U.S. patent application number 13/913898 was filed with the patent office on 2013-10-17 for method and apparatus for selecting link adaptation parameters for cdma-based wireless communication systems.
The applicant listed for this patent is InterDigital Technology Corporation. Invention is credited to Philip J. Pietraski.
Application Number | 20130272270 13/913898 |
Document ID | / |
Family ID | 38573834 |
Filed Date | 2013-10-17 |
United States Patent
Application |
20130272270 |
Kind Code |
A1 |
Pietraski; Philip J. |
October 17, 2013 |
METHOD AND APPARATUS FOR SELECTING LINK ADAPTATION PARAMETERS FOR
CDMA-BASED WIRELESS COMMUNICATION SYSTEMS
Abstract
A method and apparatus for selecting a transport format resource
combination (TFRC) using an information theoretic approach are
provided. The TFRC may be selected in a medium access control (MAC)
layer for transmission of data in a code division multiple access
(CDMA) wireless communication system. The maximum number of
spreading codes available for transmission and the set of possible
TFRCs may be determined based on channel characteristics. For each
transport block set size (TBSS) in the set of possible TFRCs, a
TFRC is selected with the largest number of spreading codes within
the maximum number of spreading codes for a corresponding minimum
coding rate. The corresponding code rate for each selected TFRCs is
compared to a threshold to select a corresponding type of
modulation. One of the selected TFRCs is selected to be provided to
the physical layer that best matches the CQI and maximizes the
TBSS.
Inventors: |
Pietraski; Philip J.;
(Huntington Station, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
InterDigital Technology Corporation |
Wilmington |
DE |
US |
|
|
Family ID: |
38573834 |
Appl. No.: |
13/913898 |
Filed: |
June 10, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13041999 |
Mar 7, 2011 |
8462738 |
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13913898 |
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11740504 |
Apr 26, 2007 |
7903614 |
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13041999 |
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60795300 |
Apr 27, 2006 |
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Current U.S.
Class: |
370/335 |
Current CPC
Class: |
H04L 1/0025 20130101;
H04J 13/16 20130101; H04W 72/085 20130101; H04L 1/0003 20130101;
H04J 13/00 20130101; H04L 1/0066 20130101; H04L 1/0015 20130101;
H04L 1/0026 20130101 |
Class at
Publication: |
370/335 |
International
Class: |
H04W 72/08 20060101
H04W072/08 |
Claims
1. A method for use in code division multiple access (CDMA)-based
wireless communication, the method comprising: receiving a channel
quality indicator (CQI) from a wireless transmit/receive unit;
selecting a transport format resource combination (TFRC) based on
the CQI, wherein the TFRC defines a transport block size, a number
of spreading codes, and a modulation type, and the TFRC is selected
with a largest number of spreading codes while maintaining a 1/3 or
greater channel coding rate within a total number of available
spreading codes; and transmitting a transport block generated in
accordance with the selected TFRC to the WTRU.
2. The method of claim 1, wherein the total number of available
spreading codes is limited by a category of the WTRU.
3. The method of claim 1, wherein the TFRC is selected by:
generating a plurality of TFRCs based on the CQI and a minimum
transport block (TB) success probability; grouping the generated
TFRCs based on a transport block set size (TBSS) associated with
each generated TFRC; determining a TFRC for each group based on a
number of spreading codes associated with each TFRC in the group
that have an acceptable associated code rate for a modulation type;
and selecting a TFRC from the determined TFRCs based on the TBSS
associated with each TFRC, the CQI, and the minimum TB success
probability.
4. The method of claim 1, wherein the modulation type is selected
to maximize the transport block size.
5. The method of claim 1, wherein the modulation type is selected
based on a code rate threshold test.
6. The method of claim 1, wherein the TFRC is selected from a
predefined CQI-to-TFRC mapping table.
7. An apparatus for use in code division multiple access
(CDMA)-based wireless communication, the apparatus comprising: a
processor configured to receive a channel quality indicator (CQI)
from a wireless transmit/receive unit, select a transport format
resource combination (TFRC) based on the CQI, and transmit a
transport block generated in accordance with the selected TFRC to
the WTRU, wherein the TFRC defines a transport block size, a number
of spreading codes, and a modulation type, and the TFRC is selected
with a largest number of spreading codes while maintaining a 1/3 or
greater channel coding rate within a total number of available
spreading codes.
8. The apparatus of claim 7, wherein the total number of available
spreading codes is limited by a category of the WTRU.
9. The apparatus of claim 7, wherein the processor is configured to
generate a plurality of TFRCs based on the CQI and a minimum
transport block (TB) success probability, group the generated TFRCs
based on a transport block set size (TBSS) associated with each
generated TFRC, determine a TFRC for each group based on a number
of spreading codes associated with each TFRC in the group that have
an acceptable associated code rate for a modulation type, and
select a TFRC from the determined TFRCs based on the TBSS
associated with each TFRC, the CQI, and the minimum TB success
probability.
10. The apparatus of claim 7, wherein the modulation type is
selected to maximize the transport block size.
11. The apparatus of claim 7, wherein the modulation type is
selected based on a code rate threshold test.
12. The apparatus of claim 7, wherein the TFRC is selected from a
predefined CQI-to-TFRC mapping table.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/041,999 filed on Mar. 7, 2011, which is a
continuation of U.S. patent application Ser. No. 11/740,504 filed
on Apr. 26, 2007, which issued as U.S. Pat. No. 7,903,614 on Mar.
8, 2011, which claims the benefit of U.S. Provisional Patent
Application No. 60/795,300 filed on Apr. 27, 2006, the contents of
which are hereby incorporated by reference herein.
FIELD OF INVENTION
[0002] The present invention is related to medium access control in
wireless communication systems. More particularly, the present
invention is a method and apparatus for selecting link adaptation
parameters in a medium access control (MAC) layer in code division
multiple access (CDMA) wireless communication systems.
BACKGROUND
[0003] Wireless communication systems are well known in the art.
Communications standards are developed in order to provide global
connectivity for wireless systems and to achieve performance goals
in terms of, for example, throughput, latency and coverage. One
current standard in widespread use, called Universal Mobile
Telecommunications Systems (UMTS), was developed as part of Third
Generation (3G) Radio Systems, and is maintained by the Third
Generation Partnership Project (3GPP).
[0004] A typical UMTS system architecture in accordance with
current 3GPP specifications is depicted in FIG. 1. The UMTS network
architecture includes a Core Network (CN) interconnected with a
UMTS Terrestrial Radio Access Network (UTRAN) via an Iu interface.
The UTRAN is configured to provide wireless telecommunication
services to users through wireless transmit receive units (WTRUs),
referred to as user equipments (UEs) in the 3GPP standard, via a Uu
radio interface. The UTRAN may have one or more radio network
controllers (RNCs) and base stations, referred to as Node Bs by
3GPP, which collectively provide for the geographic coverage for
wireless communications with UEs. One or more Node Bs may be
connected to each RNC via an Iub interface. RNCs within a UTRAN
communicate via an Iur interface.
[0005] One type of air interface defined in the UMTS standard is
wideband code division multiple access (W-CDMA). In a W-CDMA system
baseband signals are spread in the frequency domain using
orthogonal spreading codes prior to transmission, and despread at a
receiver using the same spreading codes.
[0006] The Uu radio interface of a 3GPP system uses transport
channels (TrCHs) for transfer of user data and signaling between
UEs and Node Bs. Uplink refers to signaling from a UE to a Node B,
and downlink transmissions are from a Node B to a UE. In 3GPP
communications, TrCH data is conveyed by one or more physical
channels defined by mutually exclusive physical resources, or
shared physical resources in the case of shared channels. In a
conventional 3GPP system, communications between a UE and a Node B
are conducted using a single data stream defined by a combination
of TrCHs called a coded composite TrCH (CCTrCH). Typically, a Node
B is concurrently communicating with several UEs using respective
CCTrCH data streams.
[0007] TrCH data is transferred in sequential groups of transport
blocks (TBs) defined as transport block sets (TBSs). Each TBS is
transmitted in a given transmission time interval (TTI) which may
span a plurality of consecutive system time frames. The number of
bits in a TBS is called the transport block set size (TBSS).
[0008] UMTS specification releases 5 and 6 pertain to high speed
downlink packet access (HSDPA) and high speed uplink packet access
(HSUPA), respectively. HSDPA is a downlink packet access protocol
for packet based UMTS wireless communication systems employing a
W-CDMA air interface with a spreading factor (SF) of 16. According
to HSDPA, up to 15 spreading codes may be allocated to data for
transmission in a common TTI. The data may be modulated using
either quadrature phase shift keying (QPSK) modulation or 16
quadrature amplitude modulation (16-QAM). In future releases of the
HSDPA standard, it is expected that additional types of higher
order modulation will also be supported, such as 64 quadrature
amplitude modulation (64-QAM). Fast retransmissions are
accomplished according to hybrid automatic repeat request (HARQ) by
retransmission combining, which enables operation at relatively
high Block Error Rates (BLER).
[0009] A the CQI mapping table, as in Table 1 for example,
indicates a preferred TFRC for a given CQI according to
conventional TFRC selection approaches for HSDPA, the TFRC
parameters of TBSS, number of spreading codes, and modulation are
mutually dependent. Therefore, multiple different TFRCs may be able
to match the desired channel characteristics corresponding to a
given CQI level, including maximum expected data rate and TB
success probability.
TABLE-US-00001 TABLE 1 CQI mapping table for UE category 10
according to 3GPP TS 125.214. Number of CQI TBSS spreading codes
Modulation 0 N/A Out of range 1 137 1 QPSK 2 173 1 QPSK 3 233 1
QPSK 4 317 1 QPSK 5 377 1 QPSK 6 461 1 QPSK 7 650 1 QPSK 8 792 2
QPSK 9 931 2 QPSK 10 1262 2 QPSK 11 1483 3 QPSK 12 1742 3 QPSK 13
2279 3 QPSK 14 2583 4 QPSK 15 3319 4 QPSK 16 3565 5 16-QAM 17 4189
5 16-QAM 18 4664 5 16-QAM 19 5287 5 16-QAM 20 5887 5 16-QAM 21 6554
5 16-QAM 22 7168 5 16-QAM 23 9719 7 16-QAM 24 11418 8 16-QAM 25
14411 10 16-QAM 26 17237 12 16-QAM 27 21754 15 16-QAM 28 23370 15
16-QAM 29 24222 15 16-QAM 30 25558 15 16-QAM
[0010] Conventional strategies for selecting a TFRC include
choosing a fewer number of spreading codes N than the maximum
number of available spreading codes M because the total allocated
power Pt is divided among the N used spreading codes. Therefore it
is believed that received signal quality is better when more power
is allocated per spreading code.
[0011] The inventor has recognized, however, that higher power for
each spreading code increases interference with the other spreading
codes in the channel, and employing fewer spreading codes does not
necessarily provide better performance results, particularly in
receivers employing advanced decoding techniques. Existing TFRC
selection procedures do not take into account the interference
effects of simultaneous transmissions using different spreading
codes or the capabilities of advanced receivers. Therefore, a
procedure for TFRC selection that improves upon the existing
techniques is desired.
SUMMARY
[0012] The present invention provides a method and apparatus for
transport format resource combination (TFRC) selection in a medium
access control (MAC) layer that enhances channel capacity. TFRC
selection includes selection of transport block set size (TBSS),
number of spreading codes, and modulation type for data
transmission. The maximum number of spreading codes available for
transmission and the set of possible TFRCs are determined based on
channel characteristics of the downlink channel provided by the
physical (PHY) layer. For each TBSS value in the set of possible
TFRCs, a TFRC is selected with the largest number of spreading
codes within the maximum number of spreading codes for which the
corresponding coding rate is preferably at least 1/3. The
corresponding code rates for the selected TFRCs are compared to
thresholds to select a type of modulation. Finally, one of the
selected TFRCs is selected to be provided to the PHY layer that
best matches downlink channel quality and preferably with the
largest TBSS in order to maximize the throughput. The present
invention is preferably used in Universal Mobile Telecommunications
Systems (UMTS) high speed downlink packet access (HSDPA)
communication systems, or in a wireless communication system
employing code division multiple access (CDMA).
[0013] Other objects and advantages will be apparent to those of
ordinary skill in the art based upon the following description of
presently preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows an overview of the system architecture of a
conventional Universal Mobile Telecommunications Systems (UMTS)
network.
[0015] FIG. 2 is a block diagram of an example communication system
100 in which a base station 110 is communicating with a WTRU 120 in
accordance with the present invention.
[0016] FIG. 3 illustrates a graph of measured channel throughput
versus signal-to-noise ratio (SNR) at a receiver for various
modulation techniques, which may be used to select a preferred type
of modulation for a transport format resource combination (TFRC) in
accordance with the present invention.
[0017] FIG. 4A is a flow diagram of a method for generating and
selecting TFRCs in accordance with the present invention.
[0018] FIG. 4B is a flow diagram of a method for selecting a type
of modulation from among quadrature phase shift keying (QPSK), 16
quadrature amplitude modulation (16-QAM) and 64 quadrature
amplitude modulation (64-QAM) to be associated with a TFRC in
accordance with the present invention.
[0019] FIG. 5 shows the measured throughput on a downlink CDMA
channel using the existing 3GPP CQI mapping as given in Table 1
compared to the measured throughput for the optimized CQI mapping
generated according to the present invention in Table 2 for a
category 10 UE for a range of average SNR values and for a TB
success probability of 0.9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] As used herein, a wireless transmit/receive unit (WTRU)
includes but is not limited to a user equipment, mobile station,
fixed or mobile subscriber unit, pager, cellular telephone,
personal digital assistant (PDA), computer, or any other type of
device capable of operating in a wireless environment. A base
station is a type of WTRU generally designed to provide network
services to multiple WTRUs and includes, but is not limited to, a
Node B, site controller, access point or any other type of
interfacing device in a wireless environment.
[0021] FIG. 2 is a block diagram of a communication system 100 in
which a base station 110 is communicating with a WTRU 120 over a Uu
air interface according to the current HSDPA standard and the
present embodiments. For illustrative purposes, protocol layers are
shown in the base station 110 including physical layer (PHY)
components 102, medium access control (MAC) layer components 104,
and higher layer components 106 which include a radio link control
(RLC) layer. Adjacent layers in the protocol stack communicate with
each other. The MAC layer components 104 are responsible for
mapping data streams 105 (also called logical channels) from the
higher layer components 106 to transport channels 107 provided to
the PHY layer 102 for physical transmission over the wireless
channel. [0031B] The MAC layer also selects the transport format
(TF) and link adaptation parameters for data transmission including
transport block set size (TBSS), number of spreading codes, and
modulation type that are collectively referred to as transport
format resource combination (TFRC). As described in accordance with
the present invention below, selection of TFRC is preferably
performed in the MAC layer by a TFRC selection function component
108 prior to each TTI. The selected TFRC parameters 109 are
provided to the PHY layer 102 so that they may be used in the
physical transmission of data 114 to the WTRU 120 in a common TTI
by way of TBs.
[0022] TFRC selection is based at least in part on characteristics
of the physical channel and is not fixed for a given data packet
size. Information regarding the physical channel is provided by
WTRU 120. WTRU 120 takes channel quality measurements, such as
determining the maximum expected data rate, of the downlink channel
and transmits a corresponding channel quality indicator (CQI) 122
to the base station 110.
[0023] The CQI is typically represented as an integer value from 1
to 30, where each CQI value has a predetermined mapping to a TFRC
including a TBSS, a corresponding number of spreading codes and a
corresponding type of modulation that is known at WTRU 120 and base
station 110. This mapping also depends on the physical capabilities
of WTRU 120, and a WTRU is typically assigned to a physical layer
category based on its receiver capabilities such as the maximum
supported number of received bits per TTI and the maximum supported
number of spreading codes per TTI. 3GPP Technical Standard (TS)
125.214 provides CQI mapping tables for different WTRU physical
layer categories (referred to as UE categories) and UE categories
are described in 3GPP TS 125.306. For illustration purposes, an
example CQI mapping for UE category 10 is given in Table 1.
[0024] The WTRU selects the CQI value for which the TFRC mapping is
determined to most closely match the maximum expected data rate on
the downlink channel while satisfying a minimum desired TB success
probability, which refers to the desired success rate of decoding
received TBs at WTRU 120. The same TB success probability is
typically known at WTRU 120 and base station 110, and is preferably
equal to 0.9 according to existing HSDPA standards. To compare a
TFRC to the maximum expected data rate of the downlink channel, a
corresponding expected data rate for the TFRCs in the mapping table
can be determined in advance, for example via simulation.
[0025] The CQI value 122 reported by WTRU 120 to base station 110
is provided to the MAC layer 104 via the PHY layer 102, and the
TFRC selection function 108 selects a TFRC 109 that best achieves
the maximum expected data rate of the channel as determined by the
CQI value and its mapping.
[0026] The present invention provides an information theoretic
approach to selecting a more optimal transport format resource
combination (TFRC) in a wireless communication system, where TFRC
includes transport block set size (TBSS), number of spreading
codes, and modulation type for data transmission over a wireless
code division multiple access (CDMA) channel on a common
transmission time interval (TTI) boundary. Although the present
invention is described herein with reference to high speed downlink
packet access (HSDPA) for downlink communications in UMTS systems,
the disclosed TFRC selection procedure has broader applicability
and is applicable to general CDMA wireless communication systems
including Third Generation Partnership Project (3GPP) CDMA2000 and
high speed packet access evolution (HSPA+) systems. The present
invention may be used in both uplink (UL) and downlink (DL)
communications, and therefore may be readily be implemented in
WTRUs configured as user equipments (UEs) or base stations, such as
Node Bs.
[0027] Preferably, an optimum number of spreading codes N is
selected for TFRC according to the number of spreading codes that
maximize the information theoretic channel capacity. To derive this
capacity, the communication channel can be assumed to be an
additive white Gaussian noise (AWGN) channel with bandwidth B such
that the power Pt of the transmitter is fixed. Data is transmitted
over bandwidth B using N different orthogonal spreading codes such
that N is less than or equal to the maximum number of available
spreading codes M. In HSDPA, the maximum number of available
spreading codes M is at most 15.
[0028] A signal transmitted over the communication channel using N
orthogonal spreading codes can be modeled by N separate
corresponding propagation channels, each with bandwidth B/M.
Because the spreading codes are orthogonal, the noise on each of
the propagation channels after the despreading of the signal is
uncorrelated and has equal power. If A represents the total
signal-to-noise ratio (SNR) of the communication channel, the SNR
of each individual propagation channel is proportional to A/N
because the total allocated power Pt is shared equally over the N
spreading codes. Adaptive modulation and coding (AMC) can be
applied individually to each propagation channel along with
advanced receiver and coding techniques to achieve a channel
capacity close to the theoretical upper limit of B/M*log(1+A/N)
(known as Shannon's capacity formula). It then follows that the
capacities of each of the propagation channels C.sub.1, . . . ,
C.sub.N are the same and proportional to (1/M)*log(1+A/N).
According to this result, propagation channel capacities C.sub.1, .
. . , C.sub.N slowly decrease as N increases. The total capacity C,
however, is proportional to N*(1/M)*log(1+A/N), which is an
increasing function of N in all practical cases. Therefore, in
accordance with the present invention, the number of spreading
codes N is preferably maximized in order to maximize the total
capacity of the channel. This is different from typical HSDPA TFRC
selection approaches that do not attempt to maximize the number of
spreading codes N used for communication with a particular
WTRU.
[0029] The number of spreading codes N is limited by the total
number of available spreading codes M such that N.ltoreq.M. In
accordance with a preferred embodiment of the present invention,
the number of spreading codes N may also be limited by the code
rate used for error correction on the physical channel. Coding
techniques, such as convolutional codes and turbo codes, are used
to add redundancy to transmitted information to correct bit errors
that occur in the channel and at the receiver, and the code rate
(also called coding rate) is the fraction of non-redundant data
bits in a transmitted packet. For example, if for each data bit,
one redundant bit is added by the encoder, the resulting code rate
is 1/2. According to HSDPA, all error correcting codes are derived
from a rate 1/3 turbo code, although the actual coding rate may be
adjusted based on the channel quality indicator (CQI) information
using puncturing or repetition techniques that involve deleting or
adding bits, respectively, at the encoder output. The selection of
coding rate and modulation based on channel quality information is
referred to as adaptive modulation and coding (AMC).
[0030] If, as a result of AMC, the code rate is less than 1/3, then
the effective coding gain is reduced because bit repetition, which
is known to be a weak coding scheme, is required to fill the coded
transport block (TB). Therefore, in accordance with a preferred
embodiment of the present invention, the number of spreading codes
N is preferably selected to be as large as possible while
maintaining a 1/3 or greater channel coding rate and without
exceeding the total number of available spreading codes M. The
unused spreading codes may preferably be used by other WTRUs
communicating over the same channel to make full use of the
physical resources. Alternatively, a greater number of spreading
codes may be used with a corresponding code rate less than 1/3 and
with a lower coding gain, however, this results in fewer spreading
codes being available for use by other WTRUs.
[0031] Referring back to the theoretical channel capacity model
above, each of the N propagation channels may experience multipath
fading and inter-code interference that is present at the output of
a despreader in the receiver. By modeling the inter-code
interference as additive noise, the inter-code interference has the
effect of decreasing the received SNR at the despreader output. The
multipath experienced on each of the N propagation channels is
identical (because the propagation channels are spread over the
same bandwidth), the equalizer used to receive each propagation
channel is identical and the power transmitted on each propagation
channel is identical. Thus, the effective noise increase as a
result of inter-code interference is the same in each propagation
channel and proportional to (N-1)/N, such that the actual value of
the proportionality constant depends on the effectiveness of the
equalizer in restoring the orthogonality of the spreading
codes.
[0032] This proportionality constant is commonly referred to as the
non-orthogonality factor (NOF), and can range from 0.0 to 1.0 but
is typically much less than 1.0 in advanced receivers. The overall
noise power in the channel resulting from inter-code interference
is equal to NOFS(N-1), where S is the received signal power on a
propagation channel. The resulting overall channel capacity is
given by
C = N M log 2 ( 1 + S N I + NOF S ( N - 1 ) ) . Equation ( 1 )
##EQU00001##
where I is the total interference on the received signal.
[0033] The capacity in Equation 1 is not necessarily monotone
increasing depending on the values of S, I, and NOF. Nonetheless,
for practical scenarios Equation 1 is maximized by maximizing the
number of codes N because the slowly decreasing logarithmic
function of N is outweighed by the linearly increasing factor of N
outside of the logarithmic function. In scenarios where the values
of S, I, and NOF are such that the logarithmic function of N
dominates the outside factor of N, choosing N=1 maximizes the
channel capacity. However, the latter scenario is likely only to
occur under very poor channel conditions in which a WTRU may have
already handed-off to another base station.
[0034] Based on the above, and in accordance with a preferred
embodiment of the present invention, the number spreading codes N
selected as part of TFRC selection is preferably maximized, as
permitted by the coding rate and the total number of available
codes M, except under very poor channel conditions in which case
only one spreading code N=1 is preferably used.
[0035] Also in accordance with the present invention, the
modulation type associated with a TFRC is preferably selected to
maximize channel capacity. In the case of 3GPP systems and in
particular HSDPA, the available modulation types are limited to,
for example, quadrature phase shift keying (QPSK) modulation, 16
quadrature amplitude modulation (16-QAM) and possibly higher order
modulation such as 64 quadrature amplitude modulation (64-QAM). In
accordance with a preferred embodiment of the present invention, a
threshold test on the code rate is preferably used to select a type
of modulation from among the available modulation types to maximize
the channel throughput, as described further below.
[0036] For any particular type of modulation, the code rate must be
proportionally increased with the desired signal-to-noise ratio
(SNR) at the receiver to maintain a channel throughput as close as
possible to the maximum throughput. Therefore, the code rate
threshold values are preferably determined by comparing the
measured throughput for a desired SNR for each of the available
modulation techniques, and a modulation scheme is selected that
achieves the highest throughput for its associated coding rate. By
way of example, consider an HSDPA downlink channel that supports
QPSK, 16-QAM and 64-QAM modulations. FIG. 3 illustrates an example
of the empirically measured channel throughput for different SNR
values at the receiver for each of the available modulation
techniques. The code rate CR.sub.QPSK where the QPSK and 16-QAM
throughput curves cross, and code rate CR.sub.16-QAM where the
16-QAM and 64-QAM throughput curves cross are determined and used
as the code rate threshold values. Given a TFRC, for each type of
available modulation a corresponding code rate is determined
according to the reported CQI information. If the corresponding
code rate for QPSK is below the CR.sub.QPSK threshold, then QPSK
modulation is selected because it achieves the highest throughput
for that coding rate compared to 16-QAM and 64-QAM as shown in FIG.
3. If the code rate associated with QPSK modulation is greater than
the threshold CR.sub.QPSK, and the code rate associated with 16-QAM
modulation is less than CR.sub.16-QAM, then 16-QAM is selected to
maximize throughput. If the code rate for 16-QAM is greater than
CR.sub.16-QAM, then 64-QAM is selected to maximize throughput. An
example of a possible code rate threshold for QPSK is
CR.sub.QPSK.apprxeq.0.74, which is greater than 1/3. Although
choosing a modulation type for TFRC selection has been described
for QPSK, 16-QAM and 64-QAM, the present invention may be extended
to select a modulation type from among any number of modulation
techniques, including, but not limited to, additional higher order
modulations, or between QPSK and 16-QAM only when 64-QAM is not
available.
[0037] FIGS. 4A and 4B illustrate the steps of a preferred method
200 for generating and selecting TFRCs in accordance with the
present invention that includes the various embodiments discussed
above for selecting a number of spreading codes and modulation
type. The TFRC selection presented in FIGS. 4A and 4B is preferably
performed by a TFRC selection function component 108 in FIG. 2
prior to a TTI.
[0038] Referring to FIG. 4A, in step 205 the maximum number of
spreading codes M available for use on the physical channel is
determined based on the resource allocation scheme and the receiver
capabilities of the receiving WTRU. Recall that for HSDPA, the
transmitting base station may use up to 15 spreading codes, and the
number of spreading codes supported by the receiving WTRU is given
by its UE physical layer category. In step 210, a set of possible
TFRCs are generated that match the channel characteristics implied
by the reported CQI value and that meet a minimum desired TB
success probability. The channel characteristics are preferably a
maximum expected data rate for the downlink channel, which may be
known at the base station for a given CQI level or explicitly
provided by the WTRU. Generating a possible TFRC includes
generating a transport block set size (TBSS), a number of spreading
codes and a modulation type which, when applied to a transmitted
TB, have an expected data rate close to the maximum expected data
rate of the downlink channel and meet the desired TB success
probability following decoding at the receiving WTRU. The expected
data rate for a given TFRC may be determined, for example, by
simulation. As discussed above, the desired TB success probability
for the downlink channel is preferably a predetermined value that
is known to both the transmitting base station and the receiving
WTRU.
[0039] In step 215, the set of possible TFRCs are grouped according
to common TBSS. In each group a TFRC is preferably selected that
has a largest number of spreading codes, up to the determined
maximum number of spreading codes M, for which the associated code
rate is at least 1/3 for at least one type of available modulation
scheme. Types of modulation may include QPSK modulation, 16-QAM
modulation and any other higher order modulations. The TFRCs that
are not selected are preferably discarded.
[0040] For every selected TFRC, beginning at step 220, a modulation
type is associated with the selected TFRC based on a code rate
threshold test in step 230. Determining code rate threshold values
is described above with respect to FIG. 3. FIG. 4B gives a
particular example of a code rate threshold test for selecting a
modulation type when QPSK, 16-QAM and 64-QAM modulations are
available. The code rate associated with QPSK modulation is
compared to a predetermined threshold in step 232. If that code
rate is below CR.sub.QPSK, QPSK modulation is associated with the
selected TFRC in step 233. If the code rate is above CR.sub.QPSK,
then the code rate associated with 16-QAM modulation is compared to
threshold CR.sub.16-QAM in step 235. If that code rate is below
CR.sub.16-QAM, then 16-QAM is associated with the selected TFRC in
step 236. Otherwise, 64-QAM is associated with the selected TFRC in
step 238. Referring to FIG. 4A, each selected TFRC is thereafter
preferably saved in a list in step 240.
[0041] When the listing of the selected TFRCs is completed, the
list of selected TFRCs is preferably sorted in order of TBSS in
step 245. The list represents all of the possible TFRCs for use
when sending data to the WTRU in accordance with the present
invention. In step 250, one TFRC from among the list of selected
TFRCs is provided to the PHY layer for data transmission that
preferably has the largest TBSS while maintaining the desired TB
success probability for the channel characteristics defined by the
reported CQI value.
[0042] In accordance with a preferred embodiment of the present
invention as described with respect to FIGS. 4A and 4B, a new
mapping of CQI values to preferred TFRCs can be created, an example
of which is illustrated in Table 2 for a HSDPA downlink channel
that supports a maximum number of 15 spreading codes, QPSK and
16-QAM modulation and a TB success probability of 0.9. A CQI of 0
is used when the receiving WTRU is out of range to receive signals
on the downlink channel.
TABLE-US-00002 TABLE 2 CQI mapping for TFRC selection according to
the present invention for a HSDPA downlink channel with a category
10 UE supporting a maximum of 15 spreading codes and QPSK or 16-QAM
modulation Number of CQI Code rate TBSS spreading codes Modulation
0 N/A N/A Out of range 1 0.14271 137 1 QPSK 2 0.18021 173 1 QPSK 3
0.21771 209 1 QPSK 4 0.26771 257 1 QPSK 5 0.33021 317 1 QPSK 6
0.40521 389 1 QPSK 7 0.56771 545 1 QPSK 8 0.39115 751 2 QPSK 9
0.35972 1036 3 QPSK 10 0.35313 1356 4 QPSK 11 0.3191 1838 6 QPSK 12
0.35149 2362 7 QPSK 13 0.34502 2981 9 QPSK 14 0.32656 3762 12 QPSK
15 0.32389 4664 15 QPSK 16 0.38056 5480 15 QPSK 17 0.45514 6554 15
QPSK 18 0.52528 7564 15 QPSK 19 0.59542 8574 15 QPSK 20 0.67493
9719 15 QPSK 21 0.75146 10821 15 QPSK 22 0.43361 12488 15 16-QAM 23
0.48278 13904 15 16-QAM 24 0.53753 15481 15 16-QAM 25 0.58788 16931
15 16-QAM 26 0.62031 17865 15 16-QAM 27 0.66639 19192 15 16-QAM 28
0.70316 20251 15 16-QAM 29 0.76899 22147 15 16-QAM 30 0.88743 25558
15 16-QAM
[0043] Table 2 includes the preferred TBSS, number of spreading
codes and modulation type as derived according to the present
invention described above, and the associated coding rate, for each
possible CQI value for a category 10 WTRU. When Table 2 is compared
to the CQI mapping table for a category 10 UE in 3GPP TS 125.214 as
reproduced in Table 1, it is observed that TFRC selection according
to the present invention generally selects a greater number of
spreading codes and a larger TBSS, whenever possible and in
particular for intermediate CQI values, in order to improve overall
channel throughput.
[0044] FIG. 5 shows the measured throughput on a downlink CDMA
channel using the existing 3GPP CQI mapping as given in Table 1
compared to the measured throughput for the optimized CQI mapping
generated according to the present invention in Table 2 for a
category 10 UE for a range of average SNR values and for a TB
success probability of 0.9. The throughput is given versus an
average SNR because of variations in the instantaneous SNR as a
result of fading in the channel. FIG. 5 illustrates the improvement
in throughput achieved by the present invention over existing TFRC
selection techniques.
[0045] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention. The methods or flow charts provided in the
present invention may be implemented in a computer program,
software, or firmware tangibly embodied in a computer-readable
storage medium for execution by a general purpose computer or a
processor. Examples of computer-readable storage mediums include a
read only memory (ROM), a random access memory (RAM), a register,
cache memory, semiconductor memory devices, magnetic media such as
internal hard disks and removable disks, magneto-optical media, and
optical media such as CD-ROM disks, and digital versatile disks
(DVDs).
[0046] The components for implementing the invention referenced
above may be implemented as separate physical devices or combined
such as in a processor that implements the functions of multiple
components. Suitable processors include, by way of example, a
general purpose processor, a special purpose processor, a
conventional processor, a digital signal processor (DSP), a
plurality of microprocessors, one or more microprocessors in
association with a DSP core, a controller, a microcontroller,
Application Specific Integrated Circuits (ASICs), Field
Programmable Gate Arrays (FPGAs) circuits, any integrated circuit,
and/or a state machine.
[0047] A processor in association with software may be used to
implement a radio frequency transceiver for in use in a wireless
transmit receive unit (WTRU), user equipment, terminal, base
station, radio network controller, or any host computer. The WTRU
may be used in conjunction with modules, implemented in hardware
and/or software, such as a camera, a video camera module, a
videophone, a speakerphone, a vibration device, a speaker, a
microphone, a television transceiver, a hands free headset, a
keyboard, a Bluetooth.RTM. module, a frequency modulated (FM) radio
unit, a liquid crystal display (LCD) display unit, an organic
light-emitting diode (OLED) display unit, a digital music player, a
media player, a video game player module, an Internet browser,
and/or any wireless local area network (WLAN) module.
* * * * *