Information
-
Patent Grant
-
6816472
-
Patent Number
6,816,472
-
Date Filed
Friday, July 14, 200023 years ago
-
Date Issued
Tuesday, November 9, 200419 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Patel; Ajit
- Blount; Steven A
Agents
- Lamb; James A.
- Garrett; Scott M.
-
CPC
-
US Classifications
Field of Search
US
- 370 331
- 370 320
- 370 335
- 370 338
- 370 342
- 370 441
- 370 447
- 370 332
- 455 436
- 455 442
- 455 450
-
International Classifications
-
Abstract
A technique is used in a spread spectrum communication system (100) such as cdma2000 or UMTS to assign a subscriber unit (120) a link for a supplemental channel, during a handoff period. The technique includes queuing a data packet for transmission to the subscriber unit at a packet transmission time (221) determined by a queue delay (220), obtaining pilot signal strength measurements of at least two active downlinks (125) of the subscriber unit that are measured substantially at the packet transmission time, and determining a strongest one of the at least two active downlinks from the pilot signal strength measurements. The pilot signal strength measurements are obtained by determining a pilot signal measurement time (PSMT) for the subscriber unit, calculating a pilot signal measurement delay (PSMD) from the PSMT and the queue delay, and transmitting a pilot signal measurement request (PSMRQ) to the subscriber unit after the PSMD.
Description
FIELD OF THE INVENTION
This invention relates in general to spread spectrum communication systems, and in particular to a method and apparatus for assigning an optimum link for data packet transmission in a supplemental channel (of a CDMA 2000 system) or a downlink shared channel (in a UMTS system) during soft handoff in a spread spectrum system.
BACKGROUND OF THE INVENTION
Communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone, personal communication systems, and other communication system types. Within a communication system, transmissions are conducted between a transmitting device and a receiving device over a communication resource, commonly referred to as a communication channel. Transmissions used to consist primarily of voice signals and low speed data signals. More recently, however, it has proposed to use radio communication systems for high-speed data signals. For ease of operation, it is preferable to have the data transmission capability overlay the existing voice communication capability, such that its operation is essentially transparent to the voice communication system while still utilizing the communication resources and other infrastructure of the voice communication system.
Two such communication systems currently being developed with transparent data transmission capabilities are spread spectrum communication systems known as the next generation Code-Division Multiple Access (CDMA) cellular communication system, or cdma2000, and the next generation Global System for Mobile Communications (GSM), or Universal Mobile Telephone System (UMTS). Within these well known spread spectrum communication systems, all subscriber unit transmissions occur simultaneously within a frequency band, and all base station transmissions occur simultaneously within a frequency band. Therefore, a received signal at a base station or a subscriber unit comprises a multiplicity of frequency and time overlapped coded signals form individual subscriber units or base station units, respectively. Each of these signals is transmitted simultaneously at the same radio frequency (RF) and is distinguishable only by its specific encoding (channel).
Within spread spectrum communication systems, a subscriber is typically assigned at least one link that is used to communicate information between the communication system and the subscriber unit. Each link comprises channels that are assigned to communicate information between the subscriber unit and a (geographic) sector of a base transmitter site. Every link includes one channel called a pilot signal that is used for several purposes, including setting up and monitoring the signal strength of the link. The link can also comprise what is named herein a “fundamental channel” that is dedicated only to one subscriber unit during the duration of a voice call and is used to transfer voice information between the subscriber unit and the communication system. The fundamental channel as named herein is called a fundamental channel in the cdma200 communication system but is called a dedicated channel in the UMTS communication system. The link can also comprise what is called herein a supplemental channel that is assigned to a subscriber unit to transfer high-speed digital information between a subscriber unit and the communication system, but the assignment lasts only as, long as needed to accomplish the transfer of the data. The supplemental channel as named herein is called a supplemental channel in the cdma200 communication system but is called a shared channel in the UMTA communication system. Although the supplemental channel of the cdma2000 and the shared channel of the UMTS system have some quite different characteristics, they also share some common characteristics. The same is true for the fundamental channel of the cdma2000 system and the dedicated channel of the UMTS system.
At some times during which a subscriber unit is being used in the communication system, only one link is assigned to the subscriber unit, because the strength of the link from the base transmitter site to the subscriber unit (the downlink) has been determined to be sufficient to provide consistent high quality service. However, at other times, the subscriber is located within the communication system at a point where no single downlink can provide consistent high quality service, but lower quality downlinks are possible to more than base transmitter site sectors. Both conditions can occur at different times during one voice call. A unique aspect of spread spectrum communication systems is that the use of the spread spectrum modulation and coding technique allows a combining of the multiple received signals that carry the same information. The combining adds together the signal strengths of the individual signals, and in many instances is capable of providing a high quality received signal from the several downlinks. This combining is typically used in spread spectrum communication systems for the fundamental channels (the voice channels) until one of the downlinks becomes strong enough to provide high quality service by itself, at which time all other links are dropped. The combining is, however, not allowed for the supplemental channels, because they occupy a substantial portion of the RF resources, in terms of power and bandwidth, and because they are typically very short compared to voice calls. The period of time during which the combining takes place is called the “soft handoff period” or “handoff period” and the operation is called the “handoff” by those of ordinary skill in the art, because it commonly occurs when a new single link is assigned and an old link is dropped. The terms “soft handoff period” and “handoff period” are also used herein for the time period during which a fundamental channel would operate in the combining mode but is not assigned to the subscriber unit (for example, because there is no voice call occurring). During a handoff period, the links that are assigned or would be assigned to a subscriber unit for combining are called the active links.
When a subscriber unit is not in operating during a handoff period, a supplemental channel is assigned to the same link as the pilot channel, and the quality of service for the supplemental channel is will be satisfactory. However, during a handoff, there is a problem in accurately determining a best downlink for transmission of a data packet using a supplemental channel because of a combination of factors: the shortness of typical data packets, an uncertainty as to when the data packet will be transmitted due to system queuing delays, and the difficulty of knowing the signal strength of the active links at them moment the data packet is transmitted.
Thus, what is needed is a technique for selecting a best link for supplemental channel assignment during a handoff period in a spread spectrum system.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings, in which:
FIG. 1
shows a block diagram of a portion of a spread spectrum communication system, in accordance with the preferred embodiment of the present invention.
FIG. 2
shows a block diagram of a computer used in a radio network controller of the spread spectrum communication system shown in
FIG. 1
, in accordance with the preferred embodiment of the present invention.
FIG. 3
is a timing diagram of pilot signal measurement messages, in accordance with the preferred embodiment of the present invention.
FIG. 4
is a flow chart of a method used in the computer of the spread spectrum communication system, in accordance with the preferred embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The definitions of terms used herein below are identical to those set forth above. While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. Further, the terms and words used herein are not to be considered limiting, but rather merely descriptive. In the description below, like reference numbers are used to describe the same, similar, or corresponding parts in the several views of the drawings.
In general, one aspect of the present invention is a technique used during a handoff period in a spread spectrum communication system to assign a subscriber unit a link for a supplemental channel. The technique includes queuing a data packet for transmission to the subscriber unit at a packet transmission time determined by a queue delay, obtaining pilot signal strength measurements of at least two active downlinks of the subscriber unit that are measured substantially at the packet transmission time, and determining a strongest one of the at least two active downlinks from the pilot signal strength measurements. The link that includes the strongest downlink is then assigned to carry the supplemental channel.
Referring now to the drawings and in particular to
FIG. 1
, a block diagram of a portion of a spread spectrum communication system
100
is shown, in accordance with the preferred embodiment of the present invention. The spread spectrum communication system
100
comprises a radio network controller (RNC)
105
that is coupled to a plurality of base transmitter sites
110
, of which three are shown in
FIG. 1
, by conventional fixed network links
115
that can include, for example, broadband cable and optical cable. The base station sites
115
include base station radio transmitters (not shown separately in
FIG. 1
) that transmit information on assigned channels of the spread spectrum communication system, the RF energy being radiated within one or more predetermined geographic sectors of each base station transmitter, as is well known to one of ordinary skill in the art. The spread spectrum communication system
100
also comprises a plurality of subscriber units
120
, of which one is shown in FIG.
1
. The subscriber units
120
are conventional cellular telephones. The subscriber unit
120
shown in
FIG. 1
is illustrated in a handoff wherein it is assigned three active links
125
for a voice call on a fundamental channel.
The RNC
105
comprises a computer
106
that performs a plurality of functions including, but not limited to, a radio link control function (RLC)
150
and a medium access control function (MAC)
160
. It will be appreciated that the computer
106
comprises a processor
107
and a memory portion (memory)
108
, as shown in
FIG. 2
, and that the functions performed by the computer
106
are performed by the processor
106
executing programmed instructions that are stored in the memory
109
. The memory
109
also stores data that is used in the performance of the functions. The processor
107
and memory
108
are conventional computer hardware assemblies. The processor comprises a central processing unit and can comprise other conventional hardware components as parallel input-output buffers, digital-to-analog and analog-to-digital converters, and serial data input-output lines. The memory comprises conventional combinations of such things as read only memory, random memory, and disk memory. The set of programming instructions and the organization of the data stored in the memory
108
are uniquely designed to perform the unique functions described herein.
The MAC
160
comprises a scheduling function (scheduler)
165
and a data packet queuing function (data packet queue)
170
. Data packets are received at or generated by the RLC
150
for delivery to subscriber units. For example, a data packet carrying a portion of a web page that has been delivered to the RNC
105
from the World Wide Web is received by the RLC
150
for delivery to the subscriber unit
120
. The RNC
105
hands the data packet off to the MAC
160
. The scheduler
165
determines which link is to be assigned to the subscriber unit
120
for transmission of the data packet (i.e., at which base transmitter site
110
a link having a supplemental channel is to be assigned), and transfers to the data packet to the data packet queue
170
for transmission after all data packets of the same priority have been transmitted.
It will be appreciated that portions of the MAC
160
can alternatively reside in the BTSs
110
, although the portions described in detail herein are preferably performed in the RNC
105
.
Referring now to
FIG. 3
, a timing diagram of pilot signal measurement messages is shown, in accordance with the preferred embodiment of the present invention. During normal operation of the spread spectrum communication system
100
, pilot signal strength measurements (PSSMs) are made by the subscriber unit
120
at various times as directed by the radio network controller
106
or the base transmitter site
110
. These PSMs can be made using functions that are conventional functions of the spread spectrum communication system
100
or they can be using the unique functionsdescribed herein that are performed in accordance with the preferred embodiment of the present invention. The PSSMs are initiated by a pilot signal measurement request (PSMRQ)
206
, which is a predetermined command. After receiving the PSMRQ
206
, the subscriber unit
120
measures the pilot signal strength for all active downlinks, the subscriber unit transmits a pilot signal measurement response (PSMRS)
207
, which is a data packet that includes the PSMs. Several pairs of these PSMRQs
206
and PSMRSs
207
are shown at the left side of
FIG. 3
, using a first time scale
201
. On a second time scale
202
, which is a less compressed time scale than the first time scale
201
, two other pairs
208
,
209
of these PSMRQs
206
and PSMRSs
207
are shown. The time between a PSMRQ
206
and a corresponding PSMRS
207
, which is called herein the pilot signal measurement time (PSMT)
205
, tends to be approximately the same for any two pairs that occur sufficiently close together in time. A typical value for the PSMT is 150 milliseconds. Another way to say this is that the correlation of a PSSM to actual signal strengths at a later time is high only for a period of time, typically 1000 to 2000 milliseconds. When the RLC
150
transfers a data packet to the MAC
160
during a handover period (this occurrence is shown as time
215
in FIG.
3
), the scheduler
165
determines a queue delay
220
, a packet transmission time
221
, and a last PSMRS time
211
for the data packet. The queue delay
220
is the delay (from time
215
to the transmission time
221
) that the data packet will experience if it is placed in the data packet queue
170
before another data packet is placed in or transmitted from the data packet queue
170
. The last PSMRS time
211
is the duration from the last PSMRS
207
to the time
215
when the data packet is received. According to the details described herein below, a PSMRQ
206
is then transmitted, under certain conditions, by the base transmitter sites
120
on the fundamental channels of the active links currently assigned to the subscriber unit
120
(or the active links that would be assigned if there were a voice call in process). This transmission of the PSMRQ
206
occurs after a pilot signal measurement delay (PSMD)
210
. The subscriber unit
120
transmits the PSMRS
207
prior to the queue delay by a duration that is approximately equal to a time margin
230
, which is described below. An allowed delay
225
is also shown in
FIG. 3
that represents a maximum amount of delay that is allowed for a data packet intended for the subscriber unit
120
, corresponding to a grade of service that is assigned to the subscriber unit
120
.
Referring now to
FIG. 4
, a flow chart of a method used in the computer
106
of the spread spectrum communication system
100
is shown, in accordance with the preferred embodiment of the present invention. At step
410
, the RNC
105
accepts PSMs as they are received from any active subscriber units and uses those from the subscriber unit
120
to update an estimated PSMT
205
for each link in active use by the subscriber unit
120
. An update time is also recorded, which is the time at which the most recent PSMRS
107
was received. The updated estimate is a weighted estimate that creates a weighted average of the most recent PSSM and other older PSSMs, with the older PSMs given more diminished weights the older they are. Other well-known weighting methods could be alternatively used. At step
415
, a data packet is received or generated by the RLC
150
, and transferred to the MAC
160
. The scheduler
165
determines whether the subscriber unit
120
is in a handoff. If the subscriber unit
120
is not in a handoff period, the scheduler
165
queues the message in the data packet queue
170
and exits this function at step
465
. If the subscriber unit
120
is in a handoff period, the scheduler
165
queues the message in the data packet queue
170
and determines the queue delay
220
(FIG.
3
). Then at step
430
the scheduler
165
compares the queue delay
220
to the allowed delay
225
and when it is not greater than the allowed delay
225
, which is the typical situation in a communication system that is performing normally, the scheduler
165
determines whether the most recent PSSM is stale. This is done by comparing the last PSMRS time
211
to a predetermined stale time. As indicated above, because a past PSSM correlates well to the actual present pilot signal strengths only within a limited time, the predetermined stale time is chosen to ensure that the correlation of the most recent PSSM is good. When the last PSMRS time
211
is greater than the stale time, the particular PSMRS
207
and the PSSM associated with it are deemed to be stale. In this case, which occurs fairly often, the scheduler
165
then determines whether the PSMT is greater than the queue delay. This will normally not be the case, in response to which the scheduler
165
determines the PSMD
210
at step
445
by subtracting the estimated PSMT
205
and the time margin
230
from the queue delay, and causes a PSMRQ
206
to be transmitted to the subscriber unit at step
446
at a delay time after the data packet—is received at time
215
, for which the delay time is the PSMD
210
just calculated. It can be seen that this unique method of transmitting a PSMRQ
206
after a data packet is received results in a PSMRS
207
being received before the time at which the data packet is to be transmitted by the amount of time margin
230
. The time margin
230
is a period during which the computer
106
makes a selection of a best downlink from the active links
125
an accomplishes a set-up of a supplemental channel on the link having the best downlink to use for the transmission of the data packet, at step
450
. The selection is made using the pilot signal measurements obtained just before the transmission of the data packet, using conventional methods of determining at the best downlink from the measurements.
When at step
430
the queue delay
220
is determined to be greater than the allowed delay
225
, the data packet is dropped from the data packet queue at step
460
. When the most recent PSMRS
207
is determined to be not stale at step
435
and the PSMT
205
is greater than the allowed delay
225
at step
455
, the data packet is dropped from the data packet queue at step
460
. When the most recent PSMRS
207
is determined to be stale at step
435
and the PSMT
205
is determined to be greater than the queue delay
220
at step
440
and the allowed delay
225
at step
455
, the data packet is dropped from the data packet queue at step
460
.
It will be appreciated that some of the steps of the flow chart described with reference to
FIG. 4
can be performed in a different order than described and the same results can be accomplished by different steps. For example, steps performed in a different order could accomplish the logical results produced by steps
430
,
435
,
440
, and
455
.
While the preferred and other embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the following claims.
Claims
- 1. In a spread spectrum communication system, a method used during a handoff period to assign a subscriber unit a supplemental channel, comprising the steps of:queuing a data packet for transmission to the subscriber unit at a packet transmission time determined by a queue delay; obtaining pilot signal strength measurements of at least two active downlinks of the subscriber unit that are measured substantially at the packet transmission time, comprising: determining a pilot signal measurement time (PSMT) for the subscriber unit; calculating a pilot signal measurement delay (PSMD) from the PSMT and the queue delay; and transmitting a pilot signal measurement request (PSMRQ) to the subscriber unit after the PSMD; and determining a strongest one of the at least two active downlinks from the pilot signal strength measurements.
- 2. The method according to claim 1, further comprising the step of assigning to the subscriber unit the supplemental channel of the active link that includes the strongest one of the at least two active downlinks.
- 3. The method according to claim 2, wherein the pilot signal measurement delay (PSMD) is further based on an time margin and is determined as:PSMD=queue delay−PSMT−time margin.
- 4. The method according to claim 1, wherein at least one of the steps of queuing, obtaining, and determining are performed by a radio network controller of the communication system.
- 5. The method according to claim 1, wherein at least one of the steps of queuing, obtaining, and determining are performed by a base transmitter site.
- 6. The method according to claim 1, further comprising the step of:dropping the data packet from the queue when the queue delay is greater than an allowed delay.
- 7. The method according to claim 2, further comprising the step of:dropping the data packet from the queue when a most recent pilot signal measurement response (PSMRS) is not stale and the PSMT is greater than an allowed delay.
- 8. The method according to claim 2, further comprising the step of:dropping the data packet from the queue when a most recent pilot signal measurement response (PSMRS) is stale and the PSMT is greater than the queue delay and greater than an allowed delay.
- 9. A computer for a spread spectrum communication system that assigns a supplemental channel to a subscriber unit during a handoff period, comprising:a central processing unit for executing programmed instructions; and a memory that stores the programmed instructions and data packets, wherein the programmed instructions control the processor to perform a function that queues a data packet for transmission to the subscriber unit at a packet transmission time determined by a queue delay, obtains pilot signal strength measurements of at least two active downlinks of the subscriber unit that are measured substantially at the packet transmission time by determining a pilot signal measurement time (PSMT) for the subscriber unit; calculating a pilot signal measurement delay (PSMD) from the PSMT and the queue delay; and transmitting a pilot signal measurement request (PSMRQ) to the subscriber unit after the PSMD, and determines a strongest one of the at least two active downlinks from the pilot signal strength measurements.
- 10. The computer according to claim 9, wherein the computer is within a radio network controller and the function is a medium access control function.
- 11. The computer according to claim 9, wherein the computer is within a base transmitter site.
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