loss of lock. It also causes the return power control information being transmitted to assume a 010101 . . . pattern, causing the SU to hold its transmit power constant. This can be performed using the signal lock check function which generates the reset signal to the acquisition and tracking circuit

1701

.

Two sets of quantile statistics are maintained, one by Quantile estimator

1723

B and the other by the scalar Quantile Estimator

1723

A. Both are used by the modem controller

1303

. The first set is the “vector” quantile information, so named because it is calculated from the vector of four complex values generated by the AUX AVC receiver

1712

. The second set is the scalar quantile information, which is calculated from the single complex value AUX signal that is output from the AUX Despreader

1707

. The two sets of information represent different sets of noise statistics used to maintain a pre-determined Probability of False Alarm (P

fa

). The vector quantile data is used by the acquisition and reaquisition algorithms implemented by the modem controller

1303

to determine the presence of a received signal in noise, and the scalar quantile information is used by the code lock check algorithm.

For both the vector and scalar cases, quantile information consists of calculated values of lambda0 through lambda2, which are boundary values used to estimate the probability distribution function (p.d.f.) of the despread received signal and determine whether the modem is locked to the PN code. The Aux_Power value used in the following C-subroutine is the magnitude squared of the AUX signal output of the scalar correlator array for the scalar quantiles, and the sum of the magnitudes squared for the vector case. In both cases the quantiles are then calculated using the following C-subroutine:

for (n=0; n<3; n++) {

lambda [n]+=(lambda [n]<Aux_Power)? CG[n]:GM[n];

}

where CG[n] are positive constants and GM[n] are negative constants (different values are used for scalar and vector quantiles).

During the acquisition phase, the search of the incoming pilot signal with the locally generated pilot code sequence employs a series of sequential tests to determine if the locally generated pilot code has the correct code phase relative to the received signal. The search algorithms use the Sequential Probability Ratio Test (SPRT) to determine whether the received and locally generated code sequences are in phase. The speed of acquisition is increased by the parallelism resulting from having a multi-fingered receiver. For example, in the described embodiment of the invention the main Pilot Rake

1711

has a total of 11 fingers representing a total phase period of 11 chip periods. For acquisition 8 separate sequential probability ratio tests (SPRTs) are implemented, with each SPRT observing a 4 chip window. Each window is offset from the previous window by one chip period, and in a search sequence any given code phase is covered by 4 windows. If all 8 of the SPRT tests are rejected, then the set of windows is moved by 8 chips. If any of the SPRT's is accepted, then the code phase of the locally generated pilot code sequence is adjusted to attempt to center the accepted SPRT's phase within the Pilot AVC. It is likely that more than one SPRT reaches the acceptance threshold at the same time. A table lookup is used cover all 256 possible combinations of accept/reject and the modem controller uses the information to estimate the correct center code phase within the Pilot Rake

1711

. Each SPRT is implemented as follows (all operations occur at 64 k symbol rate): Denote the fingers' output level values as I_Finger[n] and Q_Finger[n], where n=0 . . . 10 (inclusive, 0 is earliest (most advanced) finger), then the power of each window is:

Power Window [i] = \sum_{n} (I_{—} {Finger}^{2} [n] + Q_{—} {Finger}^{2} [n]

To implement the SPRT's the modem controller then performs for each of the windows the following calculations which are expressed as a pseudo-code subroutine:

/* find bin for Power */

tmp=SIGMA[0];

for (k=0; k<3; k++) {

if (Power>lambda [k]) tmp=SIGMA[k+1];

}

test_statistic+=tmp; /* update statistic */

if(test_statistic>ACCEPTANCE_THRESHOLD)you've got ACQ;

else if (test_statistic<DISMISSAL_THRESHOLD) {

forget this code phase;

} else keep trying—get more statistics;

where lambda[k] are as defined in the above section on quantile estimation, and SIGMA[k], ACCEPTANCE_THRESHOLD and DISMISSAL_THRESHOLD are predetermined constants. Note that SIGMA[k] is negative for values for low values of k, and positive for right values of k, such that the acceptance and dismissal thresholds can be constants rather than a function of how many symbols worth of data have been accumulated in the statistic.

The modem controller determines which bin, delimited by the values of lambda[k], the Power level falls into which allows the modem controller to develop an approximate statistic.

For the present algorithm, the control voltage is formed as ε=y

T

By, where y is a vector formed from the complex valued output values of the Pilot Vector correlator

1711

, and B is a matrix consisting of the constant values pre-determined to maximixe the operating characteristics while minimizing the noise as described above with reference to the Quadratic Detector.

To understand the operation of the Quadratic Detector, it is useful to consider the following. A spread spectrum (CDMA) signal, s(t) is passed through a multipath channel with an impulse response h

c

(t). The baseband spread signal is described by equation (27).

\begin{matrix} s (t) = \sum_{i} C_{i} p (t - {iT}_{c}) & (27) \end{matrix}

where C

i

is a complex spreading code symbol, p(t) is a predefined chip pulse and T

c

is the chip time spacing, where T

c

=1/R

c

and R

c

is the chip rate.

The received baseband signal is represented by equation (28)

\begin{matrix} r (t) = \sum_{i} C_{i} q (t - {iT}_{c} - τ) + n (t) & (28) \end{matrix}

where q(t)=p(t)*h

c

(t), τ is an unknown delay and n(t) is additive noise. The received signal is processed by a filter, h

R

(t), so the waveform, x(t), to be processed is given by equation (29).

\begin{matrix} x (t) = \sum_{i} C_{i} f (t - {iT}_{c} - τ) + z (t) & (29) \end{matrix}

where f(t)=q(t)*h

R

(t) and z(t)=n(t)*h

R

(t).

In the exemplary receiver, samples of the received signal are taken at the chip rate, that is to say, 1/T

c

. These samples, x(mT

c

+τ′), are processed by an array of correlators that compute, during the r

th

correlation period, the quantities given by equation (30)

\begin{matrix} v_{k}^{(r)} = \sum_{m = rL}^{rL - L - 1} x ({mT}_{c} + τ^{'}) C_{m - k}^{*} & (30) \end{matrix}

These quantities are composed of a noise component w

k

(r)

and a deterministic component y

k

(r)

given by equation (31).

y

k

(r)

=E[v

k

(r)

]=Lf

(

kT

c

÷τ′−τ) (31)

In the sequel, the time index r may be suppressed for ease of writing, although it is to be noted that the function f(t) changes slowly with time.

The samples are processed to adjust the sampling phase, τ′, in an optimum fashion for further processing by the receiver, such as matched filtering. This adjustment is described below. To simplify the representation of the process, it is helpful to describe it in terms of the function f(t+τ), where the time-shift, τ, is to be adjusted. It is noted that the function f(t+τ) is measured in the presence of noise. Thus, it may be problematical to adjust the phase τ′based on measurements of the signal f(t+τ). To account for the noise, the function v(t): v(t)=f(t)+m(t) is introduced, where the term m(t) represents a noise process. The system processor may be derived based on considerations of the function v(t).

The process is non-coherent and therefore is based on the envelope power function |v(t+τ)|

2

. The functional e(τ′) given in equation (32) is helpful for describing the process.

\begin{matrix} e (τ^{'}) = \int_{- \infty}^{0} {&LeftBracketingBar; v (t + τ^{'} - τ) &RightBracketingBar;}^{2} ⅆ t - \int_{- 0}^{\infty} {&LeftBracketingBar; v (t + τ^{'} - τ) &RightBracketingBar;}^{2} ⅆ t & (32) \end{matrix}

The shift parameter is adjusted for e(τ′)=1, which occurs when the energy on the interval (−∞,τ′−τ] equals that on the interval [τ′−τ, ∞). The error characteristic is monotonic and therefore has a single zero crossing point. This is the desirable quality of the functional. A disadvantage of the functional is that it is ill-defined because the integrals are unbounded when noise is present. Nevertheless, the functional e(τ′) may be cast in the form given by equation (33).

\begin{matrix} e (τ^{'}) = \int_{- \infty}^{\infty} w (t) {&LeftBracketingBar; v (t + τ^{'} - τ) &RightBracketingBar;}^{2} ⅆ t & (33) \end{matrix}

where the characteristic function w(t) is equal to sgn(t), the signum function.

To optimize the characteristic function w(t), it is helpful to define a figure of merit, F, as set forth in equation (34).

\begin{matrix} F = \frac{{[\overline{e (τ_{0}^{'} + T_{A}) - e (τ_{0}^{'} - T_{A})}]}^{2}}{VAR {e (τ_{0}^{'})}} & (34) \end{matrix}

The numerator of F is the numerical slope of the mean error characteristic on the interval [−T

A

,T

A

] surrounding the tracked value, τ

0

′. The statistical mean is taken with respect to the noise as well as the random channel, h

c

(t). It is desirable to specify a statistical characteristic of the channel in order to perform this statistical average. For example, the channel may be modeled as a Wide Sense Stationary Uncorrelated Scattering (WSSUS) channel with impulse response h

c

(t) and a white noise process U(t) that has an intensity function g(t) as shown in equation (35).

h

c

(

t

)={square root over (

g

+L (

t

+L ))}

U

(

t

) (35)

The variance of e(τ) is computed as the mean square value of the fluctuation

e

′(τ)=

e

(τ)−<

e

(τ)> (36)

where <e(τ)> is the average of e(τ) with respect to the noise.

Optimization of the figure of merit F with respect to the function w(t) may be carried out using well-known Variational methods of optimization.

Once the optimal w(t) is determined, the resulting processor may be approximated accurately by a quadratic sample processor which is derived as follows.

By the sampling theorem, the signal v(t), bandlimited to a bandwidth W may be expressed in terms of its samples as shown in equation (37).

v

(

t

)=Σ

v

(

k/W

)sinc[(

Wt−k

)π] (37)

substituting this expansion into equation (z+6) results in an infinite quadratic form in the samples v(k/W+τ′−τ). Making the assumption that the signal bandwidth equals the chip rate allows the use of a sampling scheme that is clocked by the chip clock signal to be used to obtain the samples. These samples, v

k

are represented by equation (38).

v

k

=v

(

kT

c

+τ′−τ) (38)

This assumption leads to a simplification of the implementation. It is valid if the aliasing error is small.

In practice, the quadratic form that is derived is truncated. An example normalized B matrix is given below in Table 12. For this example, an exponential delay spread profile g(t)=exp(−t/τ) is assumed with τ equal to one chip. An aperture parameter T

A

equal to one and one-half chips has also been assumed. The underlying chip pulse has a raised cosine spectrum with a 20% excess bandwidth.

TABLE 12

Example B matrix

0

0

0

0

0

0

0

0

0

0

0

0

0

−0.1

0

0

0

0

0

0

0

0

0

−0.1

0.22

0.19

−0.19

0

0

0

0

0

0

0

0

0.19

1

0.45

−0.2

0

0

0

0

0

0

0

−0.19

0.45

0.99

0.23

0

0

0

0

0

0

0

0

−0.2

0.23

0

−0.18

0.17

0

0

0

0

0

0

0

0

−0.18

−0.87

−0.42

0.18

0

0

0

0

0

0

0

0.17

−0.42

−0.92

−0.16

0

0

0

0

0

0

0

0

0.18

−0.16

−0.31

0

0

0

0

0

0

0

0

0

0

0

−0.13

0

0

0

0

0

0

0

0

0

0

0

0

Code tracking is implemented via a loop phase detector that is implemented as follows. The vector y is defined as a column vector which represents the 11 complex output level values of the Pilot AVC

1711

, and B denotes an 11×11 symmetric real valued coefficient matrix with pre-determined values to optimize performance with the non-coherent Pilot AVC output values y. As described above, the phase detector output is given by equation (39):

ε=

y

T

By

(39)

The following calculations are then performed to implement a proportional plus integral loop filter and the VCO:

x[n]=x[n−

1]+βε,

z[n]=z[n−

1

]+x[n]+αε

for β and α which are constants chosen from modeling the system to optimize system performance for the particular transmission channel and application, and where x[n] is the loop filter's integrator output value and z[n] is the VCO output value. The code phase adjustments are made by the modem controller the following pseudo-code subroutine:

if (z>zmx) {

delay phase {fraction (1/16)} chip;

z−=zmax;

} else if (z<−zmax){

advance phase {fraction (1/16)} chip;

z+=zmax;

}

A different delay phase could be used in the above pseudo-code subroutine consistant with the present invention.

The AMF Tap-Weight Update Algorithm of the AMF Weight Gen

1722

(shown in

FIG. 15

) occurs periodically to de-rotate and scale the phase each finger value of the Pilot Rake

1711

by performing a complex multiplication of the Pilot AVC finger value with the complex conjugate of the current output value of the carrier tracking loop and applying the product to a low pass filter to produce AMF tap-weight values, which are periodically written into the AMF filters of the CDMA modem.

The Code lock check algorithm, shown in

FIG. 15

) is implemented by the modem controller

1303

performing SPRT operations on the output signal of the scalar correlator array. The SPRT technique is the same as that for the acquisition algorithms, except that the constants are changed to increase the probability of detection of lock.

Carrier tracking is accomplished via a second order loop that operates on the pilot output values of the scalar correlated array. The phase detector output is the hard limited version of the quadrature component of the product of the (complex valued) pilot output signal of the scalar correlated array and the VCO output signal. The loop filter is a proportional plus integral design. The VCO is a pure summation, accumulated phase error φ, which is converted to the complex phasor cos φ+j sin φ using a look-up table in memory.

The previous description of acquisition and tracking algorithm focuses on a non-coherent method because the acquisition and tracking algorithm described uses non-coherent acquisition following by non-coherent tracking. This is done because, during acquisition, a coherent reference is not available until the AMF, Pilot AVC, Aux AVC, and DPLL are in an equilibrium state. It is, however, known in the art that coherent tracking and combining is preferred because in non-coherent tracking and combining the output phase information of each Pilot AVC finger is lost. Consequently, another embodiment of the invention employs a two step acquisition and tracking system, in which the previously described non-coherent acquisition and tracking algorithm is implemented first, and then the system switches to a coherent tracking method. The coherent combining and tracking method is similar to that described previously, except that the error signal tracked is of the form:

ε=

y

T

Ay

(40)

where y is defined as a column vector which represents the 11 complex output level values of the Pilot AVC

1711

, and A denotes an 11×11 symmetric real valued coefficient matrix with pre-determined values to optimize performance with the coherent Pilot AVC outputs y. An exemplary A matrix is shown below.

\begin{matrix} A = \begin{matrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & - 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & - 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & - 1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & - 1 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & 0 & - 1 \end{matrix} & (41) \end{matrix}

Although the invention has been described in terms of multiple exemplary embodiments, it is understood by those skilled in the art that the invention may be practiced with modifications to the embodiments which are within the scope of the invention defined by the following claims.

Claims

1. A Code Division Multiple Access (CDMA) modem that transmits and receives telecommunication signals over a transmit and a receive channel having respective first and second pilot code signals, the second pilot code signal being synchronous with the first pilot code signal and the telecommunication signals including information signals, the modem comprisinga modem transmitter having: a) code generation means comprising a first pilot code means for providing the first pilot code signal selected from a plurality of distinct pilot code signals and a message means for generating a plurality of distinct transmit message code signals, related to and synchronous with the first pilot code signal; b) spreading means coupled to the message means for combining each of the information signals, with a respective one of the plurality of transmit message code signals to generate a plurality of spread-spectrum processed message signals comprising a transmit Code Division Multiplex (CDM) signal for transmission over the transmit channel, and the first pilot signal wherein each of the plurality of transmit message code signals is synchronous with the pilot code signal; and a modem receiver having: a) local code generation means comprising a local second pilot code means for providing a local version of the second pilot code signal related to the first pilot code signal and a local message code means for generating a plurality of local receive message code signals related to the second pilot code signal, the second pilot code means being synchronous with the local message code means; b) a pilot code acquisition means for acquiring the second pilot code signal comprising a pilot vector correlator for estimating a channel impulse response associated with the second pilot code signal wherein the channel impulse response estimate is determined by estimating a complex channel response affecting each multipath component of the second pilot code signal, and for correlating a plurality of phase-delayed versions of the second pilot code signal with a received CDM signal from the receive channel using the channel impulse response estimate to produce a despread second pilot signal, the code phase of the second pilot code signal being changed responsive to an acquisition signal; and detection means for detecting the presence of the despread associated pilot signal to produce an acquisition signal, the acquisition signal indicating a degree of synchronization between the second pilot code signal, the received second pilot code signal and the first pilot code signal; c) a second pilot code tracking means including means for adjusting the local second pilot code signal in phase responsive to the acquisition signal in a sense which tends to increase the level of the despread second pilot signal; and d) message signal acquisition means for providing a plurality of despread receive message signals, the message signal acquisition means correlating the plurality of local receive message code signals with the received CDM signal using the channel response estimate of the second pilot code signal to produce the plurality of despread receive message signals.
2. The CDMA modem as recited in claim 1, wherein the telecommunication signals further include connection control signals and the spreading means of the modem transmitter further combines the call control signals with a respective one of the plurality of message code signals to form a call control channel.
3. The CDMA modem as recited in claim 2 wherein the call control signals include a user identification signal which identifies the user for the respective information signal and a message type signal which indicates a type and an information rate for the information signal.
4. The CDMA modem as recited in claim 1, wherein the pilot vector correlator of the pilot code acquisition means further includes:A) delay means for generating the plurality of phase-delayed versions of the second pilot code signal and a plurality of second pilot code-phase delayed correlation means each correlating a respective one of the plurality of phase-delayed versions of the second pilot code signal with the received CDM signal to produce a plurality of despread multipath pilot signals, and each providing a respective multipath weight signal corresponding to a respective multipath carrier of a respective one of the despread multipath pilot signals; and B) a pilot adaptive matched filter including a plurality of signal weighting means for scaling, in magnitude, and rotating, in phase, the respective despread multipath pilot signals and each responsive to the respective multipath weighting signal, the plurality of signal weighting means providing a plurality of weighted despread multipath pilot signals, and a summing means for summing the plurality of weighted despread multipath pilot signals to form a despread second pilot signal; and wherein the detection means, being coupled to the pilot vector correlator, produces the acquisition signal having a magnitude which is proportional to the despread second pilot code signal in signal energy, the magnitude of the acquisition signal indicating a degree of synchronization between the associated pilot code signal and the first pilot code signal; and the second pilot code tracking means includes means for adjusting the second pilot code signal in code phase, responsive to the acquisition signal in a sense which tends to increase in signal energy the despread second code pilot signal; and wherein the message signal acquisition means includes: A) a plurality of receive message signal correlators for correlating the receive message code signals with the CDM signal to produce a plurality of despread multipath receive message signals respectively; B) a message adaptive matched filter including a plurality of signal weighting means for aligning the respective despread multipath receive message signals in carrier phase responsive to the multipath weighting signals, the plurality of signal weighting means providing a plurality of weighted despread multipath receive message signals, and a summing means for summing the plurality of weighted despread multipath receive message signals to form a despread receive message signal.
5. The CDMA modem as recited in claim 3, wherein the pilot vector correlator further comprises:a phase locked loop (PLL) which measures a carrier phase error of each respective despread multipath pilot signal and produces a composite carrier phase error signal; wherein the phase error signal and each one of the plurality of despread multipath pilot signals and the respective multipath signal weighting value are applied to a respective one of a plurality of multipliers, and each despread multipath pilot signal is multiplied by the respective weighting value and the phase error signal to produce one scaled and phase rotated second pilot signal component of a plurality of scaled and phase rotated pilot signal components having substantially equal carrier phases.
6. The CDMA modem as recited in claim 1, wherein:the message signal acquisition means includes an adaptive matched filter (AMF) means for collecting signal power of a spread data channel of each of the plurality of receive message signals in the received CDM signal having a plurality of multipath signal components, each of said multipath signal components having a carrier phase, and the received CDM signal includes at least one spread data channel employing a second predetermined spreading code sequence corresponding to one of the plurality of local receive message code signals, and the spread data channel is data-modulated; the pilot vector correlator means provides a plurality of multipath signal weighting values, each weighting value being determined from the respective one of the plurality of phase-delayed versions of the second pilot code correlated with the received CDM signal, and each multipath signal weighting value corresponding to a respective multipath signal carrier of the respective received multipath signal component; the local code generation means generates a plurality of code phase-shifted versions of the one of the plurality of local receive message code signals; and the data AMF means, coupled to receive the spread signal, provides a data value determined from one of the despread receive message signals from the received CDM signal, the data AMF means comprising: a) a plurality of spreading code correlators, each spreading code correlator correlating a respective one of the code phase-shifted versions of the one of the plurality of local receive message code signals with the received CDM signal to produce a respective despread multipath data signal component having a carrier phase value; b) weighting means for scaling the data value and for aligning the carrier phase value of the despread multipath data responsive to the respective multipath weighting value; and c) first combining means for combining each one of the scaled and aligned data signal components to produce the data value.
7. The CDMA modem as recited in claim 6, wherein:a) each of the plurality of spreading code correlators of the data AMF means further includes multiplication means for multiplying the received CDM signal with a respective one of the code phase-shifted versions of the one of the plurality of local receive message code signals to produce a correlated signal value and accumulation means for accumulating the correlated signal value for a predetermined period to produce a despread multipath data signal component having a carrier phase which corresponds to the carrier signal phase of the respective received multipath signal component; and b) the aligning means comprises a plurality of multipliers, each multiplier multiplying a respective one of the despread multipath data signal components with a respective one of the multipath signal weighting values, and each multiplier producing one weighted data signal component of a plurality of weighted data signal components.
8. The adaptive matched filter (AMF) apparatus of claim 6, wherein the pilot vector correlator means further comprises:local pilot code sequence generator means for generating a plurality of local code sequences, each of the code sequences being a code phase-shifted version of the pilot spreading code sequence; a plurality of pilot spreading code correlators, each pilot spreading code correlator correlating a respective one of the local code sequences with the spread signal, each spreading code correlator comprising multiplication means for multiplying the spread signal with the respective one of the local code sequences to produce a correlated pilot signal value and accumulator means for accumulating the correlated pilot signal value for a predetermined period to produce a despread multipath pilot signal component with a carrier signal phase; wherein each one of the plurality of despread multipath pilot signal components is applied to a respective one of a plurality of low pass filters to produce a multipath signal weighting value corresponding to the carrier signal phase of the respective received multipath signal component; each one of the plurality of despread multipath pilot signal components and the respective multipath signal weighting value is applied to a respective one of a plurality of multipliers; and each multipath pilot signal component is multiplied by the respective multipath signal weighting value to produce one scaled and phase rotated pilot signal component of a plurality of scaled and phase rotated pilot signal components having substantially equal carrier phases; and second combining means for combining the plurality of weighted pilot signal components to produce a pilot data value.
9. The CDMA modem as recited in claim 1, wherein the respective code generation means of the modem transmitter and of the modem receiver each generates the respective pilot code signal from a master spreading code sequence and the plurality of message code signals from a plurality of spreading code sequences having relatively low mutual cross correlation, and having a predetermined mutual code phase relationship, the code sequence generator apparatus comprising:clock generator means for generating a clock signal; a linear feedback shift register (LFSR), responsive to the clock signal and having a plurality of stages including a first stage and a last stage, each stage defining a respective tap, each tap producing a tap signal; wherein a predetermined group of the tap signals including the tap signal of the last stage are applied to logic circuitry which combines the tap signals to produce a feedback spreading-code signal, said feedback spreading-code signal being applied as an input signal to the first stage of the LFSR; first memory means for storing a plurality of spreading-code seeds, each spreading-code seed comprising a set of spreading-code sequence bit values, and said first memory being connected to the LFSR and being responsive to a load signal for transferring each one of a predetermined set of the spreading-code sequence bit values of a selected one of a plurality of spreading-code seeds into a respective one of the shift register stages of the LSFR; code generator controller means for selecting one of the plurality of spreading-code seeds to determine the plurality of spreading code sequences and for providing the load signal indicating said one spreading-code seed; wherein said LSFR is responsive to the clock signal to sequentially transfer each respective tap signal from one stage to the next stage, from the first stage to the last stage and for transferring the feedback spreading-code value to the first stage, and each successive one of the tap values of the last stage defines the master spreading code sequence; second memory means being responsive to the clock signal for providing a repetitive even code sequence, said even code sequence having relatively low cross correlation with the master spreading sequence and having an even number of chip spreading values; a plurality of cascade connected feedforward means, coupled to receive the master spreading code sequence, for providing a plurality of code sequences, each code sequence being a distinct spreading code sequence of said plurality of spreading code sequences, said feedforward means being responsive to the clock signal, to provide a plurality of spreading code sequences; and a plurality code sequence combining means, each code sequence combining means for combining a respective spreading code sequence with said even code sequence to produce a plurality of relatively long spreading codes.
10. The CDMA modem as recited in claim 9, wherein the plurality of cascade connected feedforward means of each code sequence generator includes:receiving means for receiving the master spreading sequence; a feedforward circuit having a plurality of cascade connected feedforward logic sections, each logic section defining a tap which provides one of the plurality of spreading code sequences, including a first feedforward logic section and a last feedforward logic section and connected sequentially from the first feedforward logic section to the last feedforward logic section, each feedforward logic section comprising a single delay element having an input terminal which receives an input signal and an output terminal which provides an output signal, and logic combining means for logically combining the input signal with the output signal to produce the respective spreading code sequence.
11. The CDMA modem as recited in claim 10, wherein the code sequence combining means comprises an EXCLUSIVE-OR logic circuit.
12. The CDMA modem as recited in claim 10, wherein the logic combining means comprises an EXCLUSIVE-OR logic circuit for performing modulo-2 addition.
13. The CDMA modem as recited in claim 1, wherein the second pilot code tracking means includes a centroid tracking circuit which tracks a centroid of a plurality of multipath spread-spectrum signals, the plurality of multipath spread-spectrum signals constituting the received CDM signal including the second pilot code signal, the centroid tracking circuit comprising:an analog-to-digital converter, responsive to a clock signal and the received CDM, to produce a sequence of sample values including a set of even numbered sample values and a set of odd-numbered sample values; wherein the even numbered samples and the odd-numbered samples are interleaved in time and the set of even-numbered sample values define a sequence of early spread-spectrum channel signal samples and said set of odd sample number values define a sequence of late spread-spectrum channel signal samples; means for combining each of said plurality of phase-delayed versions of the second pilot code signal with the sequence of early received spread-spectrum channel signal samples to produce a plurality of early despread multipath signals, and for combining each of said plurality of local code sequences with the sequence of late received spread-spectrum channel signal samples to produce a plurality of late despread multipath signals, wherein each of said plurality of phase-delayed versions of the second pilot code signal has a respective a code phase and code symbol period, and each being a code phase-shifted version of a code sequence corresponding to the second pilot code signal; means for processing the plurality of early despread multipath signals to produce an early tracking value, and for processing the plurality of late despread multipath signals to produce a late tracking value; and a subtracter which determines the difference between the early tracking value and the late tracking value to produce an error signal value corresponding to the acquisition signal.
14. The CDMA modem as recited in claim 13, wherein the means for processing the plurality of early despread multipath signals and for processing the plurality of late despread multipath signals comprises:a first plurality of accumulators which accumulate the plurality of early despread multipath signals to produce a respective plurality of early signal samples, and a second plurality of accumulators which accumulate said plurality of late despread multipath signals to produce a respective plurality of late signal samples; and a first summing network for summing ones of the early signal samples to produce said early tracking value, and a second summing network for summing ones of the late signal samples to produce said late tracking value.
15. The CDMA modem as recited in claim 14, further including a plurality of signal scalers which multiply each of said early signal samples and each of said late signal samples by a respective predetermined weighting value and which apply the weighted odd and even signal samples to the respective first and second summing networks.

Parent Case Info

This is a divisional application of prior application Ser. No. 08/669,769 filed on Jun. 27, 1996, now U.S. Pat. No. 5,796,776, entitled CODE SEQUENCE GENERATOR IN A CDMA MODEM. This application claims the benefit of U.S. Provisional application Ser. No. 60/000,775 filed Jun. 30, 1995.

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Provisional Applications (1)

	Number	Date	Country
	60/000775	Jun 1995	US

Code sequence generator in a CDMA modem

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications