U.S. patent application number 11/417946 was filed with the patent office on 2006-11-23 for method and apparatus for controlling uplink transmissions of a wireless communication system.
This patent application is currently assigned to QUALCOMM Incorporated. Invention is credited to Franklin Peter Antonio, Jack M. Holtzman, Mark S. Wallace, Jay Rodney Walton.
Application Number | 20060262750 11/417946 |
Document ID | / |
Family ID | 25304663 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060262750 |
Kind Code |
A1 |
Walton; Jay Rodney ; et
al. |
November 23, 2006 |
Method and apparatus for controlling uplink transmissions of a
wireless communication system
Abstract
Techniques to partition and allocate the available system
resources among cells in a communication system, and to allocate
the resources in each cell to terminals for data transmission on
the uplink. In one aspect, adaptive reuse schemes are provided
wherein the available system resources may be dynamically and/or
adaptively partitioned and allocated to the cells based on a number
of factors such as the observed interference levels, loading
conditions, system requirements, and so on. A reuse plan is
initially defined and may be redefined to reflect changes in the
system. In another aspect, the system resources may be partitioned
such that each cell is allocated a set of channels having different
performance levels. In yet another aspect, terminals in each cell
are scheduled for data transmission (e.g., based on their priority
or load requirements) and assigned channels based on their
tolerance to interference and the channels' performance.
Inventors: |
Walton; Jay Rodney;
(Carlisle, MA) ; Wallace; Mark S.; (Bedford,
MA) ; Holtzman; Jack M.; (San Diego, CA) ;
Antonio; Franklin Peter; (Del Mar, CA) |
Correspondence
Address: |
QUALCOMM INCORPORATED
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Assignee: |
QUALCOMM Incorporated
|
Family ID: |
25304663 |
Appl. No.: |
11/417946 |
Filed: |
May 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09848937 |
May 3, 2001 |
7042856 |
|
|
11417946 |
May 3, 2006 |
|
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Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04W 52/343 20130101;
H04W 72/06 20130101; H04W 16/04 20130101 |
Class at
Publication: |
370/329 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00 |
Claims
1. A method for operating an uplink of a wireless communication
system, comprising: partitioning available system resources into a
plurality of channels; defining a reuse pattern for the
communication system, wherein the reuse pattern includes a
plurality of cells; determining one or more characteristics for
each cell in the communication system; allocating a set of channels
to each cell based at least in part on the determined one or more
characteristics for the cell, wherein each allocated channel may be
assigned to a terminal for data transmission on the uplink; and
repeating the determining and allocating to reflect changes in the
communication system.
2. The method of claim 1, wherein each cell in the reuse pattern is
allocated a respective set of channels that includes one or more
channels available for transmission at full power level and one or
more channels available for transmission at reduced power
levels.
3. The method of claim 1, wherein the set of channels allocated to
each cell is determined based in part on estimated loading
conditions in the cell.
4. A method for operating an uplink in a communication system,
comprising: defining a reuse scheme to be used for data
transmission by a plurality of terminals, wherein the defined reuse
scheme identifies a particular reuse pattern, an initial allocation
of available system resources, and a set of operating parameters;
scheduling terminals for data transmission in accordance with the
defined reuse scheme; receiving transmission from scheduled
terminals; evaluating performance of the communication system;
determining whether the evaluated system performance is within
particular thresholds; and if the evaluated system performance is
not within the particular thresholds, redefining the reuse
scheme.
5. The method of claim 4, wherein the defining the reuse scheme
includes developing characterization of interference received at
each cell in the communication system, partitioning the available
system resources into a plurality of channels, and allocating a set
of channels to each cell based at least in part on the developed
interference characterization for the cell.
6. The method of claim 5, wherein the defining the reuse scheme
further includes defining a set of back-off factors to be
associated with each allocated set of channels.
7. A base station in a communication system, comprising: a resource
allocation processor configured to receive data defining a reuse
plan to be used for uplink data transmissions by a plurality of
terminals, wherein the defined reuse plan identifies a particular
reuse pattern, an allocation of available system resources to a
cell covered by the base station, and a set of operating
parameters, wherein the resource allocation processor is further
configured to schedule one or more terminals for data transmission
and to assign a channel to each scheduled terminal; at least one
front-end processor configured to process one or more received
signals from the one or more scheduled terminals to provide one or
more received symbol streams; and at least one receive processor
configured to process the one or more received symbol streams to
provide one or more decoded data streams and to estimate one or
more characteristics for the cell, wherein the resource allocation
processor is further configured to receive channel state
information (CSI) indicative of the one or more characteristics and
to schedule terminals and assign channels based on the CSI.
8. The base station of claim 7, wherein the allocated system
resources comprise a plurality of channels, and wherein the
resource allocation processor is further configured to determine a
plurality of back-off factors for the plurality of channels based
at least in part on the CSI.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present Application for Patent is a Continuation of
patent application Ser. No. 09/848,937 entitled "Method and
Apparatus for Controlling Uplink Transmissions of a Wireless
Communication System" filed May 3, 2001, pending, and assigned to
the assignee hereof and hereby expressly incorporated by reference
herein.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates to data communication, and
more specifically to a novel and improved method and apparatus for
controlling uplink transmissions of a wireless communication system
to increase efficiency and improve performance.
[0004] 2. Background
[0005] Wireless communication systems are widely deployed to
provide various types of communication such as voice, data, and so
on, for a number of users. These systems may be based on code
division multiple access (CDMA), time division multiple access
(TDMA), frequency division multiple access (FDMA), or some other
multiple access techniques.
[0006] In a wireless communication system, communication between
users is conducted through one or more base stations. A first user
on one terminal communicates with a second user on a second
terminal by transmitting data on the uplink to a base station. The
base station receives the data and can route the data to another
base station. The data is then transmitted on the downlink from the
base station to the second terminal. The downlink refers to
transmission from the base station to the terminal and the uplink
refers to transmission from the terminal to the base station. In
many systems, the uplink and the downlink are allocated separate
frequencies.
[0007] In a wireless communication system, each transmitting source
(e.g., terminal) acts as potential interference to other
transmitting sources in the system. To combat the interference
experienced by the terminals and base stations and to maintain the
required level of performance, conventional TDMA and FDMA systems
resort to reuse techniques whereby not all frequency bands or time
slots are used in each cell. For example, a TDMA system may employ
a 7-cell reuse pattern in which the total operating bandwidth, W,
is divided into seven equal operating frequency bands (i.e., B=W/7)
and each cell in a 7-cell cluster is assigned to one of the
frequency bands. Thus, in this system every seventh cell reuses the
same frequency band. With reuse, the co-channel interference levels
experienced in each cell are reduced relative to that if all cells
are assigned the same frequency band. However, reuse patterns of
more than one cell (such as the 7-cell reuse pattern used in some
conventional TDMA systems) represent inefficient use of the
available resources since each cell is allocated and able to use
only a fraction of the total system resources (e.g., operating
bandwidth).
[0008] CDMA systems are capable of operating with a 1-cell reuse
pattern (i.e., adjacent cells can use the same operating
bandwidth). First-generation CDMA systems are primarily designed to
carry voice data having a low data rate (e.g., 32 kbps or less).
Using code division spread spectrum, the low-rate data is spread
over a wide (e.g., 1.2288 MHz) bandwidth. Because of the large
spreading factor, the transmitted signal can be received at a low
or negative carrier-to-noise-plus-interference (C/I) level,
despread into a coherent signal, and processed. Newer generation
CDMA systems are designed to support many new applications (voice,
packet data, video, and so on) and are capable of data transmission
at high data rates (e.g., over 1 Mbps). However, to achieve the
high data rates, high C/I levels are required and the need to
control interference becomes more critical.
[0009] There is therefore a need in the art for techniques to
control uplink transmissions to support data transmission at high
data rates and achieve better utilization of the available
resources.
SUMMARY
[0010] Aspects of the invention provide techniques to (1) partition
and allocate the available system resources (e.g., the spectrum)
among cells in a communication system, and (2) allocate the
resources in each cell to terminals for data transmission on the
uplink. Both of these may be performed such that greater efficiency
is achieved while meeting system requirements.
[0011] In one aspect, adaptive reuse schemes are provided wherein
the available system resources may be dynamically and/or adaptively
partitioned and allocated to the cells based on a number of factors
such as, for example, the observed interference levels, loading
conditions, system requirements, and so on. A reuse plan is
initially defined and each cell is allocated a fraction of the
total available system resources. The allocation may be such that
each cell can simultaneously utilize a large portion of the total
available resources, if desired or necessary. As the system
changes, the reuse plan may be redefined to reflect changes in the
system. In this manner, the adaptive reuse plan may be capable of
achieving very low effective reuse factor (e.g., close to 1) while
satisfying other system requirements.
[0012] In another aspect, the system resources may be partitioned
such that each cell is allocated a set of channels having different
performance levels. Higher performance may be achieved, for
example, for lightly shared channels and/or those associated with
low transmit power levels in adjacent cells. Conversely, lower
performance may result, for example, from low transmit power levels
permitted for the channels. Channels having different performance
levels may be obtained by defining different back-off factors for
the channels, as described below.
[0013] In yet another aspect, terminals in each cell are assigned
to channels based on the terminals' tolerance levels to
interference and the channels' performance. For example,
disadvantaged terminals requiring better protection from
interference may be assigned to channels that are afforded more
protection. In contrast, advantaged terminals with favorable
propagation conditions may be assigned to channels that are more
heavily shared and/or have the greater interference levels
associated with their use.
[0014] The ability to dynamically and/or adaptively allocate
resources to the cells and the ability for the cells to
intelligently allocate resources to the terminals enable the system
to achieve high level of efficiency and performance not matched by
systems that employ conventional non-adjustable, fixed reuse
schemes.
[0015] The invention further provides methods, systems, and
apparatus that implement various aspects, embodiments, and features
of the invention, as described in further detail below
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The features, nature, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly throughout
and wherein:
[0017] FIG. 1 is a diagram of a communication system that supports
a number of users and is capable of implementing various aspects
and embodiments of the invention;
[0018] FIG. 2 shows cumulative distribution functions (CDFs) of the
C/I achieved for a number of fixed reuse patterns for a particular
communication system;
[0019] FIG. 3 is a flow diagram of a specific implementation of an
adaptive reuse scheme, in accordance with an embodiment of the
invention;
[0020] FIG. 4 is a diagram of an embodiment of a resource
partitioning and allocation for a 3-cell reuse pattern;
[0021] FIG. 5 shows a CDF of the achieved C/I for a 1-cell reuse
pattern with all cells transmitting at full power;
[0022] FIG. 6 is a flow diagram of an embodiment of a scheme to
schedule data transmissions;
[0023] FIG. 7 is a flow diagram of an embodiment of a
priority-based channel assignment scheme;
[0024] FIG. 8 is a flow diagram of an embodiment of a channel
upgrade scheme; and
[0025] FIG. 9 is a block diagram of a base station and terminals in
a communication system, which are capable of implementing various
aspects and embodiments of the invention.
DETAILED DESCRIPTION
[0026] FIG. 1 is a diagram of a communication system 100 that
supports a number of users and is capable of implementing various
aspects and embodiments of the invention. System 100 provides
communication for a number of coverage areas 102a through 102g,
each of which is serviced by a corresponding base station 104. Each
base station's coverage area may be defined, for example, as the
area over which the terminals can achieve a particular grade of
service (GoS). The base station coverage areas are organized in a
manner to achieve overall coverage for a designated geographic
area. The base station and its coverage area are often referred to
as a "cell".
[0027] As shown in FIG. 1, various terminals 106 are dispersed
throughout the system. The terminals in the coverage area may be
fixed (i.e., stationary) or mobile, and are generally served by a
primary (i.e., serving) base station. Each terminal communicates
with at least one and possibly more base stations on the downlink
and uplink at any given moment depending on whether "soft handoff"
is employed and/or whether the terminal is designed and operated to
(concurrently or sequentially) transmit/receive multiple
transmissions to/from multiple base stations. The downlink refers
to transmission from the base station to the terminal, and the
uplink refers to transmission from the terminal to the base
station.
[0028] In FIG. 1, base station 104a receives data transmission from
terminals 106a and 106b on the uplink, base station 104b receives
data transmissions from terminals 106b, 106c, 106d, and 106i, base
station 104c receives data transmissions from terminals 106a, 106e,
106f, and 106g, and so on. On the uplink, the transmission from
each communicating terminal represents potential interference to
other terminals in the system. The downlink transmissions are not
shown in FIG. 1 for simplicity.
[0029] System 100 may be a multiple-input multiple-output (MIMO)
system that employs multiple (N.sub.T) transmit antennas and
multiple (N.sub.R) receive antennas for data transmission. A MIMO
channel formed by the N.sub.T transmit and N.sub.R receive antennas
may be decomposed into N.sub.C independent channels, with
N.sub.C.ltoreq.min {N.sub.T, N.sub.R}. Each of the N.sub.C
independent channels is also referred to as a spatial subchannel of
the MIMO channel. The MIMO system can provide improved performance
(e.g., increased transmission capacity) if the spatial subchannels
created by the multiple transmit and receive antennas are
utilized.
[0030] An example MIMO system is described in U.S. patent
application Ser. No. 09/532,492, entitled "HIGH EFFICIENCY, HIGH
PERFORMANCE COMMUNICATION SYSTEM EMPLOYING MULTI-CARRIER
MODULATION," filed Mar. 30, 2000, assigned to the assignee of the
present invention and incorporated herein by reference. System 100
may also be designed to implement any number of standards and
designs for CDMA, TDMA, FDMA, and other multiple access schemes.
The CDMA standards include the IS-95, cdma2000, W-CDMA standards,
and the TDMA standards include Global System for Mobile
Communications (GSM). These standards are known in the art.
[0031] In system 100, a large number of terminals share a common
system resource, namely the total operating bandwidth. To achieve
the desired level of performance for a particular terminal in the
system, the interference from other transmissions needs to be
reduced to an acceptable level. Also, to reliably transmit at high
data rates for a given operating bandwidth, it is necessary to
operate at or above a particular carrier-to-noise-plus-interference
(C/I) level. Reduction in interference and attainment of the
required C/I are conventionally achieved by dividing the total
available resource into fractions, each of which is then assigned
to a particular cell in the system.
[0032] For example, the total operating bandwidth, W, can be
divided into N.sub.r equal operating frequency bands (i.e.,
B=W/N.sub.r), and each cell can then be assigned to one of the
N.sub.r frequency bands. The frequency bands are periodically
reused to achieve higher spectral efficiency. For a 7-cell reuse
pattern such as that supported by FIG. 1, cell 102a may be assigned
the first frequency band, cell 102b may be assigned the second
frequency band, and so on, and cell 102g may be assigned the
seventh frequency band.
[0033] A communication system is typically designed to conform to a
number of system requirements that may include, for example,
quality of service (QoS), coverage, and performance requirements.
Quality of service is typically defined as every terminal in the
coverage area being able to achieve a specified minimum average bit
rate a prescribed percentage of the time. For example, the system
may be required to support any terminal within the coverage area
with a minimum average bit rate of at least 1 Mbps for 99.99% of
the time. The coverage requirement may dictate that a particular
percentage (e.g., 99%) of the terminals with received signal levels
exceeding a particular C/I threshold be able to achieve the
specified grade of service. And the performance requirements may be
defined by some particular minimum average bit rate, bit-error-rate
(BER), packet-error-rate (PER), frame-error-rate (FER), or some
other requirements. These requirements impact the allocation of the
available resources and the system efficiency, as described
below.
[0034] FIG. 2 shows example cumulative distribution functions
(CDFs) of the C/I achieved for terminals in a communication system
based on a number of reuse patterns obtained from simulation of
terminals randomly distributed throughout the coverage area. The
horizontal axis, x, represents C/I, and the vertical axis
represents the probability that the C/I achieved for a particular
terminal is less than the value shown in the horizontal axis, i.e.,
P(C/I<x). As shown in FIG. 2, virtually no terminals achieve a
C/I worse than 0 dB. FIG. 2 also shows that the probability of
greater C/I increases with greater reuse. Thus, the P(C/I>x) for
the 7-cell reuse pattern is greater than the P(C/I>x) for the
1-cell reuse pattern.
[0035] The C/I CDFs in FIG. 2 may be used to characterize the
potential performance of the system. As an example, assume that a
C/I of at least 10 dB is required to meet a minimum instantaneous
bit rate of 1 Mbps for 99.99% of the time. Using a reuse factor of
one (i.e., N.sub.r=1, every cell reuses the same channel), the
probability of not achieving the required performance (i.e., the
outage probability) is approximately 12%. Similarly, cell reuse
factors of three, four, and seven correspond to outage
probabilities of 5.4%, 3.4%, and 1.1%, respectively. Thus in order
to achieve a 10 dB C/I for 99% of the terminals, a reuse factor of
at least seven (N.sub.r.gtoreq.7) is required in this example.
[0036] A number of modulation schemes may be used to modulate data
prior to transmission. Such modulation schemes include M-ary phase
shift keying (M-PSK), M-ary quadrature amplitude modulation
(M-QAM), and others. In general, bandwidth-efficient modulation
schemes such as M-QAM are able to transmit a higher number of
information bits per modulation symbol, but require high C/I to
achieve the desired level of performance. Table 1 lists the
spectral efficiency of a number of bandwidth-efficient modulation
schemes, which is quantified by the number of information bits
transmitted per second per Hertz (bps/Hz). Table 1 also lists the
assumed required C/I to achieve 1% bit error rate for these
modulation schemes. TABLE-US-00001 TABLE 1 Required C/I Modulation
Modulation (in dB) Scheme Efficiency (bps/Hz) for 1% BER BPSK 1 4.3
QPSK 2 7.3 8-PSK 3 12.6 16-QAM 4 14.3 32-QAM 5 16.8 64-QAM 6
20.5
[0037] The average channel efficiency, E.sub.CH, of a particular
reuse scheme may be determined based on the CDF of the achievable
C/I for the scheme (as shown in FIG. 2) and the achievable
modulation efficiency as a function of C/I (as shown in Table 1).
If the most efficient modulation scheme is used whenever possible,
then the average channel efficiency, E.sub.CH, may be derived as a
weighted sum of the modulation efficiencies, with the weighting
being determined by the probability of achieving the required C/I.
For example, if BPSK through 64-QAM are employed by the system
wherever possible, the average channel efficiency can be computed
as follows: E CH = .times. 1 P .function. ( 4.3 < C / I < 7.3
) + 2 P .function. ( 7.3 < C / I < 12.6 ) + .times. 3 P
.function. ( 12.6 < C / I < 12.6 ) + 4 P .function. ( 14.3
< C / I < 16.8 ) + .times. 5 P .function. ( 16.8 < C / I
< 20.3 ) + 6 P .function. ( 20.5 < C / I ) . ##EQU1##
[0038] Table 2 lists (in column 2) the average channel efficiencies
for various reuse factors (e.g., 1-cell, 3-cell, 5-cell, and
7-cell). Table 2 also provides (in column 3) the average spectral
(i.e., overall) efficiencies for these reuse factors, which are
derived by dividing the average channel efficiencies by the reuse
factors. From Table 2, it can be observed that the average channel
efficiency increases as reuse increases. However, this gain in
channel efficiency with increasing reuse is more than offset by the
loss in overall spectral efficiency that results from allowing each
cell to use only a fraction of the total available resources for
the system. Thus, the overall spectral efficiency decreases with
increasing reuse. TABLE-US-00002 TABLE 2 Average per Channel
Average Spectral Cell Reuse Factor Efficiency Efficiency N.sub.r
(bps/channel) (bps/Hz/cell) 1 4.4 4.4 3 5.18 1.73 4 5.4 1.35 7 5.75
0.82
[0039] As indicated in FIG. 2 and Table 2, the C/I for a given
terminal may be improved if the interference from terminals in
neighboring cells is reduced by employing a higher reuse factor.
However, in a multiple access system composed of many cells,
maximizing the C/I for a single terminal in one cell typically
implies that the resource cannot be reused in some other cells in
the system. Thus, although higher C/I and higher throughout may be
achieved for some of the terminals with higher reuse factor, the
overall system throughput can decrease since the number of
terminals allowed to transmit simultaneously using the same channel
decreases with higher reuse factor.
[0040] Conventionally, systems that require high C/I operating
points employ fixed reuse schemes. In these fixed-reuse systems, a
"channel" made available for use by a terminal in one cell may only
be reused in another cell with the same channel reuse pattern. For
example, consider a 3-cell reuse cluster containing cells 1, 2 and
3. In this scheme different channel sets are allocated to each cell
in this first reuse cluster. The channels in the set allocated to
any one cell in a reuse cluster are orthogonal to the channels in
the other sets allocated to the other cells in the cluster. This
strategy reduces or eliminates mutual interference caused by
terminals within a reuse cluster. The reuse cluster is repeated
throughout the network in some prescribed fashion. So for example,
a second reuse cluster of cells 4, 5 and 6 would be permitted to
use the same channel set as cells 1, 2 and 3, respectively. The
interference to terminals in the cells in the first reuse cluster
caused by terminals in the second reuse cluster is reduced due to
the increased separation between cells using the same channel set.
The increased separation implies increased path loss, and lower
interference power. While fixed reuse schemes may be used to
maximize the percentage of terminals meeting the minimum required
C/I, they are generally inefficient because they employ a high
reuse factor.
[0041] Aspects of the invention provide techniques to (1) partition
and allocate the available system resources (e.g., the spectrum)
among cells in a communication system, and (2) allocate the
resources in each cell to terminals for data transmission on the
uplink. Both of these may be performed such that greater efficiency
than fixed reuse schemes is achieved while meeting system
requirements. Certain aspects of the invention are based on several
key observations.
[0042] First, the uplink is different from the downlink since the
transmissions from the terminals may be coordinated by the system
for increased efficiency. The system (e.g., cells) receives
information that describes certain characteristics of the terminals
in the system (e.g., their path losses to the serving cells). This
information may then be used to determine how to best schedule
terminals for data transmission on the uplink. Coordination of the
uplink data transmission allows for various benefits such as (1)
increased uplink throughput on a system-wide basis, and (2) smaller
variations in performance observed by terminals in the system,
which implies that a more uniform quality of service (QoS) may be
delivered to the terminals.
[0043] Second, the terminals in the system typically have different
tolerance levels for interference. Disadvantaged terminals such as
those near the cell borders, with poor shadowing/geometry, must
transmit at higher power to overcome their large path loss. In
essence, these terminals have small link margins, where link margin
is defined as the difference between their peak power constraint
and the transmitted power needed to achieve a desired C/I operating
point at the cell site. As a consequence, these terminals are more
vulnerable to interference from other terminals and also tend to
cause greater levels of interference to terminals in nearby cells.
In contrast, advantaged terminals such as those closer to the cell
site, with favorable propagation loss and shadowing, are more
tolerant to interference since they have larger link margins. In
addition, these advantaged terminals tend to contribute less to the
interference power seen by terminals in other cells.
[0044] In a typical system, a large percentage of the terminals in
the system are able to achieve a C/I that equals or exceeds a
setpoint. The setpoint is a particular C/I required to achieve the
desired level of performance, which may be quantified as, e.g., a
particular average data rate at 1% BER or 0.01% outage probability,
or some other criterion. For these terminals, a unity reuse pattern
may be employed to achieve high efficiency for the system. Only a
fraction of the terminals in the system are typically disadvantaged
at any given time. For the fraction of terminals that achieve a C/I
below the setpoint, some other reuse schemes and/or some other
techniques may be employed to provide the required performance.
[0045] In one aspect, adaptive reuse schemes are provided wherein
the available system resources may be dynamically and/or adaptively
partitioned and allocated to the cells based on a number of factors
such as, for example, the observed loading conditions, system
requirements, and so on. A reuse plan is initially defined and each
cell is allocated a fraction of the total available system
resources. The allocation may be such that each cell can
simultaneously utilize a large portion of the total available
resources, if desired or necessary. As the system changes, the
reuse plan may be redefined to reflect changes in the system. In
this manner, the adaptive reuse plan may be capable of achieving
very low effective reuse factor (e.g., close to 1) while satisfying
other system requirements.
[0046] In another aspect, the system resources may be partitioned
such that each cell is allocated a set of channels having different
performance levels. Higher performance may be achieved, for
example, for lightly shared channels and/or those associated with
low transmit power levels in adjacent cells. Conversely, lower
performance may result, for example, from low transmit power levels
permitted for the channels. Channels having different performance
levels may be obtained by defining different back-off factors for
the channels, as described below.
[0047] In yet another aspect, terminals in each cell are assigned
to channels based on the terminals' tolerance levels to
interference and the channels' performance. For example,
disadvantaged terminals requiring better protection from
interference may be assigned to channels that are afforded more
protection. In contrast, advantaged terminals with favorable
propagation conditions may be assigned to channels that are more
heavily shared and/or have the greater interference levels
associated with their use.
[0048] The ability to dynamically and/or adaptively allocate
resources to the cells and the ability for the cells to
intelligently allocate resources to the terminals enable the system
to achieve high level of efficiency and performance not matched by
systems that employ conventional non-adjustable, fixed reuse
schemes. The techniques described herein may be applied to any
communication systems that experience interference such as, for
example, wireless (e.g., cellular) communication systems, satellite
communication systems, radio communication systems, and other
systems in which reuse can improve performance. In one specific
implementation, these techniques may be advantageously used to
improve the spectral efficiency of a fixed-terminal, multiple
access communication system designed to accommodate high data rate
services.
Adaptive Reuse Schemes
[0049] The adaptive reuse schemes may be designed to exploit
certain characteristics of the communication system to achieve high
system performance. These system characteristics include loading
effects and the terminal's different tolerance to interference.
[0050] The loading at the cells affects the overall performance
(e.g., throughput) of the system. At low loads, the available
system resources may be divided into sets of "orthogonal" channels,
which may then be assigned to the cells, one channel set per cell
in a reuse cluster. Because the channels in each set are orthogonal
to the channels in other sets, interference on these orthogonal
channels is low and high C/I values may be achieved. As the load
increases, the number of orthogonal channels in each set may be
insufficient to meet demands, and the cells may be allowed to
deviate from the use of only the orthogonal channels. The
transmissions on non-orthogonal channels increase the average
interference levels observed in the channels used. However, by
properly controlling the transmission levels on non-orthogonal
channels, the amount of interference may be controlled and high
performance may be achieved even at higher loads.
[0051] As the load increases, the number of active terminals
desiring to transmit data also increases, and the pool of terminals
from which a cell may select to schedule for data transmission and
to assign channels also increases. Each terminal in the pool
presents interference to other terminals in the system, and this
level may be dependent (in part) on the particular location of the
terminal to the serving cell as well as other neighbor cells. In
addition, terminals with greater link margin have a greater
tolerance to other-user interference. The terminals' different
interference characteristics can be exploited in scheduling
terminals and assigning channels to achieve tight reuse (i.e.,
close to unity). In particular, as the load increases, terminals
with higher tolerance to interference may be assigned to channels
having greater likelihood of receiving high interference
levels.
[0052] FIG. 3 is a flow diagram of an adaptive reuse scheme in
accordance with an embodiment of the invention. The development of
a reuse plan and the adaptation of the plan to changing system
conditions may be performed concurrent with normal operation of the
communication system.
[0053] Initially, the system is characterized, at step 310, for one
or more parameters and based on information collected for the
system and which may be stored in a database 330. For example, the
interference experienced by the terminals, as observed at each
cell, may be determined and an interference characterization may be
developed, as described below. The interference characterization
may be performed on a per cell basis, and may involve developing a
statistical characterization of the interference levels such as a
power distribution. The information used for the characterization
may be updated periodically to account for new cells and terminals,
and to reflect changes in the system.
[0054] A reuse plan is then defined using the developed system
characterization and other system constraints and considerations,
at step 412. The reuse plan encompasses various components such as
a particular reuse factor N.sub.r and a particular reuse cell
layout based on the reuse factor N.sub.r. For example, the reuse
factor may correspond to a 1-cell, 3-cell, 7-cell, or 19-cell reuse
pattern or cluster. The selection of the reuse factor and the
design of the reuse cell layout may be achieved based on the data
developed in step 310 and any other available data. The reuse plan
provides a framework for operation of the system.
[0055] Additional system parameters and/or operational conditions
are then defined, at step 314. This typically includes partitioning
the total available system resources into channels, with the
channels corresponding to time units, frequency bands, or some
other units, as described below. The number of channels, N.sub.c,
to be employed may be determined based on the reuse plan defined in
step 312. The available channels are then associated into sets and
each cell is allocated a respective channel set. The sets can
include overlapping channels (i.e., a particular channel may be
included in more than one set). Resource partition and allocation
are described in further detail below.
[0056] Other parameters may also be defined in step 314 such as,
for example, the scheduling interval, the operating points or
setpoints of the cells in the system, the back-off factors
associated with the allocated channel set, the back-off factor
limits, the step sizes for adjustments to the back-off factors, and
others. The back-off factors determine the reductions in the peak
transmit power levels for the channels. These parameters and
conditions, which are described in further detail below, are akin
to a set of operating rules to be followed by the cells during
normal operation.
[0057] The system then operates in accordance with the defined
reuse plan and the cells receive transmissions from terminals
scheduled for data transmission. During the course of normal
operation, the system performance is evaluated for the defined
reuse plan, at step 316. Such evaluation may include, for example,
determining the effective path losses from each terminal to several
nearby cells and the associated link margins, the throughputs, the
outage probabilities, and other measures of performance. For
example, the effective link margin for each scheduled terminal in
each channel in each cell may be computed. Based on the computed
link margins, an estimate of the average throughput of the system
can be developed as well as the individual performance of the
terminals.
[0058] Once the system performance has been evaluated, a
determination is made on the effectiveness (i.e., the performance)
of the defined reuse plan, at step 318. If the system performance
is not acceptable, the process returns to step 312 and the reuse
plan is redefined. The system performance may be unacceptable if it
does not conform to a set of system requirements and/or does not
achieve the desired performance level. The redefined reuse plan may
include changes to various operating parameters, and may even
include the selection of another reuse pattern and/or reuse cell
layout. For example, if excessive interference is encountered, the
reuse pattern may be increased (e.g., from 3-cell to 7-cell). Steps
312 through 318 are performed iteratively until the system goals
are achieved (e.g., maximized throughput while simultaneously
satisfying the minimum performance requirements for the terminals
in the coverage area). Steps 312 through 318 also represent an
ongoing process while the system is operational.
[0059] If the system performance is acceptable (i.e., does conform
to the system requirements), a determination is then made whether
the system has changed, at step 320. If there are no changes, the
process terminates. Otherwise, database 330 is updated, at step
324, to reflect changes in the system, and the system is
recharacterized. The steps in FIG. 3 are described in further
detail below.
[0060] The process shown in FIG. 3 may be performed periodically or
whenever system changes are detected. For example, the process may
be performed as the system grows or changes, e.g., as new cells and
terminals are added and as existing cells and terminals are removed
or modified. The process allows the system to adapt to changes, for
example, in the terminal distribution, topology, and
topography.
Channels
[0061] The resource sharing among cells and terminals may be
achieved using time division multiplexing (TDM), frequency division
multiplexing (FDM), code division multiplexing (CDM), other
multiplexing schemes, or any combinations thereof. The available
system resources are partitioned into fractions using the selected
multiplexing scheme(s).
[0062] For TDM-based schemes, the transmission time is partitioned
into time units (e.g., time slots or frames), and each cell is
allocated a number of time units. For each time unit, the total
operating bandwidth of the system can be assigned to one or more
terminals by the cell allocated with that time unit. For FDM-based
schemes, the total operating bandwidth can be divided into
frequency bands, and each cell is allocated a set of frequency
bands. Each cell can then assign the allocated frequency bands to
terminals within its coverage areas, and thereafter
(simultaneously) receive data transmission from the terminals via
these frequency bands. For CDM-based schemes, codes can be defined
for the system and each cell may be allocated a set of codes. Each
cell can then assign the allocated codes to terminals within its
coverage areas, and thereafter (simultaneously) receive data
transmission via these codes. Furthermore, combinations of these
schemes can also be used in the partitioning process. For example,
certain code channels within a CDMA system may be associated with a
particular time slot or frequency channel. Common rules governing
the use of these partitioned channels are then defined.
[0063] FIG. 4 is a diagram of an embodiment of a resource
partitioning and allocation for a 3-cell reuse pattern (i.e.,
N.sub.r=3). In this example, the system resource is divided into 12
fractions. The division can be implemented in the time, frequency,
or code domain, or a combination of these. Thus, the horizontal
axis in FIG. 4 can represent either time of frequency, depending on
whether TDM or FDM is employed. For example, the 12 fractions can
represent 12 time division multiplexed time slots for a TDM-based
scheme or 12 frequency bands for an FDM-based scheme. Each of the
fractions is also referred to herein as a "channel", and each
channel is orthogonal to the other channels.
[0064] For the 3-cell reuse pattern, the system resources may be
partitioned by grouping the available channels into three sets, and
each cell in a 3-cell cluster can be allocated one of the channel
sets. Each channel set includes some or all of the 12 available
channels, depending on the particular reuse scheme being employed.
For the embodiment shown in FIG. 4, each cell is allocated an equal
number of channels, with cell 1 being allocated channels 1 through
4 for transmission at full power, cell 2 being allocated channels 5
through 8, and cell 3 being allocated channels 9 through 12. In
some other embodiments, each cell may be allocated a respective
channel set that can include any number of channels, some of which
may also be allocated to other cells.
Back-Off Factors
[0065] In an aspect, a channel structure is defined and employed by
the system such that as the load increases, reliable performance is
achieved using the channels a large percentage of the time. For a
particular cell, it is likely that some terminals are more immune
to other-cell interference than some other terminals. By providing
a channel structure that takes advantage of this fact, improvement
in the system throughput and performance may be realized.
[0066] For the channel structure, each cell in a reuse cluster is
allocated a respective set of channels that may then be assigned to
terminals in its coverage area. Each cell is further assigned a set
of back-off factors for the set of allocated channels. The back-off
factor for each allocated channel indicates the maximum percentage
of full transmit power that may be used for the channel. The
back-off factor may be any value ranging from zero (0.0) to one
(1.0), with zero indicating no data transmission allowed on the
channel and one indicating data transmission at up to full transmit
power. The back-off factors result in channels capable of achieving
different performance levels.
[0067] The back-off from full transmit power can be applied in one
or more selected channels, at one or more selected time slots, by
one or more selected cells, or any combination thereof. The
back-off can additionally or alternatively be applied to selected
terminals in the cell. In an embodiment, each cell applies a
back-off for each channel assigned for data transmission, with the
specific value for the back-off being based on the operating
conditions of the cell such that the desired performance is
achieved while limiting the amount of interference to terminals in
other cells
[0068] The back-off factors for each cell can be determined based
on a number of factors. For example, the back-off factors can be
determined to take into consideration the characteristics of the
terminals, the loading conditions at the cells, the required
performance, and so on. The set of back-off factors assigned to
each cell may be unique, or may be common among different cells in
the system. In general, the channels allocated to each cell and the
assigned back-off factors may change dynamically and/or adaptively
based on, for example, the operating conditions (e.g., the system
load).
[0069] In one embodiment, the back-off factors for each cell are
determined based on the distribution of the achievable C/I values
for the total ensemble of (active) terminals in the cell. A
non-uniform weighting of these terminals may be applied, for
example, based on their profile, as described below. This weighting
may be made adaptive and/or dynamic, e.g., time-of-day
dependent.
[0070] On the uplink, the C/I for a specific terminal may be
determined at the cell based on, for example, a pilot signal
transmitted by the terminal. The C/I for the terminal is dependent
on various factors including (1) that terminal's path loss to the
serving (or home) cell and (2) the other-cell interference level.
In a fixed-terminal system, the path loss for a terminal does not
change appreciably and the prediction of the terminal's signal
level ("C") may be accurately made. The other-cell interference
level (i.e., a portion of "I") depends on the path losses from
other interfering terminals to their serving cells as well as the
path losses from these terminals to the cell of interest. Accurate
estimation of the other-cell interference levels typically requires
the instantaneous knowledge of which terminals in other cells are
transmitting and their power levels.
[0071] A number of assumptions may be made to simplify the
interference characterization. For example, each cell may place an
upper bound on the other-cell interference levels. This may be
accomplished by assuming that one terminal in each cell is allowed
to transmit on each channel, in which case the worst case
other-cell interference levels may be determined based on the
assumption that the interfering terminals will transmit at full
power. Correspondingly, the worst-case C/I for each terminal in
each cell may be estimated based on the assumption that this
terminal and other interfering terminals will be transmitting at
full power. The C/I values for the terminals in each cell may be
collected and used to characterize an effective C/I CDF for the
cell.
[0072] FIG. 5 is an example of a CDF of the C/I achieved by
terminals in a cell for a 1-cell reuse pattern with one terminal
transmitting at full power on each channel in each cell. The C/I
CDF provides an indication of the percentage of terminals in the
cell that have a C/I greater than a particular C/I value when the
terminals are transmitting at full power. From FIG. 5, it can be
seen that terminals within the cell have different C/I
characteristics. These terminals may be able to achieve different
levels of performance or, for a particular level of performance,
may need to transmit at different power levels. Terminals with
smaller path losses to the serving cell typically have higher C/I,
which implies that they will be able to achieve higher
throughput.
[0073] In an aspect, the terminals in each cell are categorized
based on their link margins, and the back-off factors are selected
based on the link margin categorization. Using the example C/I
distribution shown in FIG. 5, the population of terminals may be
categorized into sets, with each set including terminals
experiencing similar other-cell interference levels (i.e., having
C/I within a range of values). As an example, the CDF shown in FIG.
5 can be partitioned into N.sub.c sets, where N.sub.c is the total
number of channels allocated per cell. The sets may be selected to
be equal size (i.e., the same percentage of terminals is included
in each set), although non-equal size set partitions may also be
defined.
[0074] Table 3 identifies the N.sub.c=12 terminal sets and (column
2) tabulates the minimum C/I for the terminals in each of the 12
terminal sets. Since there are 12 terminal sets and each set is
equal size, each set includes approximately 8.3% of the terminals
in the cell. The first set includes terminals having C/I of 10 dB
or less, the second set includes terminals having C/I ranging from
10 dB to 13 dB, the third set includes terminals having C/I ranging
from 13 dB to 15 dB, and so on, and the last set includes terminals
having C/I greater than 34.5 dB. TABLE-US-00003 TABLE 3 Minimum C/I
s(n) Terminal Set in Range (dB) (dB) .beta.(n) 1 <10 <-5
1.0000 2 10 -5 1.0000 3 13 -2 1.0000 4 15 0 1.0000 5 17 2 0.6310 6
18.5 3.5 0.4467 7 20.5 5.5 0.2818 8 22 7 0.1995 9 24 9 0.1259 10 26
11 0.0794 11 29.5 14.5 0.0355 12 >34.5 >19.5 0.0112
[0075] The cells may be designed to support a particular setpoint y
(or operating point), which is the minimum required C/I in order to
operate at a desired data rate with an acceptable error rate. In
typical systems, the setpoint is a function of the instantaneous
data rate selected by the terminals, and may thus vary from
terminal to terminal. As a simple example, it is assumed that a
setpoint of 15 dB is required by all terminals in the cell.
[0076] The minimum link margin, s(n), for each set of terminals can
then be computed as: s(n)=min{C/I(n)}-.gamma.; n=1, 2, . . . ,
N.sub.c. Eq (1)
[0077] The minimum link margin, s(n), for each set of terminals is
the difference between the minimum C/I of the terminals in the set
and the setpoint .gamma.. The minimum link margin s(n) represents
the deviation from the required transmit power to the setpoint
based on the assumption of full transmit power from all terminals
in the system. A positive link margin indicates that the C/I is
greater than necessary to achieve the desired level of performance
defined by the setpoint. Thus, the transmit power of these
terminals may be reduced (i.e., backed-off) by the amount
proportional to their link margin and still provide the desired
level of performance.
[0078] The back-off factors for each cell may then be derived based
on knowledge of the path losses to the terminals served by the cell
and the characterization of the other-cell interference levels. If
the maximum transmit power level is normalized as 1.0, the
normalized back-off factor for each set of terminals can be
expressed as: .beta.(n)=min(1.0, 10.sup.-0.1(n)); n=1, 2, . . . ,
N.sub.c. Eq (2)
[0079] The back-off factor associated with a particular terminal
set represents the reduction in the transmit power that can be
applied to that set of terminals while still maintaining the
desired setpoint .gamma., and thus the desired level of
performance. The back-off in transmit power is possible because
these terminals enjoy better C/I. By reducing the transmit power of
a scheduled terminal by the back-off factor, the amount of
interference to terminals in other cells can be reduced without
impacting the performance of this terminal.
[0080] Table 3 lists the minimum link margin s(n) (in column 3) and
the back-off factor (in column 4) for each set of terminals for a
setpoint .gamma. of 15 dB. As shown in Table 3, channels 1 through
4 have link margins of 0 dB or less and channels 5 through 12 have
progressively better link margins. Consequently, channels 1 through
4 are operated at full power and channels 5 through 12 are operated
at progressively reduced power. The back-off factors may be imposed
on transmissions from terminals in the associated terminal sets.
For example, since the terminals in set 5 have C/I of 17 dB or
better and a minimum link margin s(n) of 2 dB, then the transmit
power from these terminals may be backed off to 63.1% of peak
transmit power.
[0081] For terminals having C/I that are below the setpoint
.gamma., a number of options may be applied. The data rate of the
transmission from these terminals may be reduced to that which can
be supported by the C/I. Alternatively, the interfering terminals
that cause the low C/I may be requested to (temporarily) reduce
their transmit power or to stop transmitting on the affected
channels until the low C/I terminals are satisfactorily served.
[0082] In an embodiment, once the back-off factors are determined
for one cell in a reuse pattern, the back-off factors for other
cells in the reuse pattern can be staggered. For example, for a
N.sub.r=3 (i.e., 3-cell) reuse pattern that operates with 12
channels and uses an N.sub.s=4 channel offset, the back-off factors
for cell 2 can be offset by four modulo-N.sub.c and the back-off
factors for cell 3 can be offset by eight modulo-N.sub.c. For this
reuse pattern, cell 1 applies the back-off factors associated with
channel set 1 (which includes the channels and their back-off
factors shown in the fourth column in Table 3), cell 2 applies the
back-off factors associated with channel set 2 (which includes the
channels and back-off factors shown in the fourth column in Table 3
but shifted down by four channels and wrapped around), and cell 3
applies the back-off factors associated with channel set 3 (which
includes the channels and back-off factors shown in Table 3 but
shifted down by eight channels and wrapped around). A 4-channel
offset is employed in the example, but other offsets may also be
used.
[0083] Table 3 tabulates the back-off factors for cells 1 through 3
using the back-off factors shown in Table 3 and a four-channel
offset. For example, for channel 1, cell 1 applies the back-off
factor associated with channel 1 of set 1, cell 2 applies the
back-off factor associated with channel 9 of set 1, and cell 3
applies the back-off associated with channel 5 of set 1.
TABLE-US-00004 TABLE 4 .beta..sub.1(n) .beta..sub.2(n)
.beta..sub.3(n) Channel, n Cell 1 Cell 2 Cell 3 1 1.0000 0.1259
0.6310 2 1.0000 0.0794 0.4467 3 1.0000 0.0355 0.2818 4 1.0000
0.0112 0.1995 5 0.6310 1.0000 0.1259 6 0.4467 1.0000 0.0794 7
0.2818 1.0000 0.0355 8 0.1995 1.0000 0.0112 9 0.1259 0.6310 1.0000
10 0.0794 0.4467 1.0000 11 0.0355 0.2818 1.0000 12 0.0112 0.1995
1.0000
[0084] At low loads, each of the cells assigns terminals to the
"better" allocated channels. For the channel allocation shown in
Table 4, the terminals in cell 1 are assigned to channels 1 through
4, the terminals in cell 2 are assigned to channels 5 through 8,
and the terminals in cell 3 are assigned to channels 9 through 12.
When the load in each cell is four terminals or less, there is no
co-channel interference from the terminals in the adjacent cells
(since the 12 channels are orthogonal to one another), and each
terminal should be able to achieve its setpoint at the cell for
uplink transmission. When the load in any of the cells exceeds four
terminals, then that cell may assign certain terminals to those
channels that are not orthogonal to those of the other cells. Since
the load typically varies independently in each cell, it is
possible that the non-orthogonal channel assigned will not be
occupied by any of the adjacent cells. The probability of this
event (i.e., the probability of "non-collision") is a function of
the load in each of the adjacent cells.
[0085] The channel structure with back-off may result an increase
in the effective margin observed by all terminals in the system.
The back-off factors shown in Table 4 are initially derived based
on the C/I CDF shown in FIG. 5, which is generated with the
assumption that terminals in other cells are transmitting at full
power. However, when the back-off factors are applied along with a
staggered channel reuse scheme as shown in Table 4, the actual C/I
values achieved by the terminals in each cell may be greater than
the minimum C/I values provided in column 2 of the Table 3 since
the interference from the terminals in other cells is reduced by
the applied back-off factors.
[0086] As an illustration, consider a case where a terminal
achieved a C/I of 17 dB in cell 1. Cell 1 may then assign channel 5
to this terminal. A terminal in cell 2 is allowed to transmit at
full power on this channel and a terminal in cell 3 is allowed to
transmit at 12.6% of full power. The 17 dB C/I for the terminal in
cell 1 was computed based on full transmit power and worst-case
interference assessment. However, since the power transmitted by
the terminal in cell 3 is reduced from 1.0 to 0.126, the effective
margin for the terminal in cell 1 will increase. The actual amount
of increase in the link margin depends on the path loss from the
backed-off interfering terminal (assigned to channel 5 in cell 3)
to cell 1.
[0087] As a simple example, the terminals in each cell may be
categorized into three different sets having 0 dB margin, 3 dB
margin, and 6 dB margin. Terminals with 0 dB margin would be
allowed to transmit at full power (when scheduled), terminals with
3 dB margin would be allowed to transmit at half power, and
terminals with 6 dB margin would be allowed to transmit at 25% of
full power. If three channels are allocated per cell, the back-off
factors assigned may be 1.0 for channel 1, 0.5 for channel 2, and
0.25 for channel three. In a 3-cell reuse pattern, the channels may
be staggered so that each cell is allocated the same three channels
but with a different set of back-off factors. Table 5 lists the
staggered channel assignment for this simple example.
TABLE-US-00005 TABLE 5 Channel, n Cell 1 Cell 2 Cell 3 1 1.00 0.25
0.50 2 0.50 1.00 0.25 3 0.25 0.50 1.00
[0088] An actual system typically does not fit the idealized system
model described above. For example, non-uniform distribution of
terminals, non-uniform base station placement, varied terrain and
morphology, and so on, all contribute to variations in the
interference levels observed in each cell. The characterization of
the cells and the normalization of performance in the cells is
typically more complicated than that described above (i.e., the C/I
CDFs for the cells are not likely to be identical). Furthermore,
the terminals in each cell typically see different levels of
interference from the terminals in other cells. Thus, more
computations may be required to normalize the effective margins to
within a particular threshold level across the cells in the
system.
[0089] The back-off factors derived for each cell may thus be
different and may not be modulo shifted versions of the back-off
factors other cells in the reuse cluster. Moreover, different
setpoints for the cells and/or channels may also be used to achieve
a level of normalized performance, if so desired. The setpoints may
also be altered to achieve non-uniform system performance. The
effect of different C/I CDFs on the back-off factors and the
adjustment of the back-off factors to improve system performance
are described in U.S. patent application Ser. No. 09/539,157,
entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSIONS OF A
COMMUNICATIONS SYSTEM," filed Mar. 30, 2000, assigned to the
assignee of the present application and incorporated herein by
reference.
[0090] A number of different schemes may be used to determine the
back-off factors for the cells. In one scheme, a procedure to
determine the back-off factors is iterated a number of times, and
the back-off factors are adjusted in each iteration such that the
maximum achievable setpoint for all channels is met. In an
embodiment, the worst-case other-cell interference is assumed in
determining the initial back-off factors. In another embodiment,
other values may be used instead of the worst-case interference
levels. For example, the average, median, or 95-percentile of the
other-cell interference distribution may be used to determine the
initial back-off factors. In yet another embodiment, the
interference levels are adaptively estimated, and the back-off
factors periodically adjusted to reflect the estimated interference
levels. The back-off factors employed by each cell may or may not
be communicated to neighboring cells.
[0091] In some embodiments, a subset of the allocated channels in a
cell may be provided with some form of "protection". The protection
may be achieved, for example, by reserving one or more channels on
a periodic basis for exclusive use by terminals in the cell. The
exclusivity may also be defined to be exercisable only when
required, and only to the extent required to satisfy disadvantaged
terminals. The protected channels may be identified to neighbor
cells by various means. For example, a cell may communicate to its
neighboring cells a list of channels that are protected. The
neighbor cells may then reduce or prevent data transmission on the
protected channels by terminals in their coverage areas. Channel
protection may be used to serve disadvantaged terminals that cannot
achieve the required C/I because of excessive interference from the
terminals in neighbor cells. For these cases, the channel
protection may be removed once the disadvantaged terminals are
served.
[0092] In some embodiments, a cell may impose "blocking" (i.e., no
transmission by terminals within its coverage areas) on certain
channels if the channel conditions deteriorate to an unacceptable
level (e.g., if the FER is above a certain percentage, or the
outage probability exceeds a particular threshold value). Each cell
can measure the performance of the channels and self-impose
blocking on poor performing channels until there is reasonable
certainty that the channel conditions have improved and that
reliable communication may be achieved.
[0093] The channel protection and blocking may be performed
dynamically and/or adaptively based on, for example, the conditions
of the cell.
Adjustment to the Default Back-Off Factors
[0094] In embodiments that employ power back-off, the back-off
factors are computed and provided to the cells in the system.
Thereafter, each cell applies the back-off factors when scheduling
terminals for data transmission on the uplink and assigning
channels to the terminals.
[0095] In an aspect, the initial back-off factors may be adjusted
dynamically and/or adaptively based on, for example, changes in
system loading, terminal characteristics, user demands, performance
requirements, and so on. The back-off factors may be adjusted using
numerous schemes, some of which are described below.
[0096] In one back-off adjustment scheme, the back-off factor(s) of
offending cell(s) are reduced during the period of time a
disadvantaged terminal is actively communicating. As noted above,
the disadvantaged terminal in many instances is not able to achieve
the desired setpoint because of excessive interference from a
limited number of terminals in other cells.
[0097] If the disadvantaged terminal is unable to achieve the
desired setpoint even when assigned to the best available channel
(a condition referred to as "soft-blocking"), terminals in other
cells that cause the interference may have their transmit power
temporarily reduced such that the disadvantaged terminal will be
able to attain the desired setpoint. As an example, if the primary
interference source for a disadvantaged terminal in cell 1 is a
terminal in cell 2, then the transmit power of the terminal in cell
2 may be backed-off by an amount necessary to allow the
disadvantaged terminal to operate at the desired setpoint (e.g., an
additional 3 dB, from .beta.(n)=x down to .beta.(n)=0.5x).
[0098] In the above example, if the back-off factor is applied to
the terminal in cell 2, then this terminal may no longer be able to
meet its setpoint either, potentially causing further reductions in
the back-off factors of other cells. Therefore, adjustments may
also be made to the setpoints employed in the specified channels of
the offending cells in addition to the back-off factors. In
addition, these adjustments may be made locally as well, so that
the setpoints of both cells 1 and 2 are reduced, e.g., to values
that effectively maximize their collective throughput while still
meeting the outage criteria of the terminals in both cells.
[0099] In another back-off adjustment scheme, the offending cell(s)
may be temporarily prevented from using a particular channel so
that the disadvantaged terminal may be served. The back-off
factor(s) .beta.(n) for the effected channel(s) may be set to 0.0
for the offending cell(s).
[0100] The primary interference for a particular terminal may be
co-channel interference from another terminal in a cell in another
reuse cluster. To reduce co-channel interference, the back-off
factors for the offending cell may be modified, e.g., shifted so
that the back-off factor is not high for the channel experiencing
high level of interference.
[0101] In another back-off adjustment scheme, one or more channels
may be reserved for exclusive use by each cell in the reuse
pattern. Other cells in the reuse pattern are then prevented (i.e.,
blocked) from transmitting on these channels. The number of
reserved channels may be based on the load or system requirements,
and may be adjusted dynamically and/or adaptively as the operating
condition changes. Also, the cells may be allocated different
number of reserved channels, again depending on the system design
and conditions.
[0102] The amount of power back-off to request from other cells may
be obtained in various manners. In some implementations, each cell
knows the back-off factors necessary to allow disadvantaged
terminals to operate at the desired setpoint. The back-off factors
may be pre-computed and saved or may be determined from prior
transmissions. When a disadvantaged terminal becomes active, the
cell knows the back-off factor(s) needed for the terminal and
communicate this to the offending cell(s).
[0103] For the embodiments in which it is desired to adjust (e.g.,
reduce or block) the transmit power of the terminals in the
offending cells, the cell requesting the back-off adjustment can
convey to the offending cells the desired adjustment to the
back-off factors to satisfy the requirements of the disadvantaged
terminals. The adjustments may also be sent to other cells in the
system, which may then use the information to improve the
performance of these cells. The offending cells would then apply
the requested back-off factors, based on a defined back-off
adjustment scheme. Such adjustment scheme may define, for example,
the time and duration for which to apply the adjustment. If an
offending cell receives back-off requests from a number of other
cells, the offending cell typically applies the maximum of the
back-off factors that it receives from the requesting cells.
[0104] The request (or directive) to temporarily reduce or block
the transmission in other cells may be communicated to the
offending cells such that the disadvantage terminals can be served.
The request may be communicated dynamically to the offending cells
as needed, or in an orderly manner (e.g., every few frames), or by
some other methods. For example, each cell may send its neighbor
cells a list of such requests at the start of each transmission
frame with the expectation that the requests would be applied at
the next transmission frame. Other methods for communicating
requests to other cells may be contemplated and are within the
scope of the present invention.
[0105] The back-off adjustment may be achieved using numerous
methods. In one method, the back-off factors are sent to the
neighbor cells on a dynamic basis and are applied shortly
thereafter (e.g., the next frame). In another method, the back-off
factors are applied at predetermined time, which is known by the
affected cells.
[0106] Restoration of a back-off factor to its original value may
also be achieved using numerous methods. In one method, the
original back-off factor can be restored by issuing a "restore"
command to the offending cell(s). In another method, the back-off
factor is gradually restores to its original value by increasing it
incrementally.
[0107] In yet another method for back-off adjustment, each cell
maintains a known step size for adjusting the back-off factor in
each channel. Each cell maintains the current value of the back-off
factor employed for each channel and a step size for increasing and
decreasing the back-off factor. Thereafter, the cell adjusts the
back-off factor in accordance with the associated step size each
time it receives a request to reduce transmit power.
[0108] In an embodiment, each channel of a particular cell may be
associated with maximum and minimum limits on the back-off factor.
As an example, assume that a scheduler operating in each cell
schedules on common frame boundaries, i=1, 2, 3 . . . Further, let
.beta..sub.m.sup.max(n) and .beta..sub.m.sup.min(n) be the maximum
and minimum values for the back-off factor for channel n in cell m,
and let .delta..sup.up(n) and .delta..sup.down(n) represent the
step sizes for increasing and decreasing the back-off factor for
channel n. The back-off adjustment at frame i in cell m for channel
n can then be expressed as:
[0109] (a) if any neighbor cells send decrease power commands at
frame i:
.delta..sub.m(n,i)=max[.beta..sub.m.sup.min(n),.beta..sub.m(n,i-1).delta-
..sup.down(n)],
[0110] (b) otherwise:
.delta..sub.m(n,i)=min[.beta..sub.m.sup.min(n),.beta..sub.m(n,i-1).delta.-
.sup.up(n)].
[0111] The maximum and minimum back-off limits may also be adjusted
as desired or necessary. For example, the maximum and minimum
limits can be adjusted based on system loading or requirements.
[0112] Dynamic adjustment of the back-off factors may be equated to
dynamic adjustment of the system setpoint or the maximum permitted
data rate for the channels, based on loading, performance, or some
other measures. As the system loading increases, the setpoint may
be adjusted (i.e., decreased) to a level that permits reliable
operation in the channels. Generally, the setpoint for each channel
may also be made adaptive. This allows the data rates associated
with the channels to be set differently as desired or necessary.
Adaptation of the setpoint in each channel may be performed locally
by each cell.
[0113] Dynamic adjustment of the back-off factors may be extended
such that the back-off factors for all channels in every cell can
be dynamically adjusted. This feature allows the system to
effectively adjust the power level in each of the channels so that
the active terminals in the specified channels are able to meet the
desired setpoint. The powers in the channels of adjacent cells can
thus become a function of, for example, a group of active terminals
in the local cell, their requirements, and so on. If the mix of
terminals in a cell is such that all can achieve their setpoints in
their assigned channels, then the default back-off factors are
employed. Otherwise, additional reductions in the back-off factors
(i.e., reduced transmit power) are applied temporarily in the
offending neighbor cells in the specified channels and for the
specified duration.
[0114] When the back-off factors are allowed to be changed
dynamically, a scheduler in a particular cell may not be certain of
the power being transmitted by the neighbor cells. This can result
in an ambiguity in the actual operating points for the terminals in
the local cell. Nevertheless, adjustments to the back-off factors
can still be performed dynamically, for example, by basing the
adjustments on the observed performance of the affected
channel.
[0115] For example, in one implementation, the cell monitors the
average frame-erasure-rate (FER) associated with a terminal in a
specific channel. If the actual C/I is lower than the setpoint,
there is a higher probability that a frame erasure will occur,
thereby resulting in a retransmission of the error frame. The cell
can then (1) reduce the data rate for the terminal, (2) request the
terminals in the offending cell(s) to reduce their transmit power
on this channel, or do both (1) and (2).
Parameters Used for Scheduling and Channel Assignment
[0116] The adaptive reuse schemes provide a structure for
allocating resources to terminals requesting to transmit data on
the uplink. During normal system operation, requests to transmit
data are received from various terminals throughout the system. The
cells then schedule terminals for data transmission and assign
channels to the terminals such that high efficiency and performance
are achieved.
[0117] The scheduling of terminals for data transmission and the
assignment of channels to the terminals may be achieved using
various scheduling schemes and based on a number of factors. Such
factors may include (1) one or more channel metrics, (2) the
priority assigned to active terminals, and (3) criteria related to
fairness. Other factors (some of which are described below) may
also be taken into account in scheduling terminals and assigning
channels and are within the scope of the invention.
[0118] One or more channel metrics may be used to schedule
terminals and/or assign channels such that more efficient use of
the system resources and improved performance may both be achieved.
Such channel metrics may include metrics based on throughput,
interference, outage probability, or some other measures. An
example of a channel metric indicative of "goodness" is described
below. However, it will be recognized that other channel metrics
may also be formulated and are within the scope of the
invention.
[0119] The channel metrics may be based on various factors such as
(1) a terminal's path loss and peak transmit power to the serving
cell, (2) other-cell interference characterization, (3) the
back-off factors, and possibly other factors. In an embodiment, a
channel metric, d.sub.m(n,k), for active terminals may be defined
as follows:
d.sub.m(n,k)=f{.beta..sub.m(n)P.sub.max(k).zeta..sub.m(k)/I.sub.m(n)},
Eq (3) where: [0120] .beta..sub.m(n) is the back-off factor
associated with channel n of cell m, with 0.ltoreq..beta..ltoreq.1
(when .beta..sub.m(n)=0, this is equivalent to preventing cell m
from using channel n;); [0121] P.sub.max(k) is the maximum transmit
power for terminal k; [0122] .zeta..sub.m(k) is the path loss from
terminal k to cell m; [0123] I.sub.m(n) is the interference power
observed by cell m on channel n; and [0124] f(x) is a function that
describes the "goodness" of the argument x, where x is proportional
to the C/I.
[0125] The exact computation of the other-cell interference,
I.sub.m(n), requires the knowledge of the path losses from each
interfering terminal (i.e., those assigned to the same channel n)
to its serving cell as well as to cell m under consideration. The
path loss to the serving cell determines the amount of power to be
transmitted by this interfering terminal, if power control is used.
And the path loss to cell m determines the amount of transmit power
from the interfering terminal will be received at cell m as
interference. Direct computation of the other-cell interference,
I.sub.m(n), is typically not practical since information about the
interfering terminals is normally not available (e.g., these
terminals are being scheduled and assigned by other cells at the
approximately same time) and the path loss characterization for
these terminals is typically not accurate (e.g., likely based on
averages and may not reflect fading).
[0126] The other-cell interference, I.sub.m(n), may thus be
estimated based on various schemes. In one interference estimation
scheme, each cell maintains a histogram of the received
interference power for each channel. The total receive power,
I.sub.o,m(n), at cell m for channel n comprises the power,
C.sub.k(n), received for the scheduled terminal k in channel n and
the interference power received from other interfering terminals in
other cells (plus thermal and other background noise). Thus, the
other-cell interference may be estimated as:
I.sub.m(n)=I.sub.o,m(n)-C.sub.k(n), Eq (4) where I.sub.m(n) is the
estimated other-cell interference for cell m in channel n. The
other-cell interference, I.sub.m(n), may be estimated for each
channel and at each scheduling interval to form a distribution of
the other-cell interference for each channel. An average value,
worst case, or some percentile of this distribution may then be
used as the other-cell interference I.sub.m(n) in equation (3).
[0127] Various functions f(x) may be used for the channel metric.
In one embodiment, the channel metric d.sub.m(n,k) represents the
outage probability for terminal k in cell m in channel n. In
another embodiment, the channel metric d.sub.m(n,k) represents the
maximum data rate that may be reliably sustained at the C/I=x.
Other functions may also be used for the channel metric and are
within the scope of the invention.
[0128] The channel metric d.sub.m(n,k) represents a "score" for
terminal k in cell m on channel n. The channel metric may be used
to schedule terminals for data transmission or to assign channels
to terminals, or both. In scheduling terminals and/or assigning
channels, a score may be computed for each active terminal for each
channel in the cell. For each terminal, the (up to N.sub.c) scores
are indicative of the expected performance associated with the
channels available for assignment. For a particular terminal, the
channel having the "best" score may be the best channel to assign
to the terminal. For example, if the channel metric d.sub.m(n,k)
represents the outage probability, then the channel with the lowest
outage probability is the best channel to assign to the
terminal.
[0129] The channel metric d.sub.m(n,k) may be computed to a degree
of confidence based on estimates of the parameters that comprise
the function f(x) (e.g., the path loss from terminal k to cell m,
the interfering power I.sub.m(n) observed by cell m, and so on).
The value of d.sub.m(n,k) may be averaged over a time period to
improve accuracy. Fluctuations in the value of d.sub.m(n,k) are
likely to occur due to small signal fading of both signal and
interference, changes in the location of interference source
causing changes in the interference power, and perhaps occasional
shadow (e.g., a truck blocking the main signal path). To account
for the fluctuations, channels with larger back-off factors may be
selected to provide some margins, and the data rates may also be
adapted based on changes in the operating conditions.
[0130] In an aspect, terminals may be scheduled for data
transmission and assigned channels based on their priority such
that higher priority terminals are generally served before lower
priority terminals. Prioritization typically results in a simpler
terminal scheduling and channel assignment process and may also be
used to ensure certain level of fairness among terminals, as
described below. The terminals in each cell may be prioritized
based on a number of criteria such as, for example, the average
throughput, the delays experienced by the terminals, and so on.
Some of these criteria are discussed below.
[0131] In one terminal prioritization scheme, terminals are
prioritized based on their average throughput. In this scheme, a
"score" is maintained for each active terminal to be scheduled for
data transmission. A cell can maintain the scores for the active
terminals it services (i.e., for a distributed control scheme) or a
central controller can maintain the scores for all active terminals
(i.e., in a centralized control scheme). The active status of a
terminal may be established at higher layers of the communication
system.
[0132] In an embodiment, a score .phi..sub.k(i) indicative of an
average throughput is maintained for each active terminal. In one
implementation, the score .phi..sub.k(i) for terminal k at frame i
is computed as an exponential average throughput, and can be
expressed as:
.phi..sub.k(i)=.alpha..sub.1.phi..sub.k(i-1)+.alpha..sub.0r.sub.k(i)/r.su-
b.max. Eq (5) where [0133] .phi..sub.k(i)=0 for i<0, [0134]
r.sub.k(i) is the data rate for terminal k at frame i (in unit of
bits/frame), and [0135] .alpha..sub.0 and .alpha..sub.1 are time
constants for the exponential averaging. Typically, r.sub.k(i) is
bounded by a particular maximum achievable data rate, r.sub.max,
and a particular minimum data rate (e.g., zero). A larger value for
.alpha..sub.1 (relative to .alpha..sub.0) corresponds to a longer
averaging time constant. For example, if .alpha..sub.0 and
.alpha..sub.1 are both 0.5, then the current data rate r.sub.k(i)
is given equal weight as the score .phi..sub.k(i-1) from the prior
scheduling interval. The scores .phi..sub.k(i) are approximately
proportional to the normalized average throughput of the
terminals.
[0136] The data rate r.sub.k(i) may be a "realizable" (i.e.,
"potential") data rate for terminal k based on the achieved (i.e.,
measured) or achievable (i.e., estimated) C/I for this terminal.
The data rate for terminal k can be expressed as:
r.sub.k(i)=c.sub.klog.sub.2(1+C/I.sub.k), Eq (6) where c.sub.k is a
positive constant that reflects the fraction of the theoretical
capacity achieved by the coding and modulation scheme selected for
terminal k. The data rate r.sub.k(i) may also be the actual data
rate to be assigned in the current scheduling period, or some other
quantifiable data rates. The use of the realizable data rate
introduces a "shuffling" effect during the channel assignment
process, which may improve the performance of some disadvantaged
terminals, as described below.
[0137] In another implementation, the score .phi..sub.k(i) for
terminal k at frame i is computed as a linear average throughput
achieved over some time interval, and can be expressed as: .PHI. k
.function. ( i ) = 1 K .times. j = i - K + 1 i .times. r k
.function. ( j ) / r max . Eq .times. .times. ( 7 ) ##EQU2## The
average (realizable or actual) throughput of the terminal can be
computed over a particular number of frames (e.g., over the latest
10 frames) and used as the score. Other formulations for the score
.phi..sub.k(i) for active terminals can be contemplated and are
within the scope of the present invention.
[0138] In an embodiment, when a terminal desires to transmit data
(i.e., becomes active), it is added to a list and its score is
initialized (e.g., to zero or a normalized data rate that the
terminal can achieve based on the current C/I). The score for each
active terminal in the list is subsequently updated in each frame.
Whenever an active terminal is not scheduled for transmission in a
frame, its data rate is set to zero (i.e., r.sub.k(i)=0) and its
score is updated accordingly. If a frame is received in error by a
terminal, the effective data rate for that frame is also set to
zero. The frame error may not be known immediately (e.g., due to
round trip delay of an acknowledgment/negative acknowledgment
(Ack/Nak) scheme used for the data transmission) but the score can
be adjusted accordingly once this information is available.
[0139] A scheduler can then use the scores to prioritize terminals
for scheduling and/or channel assignment. In a specific embodiment,
the set of active terminals is prioritized such that the terminal
with the lowest score is assigned the highest priority, and the
terminal with the highest score is assigned the lowest priority.
The scheduling processor may also assign non-uniform weighting
factors to the terminal scores in performing the prioritization.
Such non-uniform weighting factors can take into account others
factors (such as those described below) to be considered in
determining terminal priorities.
[0140] In certain embodiments (e.g., if the realizable data rate is
used), the score .phi..sub.k(i) for a particular terminal is not
necessarily indicative of what is supportable by the terminal
(i.e., may not reflect the terminal's potential data rate). For
example, two terminals may be assigned the same data rate, even
though one terminal may be capable of supporting a higher data rate
than the other. In this case, the terminal with the higher
potential data rate can be given a higher score and thus will have
a lower priority.
[0141] The priority of a terminal may also be made a function
various other factors. These factors may include, for example,
payload requirements, the achievable C/I and the required setpoint,
the delays experienced by the terminals, outage probability,
interference to adjacent cells, interference from other cells, data
rates, the maximum transmit powers, the type of data to be
transmitted, the type of data services being offered, and so on.
The above is not an exhaustive list. Other factors may also be
contemplated and are within the scope of the invention.
[0142] A terminal's payload may be used to determine priority. A
large payload typically requires a high data rate that may be
supported by a smaller number of the available channels. In
contrast, a small payload can typically be supported by more of the
available channels. The small payload may be assigned to a channel
having a large back-off factor that may not be able to support a
high data rate needed for a large payload. Since it is more
difficult to schedule data transmission for a large payload, a
terminal with the large payload can be assigned a higher priority.
In this way, the terminal with the large payload may be able to
enjoy comparable level of performance as a terminal with a small
payload.
[0143] A terminal's achieved C/I may also be used to determine
priority. A terminal having a lower achieved C/I can only support a
lower data rate. If the available resources are used for
transmission to a terminal having a higher achieved C/I, the
average system throughput would likely increase, thereby improving
the efficiency of the system. Generally, it is more preferable to
transmit to terminals having higher achieved C/I.
[0144] The amount of delay already experienced by a terminal may
also be considered in determining priority. If resource allocation
is achieved based on priority, a low priority terminal is more
likely to experience longer delays. To ensure a minimum level of
service, the priority of the terminal can be upgraded as the amount
of delay experienced by the terminal increases. The upgrade
prevents a low priority terminal from being delayed for an
intolerable amount of time or possibly indefinitely.
[0145] The type of data to be transmitted by a terminal may also be
considered in determining priority. Some data types are time
sensitive and require quick attention. Other data types can
tolerate longer delay in transmission. Higher priority may be
assigned to data that is time critical. As an example, data being
retransmitted may be given higher priority than data transmitted
for the first time. The retransmitted data typically corresponds to
data previously transmitted and received in error. Since other
signal processing at the cell may be dependent on the data received
in error, the retransmitted data may be given higher priority.
[0146] The type of data services being provided may be considered
in assigning terminal priority. Higher priority may be assigned to
premium services (e.g., those charged higher prices). A pricing
structure may be established for different data transmission
services. Through the pricing structure, the terminal can
determine, individually, the priority and the type of service the
terminal can expect to enjoy.
[0147] The factors described above and other factors may be
weighted and combined to derive the priorities of the terminals.
Different weighting schemes may be used depending on the set of
system goals being optimized. As an example, to optimize the
average throughput of the cell, greater weight may be given to the
terminals' achievable C/I. Other weighting schemes may also be used
and are within the scope of the invention.
[0148] A fairness criterion may be imposed in scheduling terminals
and assigning channels to ensure (or maybe even guarantee) a
minimum grade of service (GOS). The fairness criterion is typically
applied to all terminals in the system, although a particular
subset of the terminals (e.g., premium terminals) may also be
selected for application of the fairness criterion. Fairness may be
achieved with the use of priority. For example, a terminal may be
moved up in priority each time it is not scheduled for data
transmission and/or with each unsuccessful transmission.
[0149] For the terminal prioritization scheme described above, the
allocation of resources may be made on the basis of the ratio of
scores. In this case, the scores of all active terminals may be
referenced to the maximum of the terminal scores to form a modified
score {circumflex over (.phi.)}.sub.n(k), which can be expressed
as: {circumflex over
(.phi.)}.sub.k(i)=.phi..sub.k(i)/max.sub.k{.phi..sub.k(i)}. Eq
(8)
[0150] The resources allocated to a particular terminal may then be
based on their modified score. For example, if terminal 1 has a
score that is twice that of terminal 2, then the scheduling
processor can allocate a channel (or a number of channels) having
the capacity necessary to equalize the data rates of these two
terminals (provided that such channel(s) are available). As a
fairness consideration, the scheduler can attempt to normalize data
rates at each scheduling interval. Other fairness criteria may also
be imposed and are within the scope of the invention.
Scheduling of Data Transmissions
[0151] The cells in the system operate using an adaptive reuse plan
formulated in the manner described above and in accordance with the
prescribed rules and conditions. During normal operation, each cell
receives requests from a number of terminals in the cell for data
transmission. The cells then schedule terminals for data
transmission to meet the system goals. The scheduling can be
performed at each cell (i.e., for a distributed scheduling scheme),
by a central scheduler (i.e., for a centralized scheduling scheme),
or by a hybrid scheme in which some of the cells schedule their own
transmissions and a central scheduler schedules transmissions for a
set of cells.
[0152] FIG. 6 is a flow diagram of an embodiment of a
priority-based scheduling scheme to schedule terminals for data
transmission. In this priority-based scheduling scheme, active
terminals are scheduled for transmission based on their priority,
one terminal at a time, from the highest priority to lowest
priority. The number of terminals that may be scheduled for data
transmission at each scheduling interval is limited by the number
of available channels. For example, up to N.sub.c terminals per
cell may be scheduled for transmission on the N.sub.c available
channels.
[0153] Initially, parameters to be used for scheduling terminals
are updated, at step 610. These parameters may include the back-off
factors, the other-cell interference characterization, the path
losses for the terminals, and possibly others. The parameters may
be used to determine the channel metrics for the terminals.
[0154] The terminals are then prioritized and ranked, at step 612.
Generally, only active terminals having data to transmit are
considered for scheduling, and these terminals are prioritized and
ranked. Prioritization of terminals may be performed using any one
of a number of terminal-rating schemes and may be based on one or
more criteria listed above such as the average throughput, payload,
and so on. The active terminals are then ranked accordingly based
on their priorities, from highest priority to lowest priority.
[0155] The available channels are then assigned to the active
terminals, at step 614. The channel assignment typically involves a
number of steps. First, one or more channel metrics are computed
for each terminal for each available channel based on the updated
parameters. Any number of channel metrics may be used, such as the
one shown in equation (3). The terminals are then assigned to the
available channels based on their priority, the computed channel
metrics, and possibly other factors such as demand requirements.
The channel assignment may be performed based on various channel
assignment schemes, some of which are described below.
[0156] A channel assignment can imply a channel assigned as well as
a data rate to be used. Each of the possible data rates may be
associated with a respective coding and modulation scheme. Each
scheduled terminal may know (e.g., a priori) the proper coding and
modulation scheme to be used based on the assigned data rate.
Alternatively, the coding and modulation scheme may be conveyed to
the scheduled terminal. This "adaptive" coding and modulation may
be used to provide improved performance.
[0157] System parameters are then updated to reflect the channel
assignments, at step 616. The system parameters to be updated may
include, for example, adjustments to the back-off factors for the
channels in the cell based on (1) the channel assignments for the
scheduled terminals in this cell, (2) requests for adjustment of
back-off factors from other cells, and so on. The cell may also
request adjustments of the back-off factors by neighbor cells.
[0158] The cell then receives data transmissions from the scheduled
terminals via the assigned channels, at step 618. From the received
transmissions, the cell estimates various quantities that may be
used for a future scheduling interval, such as the interference for
each channel. Generally, steps 610 through 618 are performed during
normal operation of the cell. At step 620, a determination is made
whether another scheduling interval has occurred. If the answer is
yes, the process returns to step 610 and the terminals are
scheduled for the next scheduling interval. Otherwise, the process
waits at step 620. Some of these steps are described in further
detail below.
Channel Assignment Schemes
[0159] The available channels may be assigned to active terminals
based on various schemes and taking into account various factors.
These channel assignment schemes can range in complexity and in the
optimality (i.e., quality) of the assignment results. A few channel
assignment schemes are described below for illustration, and these
include (1) a priority-based channel assignment scheme, (2) a
demand-based channel assignment scheme, and (3) a channel
assignment with upgrade scheme. Other schemes can also be
implemented and are within the scope of the invention.
[0160] In a priority-based channel assignment scheme, channel
assignment is performed for one terminal at a time, with the
highest priority terminal being considered first for channel
assignment and the lowest priority terminal being considered last
for channel assignment. All active terminals in the cell are
initially prioritized based on a number of factors such as those
described above.
[0161] FIG. 7 is a flow diagram of an embodiment of a
priority-based channel assignment scheme. Initially, channel
metrics are computed for the active terminals and for the available
channels, at step 710. Various channel metrics may be used, such at
those described above. The active terminals are then prioritized
and ranked based on the factors described above, at step 712. The
prioritization may also be based on the computed metrics computed
in step 710. The terminal priority and channel metrics are then
used to perform channel assignment.
[0162] At step 714, the highest priority terminal is selected from
the list of active terminals, and is assigned an available channel,
at step 716. In one embodiment, the selected terminal is given the
first choice of channel and is assigned an available channel with
the best channel metric. In another embodiment, the selected
terminal is assigned an available channel with the worst metric
that still meets the terminal's requirements. The selected terminal
is also assigned a particular data rate determined based on (1) the
maximum rate required by the terminal, (2) the terminal's available
transmit power and the back-off factor associated with the assigned
channel, and (3) the terminal's requirements (e.g., outage
criterion), at step 718.
[0163] The assigned terminal is then removed from the list of
active terminals, at step 720. A determination is then made whether
the active terminal list is empty, indicating that all active
terminals have been assigned channels, at step 722. If the list is
not empty, the process returns to step 714 and the highest
priority, unassigned terminal in the list is selected for channel
assignment. Otherwise, if all terminals have been assigned
channels, the process terminates.
[0164] In an embodiment, if there is a tie during the channel
assignment (i.e., if more than one channels are associated with the
same or similar channel metrics), the channels are not assigned
immediately. Instead, those channels that resulted in the tie are
tagged and the evaluation of other lower priority terminals
continues. If the next terminal has its largest metric associated
with any one of the tagged channels, then that channel may be
assigned to that terminal and removed from the list of available
channels. When the list of tagged channels for a particular
terminal is reduced to one, the remaining channel is assigned to
the highest priority terminal that tagged that channel.
[0165] If the channel assignments result in a terminal having
additional link margin over that required for the assigned data
rate (i.e., the C/I of the terminal on the assigned channel is
greater than the setpoint), then (1) the data rate of the terminal
may be increased to a level that satisfies the required level of
performance, or (2) the transmit power of the terminal may be
reduced (e.g., by lowering the back-off factor) by up to the amount
of the link margin to reduce interference in the system. The
increased data rate of the terminal, as supported by the effective
link margin, increases the throughput for the terminal as well as
the system. Power control is thus effectively exercised for the
scheduled terminal. The adjustment in data rate and/or back-off
factor may be made for each scheduled terminal based on its channel
assignment.
[0166] If a terminal is assigned a channel not capable of
supporting the desired data rate, several options may be applied.
In one option, the terminal is scheduled to transmit at a reduced
data rate (a condition referred to herein as "dimming"). In another
option, the terminal is not permitted to transmit in the current
scheduling interval (a condition referred to herein as "blanking"),
and the channel is made available to another active terminal. In
either case, the priority of a terminal that is dimmed or blanked
may be increased, improving the terminal's chances for earlier
consideration in the next scheduling interval.
[0167] If the priority of a terminal is updated according to its
average throughput, then the channel assignment scheme may consider
the terminal's achievable data rate when assigning channel. In one
embodiment, the particular channel assigned to a terminal is the
one that maximizes the terminal's throughput at a given outage
level. The channel assignment scheme can first evaluate the best
channel for the terminal from the list of available channels. The
maximum data rate that satisfies the required outage criteria is
then assigned to the terminal for that channel.
[0168] In a demand-based channel assignment scheme, the demand or
payload requirements of the terminals are considered when making
channel assignments such that the available system resources may be
better utilized. For a particular set of available channels, a
terminal having lower payload requirements (which may be satisfied
with a lower data rate) may be serviced by a number of available
channels whereas a terminal having higher payload requirements
(which may require a higher data rate) may be serviced by a reduced
number of available channels. If the terminal with the lower
payload requirements has higher priority and is assigned the best
available channel (among many channels that also fulfill the
terminal's requirements), and if that channel is the only one that
can fulfill the requirements of the terminal with the higher
payload, then only one terminal will be served and the resources
are not effectively used.
[0169] As an example, consider a situation where three channels are
available for assignment to two terminals and that terminal 1 has a
payload requirement of 1 Kbyte and terminal 2 has a payload
requirement of 10 Kbytes. Further, assume that only one of the
three channels will satisfy the requirement of terminal 2 whereas
all three channels will satisfy the requirement of terminal 1. The
channels may be assigned as follows: [0170] (a) If terminal 2 has
higher priority than terminal 1, terminal 2 is assigned the channel
that maximizes its throughput. Terminal 1 is then assigned the next
best channel by default. Both terminals are served by their channel
assignments. [0171] (b) If terminal 1 has higher priority than
terminal 2, and if the payload requirements of the terminals are
not considered in making the channel assignment, terminal 1 may be
assigned the channel that has the largest effective margin even
though any one of the available channels would have satisfied
terminal 1's requirement. Terminal 2 would be assigned the next
best channel by default, which may not satisfy its requirement.
Terminal 2 would then be served at a lower data rate or remain in
the queue until the next scheduling period.
[0172] Several assignment options are available for case (b). If
the channel assignment is performed as described above, the power
used in the channel assigned to terminal 1 can be reduced to the
level required for reliable communications at the desired data
rate. Another assignment option in case (b) is to assign terminal 1
the channel having the lowest margin that satisfies the
requirements of terminal 1. With this channel assignment, other
better channels are made available for other terminals that may
need them (e.g., because of higher payload requirements or lower
achieved C/I). Using this demand or payload-based channel
assignment, channels with larger margins are available for
assignment to subsequent terminals that may require the additional
margins. Payload-based channel assignment may thus maximize the
effective throughput in a scheduling interval.
[0173] A flow diagram for the demand-based channel assignment
scheme may be implemented similar to that shown for the
priority-based channel assignment scheme in FIG. 7. In one
embodiment, each terminal selected for channel assignment is
assigned an available channel with the worst metric that still
meets the terminal's requirements. In another embodiment, the
priorities of the terminals may be modified such that terminals
with larger payloads are considered for assignment earlier.
Numerous other variations are also possible and are within the
scope of the invention.
[0174] In a channel assignment with upgrade scheme, the active
terminals are initially assigned channels (e.g., based on their
priorities or demands as described above) and thereafter upgraded
to better channels if any are available. In certain embodiments of
the schemes described above, higher priority terminals may be
initially assigned to the worst channels that still satisfy their
requirements, and better channels are saved for lower priority
terminals in case they are needed. These schemes may result in
successively lower priority terminals being assigned to
successively better channels associated with back-off factors that
are closer to unity (i.e., greater transmit power).
[0175] If the number of active terminals is less than the number of
available channels, it is possible to upgrade the terminals. A
terminal may be upgraded to another unassigned channel that has a
higher margin than the initial assigned channel. The reason for
upgrading the terminal is to increase reliability and/or lower the
effective transmit power required to support the transmission. That
is, since a number of unassigned channels satisfies the terminal's
requirements, reassigning the terminal to the channel with higher
margin allows for reduction in the transmit power by the amount of
margin.
[0176] Various schemes may be used to upgrade channels, some of
which are described below. Other channel upgrade schemes may also
be implemented and are within the scope of the invention.
[0177] In one channel upgrade scheme, terminals are reassigned to
better available channels, if these channels meet the requirements
of the terminals and can provide larger link margins. The channel
upgrade may be performed based on priority such that higher
priority terminal are upgraded first and lower priority terminals
are upgraded later if channels are available. This upgrade scheme
allows some or all of the active terminals to enjoy better channels
having higher link margins.
[0178] FIG. 8 is a flow diagram of an embodiment of a channel
upgrade scheme whereby terminals are upgraded based on their
priorities. Prior to commencing the upgrade process shown in FIG.
8, the active terminals are assigned to their initial channel
assignments, which can be achieved using the channel assignment
scheme described above in FIG. 7. At step 810, a determination is
made whether all available channels have been assigned to active
terminals. If all channels have been assigned, no channels are
available for upgrade and the process proceeds to step 828.
Otherwise, the terminals are upgraded to the available channels, if
these channels are better (i.e., associated with better channel
metrics) than the original assigned channels.
[0179] At step 812, the highest priority terminal from the list of
active terminals is selected for possible channel upgrade. For the
selected terminal, the "best" channel from the list of unassigned
channels is selected. The best channel may correspond to the
channel having the "best" channel metric for the selected
terminal.
[0180] A determination is then made whether an upgrade is possible
for the selected terminal, at step 816. If the channel metric of
the best available channel is worse than that of the channel
originally assigned to the selected terminal, then no upgrade is
performed and the process proceeds to step 824. Otherwise, the
selected terminal is upgraded to the best available channel, at
step 818, which is then removed from the list of available
channels, at step 820. The channel initially assigned to the
selected terminal may be placed back on the list of available
channels for possible assignment to some other lower priority
terminal, at step 822. The selected terminal is then removed from
the list of active terminals, at step 824, regardless of whether a
channel upgrade was performed or not.
[0181] At step 826, a determination is made whether the list of
active terminals is empty. If the terminal list is not empty, the
process returns to step 810 and the highest priority in the list is
selected for possible channel upgrade. Otherwise, if no channels
are available for upgrade or if all active terminals have been
considered, the process proceeds to step 828 and the back-off
factors for all channels are adjusted to reduce the transmit powers
of the scheduled and assigned terminals. The process then
terminates.
[0182] The upgrade process in FIG. 8 effectively upgrades active
terminals to the available channels that are more likely to provide
improved performance. The channel upgrade scheme shown in FIG. 8
may be modified to provide improved channel upgrades. For example,
for a particular terminal, it may be possible that a channel freed
up by a lower priority terminal is better for this terminal.
However, the terminal is not assigned to this channel because it
has already been removed from the terminal list by the time the
lower priority terminal is considered. The process in FIG. 8 may
thus be iterated a number of times, or other tests may be performed
to account for this situation.
[0183] In another channel upgrade scheme, the assigned terminals
are upgraded by the number of available channels. For example, if
three channels are available, each scheduled and assigned terminals
move up by three slots. This upgrade scheme allows most (if not
all) terminals to enjoy better channels. For example, if channels 1
through 12 having progressively worse performance are available for
assignments and nine terminals are initially assigned to channels 4
through 12, then each terminal may be upgraded by three channels.
The nine terminals then occupy channels 1 through 9 and channels 10
through 12 may be disabled.
[0184] In another channel assignment scheme, the differences
between the channel metrics associated with the channels may be
taken into account in the channel assignment. In some instances, it
may be better to not assign the highest priority terminal the
channel with the best channel metric. For example, a number of
channels may be associated with approximately similar metrics for a
particular terminal, or a number of channels may provide the
required C/I. In these instances, the terminal may be assigned one
of several channels and still be properly served. If a lower
priority terminal has as its best channel the same one selected by
a higher priority terminal, and if there is a large disparity
between the lower priority terminal's best and second best
channels, then it may be more optimal to assign the higher priority
terminal its second best channel and assign the lower priority
terminal its best channel. For example, if terminal 1 has similar
channel metrics for channels 2 and 3 and the next lower priority
terminal 2 has a much larger channel metric for channel 3 than
channel 2, then terminal 1 may be assigned channel 2 and terminal 2
may be assigned channel 3.
[0185] In yet another channel assignment scheme, the highest
priority terminal tags the available channels that provide the
required performance (similar to the tagging of tied channels
described above). The next lower priority terminal then tags its
acceptable channels. The channel assignment is then performed such
that lower priority terminals are assigned channels first but
channels needed by higher priority terminals are reserved.
[0186] In yet another channel assignment scheme, the channels are
more optimally assigned to active terminals in the cell by
considering a large number of permutations of channel assignments
over the group of active terminals in the cell. In this case, the
channel assignment decision for a particular terminal is not made
on the basis of the terminal's metrics and priority alone. In an
implementation, the terminal's priority can be converted into a
weight that is used to scale the metrics in the computation of the
channel assignments in the cell.
[0187] Active terminals may be scheduled for transmission and
assigned channels based on their priorities, demand, scores (e.g.,
as computed in equation (3)), and so on, as described above. Some
other considerations for scheduling terminals for data transmission
and assigning channels are described below.
[0188] First, a particular terminal may be assigned to multiple
channels if such channels are available and if one channel is not
capable of meeting the terminal's requirements. For example, a
terminal may be assigned a first channel capable of supporting 50%
of the terminal's requirements, a second channel capable of
supporting 35% of the terminal's requirements, and a third channel
capable of supporting the remaining 15% of the terminal's
requirements. If this particular allocation of resources prevents
other terminals from achieving their requirements, then the
priorities of the underserved terminals may improve such that they
will be considered earlier for the allocation of resources in
subsequent scheduling intervals.
[0189] Second, a particular terminal may be assigned to different
channels for different scheduling intervals to provide a
"shuffling" effect. This shuffling of assigned channels may provide
interference averaging in certain instances, which may improve the
performance of a disadvantaged terminal.
[0190] Third, the probabilities of other terminals transmitting on
a particular channel can be taken into account. If a number of
channels have nearly equal channel metrics without taking into
account the occupancy probabilities, then the better channel to
assign is the one that has the lowest probability of being used.
Thus, the probability of channel occupancy may be used to determine
the best channel assignment.
[0191] Fourth, excessive outage probability may be considered in
making the channel assignments. In some instances, it is possible
that assignment of a channel to a particular terminal is
unwarranted or unwise. For example, if a terminal's expected outage
probability for a particular channel is excessive, there may be a
reasonable likelihood that the entire transmission on that channel
will be corrupted and would need to be re-transmitted. Furthermore,
assignment of the channel may increase the likelihood that
transmissions by terminals in adjacent cells are also corrupted by
the additional interference. In such instances, assignment of the
channel to the terminal may be unwise, and it may be better to not
assign the channel at all or to assign the channel to another
terminal that may make better use of it.
[0192] The available channels may also be assigned to terminals
with zero or more conditions or constraints on usage. Such
conditions may include, for example (1) limitation on the data
rate, (2) maximum transmit power, (3) restriction on the setpoint,
and so on.
[0193] A maximum data rate may be imposed on a channel assigned to
an active terminal. For example, if the expected C/I is not able to
support the required data rate, then the data rate may be reduced
to achieve the requirement.
[0194] Maximum transmit power constraints may be placed on certain
assigned channels. If the cells in the system have knowledge of the
power constraints for the channels in other cells, then the
interference levels may be computed locally with higher degree of
certainty and better planning and scheduling may be possible.
[0195] A particular setpoint may be imposed on an assigned channel,
for example, in heavily loaded situations. A (e.g., low priority)
terminal may be assigned a channel that does not meet the required
minimum outage probability (i.e., the assigned channel has an
expected C/I that is lower than required). In this case, the
terminal may be required to operate using the assigned channel at a
lower setpoint that satisfies the required performance criteria.
The setpoint employed may be static or adjustable with system
loading. Also, the setpoint may be imposed on a per channel
basis.
Control Schemes
[0196] The adaptive reuse schemes, the scheduling of terminals for
data transmission, and the assignment of channels may be
implemented in various manners and using numerous control schemes
such as centralized, distributed, and hybrid control schemes. Some
of these control schemes are described below.
[0197] In a centralized control scheme, information from the active
terminals in all cells to be commonly controlled is provided to a
central processor that processes the information, schedules data
transmissions, and assigns channels based on the received
information and a set of system goals. In a distributed control
scheme, information from the active terminals in each cell is
provided to a cell processor that processes the information,
schedules data transmissions, and assigns channel for that cell
based on the information received from the terminals in that cell
and possibly other information received from other cells.
[0198] A distributed control scheme performs scheduling of
terminals for data transmission and channel assignment at the local
level. The distributed control scheme may be implemented at each
cell and involved coordination between cells is not required.
[0199] In the distributed control scheme, local information may be
shared dynamically with other cells in the system even though the
scheduling and channel assignment may be performed locally at each
cell. The shared information may include, for example, the loading
at a particular cell, a list of active terminals at the cell,
channel availability information, the assigned back-off factors,
and so on. In the distributed control scheme, this information need
not be shared in a dynamic manner and may be "static" information
available to the cells in the system. The shared information can be
used by the cells to help decide how to best allocate resources
locally.
[0200] The distributed control scheme may be advantageously used
under both low and high load conditions, and is simpler to
implement than the centralized control scheme. At low load, the
terminals in the cells are more likely to be able to transmit using
orthogonal channels, which results in minimal interference to
terminals in other cells. As the load increases, the interference
levels in the system will generally increase and there is a higher
likelihood that the terminals will be assigned to non-orthogonal
channels. However, as the load increases, the group of terminals
the cell can select from for scheduling also increases. Some of
these terminals may be more tolerant of other-cell interference
than others. A distributed control scheme exploits this fact
scheduling terminals and assigning channels.
[0201] Distributed, centralized, and hybrid scheduling schemes are
described in further detail in U.S. Pat. No. 5,923,650, entitled
"METHOD AND APPARATUS FOR REVERSE LINK RATE SCHEDULING," issued
Jul. 13, 1999, U.S. Pat. No. 5,914,950, also entitled "METHOD AND
APPARATUS FOR REVERSE LINK RATE SCHEDULING," issued Jun. 22, 1999,
and U.S. patent application Ser. No. 08/798,951, entitled "METHOD
AND APPARATUS FOR FORWARD LINK RATE SCHEDULING," filed Sep. 17,
1999, all assigned to the assignee of the present invention and
incorporated herein by reference.
Power Control
[0202] Power control may be exercised by the cells for the assigned
channels. If a terminal is assigned a channel and has positive link
margin (i.e., the difference between the expected C/I and the
setpoint is positive), the transmit power of the terminal may be
reduced based on the determined link margin. Even if other cells in
the system are not aware of the reduced back-off for a particular
channel, the overall effect is to reduce interference levels and
improve the probability of successful transmission. Power control
may be performed dynamically, possibly in similar manner as that
performed for the uplink power control in CDMA systems.
Combination with Other Reuse Structures
[0203] The adaptive reuse schemes described herein may also be
implemented within or in combination with other reuse structures.
One such structure is disclosed by T. K. Fong et al. in a paper
entitled "Radio Resource Allocation in Fixed Broadband Wireless
Networks," IEEE Transactions on Communications, Vol. 46, No. 6,
June 1998, which is incorporated herein by reference. This
reference describes partitioning each cell into a number of sectors
and transmitting to each sector at designated (and possibly
non-designated) and staggered time slots selected to reduce the
amount of interference.
[0204] Another reuse structure is disclosed by K. K. Leung et al.
in a paper entitled "Dynamic Allocation of Downlink and Uplink
Resource for Broadband Services in Fixed Wireless Networks," IEEE
Journal on Selected Areas in Communications, Vol. 17, No. 5, May
1999, which are incorporated herein by reference. This reference
describes partitioning each cell into a number of sectors and
transmitting to each sector at designated (and possibly
non-designated) and staggered time slots and sub-time slots
selected to reduce the amount of interference. The C/I of the
terminals are determined, and terminals are classified into groups
based on their tolerance for up to q concurrent transmissions. The
transmission pattern is then selected and data transmissions are
scheduled to ensure conformance with the requirements of the
terminals.
[0205] Yet another reuse structure is disclosed by K. C. Chawla et
al. in a paper entitled "Quasi-Static Resource Allocation with
Interference Avoidance for Fixed Wireless Systems," IEEE Journal on
Selected Areas in Communications, Vol. 17, No. 3, March 1999, which
is incorporated herein by reference. This reference describes
assigning each cell with a "beam-off" sequence and allowing the
terminals to inform the cell the best time slots for its data
transmissions.
System Design
[0206] FIG. 9 is a block diagram of base station 104 and terminals
106 in communication system 100, which is capable of implementing
various aspects and embodiments of the invention. At each scheduled
terminal 106, a data source 912 provides data (i.e., information
bits) to a transmit (TX) data processor 914. TX data processor 914
encodes the data in accordance with a particular encoding scheme,
interleaves (i.e., reorders) the encoded data based on a particular
interleaving scheme, and maps the interleaved bits into modulation
symbols for one or more channels assigned to the terminal for data
transmission. The encoding increases the reliability of the data
transmission. The interleaving provides time diversity for the
coded bits, permits the data to be transmitted based on an average
C/I for the assigned channels, combats fading, and further removes
correlation between coded bits used to form each modulation symbol.
The interleaving may further provide frequency diversity if the
coded bits are transmitted over multiple frequency subchannels. In
an aspect, the coding and symbol mapping may be performed based on
information provided by the base station.
[0207] The encoding, interleaving, and signal mapping may be
achieved based on various schemes. Some such schemes are described
in U.S. patent application Ser. No. 09/532,492, entitled "HIGH
EFFICIENCY, HIGH PERFORMANCE COMMUNICATIONS SYSTEM EMPLOYING
MULTI-CARRIER MODULATION," filed Mar. 22, 2000, U.S. patent
application Ser. No. 09/826,481, entitled "METHOD AND APPARATUS FOR
UTILIZING CHANNEL STATE INFORMATION IN A WIRELESS COMMUNICATION
SYSTEM," filed Mar. 23, 2001, and U.S. patent application Ser. No.
09/776,075, entitled "CODING SCHEME FOR A WIRELESS COMMUNICATION,"
filed Feb. 1, 2001, all assigned to the assignee of the present
application and incorporated herein by reference.
[0208] A TX MIMO processor 920 receives and demultiplexes the
modulation symbols from TX data processor 914 and provides a stream
of modulation symbols for each transmission channel (e.g., each
transmit antenna), one modulation symbol per time slot. TX MIMO
processor 920 may further precondition the modulation symbols for
each assigned channel if full channel state information (CSI) is
available (e.g., a channel response matrix H). MIMO and full-CSI
processing is described in the aforementioned U.S. patent
application Ser. No. 09/532,492.
[0209] If OFDM is not employed, TX MIMO processor 920 provides a
stream of modulation symbols for each antenna used for data
transmission. And if OFDM is employed, TX MIMO processor 920
provides a stream of modulation symbol vectors for each antenna
used for data transmission. And if full-CSI processing is
performed, TX MIMO processor 920 provides a stream of
preconditioned modulation symbols or preconditioned modulation
symbol vectors for each antenna used for data transmission. Each
stream is then received and modulated by a respective modulator
(MOD) 922 and transmitted via an associated antenna 924.
[0210] At base station 104, a number of receive antennas 952
receive the signals transmitted by the scheduled terminals, and
each receive antenna provides a received signal to a respective
demodulator (DEMOD) 954. Each demodulator (or front-end unit) 954
performs processing complementary to that performed at modulator
922. The modulation symbols from all demodulators 954 are then
provided to a receive (RX) MIMO/data processor 956 and processed to
recover one or more data streams transmitted for the terminal. RX
MIMO/data processor 956 performs processing complementary to that
performed by TX data processor 914 and TX MIMO processor 920 and
provides decoded data to a data sink 960. The processing by base
station 104 is described in further detail in the aforementioned
U.S. patent application Ser. No. 09/776,075.
[0211] RX MIMO/data processor 956 further estimates the link
conditions for the active terminals. For example, RX MIMO/data
processor 956 may estimate the path loss for each active terminal,
the interference on each channel, and so on, which comprise channel
state information (CSI). This CSI may be used to develop and adapt
the reuse plan and to schedule active terminals and assign
channels. Methods for estimating a single transmission channel
based on a pilot signal or a data transmission may be found in a
number of papers available in the art. One such channel estimation
method is described by F. Ling in a paper entitled "Optimal
Reception, Performance Bound, and Cutoff-Rate Analysis of
References-Assisted Coherent CDMA Communications with
Applications," IEEE Transaction On Communication, October 1999.
[0212] A cell processor 964 at base station 104 uses the CSI to
perform a number of functions including (1) developing and adapting
a reuse plan, (2) scheduling the best set of terminals for data
transmission, (3) assigning channels to the scheduled terminals,
and (4) determining the data rate and possibly the coding and
modulation scheme to be used for each assigned channel. Cell
processor 964 may schedule terminals to achieve high throughput or
based on some other performance criteria or metrics, as described
above. For each scheduling interval, cell processor 964 provides a
list of terminals scheduled to transmit on the uplink and their
assigned channels and (possibly) data rates (i.e., scheduling
information). In FIG. 9, cell processor 964 is shown as being
implemented within base station 104. In other implementation, the
functions performed by cell processor 964 may be implemented within
some other element of communication system 100 (e.g., a central
controller located in a base station controller that couples to and
interacts with a number of base stations).
[0213] A TX data processor 962 then receives and processes the
scheduling information, and provides processed data to one or more
modulators 954. Modulator(s) 954 further condition the processed
data and transmit the scheduling information back to terminals 106
via a downlink channel. The scheduling information may be sent to
the scheduled terminals by the base station using various signaling
techniques, as described in the aforementioned U.S. patent
application Ser. No. 09/826,481, For example, the scheduling
information may be sent on a designated downlink channel (e.g., a
control channel, paging channel, or some other type of channel).
Since the active terminals request the base station for data
transmission on the uplink, these terminals would know to monitor
the designated downlink channel for their schedules, which would
identify the times they are scheduled to transmit and their
assigned channels and (possibly) data rates.
[0214] At terminal 106, the transmitted feedback signal is received
by antennas 924, demodulated by demodulators 922, and provided to a
RX data/MIMO processor 932. RX data/MIMO processor 932 performs
processing complementary to that performed by TX data processor 962
and recovers a schedule, which is then used to direct the
processing and transmission of data by the terminal. The schedule
determines when and on which channel the terminal is allowed to
transmit on the uplink, and typically further identifies the data
rate and/or coding and modulation scheme to be used for the data
transmission. If the terminal is not provided with information
regarding which data rates to use on which channel, then the
terminal may use "blind" rate selection and determine the coding
and modulation scheme. In this case, the base station may perform
blind rate detection to recover the data transmitted by the
terminal.
[0215] The elements of the base station and terminals may be
implemented with one or more digital signal processors (DSP),
application specific integrated circuits (ASIC), processors,
microprocessors, controllers, microcontrollers, field programmable
gate arrays (FPGA), programmable logic devices, other electronic
units, or any combination thereof. Some of the functions and
processing described herein may also be implemented with software
executed on a processor.
[0216] Certain aspects of the invention may be implemented with a
combination of software and hardware. For example, the processing
to schedule (i.e., select terminals and assign transmit antennas)
may be performed based on program codes executed on a processor
(e.g., cell processor 964 in FIG. 9).
[0217] Headings are included herein for reference and to aid in
locating certain sections. These headings are not intended to limit
the scope of the concepts described therein under, and these
concepts may have applicability in other sections throughout the
entire specification.
[0218] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention. Thus,
the present invention is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *