Incremental redundancy (IR) schemes for WCDMA HSDSCH
ABSTRACT The paper compares two hybrid automatic repeat request (HARQ) schemes using incremental redundancy (IR) which have been proposed for the UMTS (WCDMA) high speed downlink shared channel (HSDSCH). Their relative throughput performance is reported for various channel conditions. It is shown that the twostage ratematching scheme has marginally worse performance compared to the alternative block interleaving scheme for certain coding rates and under some channel conditions.

Conference Paper: Performance of 3GPP high speed downlink packet access (HSDPA)
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ABSTRACT: High speed downlink packet access (HSDPA) technology was standardized for 3GPP WCDMA release5. HSDPA includes advanced techniques such as adaptive modulation and coding (AMC), hybrid ARQ (HARQ), and fast scheduling. HSDPA can deliver a 3× increase in sector and a 6× increase in user throughput for some traffic models. Additionally HSDPA can support almost a 4× increase in the number of users compared to UMTS release99 for the same service. With advanced receivers, HSDPA can support broadband quality service with 60 users per site. This paper provides an overview of the physical layer aspects of HSDPA and discusses the improvements in sector and user throughput (with respect to release99 UMTS) resulting from HSDPA with joint scheduling and resource allocation.Vehicular Technology Conference, 2004. VTC2004Fall. 2004 IEEE 60th; 10/2004  SourceAvailable from: Djamal Zeghlache[Show abstract] [Hide abstract]
ABSTRACT: HSDPAbased UMTS networks should offer high bit rate connections in the next generation wireless networks. However, radio resources available to achieve them are limited. This paper presents suggestions to optimize some HSDPA techniques such as scheduling in single and multiservice cases while distinguishing terminal capabilities.
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INCREMENTAL REDUNDANCY (IR) SCHEMES FOR WCDMA HSDSCH
Amitava Ghosh1, Kenneth Stewart2 , Rapeepat Ratasuk1, Eoin Buckley2, and Raja Bachu2
1 Advanced Radio Technology, GTSS, Motorola, Arlington Heights, IL, USA 60004
2 Personal Communications Sector, Motorola, Libertyville, IL, USA 60048
email: {qa0047, qa2191, rratasu1, w50105, frb024c}@email.mot.com
Abstract – This paper compares two Hybrid Automatic
Repeat Request (HARQ) schemes using Incremental
Redundancy (IR) which have been proposed for the UMTS
(WCDMA) High Speed Downlink Shared Channel (HS
DSCH). Their relative throughput performance is reported
for various channel conditions. It is shown that the two
stage ratematching scheme
performance compared to the alternative scheme for certain
coding rates and under some channel conditions.
has marginally worse
I. INTRODUCTION
The two fundamental forms of HARQ are Chase
combining and incremental redundancy (IR). In Chase
combining [1], each retransmission repeats the first
transmission or part of it. In IR, each retransmission
provides new code bits from the mother code to build a
lower rate code [2]. While Chase combining is sufficient to
make Adaptive Modulation and Coding (AMC) robust, IR
offers the potential for better performance with high initial
and successive code rates, at higher SNR estimation error
and FER operating points (i.e., a greater probability that a
transmission beyond the first will be needed), albeit at the
cost of additional memory and decoding complexity. The
consensus in the 3GPP UMTS standards bodies is to
explicitly define and allow IR, while retaining Chaselike
operation as a subset of IR since the memory requirements
at the User Equipment (UE) for the highest data rate is
derived based on Chase combining.
In this paper, two types of IR schemes for WCDMA
HSDSCH are discussed: a) IR based on twostage rate
matching and b) IR based on block interleaving. The IR
based on twostage rate matching [3][4][9] is the agreed
upon IR scheme for HSDSCH. It uses two stages of rate
matching where the first stage is used to match the amount
of coded bits to the UE buffering capability and the second
stage is used to generate the different redundancy versions.
It may be noted that the first stage will be bypassed in case
the UE has full buffering capability. IR based on block
interleaving was proposed in [5][6]. The approach uses a
block interleaver in the channel coding chain followed by a
redundancy version selector and a virtual bit priority
mapper. The block interleaver rows and columns are
permuted according to UMTSRelease ’99 internal turbo
interleaver and 1st interleaver algorithms. This technique
simplifies the Release’99 channel coding chain by
removing the ratematching block. The block interleaver
acts both as a ratematcher and channel interleaver followed
by a virtual bit priority mapper. In this scheme, orthogonal
retransmissions (i.e., each transmission contains unique
parity bits) are guaranteed and as such the performance of
this scheme is optimal. Section II describes the WCDMA
HSDSCH IR scheme based on twostage rate matching.
Section III provides a brief description of the IR scheme
based on block interleaving. Simulation results comparing
these two methods are then presented in Section IV.
Finally, conclusions are drawn in Section V.
II. HARQ BASED ON TWOSTAGE RATE
MATCHING
The 3GPP HARQ scheme for HSDSCH is shown in
Figure 1. This scheme is based on the ratematching
algorithm defined in Section 4.2.7 of [7] and can support
both selfdecodable and non selfdecodable transmissions.
The First Rate Matching block is used to adjust the number
of available coded bits at the NodeB to the virtual UE’s
buffer size. The maximum number of soft bits available in
the virtual IR buffer is signaled from higher layers for each
HARQ process. The Second Rate Matching block is then
used to ratematch and select the set of coded bits for
transmission, given the redundancy version (RV) selected.
Two parameters control the generation of the
redundancy versions, a selfdecodability parameter s and eini
variation parameter r. The goal of the eini variation
parameter is to enable robust retransmissions by
orthogonalizing available redundancy versions that contain
the same selfdecodability parameter s. The number of
RV’s allowed for HSDSCH is limited to 8. HARQ second
stage rate matching is done with the general method
described in 4.2.7.5 of [7]. The initial (eini), increment (eplus)
and decrement (eminus) value of variable e in the rate
matching pattern determination algorithm are based on the
RV parameters s and r. The parameter s can take the value
Systematic
bits
Parity 1
bits
Parity2
bits
RM_P1_1
RM_P2_1
RM_P1_2
RM_P2_2
RM_S
First Rate Matching
Second Rate MatchingVirtual IR Buffer
Nsys
Np1
Np2
Nt,sys
Nt,p1
Nt,p2
bit
separation
NTTI
bit
collection
Ndata
C
W
Figure 1. IR Scheme for HSDSCH based on two
stage ratematching.
0780375890/02/$17.00 ©2002 IEEE PIMRC 2002
Page 2
0 or 1 to distinguish between prioritizing systematic and
parity bits. The parameter r (range 0 to rmax) changes the
initial error variable eini in the case of puncturing. In case of
repetition both parameters r and s change the initial error
variable eini. The parameters Xi, eplus and eminus are calculated
as per Table 1 shown below.
Systematic
RM S
Parity 1
RM P1_2
Parity 2
RM P2_2
Xi
eplus
eminus
N
−
sys
N
sys
N
syst sys
N
,
1
p
N
1
p
Na⋅
1,1
ptp
NNa
−⋅
2
p
N
2
p
Na⋅
2,2
ptp
NNa
−⋅
Table 1. Parameters for the secondstage rate matching.
Denote the number of bits before second rate matching
as Nsys for the systematic bits, Np1 for the parity 1 bits, and
Np2 for the parity 2 bits, respectively. Denote the number of
physical channels used for the Coded Composite Transport
Channel (CCTrCH) by P. Ndata is the number of bits
available to the CCTrCH in one radio frame (defined as
Ndata=P×3×Ndata1, where Ndata1 is given in [7]). The rate
matching parameters are determined as follows.
For
21
pp sys data
NNNN
++≤
, puncturing is performed
in the second rate matching stage. The number of
transmitted systematic bits in a retransmission is
{
datasyssyst
NNN
, min
,
for a transmission of selfdecodable
type and
(
max
,
data syst
NNN
decodable case. For
>
data
N
N
performed in the second rate matching stage. A similar
repetition rate in all bit streams is achieved by setting the
number of transmitted systematic bits as follows
}
=
)
0 ,
+
{}
N
21
pp
N
+
+
−=
in the non self
, repetition is
21
ppsys
N
+
=
2
,
2
psys
data
syssyst
NN
N
NN
(Eq. 1)
The number of parity bits in a transmission is then
−
2
=
,
1,
systdata
pt
NN
N
(Eq. 2)
and
−
2
=
,
2,
syst data
pt
NN
N
(Eq. 3)
for the first and second parity stream, respectively. The
parameter a in Table 1 is chosen using a = 2 for the first
parity stream and a = 1 for the second parity stream.
The rate matching parameter eini is calculated for each
bit stream according to the redundancy version parameters r
and s using
1 mod
1)(
max
r
+
−
−=
plus
plus
i ini
e
e
rXre
(Eq. 4)
in case of puncturing, and
()
1 mod
1
2
2)(
max
r
+
−
+−=
plus
plus
iini
e
e
e
rsXr
(Eq. 5)
in case of repetition, where r ∈{0,1,…,
the total number of redundancy versions allowed. Note that
r
varies depending on the modulation mode (for 16
r
= 2 and for QPSK,
max
r
1} and
max
r
is
max
QAM,
maxmax
r
= 4).
Subsequently, the bits out of the 2nd ratematching stage
are mapped based on priority via the HARQ bit collection
unit, and interleaved via one or more Release ’99
interleavers based on the modulation level (QPSK or 16
QAM). The HARQ bit collection is achieved using a
rectangular interleaver of size
rows and columns are determined from:
colrow
NN
×
. The number of
rowcol
row
NFN
MN
/
)( log2
=
=
(Eq. 6)
where M is the modulation size and F is the number of
coded and ratematched bits to be transmitted. Data is
written into the interleaver column by column, and read out
of the interleaver column by column. The parameter Nt,sys is
the number of transmitted systematic bits. Intermediate
values Nr and Nc are calculated using:
=
col
syst
r
N
N
N
,
(Eq. 7)
and
colr
col
syst
c
NN
N
N
N
⋅
−=
,
(Eq. 8)
If Nc=0, the systematic bits are written into rows 1…Nr.
Otherwise systematic bits are written into rows 1…Nr+1 in
the first Nc columns and rows 1…Nr in the remaining Nc
columns. The remaining space is filled with parity bits. The
parity bits are written column wise into the remaining rows
of the respective columns. Parity 1 and 2 bits are written in
alternating order. In the case of 16QAM for each column
the bits are read out of the interleaver in the order row 1,
Page 3
row 2, row 3, row 4. In the case of QPSK for each column
the bits are read out of the interleaver in the order row1,
row2, row 3, row 4. The interleaving after the bit collection
unit is done using two Release ’99 2nd interleaver with fixed
size (32x30). For QPSK, only one interleaver is used
whereas for 16QAM, there are two identical interleavers of
the same fixed size. For 16QAM, the first two bits out of
the bit collection unit are read into the first interleaver and
the next two bits are read into the 2nd interleaver and so on.
III. HARQ BASED ON BLOCKINTERLEAVING
This scheme was proposed in [5][6]. A block diagram
illustrating the functionality of this simpler scheme is shown
in Figure 2. It consists of a slightly modified Release ’99
interleaver, a redundancy version selector, and a Bit Priority
Mapper. The HARQ functionality is implemented by the
blockinterleaver and the redundancy version (RV) selector.
Turbo
Coder
Systematic
Parity
Bit Priority
Mapper
R’99
interleaver + Row
rearrangement
R’99
interleaver + Row
rearrangement
RV selector
RV selector
Figure 2. Simpler IR/BPM Scheme.
A. Block Interleaver
The encoder used here is the rate1/3 Turbo encoder
specified in [7]. The unpunctured codeword bits are
separated into a systematic and a parity stream denoted by
S x,,
k
p, 1,
k
p, 2
} (i.e.,
∈
k
{{0,0}, {0,1}, {1,0}, {1,1}}).
Each stream is read into an
interleaver while the tail bits are buffered separately and are
later appended onto the unpunctured instantaneous
codeword (where the instantaneous codeword is the
codeword transmitted in a specific transmission time
interval). Thus, the 12 buffered tail bits are always
transmitted. As an example, if the first transmission is at
R=3/4 and the number of information bits is 600, then 600
systematic, 188 parity are read from the Systematic and
Parity interleaver respectively into the Virtual Bit Priority
mapper. Finally the 12 tail bits are read. The number of
columns in each interleaver is always fixed at 30 while the
number of rows is variable (dependent on the number of
information bits) and is determined in exactly the same
manner as the turbo code internal interleaver defined for
Release ’99 in step 2 of section 4.2.3.2.3.1 of 25.212.
kP x, respectively where
∈
kS x,
{0,1} and
∈
kP x,
{
kP x,
colrow
NN
×
block matrix
Data is read rowwise into each interleaver, with
dummy bits padded if
N
<
????
to the procedure used for the Release ’99 second interleaver.
To facilitate flexibility in supporting variable coding rates,
both columns and rows are permuted prior to reading out of
the block matrix contents. The same column permutation is
proposed for both the systematic and parity interleavers and
is defined in [7]. The proposed row permutations are derived
directly from the intrarow permutations of the Release ’99
Turbo code internal interleaver.
colrow
NN
×
. This is identical
B. Redundancy Version Selector
The redundancy version selector determines the subset
of systematic and parity bits to be transmitted over the air.
A salient feature of this method is that the selector does not
suggest a particular order of transmission. Using this simple
approach the selected redundancy version sequence may be
chosen to support Chase, partial and full IR schemes. Each
redundancy version j contains a starting column
either the systematic or parity interleaver (i.e.
∈
j
{0,…7},
},{
PSi∈
and
procedure for computing
j
α is as follows. First, compute
j
α in
},{ ki
). The
j∈α
for
∈
k
},, 1 {
max_,
coli
N
?
colP colS vr
NNN
max_, max_,
2×+=
(Eq. 9)
then
) 4 ,
j
mod(2
8
) 4 ,
j
mod(2
N
VR
vr
j
⋅−
⋅⋅
=
(Eq.10)
for ∈
j
{0,…7}. Finally, calculate
j
α via
},{
jj
VRS
=α
(Eq. 11)
if
colSj
N VR
max_,
<
and
−
=
2
,
_,
colSj
j
N VR
P
???
α
(Eq. 12)
otherwise. This gives four unique starting columns evenly
spaced throughout the lowest code rate supported by the
mobile. Coded bits are read from the selected starting
columns columnwise either
(
) 1,(
+×
row
Nk
,
) 2 ,(
+×
row
Nk
, …) for
decreasing order (
) 1,(
−×
row
Nk
{4,5,6,7}. Coded bits continue to be read until the end
(
),(
row
NNk
××
???????
for
∈
j
{0,1,2,3}) or start (
{4,5,6,7}) of the interleaver is reached. Reading then
continues column wise from the other interleaver iˆ,
(
},{
ˆ
PSi ∈
,
ii ≠
ˆ
) either from the first column or the last
column.
in increasing
{0,1,2,3}) or in
order
′
iy
′
iy
∈
j
′
iy
,
) 2
−
,(
×
′
row
Nkiy
, …) for
∈
j
iy
′
1 , iy′ for
∈
j
Page 4
As an example, consider the case shown in Figure 3
where the encoded message consists of 720 bits with the UE
able to support the full rate 1/3 code. Assuming the
redundancy versions chosen for transmission are RV0 and
RV2 and each transmission consisting of 960 bits, then the
first transmission will consist of codeword bits followed by
the 12 tail bits and the second transmission will consist of
the codeword bits followed by the 12 tail bits.
C. Virtual Bit Priority Mapper
Priority bit mapping is based on utilizing the differing
bit reliability offered by higher order constellations (16
QAM or higher). Since the systematic portions of a
codeword are of greater importance to decoder performance
than the parity portions, they should be placed in positions
of high reliability when higher order constellation is used.
Since channel interleaving has already implicitly been
performed, codeword bits can be read directly from the
Systematic and Parity puncturing block interleavers, to form
the desired 16QAM symbol in a simple and straightforward
manner.
IV. SIMULATION RESULTS
The effective code rate is defined as ni/ni+np where ni is
the number of information bits and np is the number of
unique parity bits, after the second transmission is plotted as
a function of the initial code rate for the above two schemes
(assuming full buffering) in Figure 4. The two schemes
have identical results for initial code rates lower than 1/2.
At higher code rates, however, the ratematching scheme
cannot select only unique parity bits in the second
transmission, thus resulting in a higher effective code rate.
The block interleaving approach, however, is able to select
all unique parity bits in subsequent transmissions. As a
result, the optimality of the block interleaving based
approach is evident from the figure since the block
interleaving approach could
transmissions as opposed to the twostage ratematching
scheme.
guarantee orthogonal
Next, the two IR schemes are compared using a
symbollevel simulator for HSDSCH. The results presented
here assume ideal channel estimation, QPSK/16QAM
modulation and non selfdecodable retransmissions. Table
2 lists additional relevant simulation parameters.
Parameter Value
Carrier Frequency 2GHz
Channel conditions AWGN, Flat Fading
HSDPA frame Length 2 msec (3 slots)
Ior/Ioc Variable
Fast fading model Jakes spectrum
Channel Coding R=1/3 Turbo
Max no. of Decoder Iter 8
Metric for Turbo Coder MaxLogMap
Turbo Interleaver Random
Table 2. Simulation Parameters.
Figure 5 compares the spectral efficiency of the two
schemes under static AWGN channel for N=720 bits, R=3/4
and QPSK modulation. In this case, the blockinterleaving
based scheme has a marginal advantage over the rate
matching scheme at Ior/Ioc greater than –1.2 dB. Figure 6
compares the spectral efficiency vs. Ior/Ioc of the two
schemes using QPSK modulation, N=720 bits, R=3/4 code
under Rayleigh fading channel at 3 km/h. For the rate
matching approach, we assume s=1, r=0 on the first
transmission, s=0, r=0 on the second transmission, s=0, r=1
on the third transmission, and s=1, r=1 on the fourth
transmission for the rate matching approach. A maximum
of 4 retransmissions were allowed for the packet for both
the above cases.
Figure 7 compares the performance using 16QAM
modulation at 120 kmph using R=3/4 code for a message
size of 1,440 bits. For the ratematching approach, we again
select s and r as outlined above. It may be observed from
both the figures that the block interleaving approach
outperforms the twostage ratematching scheme by
approximately 0.5 dB under fading channel conditions.
V. CONCLUSIONS
The blockinterleaving scheme has the following
advantages over the 3GPP twostage ratematching scheme.
First, the systematic and parity interleavers in Figure 2 are
analogous to the second ratematching block combined with
the Release ’99 interleavers in that they interleave as well as
rate match. As such, a single set of backwardscompatible
interleavers serve as a simple approach for RV selection,
Systematic Parity
Nrow=24
NS,max_col=30
NP,max_col=30
RV0
RV1 RV2 RV3
RV5
RV6RV7
RV4
Column 1 Column 20 Column 5
Column 15
Figure 3. Example of Redundancy Version Selector
and Codeword Selection.
Page 5
symbol mapping etc. Second, the performance of the block
interleaving approach is superior to IR based on twostage
ratematching by approximately 0.5dB. Third, for the two
stage rate matching scheme, the ratematching patterns must
be determined for every redundancy version selected,
whereas in the blockinterleaving approach only the index of
the starting location is needed. Lastly, not all re
transmissions are guaranteed to have unique parity bits due
to the ratematching algorithm, which degrades incremental
redundancy performance.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Bob Love and Brian
Classon for their invaluable comments.
REFERENCES
[1] D. Chase, “Code combining—A maximumlikelihood
decoding approach for combining an arbitrary number
of noisy packets”, IEEE Trans. Comm., Vol. COM33,
No. 5, pp 385393, May 1985.
[2] S. Lin and D. J. Costello, Error Control Coding:
Fundamentals and Applications. New York: Prentice
Hall, 1983, pp. 477494.
[3] R1011045, Ericsson, “Physicallayer HybridARQ
functionality”, 3GPP RAN1#22, Korea.
[4] R1011196, Siemens, “Rate Matching and Incremental
Redundancy for HSDPA”, 3GPP RAN1#22, Korea.
[5] R1011244, Motorola, “Revised:HARQ Scheme Using
Incremental Redundancy”, 3GPP RAN1#22, Korea.
[6] R1020285, Motorola, “Enhancement for IR for
HSDPA”, 3GPP RAN1#24, USA.
[7] 3GPP “HSDPAPhysical Layer Aspects”, 3GPP
TS25.212, V5.0.0.
[8] R1020234, Motorola, “Enhancement of IR for
HSDPA”, 3GPP RAN1#24, USA.
[9] R1011026, TI, “Two Stage Rate Matching for 3GPP
HSDPA”, 3GPP RAN1 AdHoc, Sophia Antipolis.
0.3 0.40.5 0.6
Initial Code Rate
0.7 0.80.91
0.32
0.34
0.36
0.38
0.4
0.42
0.44
0.46
0.48
0.5
Effective Code Rate
Block Interleaver Based
Two−Stage Rate Matching Scheme
Figure 4. Effective Code Rate Comparison of IR based
on twostage ratematching and block interleaving.
−2−1.8 −1.6 −1.4−1.2 −1−0.8−0.6−0.4−0.20
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Ior/Ioc (dB)
Spectral Efficiency
Spectral Efficiency for N=720, QPSK, R=3/4, AWGN Channel
Block Interleaver Based
Two−Stage Rate Matching Based
Figure 5. Spectral Efficiency for N=720, QPSK, R=3/4,
AWGN Channel.
−10−505 1015
0
0.5
1
1.5
2
2.5
3
Ior/Ioc (dB)
Spectral Efficiency
Spectral Efficiency for N=720, QPSK, R=3/4, Flat Fading (3 km/h) Channel
Block Interleaver Based
Two−Stage Rate Matching Based
Figure 6. Spectral Efficiency for N=720, QPSK, R=3/4,
Flat Fading channel (3 km/h).
−12−10 −8−6−4−202
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Ior/Ioc (dB)
Spectral Efficiency
Spectral Efficiency for N=1440, 16−QAM, R=3/4, Fading (120 km/h) Channel
Block Interleaver Based
Two−Stage Rate Matching Based
Figure 7. Spectral Efficiency for N=1440, R=3/4, 16
QAM, Flat Fading channel (120 km/h).