Information
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Patent Grant
-
H2106
-
Patent Number
H2,106
-
Date Filed
Thursday, September 24, 199826 years ago
-
Date Issued
Tuesday, July 6, 200420 years ago
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Inventors
-
Original Assignees
-
Examiners
-
-
US Classifications
Field of Search
US
- 370 276
- 370 277
- 370 280
- 370 281
- 370 294
- 370 295
- 370 328
- 370 330
- 370 334
- 370 344
- 370 347
- 370 436
- 370 458
- 370 912
- 370 913
- 709 248
- 709 249
- 709 250
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International Classifications
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Abstract
A virtual FDD base station comprises two base station sub-units each of which comprise a base station transmitter and a base station receiver. The first base station sub-unit transmits base-to-user messages to user stations only during a first half of a repeating time frame, and receives user-to-base messages from the user stations only during a second half of the time frame. The second base station sub-unit is preferably collocated with the first base station sub-unit and synchronized thereto, and transmits base-to-user messages to user stations only during half of the time frame, while receiving user-to-base messages from the user stations only during the other half of the time frame. Duplex communication channels are preferably defined by correlating a base transmit time slot with a user transmit time slot, which are separated by a sufficient amount of time to allow transmit/receive switching by a user station. The base station sub-units may be configured so that one of the sub-units transmits continuously and the other receives continuously, thereby providing full FDD functionality. In another embodiment, a TDD base station is adapted to support FDD communication. An over-the-air controller switches the transmit and receive operating frequency in accordance with a defined time slot communication pattern comprising base transmit time slots and user transmit time slots, and at the same time ensures that the base transmitter and base receiver are appropriately switched back and forth for connection with the base station antenna.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the present invention relates to a method and apparatus for multiple access communication.
2. Background
A variety of techniques are known for allowing multiple users to communicate with one or more fixed stations (i.e., base stations) by making use of shared communication resources. Examples of multiple access communication systems include, for example, cellular telephone networks and local wireless communication systems, such as wireless private branch exchange (PBX) networks. In such multiple access communication systems, transmissions from different sources may be distinguished in a variety of manners, such as on the basis of different frequencies, time slots, and/or codes, for example.
As used herein, a communication system in which transmissions are distinguished according to the transmission frequency may be referred to as a frequency division multiple access (FDMA) communication system. A communication system in which a forward link transmission over one frequency is paired with a reverse link transmission over a different frequency may be referred to as a frequency division duplex (FDD) communication system.
A communication system in which transmissions are distinguished according to the relative timing of the transmission (i.e., by use of time slots) may be referred to as a time division multiple access (TDMA) communication system. A communication system in which a forward link transmission during one time slot (or time segment) is paired with a reverse link transmission occurring during a different time slot (or time segment) may be referred to as a time division duplex (TDD) communication system. The DECT system is an example of a well known type of TDD communication system.
A communication system in which transmissions are distinguished according to which code is used to encode the transmission may be referred to as a code division multiple access (CDMA) communication system. In a CDMA communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be “spread” over a broader frequency range and also typically causes the signal power to decrease as the frequency bandwidth is spread. At the receiver, the signal is decoded, which causes it to be “despread” and allows the original data to be recovered. Distinct codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication, albeit over a broader frequency band and generally at a lower power level than “narrowband” FDMA or TDMA systems. Different users may thereby transmit simultaneously over the same frequency without necessarily interfering with one another.
Various “hybrid” communication systems incorporating aspects of more than one multiple access communication technique have been developed or proposed. For example, a GSM system may be viewed as a “hybrid” communication system utilizing aspects of both FDD and TDMA. In a GSM system, each base station is assigned a transmission frequency band and reception frequency band. The base station transmits to each of its mobile stations using a transmission frequency within its assigned frequency band, and the mobile stations transmit to the base station using a frequency within the base station's reception frequency band. The transmissions to the user stations are sent in assigned time slots over the base station's transmission frequency, and the transmissions from the user stations are sent in corresponding assigned time slots over the base station's reception frequency.
While multiple access communication may be achieved using techniques of either FDMA, TDMA or CDMA, or certain variations (e.g., FDD or TDD) or combinations thereof, problems can occur if an equipment manufacturer or operator desires to migrate from one type of multiple access communication to a different type. This problem results from the fact that equipment manufactured specifically for any one type of multiple access communication system typically cannot be used with another type of multiple access system because of inherent differences in the nature of the communication techniques, leading to incompatibilities between the physical hardware as well as the communication protocols employed by the two communication systems. For example, a base station designed for TDD communication cannot be expected to communicate properly with an FDD handset, nor can it be expected that a TDD handset will communicate properly with a base station designed for FDD communication.
It may nevertheless be desired by equipment manufacturers or service providers to deploy or offer systems using different multiple access communication techniques or protocols, in order to serve different markets, geographical regions, or clientele, or for other reasons. However, to develop separate equipment for operation in different multiple access communication environments can substantially increase equipment design and manufacturing costs. Such a development process can also lead to the creation of different and incompatible protocols, which can require, for example, different types of backhaul service, leading to greater design expense to support the different backhaul formats and possibly duplicative base station controllers in the same local area, each servicing a different type of base station (i.e., FDD vs. TDD). Furthermore, an equipment manufacturer or service provider may desire to migrate from one type of multiple access communication and protocol to another type, without incurring substantial redesign costs.
It would therefore be advantageous to provide an apparatus and method allowing communication in more than one multiple access communication environment. It would further be advantageous to provide a method and apparatus for converting or adapting equipment from one type of multiple access communication service (e.g., TDD) to a different type (e.g., FDD).
SUMMARY OF THE INVENTION
The invention provides in certain aspects techniques for using or converting multiple access communication equipment to serve in a different multiple access communication environment.
In one embodiment, a base station within a communication system comprises two base station sub-units, preferably collocated, for performing virtual FDD communication. Each of the two base station sub-units comprises a base station transmitter and a base station receiver. The first base station sub-unit transmits base-to-user messages to user stations only during a first half of a repeating time frame, and receives user-to-base messages from the user stations only during a second half of the time frame. The second base station sub-unit is preferably collocated with the first base station sub-unit, and transmits base-to-user messages to user stations only during the second half of the time frame, while receiving user-to-base messages from the user stations only during the first half of said time frame. The two base station sub-units are preferably synchronized so as to maintain proper alignment of the time frame and of the time slots within the time frame.
In a second embodiment, a base station also comprises two base station sub-units. In the second embodiment, a time frame comprises a plurality of base transmit time slots defined with respect to a base transmit frequency band and a plurality of user transmit time slots defined with respect to a user transmit frequency band. The time frame is divided between the two base station sub-units such that the first base station sub-unit and second base station sub-unit each are assigned one half of the base transmit time slots and one half of the user transmit time slots. The base transmit time slots assigned to each base station sub-unit may form a contiguous block, or may alternate with one or more base transmit time slots assigned to the other base station sub-unit. Duplex communication channels are preferably defined by correlating a base transmit time slot with a user transmit time slot, with the base transmit time slots and user transmit time slot preferably separated by a sufficient amount of time to allow transmit/receive switching by a user station between the base transmit time slot and the user transmit time slot. Multiple time slots may be aggregated to a single user station in certain embodiments.
In another embodiment, a base station comprises a pair of modified TDD base station sub-units. One of the modified TDD base station sub-units is adapted to transmit continuously over a base transmit frequency band using its base station transmitter, while the other of the modified TDD base station sub-units is adapted to receive continuously over a user transmit frequency band using its base station receiver. A backhaul interface transmits information over a backhaul line from the modified TDD base station sub-unit that receives continuously, and transmits information from the backhaul line to the modified TDD base station sub-unit that transmits continuously, so as to support a plurality of duplex communication channels. The two modified TDD base station sub-units may, in one embodiment, pass appropriate synchronization and error correction information to one another over a signal interface.
In another embodiment, a TDD base station is adapted to support FDD communication. The TDD base station comprises a radio transceiver, an over-the-air controller, a memory buffer and backhaul interface. The over-the-air controller switches the transmit and receive operating frequency between a base transmit frequency band and a user transmit frequency band in accordance with a defined time slot communication pattern comprising base transmit time slots and user transmit time slots, and at the same time ensures that the base transmitter and base receiver are appropriately switched back and forth for connection with the base station antenna (or antennas). The modified TDD base station may toggle back and forth between base transmit time slots and user transmit time slots on a slot-by-slot basis, or else may switch the base transmit frequency band and user transmit frequency band after a predefined number of transmit time slots or user transmit time slots.
Further embodiments, modifications, variations and enhancements of the invention are also disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a diagram of a cellular system.
FIG. 2
is a diagram of an exemplary TDD frame structure as known in the art.
FIG. 3
is a diagram of a GSM frame structure.
FIG. 4
is a block diagram of a base station as known in the art for carrying out time division duplex communication.
FIG. 5
is a diagram of a frame structure for half-capacity FDD communication that can be supported by modifying the base station shown in FIG.
4
.
FIG. 6
is a diagram of an alternative frame structure for half-capacity FDD communication that can be supported by modifying the base station shown in FIG.
4
.
FIG. 7
is a block diagram of one embodiment of a base station comprising two base station sub-units for achieving “virtual” FDD communication.
FIG. 8
is a diagram of a frame structure that can be supported by the base station shown in FIG.
7
.
FIG. 9
is a diagram of an alternative frame structure that can be supported by the base station shown in FIG.
7
.
FIG. 10
is a diagram showing additional details of one of the base station sub-units that may be utilized in the base station shown in FIG.
7
.
FIG. 11
is a block diagram of another embodiment of a base station for achieving FDD communication.
FIG. 12
is a diagram of another frame structure for FDD communication that can be supported by modifying a TDD base station.
FIG. 13
is a diagram for a frame structure for FDD communication illustrating slot aggregation.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1
is a diagram of a cellular communication system
101
having base stations and user stations. In
FIG. 1
, a communication system
101
for communication among a plurality of user stations
102
includes a plurality of cells
103
, each with a base station
104
, typically located at or near the center of the cell
103
. Each station (both the base stations
104
and the user stations
102
) may generally comprise a receiver and a transmitter. The user stations
102
and base stations
104
preferably communicate using frequency division duplex (FDD) techniques as further described herein, in which base stations
104
communicate over one frequency band and user stations
102
communicate over another frequency band. Communication is also conducted such that different user stations
102
transmit at different times (i.e., during different time slots), as further described herein.
As further shown in
FIG. 1
, the communication system
101
may also comprise a base station controller
105
which connects to the base stations
104
in a particular geographic region. The base station controller
105
aggregates inputs from multiple base stations
104
and relays information from the base stations
104
to a mobile switching center (MSC) (not shown) and ultimately to a public switched telephone network (PSTN, or “network”) (not shown). The base station controller
105
also relays information from the network to the individual base stations
104
. The base station controller
105
may, if necessary, perform conversion of signaling messages relating to such things as mobility management and call control, to make the signaling messages compatible with the communication protocol used by the base stations
104
.
FIG. 2
is a diagram of a particular TDD frame structure as known in the art. In
FIG. 2
, a repeating major time frame
201
comprises a plurality of time slots (or minor time frames)
202
. Each time slot
202
can be assigned by the base station
104
to a user station
102
. User stations
102
can be assigned more than one time slot
202
if desired, and the time slots
202
so assigned may or may not be contiguous.
As further shown in
FIG. 2
, each time slot
202
comprises two time segments
205
,
206
. In the first (i.e., user transmission) time segment
205
, the user station
102
to which the time slot
202
is assigned transmits a user-to-base message
211
to the base station
104
. In the second (i.e., base transmission) time segment
206
, the base station
104
transmits a base-to-user message
212
to the user station
102
to which the time slot
202
is assigned. Each user station
102
thereby transmits and receives in its assigned time slot
202
, thus allowing multiple user stations
102
to communicate with the same base station
104
.
FIG. 4
is a block diagram of a base station
401
as known in the art for communicating according to an over-the-air TDD protocol such as shown in FIG.
2
. As shown in
FIG. 4
, the base station
401
comprises a radio transceiver
405
(comprising a transmitter
415
and a receiver
416
), an antenna
406
connected to the radio transceiver
405
, and an over-the-air controller
410
also connected to the radio transceiver
405
. The over-the-air controller
410
is connected to a memory buffer
411
, which the over-the-air controller
410
shares with a backhaul line controller
412
. The over-the-air controller
410
oversees retrieval of information from the memory buffer
411
by the radio transceiver
405
for transmission to the various user stations
102
with which the base station
401
communicates, and storage of information into the memory buffer
411
by the radio transceiver
405
when such information is received from the user stations
102
. The backhaul line controller
412
removes information from the memory buffer
411
to transmit over a backhaul line
430
to the network, and stores information from the backhaul line
430
received from the network in the memory buffer
411
, so as to make it available for the radio transceiver
405
. In this manner, information is passed from the user stations
102
to the network, and back, so that telephone calls or similar communication links can be supported.
FIG. 4
also shows further details of the over-the-air controller
410
. As shown therein, the over-the-air controller
410
comprises a clock
420
connected to a time frame counter
421
and a time slot counter
422
. The time frame counter
421
and time slot counter
422
are connected to control logic
423
, which uses outputs from the time frame counter
421
and time slot counter
422
to format messages for over-the-air communication. Under control of the over-the-air controller
410
, the radio transceiver
405
stores and removes information from the memory buffer
411
.
The radio transceiver
405
further comprises a transmit/receive (T/R) switch
417
to allow selection between a transmission mode and a reception mode. The control logic
423
of the over-the-air controller
410
controls the T/R switch
417
, and thereby selects between the transmission mode and reception mode based, for example, upon the current portion of the time frame. Thus, if the base station
401
is operating using the time frame
201
of
FIG. 2
, then the over-the-air controller
410
selects a reception mode during the user transmission time segment
205
of each time slot
202
, and selects the position of the T/R switch
417
accordingly. Similarly, the over-the-air controller
410
selects a transmission mode during the base transmission time segment
206
of each time slot
202
, and selects the position of the T/R switch
417
accordingly.
To facilitate rapid or convenient storage and extraction of data, the memory buffer
411
may be partitioned into memory segments
429
, each memory segment
429
corresponding to one time slot
202
. In one embodiment, for example, the current time slot (as output from, for example, the slot counter
422
) can be used as a pointer offset to control which memory segment
429
the radio transceiver
405
is accessing at a given time. The memory segments
429
can be organized such that the data for the user transmission time segment
206
and data for the base transmission time segment
205
are stored adjacent to one another. Alternatively, the memory segments
429
can be organized such that the data for all of the user transmission time segments
206
are stored in one half of the memory buffer
411
, and the data for all of the base transmission time segments
205
are stored in the other half of the memory buffer
411
. In such a case, the control signal for the T/R switch
417
can be used as an additional pointer offset to control whether the radio transceiver
405
will access the “upper” half of the memory buffer
411
or the “lower” half of the memory buffer
411
(i.e., the user transmission data or the base transmission data).
The base station
401
shown in
FIG. 4
may also provide for selection of transmission and reception frequency, so as to allow deployment of the base station
401
in a cellular environment in which different cells
103
(see
FIG. 1
) are assigned a different frequencies (consistent with a repeating pattern, such as a three-cell or seven-cell repeating pattern, as disclosed, for example, in U.S. Pat. No. 5,402,413, incorporated herein by reference as if set forth fully herein). The base station
401
can be deployed with the desired frequency by, for example, selecting external switches on the base station
401
or programming the desired frequency using software or firmware of the over-the-air controller
410
. In the base station
401
shown in
FIG. 4
, the radio transceiver
405
comprises a programmable voltage-controlled oscillator
418
, which is responsive to a control signal (e.g., control bits) from the over-the-air controller
410
and generates an output frequency according to such a control signal. Because the base station
401
implements a TDD time frame
201
such as shown in
FIG. 2
, it uses the same frequency for transmission and reception.
FIG. 3
is a diagram showing a different over-the-air frame structure
301
, commonly associated with a conventional GSM system. As shown in
FIG. 3
, a base transmission time frame
302
is defined over a base transmission frequency
311
, and a user transmission time frame
303
is defined over a base reception frequency
312
. The base transmission frequency
311
and base reception frequency
312
are separated by a predefined frequency separation (e.g., 45 MHz).
The base transmission time frame
302
comprises a number of base transmission time slots
306
of equal duration. Likewise, the user transmission time frame
303
comprises a number of user transmission time slots
307
of equal duration. Both the base transmission time frame
302
and the user transmission time frame
303
have the same number of time slots
306
,
307
, such as eight time slots
306
,
307
apiece.
In operation, a GSM base station transmits forward-link transmissions during the base transmission time slots
306
and receives reverse-link transmissions during the user transmission time slots
307
. The user transmission time frame
303
is “offset” by a predefined duration
305
(e.g., three time slots
306
or
307
) from the base transmission time frame
302
, so as to allow the user stations
302
a sufficient “turn-around” switching time and information processing time, and also to allow propagation of the forward-link messages to the user stations
102
.
According to one embodiment disclosed herein, a TDD base station (such as base station
401
shown in
FIG. 4
) which is otherwise capable of supporting a TDD time frame (such as time frame
201
shown in
FIG. 2
) is adapted to operate in an FDD environment and in general accordance with an FDD frame structure such as shown in, for example,
FIG. 3
(or other suitable FDD frame structure). In one embodiment, the TDD base station
401
is modified so as to transmit and receive in a repeating pattern as shown by the frame structure
501
in FIG.
5
. As shown in
FIG. 5
, a time frame
502
comprises a plurality of base transmit time slots
505
over a base transmission frequency band
511
, and a plurality of base receive time slots
506
over a user transmission frequency band
512
(also referred to as a base reception frequency band). Transmissions from user stations
102
, conducted over the user transmission frequency band
512
, alternate in time with transmissions from the base station
104
, conducted over the base transmission frequency band
511
. Thus, in the embodiment shown in
FIG. 5
, the user stations
102
transmit in the odd time slots
506
a
over the user transmission frequency band
512
, and the base station
104
transmits in the even time slots
505
b
over the base transmission frequency band
511
. The even time slots
506
b
for the user transmission frequency band
512
and the odd time slots
505
a
for the base transmission frequency band
511
remain “dark” or unused.
In the particular embodiment shown in
FIG. 5
, a duplex pairing of transmissions occurs in adjacent time slots. As shown in
FIG. 5
, a first user station (designated “M
1
”) transmits to the base station (designated “B”) in a first odd time slot
506
a
, and the base station B transmits to the first user station M
1
in the first even time slot
505
b
(i.e., the second base transmit time slot
505
, the first one being “dark”). Likewise, the second user station (designated “M
2
”) transmits to the base station B in a second odd time slot
506
a
(i.e., the third base receive time slot
506
), and the base station B transmits to the second user station M
2
in the second even time slot
505
b
(i.e., the fourth base transmit time slot
505
, the third one being “dark”). This pattern is repeated for the entirety of the time frame
502
, and again for each succeeding time frame
502
.
A TDD base station (such as the base station
401
shown in
FIG. 4
) may be adapted to support the frame structure
501
shown in
FIG. 5
by certain adjustments or modifications, including adjustments or modifications (in hardware, software or both) to the over-the-air controller
410
. For example, the over-the-air controller
410
may be modified such that it toggles the programmable VCO
418
between the base transmission frequency band
511
and the base reception frequency band
512
, synchronized with the timing of the base transmit time slots
505
and base receive time slots
506
. Via a control signal, the over-the-air controller
410
selects the base transmission frequency band
511
for the even time slots
505
b
and the base reception frequency band
512
for the odd time slots
506
a
. The over-the-air controller
410
controls the T/R switch
417
of the base station
401
in the same manner as for the frame structure
201
shown in
FIG. 2
, by selecting it to be in a transmission mode during the even time slots
505
b
and in a reception mode during the odd time slots
506
a.
If the frame structure
501
of
FIG. 5
is not compatible with transmit/receive switching speeds at the user stations
102
(in other words, a user station
102
is not able to transmit in one time slot
506
a
of the user transmission frequency band
512
, and then receive in the immediately following time slot
505
b
over the base transmission frequency band
511
), an alternative frame structure
1201
is depicted in
FIG. 12
which addresses this problem of limited transmit/receive switching time in the user stations
102
. The frame structure
1201
shown in
FIG. 12
is quite similar to the frame structure
501
shown in
FIG. 5
, in that user stations
102
transmit in odd time frames
1206
a
and the base station
104
transmits in even time frames
1205
b
. However, a user station
102
does not receive a base transmission from the base station
104
in the base transmit time slot
505
b
immediately following the user station's user transmit time slot
506
a
. Rather, the user transmit time slot
506
a
for a particular user station
102
is paired with a base transmit time slot
505
b
occurring more than one time slot
505
later, so as to allow the user station
102
time to switch between its transmission frequency and its reception frequency.
According to the frame structure
1201
shown in
FIG. 12
, a user station
102
transmits during its assigned user transmit time slot
506
a
, and later receives during a base transmit time slot
505
b
occurring, for example, three time slots
505
later, giving the user station
102
a time period equal to two time slots
505
to switch between the transmit and receive frequencies. This same principle can be extended, by pairing the user transmit time slot
506
a
with a base transmit time slot
505
b
occurring even later in the time frame
1201
.
As a consequence of the splitting apart the forward link and reverse link transmissions from one another in the manner described above, the over-the-air controller
410
of the base station
401
is preferably modified in this embodiment so that the mapping of information into and out of the memory buffer
411
carried out by the radio transceiver
405
(under control of the over-the-air controller
410
) is adjusted to account for the time separation between the forward link and reverse link transmissions. To this end, the over-the-air controller
410
may be configured so that it causes the radio transceiver
405
to store and extract packet data in the proper memory segment
429
of the memory buffer
411
corresponding to the particular user station
102
. One possible way this can be achieved is through software, by use of a slot offset parameter. In such an embodiment, when the over-the-air controller
410
instructs radio transceiver
405
to extract information from the memory buffer
411
for the base transmit time slot
505
b
, the slot offset parameter is applied such that the information is extracted from the proper location (i.e., proper memory segment
429
) in the memory buffer
411
. In such a manner, no modifications are necessary for the backhaul line controller
412
(with the possible exception of a timing adjustment to account for the increase in delay between the forward and reverse link information).
Alternatively, a similar result may be achieved by modifying the backhaul line controller
412
in addition or as opposed to the over-the-air controller
410
, so as to obtain the desired memory management. In this alternative embodiment, the backhaul line controller
412
may be configured so that it stores information received from the network to be transmitted to a particular user station
102
in the appropriate memory segment
429
of the memory buffer
411
. For example, the backhaul line controller
412
would store information received from the network, not in the memory segment
429
for the immediately following base time slot
505
b
, but in the memory segment
429
for the next occurring base time slot
505
b.
The over-the-air controller
410
then causes the radio transceiver
405
to transmit the information in the correct base transmit time slot
505
b.
However, the over-the-air controller
410
is still preferably modified or configured to associate the proper user transmit time slot
506
a
and base transmit time slot
505
a
pair as a single duplex channel, so that the over-the-air controller
410
knows when to instruct the radio transceiver
405
to transmit (or receive) and when to remain dormant or inactive (or to otherwise transmit a dummy pattern) because no user station
102
is assigned to a particular time slot
505
or
506
.
FIG. 6
shows an alternative inventive frame structure
601
that can be supported using a single TDD base station (such as the base station
401
shown in
FIG. 4
) with suitable modifications. In the frame structure
601
shown in
FIG. 6
, a repeating time frame
602
comprises a plurality of base transmit time slots
605
and a plurality of user transmit time slots
606
. Each of the base transmit time slots
605
is preferably paired with a corresponding one of the user transmit time slots
606
, with such a pair defining a duplex channel for communication (up to N total duplex channels). During the first half
602
a
of the time frame
602
, the base station
104
transmits over a base transmission frequency band
611
in each of the base transmit time slot
605
in succession. With respect to the user transmission frequency band
612
, the first half
602
a
of the time frame
602
is “dark” or unused. During the second half
602
b
of the time frame
602
, the user stations
102
transmit in succession over the user transmission frequency band
612
. With regard to the base transmission frequency band
611
, the second half
602
b
of the time frame
602
is “dark” or unused.
Certain modifications can be made to a TDD base station (such as the base station
401
shown in
FIG. 4
) so as to accommodate the frame structure
601
shown in FIG.
6
. For example, the over-the-air controller
410
would be modified such that it causes the programmable VCO
418
to toggle between the base transmission frequency
611
and the base reception frequency
612
each half of the time frame
602
. The over-the-air controller
410
may, for example, use the output of the time slot counter
422
to determine when to switch between frequencies. The over-the-air controller
410
may further be modified such that it causes the radio transceiver
405
to extract data from the appropriate memory segments
429
of the memory buffer
411
during successive base transmit time slots
605
, and to store data in the appropriate memory segments
429
of the memory buffer
411
during successive user transmit time slots
606
.
One benefit of the frame structure
601
shown in
FIG. 6
is that the user stations
102
should have more than adequate time to switch between their reception and transmission frequencies of the forward link and reverse link. The backhaul line controller
412
of the base station
401
may, however, need to be modified to account for delays introduced by the separation of the forward link and reverse link transmissions over the TDD time frame
201
of FIG.
2
.
The frame structure
601
shown in
FIG. 6
should be capable of supporting at least as many user stations
102
as the frame structure
501
shown in FIG.
5
. However, when considering the transmit/receive switching time of the base station
104
, the frame structure
601
of
FIG. 6
actually has increased capacity over the frame structure
501
shown in FIG.
5
. For the frame structure
501
of
FIG. 5
, the base station
104
needs to switch between transmit and receive frequencies in between each time slot
506
a
,
505
b.
The transmit/receive switch time results in either less data being transmitted during each time slot
506
a
,
505
b
, or else fewer total user stations
102
being supported for a given time frame
502
(i.e., fewer total time slots
505
,
506
). For the frame structure
601
shown in
FIG. 6
, the base station
104
needs to switch between transmit and receive frequencies only twice during an entire time frame
602
(i.e., at the end of the first half
602
a
of the time frame
602
and at the end of the second half
602
b
of the time frame
602
). The base transmit time slots
605
and/or user transmit time slots
606
can be made slightly shorter, if necessary, to accommodate the base station transmit/receive turnaround time. If the transmit/receive switch time is significant, an entire base transmit time slot
605
and/or a user transmit time slot
606
of time slot
602
can be made “dark” or unavailable for communication to allow the base station
104
time to switch frequencies during that time slot
605
or
606
.
If only one time slot (for example, a user transmit time slot
606
) needs to be made dark in order to meet the frequency switching timing requirements, then the base station
104
can use the free base transmit time slot
605
to transmit control or signaling information, or to broadcast current traffic conditions, or for other similar purposes.
In addition to saving time by reducing the number of transmit/receive frequency switches (in comparison to the frame structure
501
shown in
FIG. 5
, for example), the frame structure
601
of
FIG. 6
also saves power, because each transmit/receive frequency switch consumes additional power.
It will be understood by those skilled in the art that other frame structures can also be supported using the same principles as described above, with the base station transmitter active over the base transmission frequency band for approximately half of the time frame, and the base station receiver active over the base reception frequency band for approximately half of the time frame. The pattern of transmit and receive time slots for the base station can vary, and need not be symmetric. For example, an asymmetric time slot pattern may comprise two base transmit time slots, followed by two user transmit time slots, followed by one base transmit time slot, followed by one user transmit time slot, and so on. Also, the number of base transmit time slots and user transmit time slots need not be equal, if it is desired to have more bandwidth in one direction than the other.
Some modifications may be necessary to the communication protocol for which the TDD base station was originally designed in order to support an FDD frame structure, such as the virtual FDD frame structures shown in
FIGS. 5
or
6
. For example, if the TDD base station as originally designed supports aggregation of time slots
202
to a single user station
102
, and such a capability is desired in the FDD system, then the over-the-air controller
410
may be modified to allow such. Assuming a transmit/receive frequency switching time of one time slot or less, the number of aggregated time slots possible in the FDD frame structures of
FIGS. 5 and 6
depends primarily upon the offset between the transmit and receive slots for the user stations
102
.
The implementation of slot aggregation may be explained by reference to an example. In an illustrative embodiment, the FDD frame structure
601
shown in
FIG. 6
may comprise a total of 32 time slots, with 16 of these time slots being base transmit time slots
605
and 16 of these time slots being user transmit time slots
606
. An offset of 16 time slots may be provided between the base transmit time slot
605
and a corresponding user transmit time slot
606
. In such an embodiment, a single user station
102
may be assigned up to 15 consecutive duplex time slots (i.e., 15 base transmit time slots
605
and their corresponding user transmit time slots
605
), with the remaining base transmit time slot
605
being set aside for the user station
102
to switch between the base transmission frequency
611
(a user reception mode) and the user transmission frequency
612
(a user transmission mode), and the remaining user transmit time slot
606
being set aside for the user station
102
to switch between the user transmission frequency
612
(a user transmission mode) and the base transmission frequency
611
(a user reception mode).
The above example assumes a timing offset of 16 time slots between the base transmit time slots
605
and the corresponding user transmit time slots
606
. In alternative frame structures, the amount of potential slot aggregation might be less. For example, if the amount of timing offset were 8 time slots between the base transmit time slot and the user transmit time slot, such as shown in
FIG. 13
, then the time slot pattern for the frame structure would involve the following: 8 base transmit time slots
1305
, followed by 8 user transmit time slots
1306
, followed by 8 additional base transmit time slots
1305
, followed by 8 additional user transmit time slots
1306
. In such a case, the maximum slot aggregation allowed to a single user station
102
is 14 duplex time slots, seven duplex time slots from the first half
1320
of the time frame
1301
, and seven duplex time slots from the second half
1321
of the time frame. One duplex time slot (i.e., one base transmit time slot
1305
and one user transmit time slot
1306
) from each half
1320
,
1321
of the time frame is set aside for the user station
102
to switch between the reception and transmission frequencies
1311
,
1312
and back again, as necessary. The time slots
1308
,
1309
designated “T/R” in
FIG. 13
are used for transmit/receive frequency switching by the user station
102
, assuming maximum slot aggregation.
Accordingly, with the frame structure
1301
of
FIG. 13
, a total of up to 14 duplex time slots may be aggregated to a single user station
102
, with two duplex time slots (four individual or half-duplex time slots
1308
,
1309
) being used for transmit/receive frequency switching.
As a general matter, the smaller the timing offset between transmit time slots and receive time slots, the fewer time slots can be aggregated to a single user station
102
. Thus, for example, an offset of four time slots between the base transmit time slot and the user transmit time slot would allow a maximum aggregation of 12 full duplex time slots, with eight half-duplex time slots being used for transmit/receive frequency switching. An offset of two time slots between the base transmit time slot and the user transmit time slot would allow a maximum aggregation of 8 full duplex time slots, with 16 half-duplex time slots being used for transmit/receive frequency switching.
Of course, a user station
102
with a radio transceiver that can transmit and receive simultaneously is not necessarily limited to the number of FDD time slots that can be aggregated in the various FDD frame structures. However, user stations
102
with such a capability are substantially more costly to build. Generally, user stations
102
(e.g., handsets) constructed for use in a TDD system do not have a capability to transmit and receive simultaneously, because such a feature is unnecessary in a TDD environment. Thus, handsets adapted from a TDD setting to an FDD environment would typically be subject to the slot aggregation limitations discussed above.
The amount of slot offset (i.e., timing offset) between base transmit time slots and user transmit time slots may also affect other aspects of the performance of the base station
104
, including control traffic. In one embodiment, the base station
104
exchanges control traffic information with a user station
102
in multiple time slots within a time frame. The control traffic can involve alternating transmissions between the base station
104
and the user station
102
. Generally, each control traffic message must be processed by the recipient before a responsive control traffic message is transmitted. In the FDD frame structure
601
shown in
FIG. 6
, control traffic exchanges may be relatively slow due to the size of the offset (i.e., 16 time slots) between the base transmit time slot
605
and the user transmit time slot
606
. Ordinarily, only one control traffic exchange will be possible between a base station
104
and a user station
102
within a time frame
602
of the
FIG. 6
FDD frame structure
601
. However, with a minimal offset (e.g., FIG.
5
), the base station
104
and user station
102
could exchange control traffic messages in as many time slots
505
,
506
as available, subject to the processing time needed to analyze each control traffic message. The base station
104
and user station
102
therefore could exchange a maximum of 16 control traffic messages, for example, in the FDD frame structure
501
shown in FIG.
5
. However, a more realistic number of control traffic exchanges might be four, allowing for transmit/receive frequency switching time and control traffic message processing time.
As a general matter, the larger the slot offset (i.e., timing offset) between the base transmit time slot and the corresponding user transmit time slot, the slower control traffic exchanges can potentially be carried out. On the other hand, maximum potential slot aggregation generally increases by the largest possible slot offset between the base transmit time slot and the corresponding user transmit time slot. Consequently, a tradeoff may need to be made in terms of control traffic speed and maximum possible slot aggregration when considering the timing offset between base transmit time slots and user transmit time slots. The slot offset may ultimately be selected according to the needs of the overall communication system, taking into account whether it is more important within a particular system to have faster control traffic or greater potential slot aggregation. One possible solution to accommodate both needs is to allow for independent slot allocation (i.e., flexible slot offset between base transmit time slot and user transmit time slot); however, this solution generally requires increased complexity in the over-the-air controller and in the backhaul line controller of the base station in terms of overhead and slot maintenance.
Converting a base station from a TDD environment to an FDD environment may affect error correction mechanisms employed at the physical (i.e., RF) layer. For example, in a TDD environment, an ARQ error correction mechanism may be implemented whereby the recipient of the most recently transmitted data packet sends, along with its next data packet transmission, an indication of whether the most recently transmitted data packet was received error free. This indication may take the form of a single field in the message header. The ARQ field may comprise as little as a single bit, with an ARQ acknowledgment (“ACK”) bit value indicating successful receipt of the data and an ARQ non-acknowledgment (“NAK”) bit value indicating unsuccessful receipt of the data. If the data was not successfully received, then the sender recognizes this fact from the ARQ field (i.e., the NAK indicator), and resends the data in the next immediate data packet transmission. The ARQ error correction method may be applied in a TDD system, regardless of whether time slots are aggregated.
In an FDD system, an ARQ error correction method may also be used, but its working may be somewhat more complicated in situations where FDD slot aggregation is permitted. In an aggregated data mode (i.e., slot aggregation mode), there should be a symmetric number of base transmit time slots and user transmit time slots assigned to the same user station
102
, similar to TDD slot aggregation. The ARQ error correction method described above for a TDD environment (i.e., using header bits to indicate successful receipt of the previously received data packet) will work so long as the sender and receiver recognize the circuit as being composed of multiple duplex channels, each of which is preferably treated independently for ARQ purposes.
Accordingly, in one embodiment supporting slot aggregation in an FDD environment, a recipient discovering a packet in error requests its retransmission using an ARQ indicator. In response, the sender retransmits the data packet in the same time slot of the next time frame. To support this approach, the receiver is preferably configured so as to allow insertion of the corrected data packet back into the received stream of data in the same time slot of the time frame following its original transmission. While such a technique allows the ARQ principles of operation to be adapted from the TDD environment to an FDD environment, there potentially can be an impact on data latency, particularly if multiple retransmissions in the same time slot are required. To address this data latency issue, the receiver is preferably configured with a buffer large enough to hold all data packets received since the oldest unresolved error.
The use of an ARQ error correction mechanism may be more difficult if asymmetric data transmission is supported. In a TDD environment, asymmetric data transmission generally involves the allocation of a greater amount of a TDD time slot to one link of the duplex channel than to the other link. For example, the forward link transmission of a TDD time slot may be allocated 75% of the time slot, while the reverse link transmission may in such a case be allocated 25% of the time slot. Asymmetric data transmission, while possible in some TDD systems, is more difficult to implement in an FDD system. This is because channels are assigned in FDD systems as duplex pairs (i.e., one base transmit time slot and one user transmit time slot), and the unused portion of the base transmit time slot cannot, by definition, be used by the user station for transmission, and vice versa, due to the frequency separation between the base station
104
and the user station
102
.
In one embodiment using dynamic time slot assignment, more base transmit time slots than user transmit time slots are assigned to a single user station
102
, or vice versa, thus allowing a form of asymmetric communication between the base station
104
and the particular user station
102
. However, in such a system the ARQ mechanism described above may have difficulty being implemented because there is no uniform match-up between base transmit time slots and user transmit time slots. In this embodiment, control traffic messages may be used to support error correction. For example, the recipient of the larger amount of data may send a control traffic message (e.g., a CT-ARQ message) to the sender providing an acknowledge or non-acknowledge (ACK/NAK) for each time slot of information that has been transmitted since the last CT-ARQ message. While the control traffic (CT-ARQ) message does take some overhead, only one such message need be used to provide error information concerning a multiplicity of time slots. The periodicity of the CT-ARQ message depends primarily on the length of the control traffic message (requiring, in the above-described embodiment, one ARQ bit for each time slot), and the size of the data message buffer at the receiver.
According to the methods and techniques described above, a TDD base station can be adapted to support an FDD frame structure, thereby allowing use of the same equipment to achieve different types of multiple access communication. Being able to employ the same equipment in different multiple access communication environments can achieve reduced cost of equipment design and manufacturing, and may allow those equipment manufacturers and/or service providers that have developed or deployed TDD systems to, in many cases, readily and rapidly convert to FDD systems without substantial re-design effort.
According to another embodiment, multiple TDD base stations are combined to support full “virtual” FDD communication capability. A preferred embodiment of a virtual FDD base station
701
is shown in
FIG. 7
, in which a pair of virtual base station sub-units
702
a
,
702
b
interact to support FDD communication. Each virtual base station sub-unit
702
a
,
702
b
may comprise a TDD base station (such as base station
401
shown in
FIG. 4
) that has been modified to provide for FDD (or virtual FDD) communication according to principles previously described herein. The virtual base station sub-units
702
a
,
702
b
may, if desired, share a common antenna
706
, or may alternatively use separate antennas. A backhaul coordinator
711
may be provided to assist in multiplexing data and control information over a common backhaul line
720
. The two virtual base station sub-units
702
a
,
702
b
are preferably synchronized, and may, for example, be connected to a common synchronization unit
710
, as shown in FIG.
7
.
FIG. 8
depicts an example of a frame structure
801
that can be supported with the virtual FDD base station
701
shown in FIG.
7
. According to the frame structure
801
illustrated in
FIG. 8
, communication is carried out over a base transmission frequency band
821
and a user transmission frequency band
822
. A time frame
802
comprises, with respect to the base transmission frequency band
821
, a first half
807
during which the first base station sub-unit
702
a
transmits, and a second half
808
during which the second base station sub-unit
702
b
transmits. The time frame
802
further comprises, with respect to the user transmission frequency band, a first half
811
during which user stations
102
transmit user-to-base messages to the second base station sub-unit
702
b
, and a second half
812
during which user stations
102
transmit user-to-base messages to the first base station sub-unit
702
a.
The first base station sub-unit
702
a
essentially communicates according to the pattern of the frame structure
601
shown in
FIG. 6
, and the second base-station sub-unit
702
b
essentially communicates in the same pattern, but offset by a half time frame such that the base transmissions from the first base station sub-unit
702
a
do not interfere with the base station transmissions from the second base station sub-unit
702
b
, and the user station transmissions for user stations in communication with either the first base station sub-unit
702
a
or the second base station sub-unit
702
b
do not interfere. In one aspect the frame structure of
FIG. 8
may therefore be viewed as an “interleaved” frame structure.
The net effect of the frame structure
801
shown in
FIG. 8
is to double the capacity over the frame structure
601
of
FIG. 6
, by adding a second “modified” TDD base station (i.e., base station sub-unit
702
b
) which is active during the periods that the first “modified” TDD base station (i.e., base station sub-unit
702
a
) is inactive, and vice versa. Together, the two base station sub-units
702
a
,
702
b
support twice as many user stations
102
as either alone could support, using the same frequency resources. If, for example, each base station sub-unit
702
a
,
702
b
supports 16 user stations
102
in full duplex, then the two base station sub-units
702
a
,
702
b
together may support up to 32 user stations in full duplex.
FIG. 9
shows an alternative frame structure
901
that can be supported by the virtual FDD base station
701
shown in FIG.
7
. According to the frame structure
901
shown in
FIG. 9
, a time frame
902
is divided into a series of base transmit time slots
905
with respect to a base transmission frequency band
921
and a series of user transmit time slots
906
with respect to a user transmission frequency band
922
. To support the time frame
902
shown in
FIG. 9
, the two base station sub-units
702
a
,
702
b
transmit and receive in alternate time slots. The first base station sub-unit
702
a
transmits during the odd-numbered base transmit time slots
905
a
, and receives during the even-numbered user transmit time slots
906
b
. Conversely, the second base station sub-unit
702
b
transmits during the even-numbered base transmit time slots
905
b
, and receives during the odd-numbered user transmit time slots
906
a
. Each of the base station sub-units
702
a
,
702
b
is essentially configured to support a frame structure similar to that of
FIG. 5
(except that the base transmission precedes, rather than follows, the corresponding user transmission), with the effective time frame of the second base station unit
702
b
offset by one time slot from that of the first base station sub-unit
702
a
. In this manner, as with the frame structure of
FIG. 8
, the virtual FDD base station
701
achieves twice the capacity over the base station configured to support the frame structure of either
FIG. 5
or
FIG. 6
alone.
In order to achieve the “virtual” FDD frame structure shown in
FIG. 8
or
9
, the virtual base station sub-units
702
a
,
702
b
are, as previously indicated, preferably synchronized such that the start of each time frame and time slot is coordinated. In one embodiment, a synchronization unit
710
is connected to each of the virtual base station sub-units
702
a
,
702
b
to maintain frame and slot synchronization between them. Alternatively, one of the virtual base station sub-units (e.g.,
702
a
) can send a frame signal, slot signal and/or clock signal to the other virtual base station sub-unit (e.g.,
702
b
), thereby achieving synchronization using a master-slave clocking method. Alternatively, the first virtual base station sub-unit
702
a
sends only a start-of-frame marker to the other virtual base station sub-unit
702
b
, which then may synchronize its own internal clock(s) using a phase-locked loop. Alternatively, synchronization may be achieved in each virtual base station sub-unit
702
a
,
702
b
by using a timing marker from an external source (such as a base station controller (not shown)) that connects to the base station sub-units
702
a
,
702
b
through the backhaul line
720
. A base station controller can also, if desired, connect to other base stations in the same geographic region. Synchronization may also be achieved by providing a GPS receiver in each base station sub-unit
702
a
,
702
b
, or using a similar external timing reference, and communicating start-of-frame information between the two base station sub-units
702
a
,
702
b
if otherwise not provided by the external timing reference.
In addition to synchronizing the virtual base station sub-units
702
a
,
702
b
of the virtual FDD base station
701
, it is also preferably to synchronize the base stations
104
(including any of which are embodied as FDD base station
701
) within a geographic region. For example, the base stations
104
(see
FIG. 1
) can be synchronized by receiving a timing marker over a backhaul connection from a common base station controller, or from some other system component connected over the backhaul path. Preferably, base stations
104
within a geographical region are both frame-synchronized and slot-synchronized, which can lead to higher potential capacity, potentially reduced interference, and faster handoffs between base stations
104
.
FIG. 10
depicts another embodiment of an FDD base station
1011
. As shown in
FIG. 10
, the FDD base station
1011
comprises a pair of base station sub-units
1012
a
,
1012
b
. The base sub-units
1012
a
,
1012
b
can be connected to a backhaul coordinator
1021
in a manner similar to the base station
701
of
FIG. 7
, and can also be synchronized using a synchronization unit
1020
similar to that shown in
FIG. 7
, or by using any other of the aforementioned synchronization methods. Each of the base station sub-units
1012
a
,
1012
b
in
FIG. 10
may comprise a TDD base station (such as base station
401
of
FIG. 4
) that has been modified to operate such that the transmitter of one of the base sub-units (e.g.,
1012
a
) operates in a continuous fashion, and the receiver of the other of the base sub-units (e.g.,
1012
b
) operates in a continuous fashion. By coordinating operation of the two base station sub-units
1012
a
,
1012
b
, full FDD can be supported.
FIG. 11
depicts an example of an FDD frame structure that can be supported by the base station
1011
of FIG.
10
. As shown in
FIG. 11
, the first base station sub-unit
1012
a
(designated “BS
1
”) transmits, over a base transmission frequency
1121
, in each of a plurality of base transmit time slots
1105
of an FDD time frame
1102
. The second base station sub-unit
1012
b
(designated “BS
2
”) receives, over a user transmission frequency
1122
, in each of a plurality of user transmit time slots
1106
of the FDD time frame
1102
. User stations
102
communicating with the base station
1011
are assigned a pair of time slots (a base transmit time slot
1105
and a user transmit time slot
1106
) in order to carry out duplex communication. The base transmit time slot
1105
is offset from the corresponding user transmit time slot
1106
in each duplex pair by a predefined duration, such as, e.g., eight time slots (or any other suitable duration). In this manner, the base station
1011
may conduct FDD communication using two “modified” TDD base stations as base station sub-units
1012
a
,
1012
b.
The backhaul coordinator
1021
interfaces with the next hardware link in the chain to the network. The backhaul coordinator
1021
sends information received over the backhaul line
1025
to the first base station sub-unit
1012
a
for transmission to the user stations
102
, and receives information received by the second base station sub-unit
1012
b
from user stations
102
for transmission over the backhaul line
1025
.
Certain software or firmware modifications may be employed in the base station sub-units
1012
a
,
1012
b
in order to achieve FDD compatibility. For example, assuming that the base station sub-units
1012
a
,
1012
b
each comprise hardware originally developed for a TDD base station
401
(such as shown in FIG.
4
), the first (transmitting) base station sub-unit
1012
a
may be modified such that all of the memory segments
429
in its memory buffer
411
are treated as transmit memory segments, and the second (receiving) base station sub-unit
1012
b
may be modified such that all of the memory segments
429
in its memory buffer
411
are treated as receive memory segments. In such a case, the backhaul interface
412
of the first base station sub-unit
1012
a
is thereby provided with capability to store information received from the backhaul coordinator
1021
in all of the memory segments
429
of the memory buffer
411
(one memory segment
429
for each base transmit time slot
1105
), and the over-the-air controller
410
is modified so that it removes information from all of the memory segments
429
of the memory buffer
411
as appropriate for the sending of data packets in the base transmit time slots
1105
. Similarly, the backhaul interface
412
of the second base station sub-unit
1012
b
is provided with the capability to remove information from all of the memory segments
429
(one memory segment
429
for each user transmit time slot
1106
) of the memory buffer
411
as appropriate for sending to the backhaul coordinator
1021
and, ultimately, to the network, and the over-the-air controller
410
is modified so that it stores information in all of the memory segments
429
of the memory buffer
411
, each data packet of information being stored in a memory segment
429
according to the user transmit time slot
1106
in which it was received.
The base station
1011
of
FIG. 10
may also comprise a mechanism for coordinating error correction between the two base station sub-units
1012
a
,
1012
b
. For example, if a data packet is received in error, the second base station sub-unit
1012
b
may send the first base station sub-unit
1012
a
an indication that an error was received and which time slot the error occurred in. The first base station sub-unit
1012
a
then may send an ARQ message (i.e., a re-transmit request) to the user station
102
in the appropriate base transmit time slot
1105
. Similarly, if the second base station sub-unit
1012
b
receives an ARQ message from a user station
102
, it will send the ARQ message and a time slot indicator to the first base station sub-unit
1012
a
, which can then re-send the data packet in the appropriate base transmit time slot
1105
.
The principles of the present invention are applicable to both mobile and fixed systems, and the embodiments disclosed herein may be deployed in a mobile communication environment or a fixed wireless local-loop system.
In a preferred embodiment, the base station
104
and user stations
102
communicate using spread spectrum communication. Each of the embodiments previously described can be configured to operate using spread spectrum communication. Suitable spread spectrum transmission and reception techniques are described, for example, in U.S. Pat. Nos. 5,016,255, 5,022,047 or 5,659,574, each of which is assigned to the assignee of the present invention, and each of which is hereby incorporated as if fully set forth herein. Different cells
103
(see
FIG. 1
) may be assigned different spread spectrum codes (or different sets of spread spectrum codes, from which individual codes may be temporarily assigned to individual user stations
102
), thereby obtaining benefits of CDMA.
While preferred embodiments of the invention have been described herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification and the drawings. The invention therefore is not to be restricted except within the spirit and scope of any appended claims.
Claims
- 1. A communication system for virtual FDD communication, comprising:a first base station sub-unit, said first base station sub-unit comprising a first transmitter for transmitting a first plurality of base-to-user messages to a first plurality of user stations only during a first half of a repeating time frame, said first base station sub-unit further comprising a first receiver for receiving a first plurality of user-to-base messages from said first plurality of user stations only during a second half of said repeating time frame; and a second base station sub-unit collocated with said first base station sub-unit, said second base station sub-unit comprising a second transmitter for transmitting a second plurality of base-to-user messages to a second plurality of user stations only during said second half of said repeating time frame, said second base station sub-unit further comprising a second receiver for receiving a second plurality of user-to-base messages from said second plurality of user stations only during said first half of said repeating time frame.
- 2. The communication system of claim 1, further comprising means for synchronizing said first base station sub-unit and said second base station sub-unit.
- 3. The communication system of claim 2, wherein said means for synchronizing comprises a synchronization unit connected to said first base station sub-unit and said second base station sub-unit, said synchronization unit providing a time frame marker and a time slot marker to said first base station sub-unit and said second base station sub-unit.
- 4. The communication system of claim 2, wherein said means for synchronizing comprises a master-slave clock connection between said first base station sub-unit and said second base station sub-unit.
- 5. The communication system of claim 2, wherein said means for synchronizing comprises a time frame marker signal from a base station controller connected to both said first base station sub-unit and said second base station sub-unit.
- 6. The communication system of claim 1, wherein said first base station sub-unit and said second base station sub-unit share at least one common antenna.
- 7. The communication system of claim 1, further comprising a backhaul interface connected to said first base station sub-unit and said second base station sub-unit, said backhaul interface providing multiplexing and de-multiplexing of communication channels over a backhaul signal line.
- 8. The communication system of claim 1, wherein said first base station sub-unit communicates with each of said first plurality of user stations in a duplex time slot assigned to the user station, and wherein said second base station sub-unit communicates with each of said second plurality of user stations in a duplex time slot assigned to the user station.
- 9. The communication system of claim 8, wherein said first plurality of base-to-user messages are interleaved with said second plurality of base-to-user messages, and wherein said first plurality of user-to-base messages are interleaved with said second plurality of user-to-base messages.
- 10. The communication system of claim 8, wherein said first plurality of base-to-user messages are consecutive without any intervening base-to-user message of said second plurality of base-to-user messages, wherein said second plurality of base-to-user messages are consecutive without any intervening base-to-user message of said first plurality of base-to-user messages, wherein said first plurality of user-to-base messages are consecutive without any intervening user-to-base message of said second plurality of user-to-base messages, and wherein said second plurality of user-to-base messages are consecutive without any intervening user-to-base message of said first plurality of user-to-base messages.
- 11. A method for FDD communication, comprising:generating a repeating time frame; generating a first plurality of duplex time slots and a second plurality of duplex time slots in said time frame, each duplex time slot of said first plurality of duplex time slots comprising a base transmit time slot within a first half of said time frame and a user transmit time slot within a second half of said time frame, and each duplex time slot of said second plurality of duplex time slots comprising a base transmit time slot within said second half of said time frame and a user transmit time slot within said first half of said time frame, and wherein the base transmit time slot of each duplex time slot bears the same temporal relationship with its respective user transmit time slot; assigning, on demand, said first plurality of duplex time slots and said second plurality of duplex time slots to user stations for communication with the base station; transmitting, over a base transmit frequency band and from a first base station sub-unit, a first plurality of base-to-user messages during the base transmit time slots of said first plurality of duplex time slots; receiving said first plurality of base-to-user messages at a first plurality of said user stations; transmitting, over a base transmit frequency band and from a second base station sub-unit, a second plurality of base-to-user messages during the base transmit time slots of said second plurality of duplex time slots; receiving said second plurality of base-to-user messages at a second plurality of said user stations; transmitting, over a user transmit frequency band and from said first plurality of user stations, a first plurality of user-to-base messages during the user transmit time slots of said first plurality of duplex time slots; receiving said first plurality of user-to-base messages at said first base station sub-unit; transmitting, over said user transmit frequency band and from said second plurality of user stations, a second plurality of user-to-base messages during the user transmit time slots of said second plurality of duplex time slots; and receiving said second plurality of user-to-base messages at said second base station sub-unit.
- 12. The method of claim 11, wherein said first half of said time frame and said second half of said time frame each comprise a contiguous portion of said time frame.
- 13. The method of claim 11, wherein said first half of said time frame and said second half of said time frame each comprise non-contiguous portions of said time frame, such that base transmit time slots of said first plurality of duplex time slots are interleaved with base transmit time slots of said second plurality of duplex time slots, and user transmit time slots of said first plurality of duplex time slots are interleaved with user transmit time slots of said second plurality of duplex time slots.
- 14. The method of claim 13, wherein said base transmit time slots of said first plurality of duplex time slots alternate with the base transmit time slots of said second plurality of duplex time slots, and wherein said user transmit time slots of said first plurality of duplex time slots alternate with the user transmit time slots of said second plurality of duplex time slots.
- 15. The method of claim 14, wherein the base transmit time slot and user transmit time slot of each duplex time slot are separated by at least one time slot.
- 16. The method of claim 11, wherein generating a repeating time frame comprises synchronizing said first base station sub-unit and said second base station sub-unit.
- 17. A method for FDD communication, comprising:generating a repetitive time frame; allocating a first portion of said time frame to a first base station sub-unit for transmitting a first plurality of base-to-user messages to a first plurality of user stations over a base transmit frequency band; allocating a second portion of said time frame to a second base station sub-unit for transmitting a second plurality of base-to-user messages to a second plurality of user stations over said base transmit frequency band, said first portion of said time frame and said second portion of said time frame comprising substantially the entirety of said time frame; allocating said second portion of said time frame to said first base station sub-unit for receiving a first plurality of user-to-base messages from said first plurality of user stations over a user transmit frequency band; and allocating said first portion of said time frame to said second base station sub-unit for receiving a second plurality of user-to-base messages from said second plurality of user stations over said user transmit frequency band.
- 18. The method of claim 17, wherein generating a repetitive time frame comprises synchronizing said first base station sub-unit and said second base station sub-unit.
- 19. The method of claim 17, wherein said first base station sub-unit and said second base station sub-unit each transmit base-to-user messages and receive user-to-base messages in duplex time slots assigned to said first plurality of user stations and said second plurality of user stations.
- 20. A method for communication, comprising:generating a time frame at a base station; generating a plurality of time slots within said time frame; assigning pairs of said time slots for duplex communication to a plurality of user stations on demand, each pair of time slots comprising a user transmit time slot and a base transmit time slot, such that no more than half of said time slots are assigned as user transmit time slots and no more than half of said time slots are assigned as base transmit time slots, and wherein each user transmit time slot from each pair of time slots is separated temporarily from its respective base transmit time slot in the pair by the same amount; transmitting, over a base transmission frequency band, base-to-user messages from the base station to said user stations during said base transmit time slots; receiving said base-to-user messages at said user stations, each user station receiving a base-to-user message in its assigned base transmit time slot; transmitting, over a user transmission frequency band separate and distinct from said base transmission frequency band, user-to-base messages from said user stations to the base station during said user transmit time slots, each user station transmitting a user-to-base message in its assigned user transmit time slot; and receiving said user-to-base messages at the base station.
- 21. The method of claim 20 wherein said base transmit time slots alternate with said user transmit time slots.
- 22. The method of claim 20 wherein all of said base transmit time slots are consecutive without any intervening user transmit time slots, and wherein all of said user transmit time slots are consecutive without any intervening base transmit time slots.
- 23. A base station, comprising:a radio transceiver, said radio transceiver comprising a base transmitter and a base receiver selectably connected to at least one base antenna by a transmit/receive switch, and said radio transceiver further comprising a voltage controlled oscillator whereby a transmission/reception frequency of said radio transceiver is set; a memory buffer connected to said radio transceiver, said memory buffer partitioned into a plurality of memory segments according to separate time division duplex communication channels, one memory segment for each such time division duplex communication channel; an over-the-air controller connected to said radio transceiver, said over-the-air controller comprising a time frame counter and a time slot counter; a toggle signal output from said over-the-air controller to said transmit/receive switch, said toggle signal causing said base transmitter and base receiver to be alternately connected to said at least one base antenna; a frequency selection signal output from said over-the-air controller to the voltage controlled oscillator of said radio transceiver, said frequency selection signal causing said radio transceiver to alternate between a base transmit frequency band and a base receive frequency band, said base transmit frequency band being selected when said base transmitter is connected to said at least one base antenna, and said base receive frequency band being selected when said base receiver is connected to said at least one base antenna; and a backhaul interface connected to said memory buffer, said backhaul interface multiplexing information from said memory buffer for transmission over a backhaul line, and demultiplexing information received over said backhaul line for storage in said memory buffer.
- 24. The base station of claim 23, wherein said toggle signal switches states with each change in state of said time slot counter.
- 25. The base station of claim 23, wherein said toggle signal switches states in response to said time slot counter, each time a predetermined number of time slots are counted.
- 26. A method for frequency division duplex (FDD) communication, comprising:generating a repeating time frame; generating a first plurality of duplex time slots and a second plurality of duplex time slots in the time frame, each duplex time slot of the first plurality of duplex time slots comprising a base transmit time slot within a first half of the time frame and a user transmit time slot within a second half of the time frame, each duplex time slot of the second plurality of duplex time slots comprising a base transmit time slot within the second half of the time frame and a user transmit time slot within the first half of the time frame; assigning at least a portion of the first plurality of duplex time slots and the second plurality of duplex time slots to user stations; transmitting a first plurality of base-to-user messages to a first plurality of the user stations from a first base station sub-unit over a base transmit frequency band during the base transmit time slots of the first plurality of duplex time slots; transmitting a second plurality of base-to-user messages to a second plurality of the user stations from a second base station sub-unit over the base transmit frequency band during the base transmit time slots of the second plurality of duplex time slots; receiving a first plurality of user-to-base messages from the first plurality of user stations at the first base station sub-unit over a user transmit frequency band during the user transmit time slots of the first plurality of duplex time slots; and receiving a second plurality of user-to-base messages from the second plurality of user stations at the second base station sub-unit over the user transmit frequency band during the user transmit time slots of the second plurality of duplex time slots.
- 27. A method for communication, comprising:generating a time frame; generating a plurality of time slots within the time frame; assigning pairs of the time slots for duplex communication to a plurality of user stations, each pair of time slots comprising a user transmit time slot and a base transmit time slot, no more than half of the time slots assigned as user transmit time slots and no more than half of the time slots assigned as base transmit time slots; transmitting base-to-user messages from a base station to the user stations over a base transmission frequency band during the base transmit time slots; and receiving user-to-base messages from the user stations at the base station over a user transmission frequency band separate and distinct from the base transmission frequency band during the user transmit time slots.
US Referenced Citations (8)