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8/9/2019 Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA
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Performance Comparison of HARQ with Chase Com bining
and Incremental Redundancy for HSDPA
PA1 Frenger: Stefan Parkvall, and Erik Da.hlman
Ericsson Research, Ericsson Radio Systems
AB,
SE-164 80 Stockholm, Sweden
Abstract-In thi s paper we compare two hybrid aut om ati c
repeat request (H AR Q) combining strategies that currently
are considered for the high speed downlink packet access
(HSD PA) evolution of WCDMA. The two HARQ combin-
ing schemes are Chase combining, where the retransmissions
are identical copies of the original transmission, and incre-
mental redundancy ( IR ), where the retransmissions consist
of
new parity bits from the channel encoder. We show in
this paper that the link-level performance of a HARQ type-
I system can be significantly better with 1R compared to
Chase combinin g. The largest gains are obtained for high
channel-coding rate s and high modu lation orders. For
low
modulation and coding schemes (M CSs ), the link-level per-
formance gains with IR are less significant. We further show
that in a system that uses link adaptation we can not expect
any large gains with IR as long as the link adaptation errors
are reasonable small. Furthermore, we show that on fading
channels there are situations when an IR system actually
performs poorer than a Chase combining system.
Keywords-
WCDMA evolution, High Speed Downlink
Packet Access (H SDP A), Chase Combining, Incremen-
tal Redundancy ( IR ), Hybrid Automatic R epeat reQuest
(HARQ).
I. INTRODUCTION
As a first step in the evolution of WCDMA, a new con-
cept denoted high speed downlink packet access (HSDPA)
is currently being developed within the 3GPP framework.
Two important design targets for the HSDPA concept are
to provide downlink peak dat a rates in th e order of 8-10
Mbit /s for best effort packet based services a,nd to signifi-
cantly reduce t he downlink transmission delays.
Some importa.nt features th at are introduced in HSDPA
are fast link adaptation, fast scheduling, and fast HARQ
with
soft
combining (i.e. type-I1 [ l ] ) .A new high speed
downlink shared channel (HS-DSCH) is introduced that is
shared in the time domain among the active users, sim-
ilar to the DSCH in WCDMA of today. Instead of fast
power control, the HS-DSCH will use fast link adaptation
that adapts the size of the modulation alphabet and the
rate of th e chaiinel encoder t.0 the fast channel fading. Th e
scheduler decides, based
on
e.g. t he instanta.neous channel
qualit ies of all users, which user shall be assigned the HS-
DSCH channel during the upcoming tra,nsmission time in-
terval. Furthermore, if a,n error is detected by the receiver,
the fast hybrid ARQ system ensures tha t the necessary re-
transmission is executed quickly.
Achieving low transmission delays for HSDPA is essential
in order to ensure good performance also together with
higher layer protocols e.g. TCP. It is important that the
bandwidth-delay product
of
the channel is in the order
of
Phone:
+46
0
7
7
1.52. Fax:
+46
8 585 314 80. Email:
pal. frengeroera ericsson.se.
the TC P window-size, or else it will not be possible to
fully utilize the radio link. Furthermore, for small da ta
packets the slow-start beha.vior of TCP, and not the data
bandwidth of the channel, will limit the performance unless
the round trip time for TCP acknowledgments is small.
Thus, in order to benefit from the increased data rates
provided by HSDPA, reducing the transmission delays is a
key concern [ a ]
In the current UMTS radio access network (UTR AN) ar-
chitec ture the scheduling of users, selection of the t ranspo rt
format (including modulation and coding parameter s), and
the ARQ retransinissions are located in the radio network
controller (RNC). Since the HSDPA fast link adaptation
and fast scheduling will adap t to t he fast fading of the radio
channel, it is necessary to move these functionalities closer
to the radio channel, i.e. to the Node-B (base station) in-
stead. Also the HARQ termination point for HSDPA needs
to be located in the Node-B in order to reduce the delays
for retransmitted packets
[ 3 ] .
In this paper we will compare two different packet com-
bining strategies th at are considered for HSDPA. These are
Chase combining, where each retransinissioii is identical to
the original transmission, and incremental redundancy ( IR)
where each retransmission consists of new redunda.ncy bits
from the channel encoder. Obviously IR have the potential
of achieving better perforimmce compared to Chase com-
bining. However,
a
HARQ system with Chase combining
will have lower complexity. Th e use of IR requires some ad-
ditional signaling since the retransinission numbers needs
to be communicated to the receiver. Furthermore, IR re-
quires larger receiver buffer size. Th e receiver buffer size
increases for each IR transmission aiid
it
is also necessary t o
buffer
soft
bits instead of soft symbols in the mobile termi-
nal
(UE).
Thus if IR is to be implemented for HSDPA the
complexity and cost of the system will be higher. Therefor
it is important to examine if there are any, large perfor-
mance gains with I R, or if Chase combining can provide
comparable performance a.t lower cost.
In this paper we show that the link-level performance of
a HARQ type-I1 system in some cases is significantly bet-
ter for IR compared to Chase combining. The largest gains
are obta,ined for high channel-coding rates a.nd high mod-
ulation orders. For low modulat ion aiid coding schemes
(r\irCSs), the link-level performance gains with IR are less
significant. Furthermore, in a.system using link adapta tion
we can not expect any significa.iit gains with IR unless the
link adaptation errors are very large. The reason for this is
that Chase combining gives
3
dB additional signal energy
in the first retransmission a.nd with reasonably good link
0-7803-7005-8/01/ 10.000 2001 IEEE 1829
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adaptation we will not need the additional coding gain th at
can be achieved by IR. To show this effect we compare in
this paper t he performance difference of Chase combining
and
IR
combining when the channel quality estimates are
poor. Poor link adaptat ion can be caused by e.g. high
Doppler shifts which makes the channel difficult to pre-
dict, or by rapid variations in the interference level. Large
Doppler will cause not only poor link adaptation, but large
errors in the receiver channel estimates
as
well. In this pa-
per we therefore investigate the sensitivity towards chan-
nel estimation errors for different
MCSs
and we show that
high MCSs can only be used when the receiver channel es-
tima tes a.re very accurate. Therefore , in cases with high
Doppler i t is likely th at only the lowest MCSs can be used,
for which the gains with IR are small. There could how-
ever be situations where poor link adaptation is caused by
large delays in the channel quality report feedback. In this
situation it is possible that the receiver could still obtain
accurate channel estimates while the transmitter is unable
to obtain accurate channel quality estimates for the link
adaptation. However, the requirements in accuracy on the
channel quality estimates in the transmi tter and the chan-
nel estimates in the receiver differs,
as
we shall see later.
by several orders of magnitude . Rapid and unpredictable
variations in the received interference also causes poor link
adaptation. For this scenario we argue tha t t he most im-
portant gain with an HARQ system is the diversity effect
it
provides. If the interference level was high for the original
transmission it is likely that the situation will improve for
the retransmission. If the interference is constantly high
it
becomes predictable and the link adaptation will then
become accurate.
N.o.
multi-codes,
L
N.o.
bits per transport block, NTrBlk
N.o.
CRC bits, ~ C R C
N.o.
decoder tail bits
Furthermore, we show that on fading channels there are
situations when ai1 IR system actually performs poorer
than
a
Chase combining system. This is due to the sys-
tematic t urbo encoder used in WCDMA and the fact th at
all systematic bits are included in the first transmission.
Therefore the retransmission when using IR consists only of
new parity bits. If th e systematic bits in the first transmis-
sion are destroyed by a fading dip th e receiver would benefit
more from a retransmission that includes the systematic
bits
(as
in Chase combining) than from a retransmission
that only contains parity bits
(as
in IR). An alternative
would be to use a partial IR scheme where all systematic
bits are included in each transmission but
a
new set of par-
ity bit s are sent in each retransmission. Th e perforinaiice of
a
partial IR scheme will be somewhere in between the per-
formance of the full IR and t he Chase combining schemes.
3
320
24
G
A I S F .
L
Fig.
1.
Block
diagram of the simulated system
MCS # Ktot
1
3
2
6
T A B L E I
S l h l U L A T l O N P A R A h I E T E R S U S E D I N T H I S
PAPER.
Ad
R
4 0.25
4 0.50
Value
3.84
x
10
0.67 ms 1 slot)
Parameter
Channel
Chip rate [Hz]
Transmission time interval (TTI)
N.o.
chips per TTI, Nchip
%reading factor. SF
4
5
15 16 0.63
21
64
0.58
I N.o.
decoder iterations 1 8
Decoder metric Log-Max
TABLE
I1
~IODULATIONN D CODING SCHENES (hICSs)
U SED
IN
THIS PAPER
3 9 16 0.38
6 27 64 0.75
11.
SYSTEM
ODEL
A dia.grain
of
the simulated system is shown in Fig. 1. In
the simulations performed,
a
number of transport blocks,
Ktot,of size N n B l k are concatenated, and a. CRC field
of size ~ C R C its is added to form an encoding block
of size Nullcoded Ktot
x
NT, .B~~m C R C . By letting
Nchip,
S F ,
L ,
an d denote the number of chips in a
HSDPA transmission time interval: the spreading factor:
the number of multi-codes, and the modulation order: re-
spectively, we obtain that the number of coded bits must
equal Ncoded = L x log,(M)
x
N,l,i,/SF. Consequently, the
rate of the turbo encoder becomes R =
Nup-oded/N,-oded.
In this paper we have used
N&ip
= 2560,
NmBlk =
320,
nxCRC
=
24, S F
4,
and L = 3 in all siniulations per-
formed. Furthermore, all simulations are performed on an
AWGN channel. The reason for this is that the channel is
a.ssuined to be constant during one transniission time in-
terval. Using link adaptation we select th e modulation and
coding scheme (MCS) based on the instantaneous channel
quality just prior to the transmission time. Th e results
presented here are thus valid for
a
range of low to mod-
era te Doppler frequencies. Th e paramete rs used for t,he
simulations in this paper are listed in Table I. Six different
modulation a.nd coding schemes (hICS1-hlCSG) are siniu-
lated and t,he parameters of these
six
MCSs are listed in
Ta.ble 11.
183
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I ,
I
1
2
Fig. 2 . Simulated slot error rate versus Zo r / I o c in dB for MCSl
Kt,t =
3) . White and black markers are used for Chase
com-
bining and
IR,
respectively.
I
First
-o- Second
hird
4
6
8
I O
12
14
16
[dB
Fig.
3 .
Simulated slot error rate versus
lo , . / Ioc
in
dB
for
h3CS6
Kt,t = 27). White and black markers are used for Chase com-
bining and
IR:
respectively.
111.
N U M E R I C A L
ESULTS
In Fig. 2 aiid Fig. 3 the results with Chase combining
aiid IR are compared in terms of the slot. error rate versus
th e ratio of the total received power Io,.) nd total interfer-
ence I o c ) .Result,s for MCS1 (Kt,ot, 3 ) are shown in Fig.
2 a,iid results for MCSG I<,,,,
= 27)
are shown in Fig. 3.
In these two figures we see th e performance of Chase coiii-
biiiiiig (white markers) aiid IR (black markers) after the
second, third aiid fourth traiismissioiis ( i.e. first: second,
and third retransmissions). We clearly see t,liat the gain
with IR is significant 0111~-for AICSG aiid not for hlCS1.
The gains mit.li
IR
coiiipared to Chase coiiibiiiiiig in
= 5 , I R
0 (J
=
5 , Chase
J =
100,
IR
I i
0 D J= 100,
Chase
2
-5
5 10
Instantaneous
I , ,rl
I
[ d B ]
15
20
Fig.
4.
Throughput versus instantaneous
I,,/I,,
in
dB.
Th e param-
eter
U
is the standard deviation of the channel quality estimate
error.
terms of Io,./Ioc required to achieve a slot error rate of
10% are listed for all MCSs in Ta.ble 111. From Ta.ble I11 we
conclude that IR gives significantly better link-level perfor-
mance compa.red to Cha.se combining for high modulatioiis
and coding schemes (MCS4-MCSG) aiid t ha t only sma.11
differences are observed for lower modulation a.nd coding
schemes (MCSl-MCSS).
Th e transmit ter will select which modulation aiid coding
scheme to use based on some cha.iiiiel qudi ty estimate.
If
the error of this chaiiiiel quality estimate is small, then th e
gains that we show in Table I11 may not be visible when
comparing the throughput of th e systems with Chase coin-
biiiing and IR respectively. Since in iiia.ny
ca.ses
only
a
single retransmission is necessary also
if
Chase combining
is
used we may not need the additional coding ga.in that
can be achieved by IR. In Fig. 4 we show the throughput
th at can be achieved with IR and Chase combining, respec-
tively. Which
MCS
to use in each transmission is based on
the channel quality estiiiiate aiid
is
selected by coiiipariiig
with predefined switching points in a lookup table. Th e
lookup table was obtained by plotting t,he throughput for
each MCS individually a,iid selecting the intersection points
of the throughput curves as switching points. Th e chaiiiiel
quality estimate is assumed to be normally distributed in
a logarithmic s a l e with a mean value equal to the true
channel quality aiid
a
standa.rd deviat.ioii of
cr
Significant
gains with IR ar e observed for the case when the scheduler
has almost. no knowledge of the actual channel quality (i.e.
=
100).
However for smaller errors in t.he link a.dapta-
tion: t.he performance difference between Chase combining
aiid IR are much smaller (i.e. c =
5 ) .
In Fig.
5
we show the relative throughput increase in
percent that can be achieved by iiitroduciiig iiicreiiieiital
redundancy. \Ire see
t.hat
when the sclieduler
has
almost
no kiiowleclge of the channel yua1it.y n
= 100)
then the
83
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TABLE
I11
G A I N
WITH IR
C:O~IP.\REDTO
CHASE OAIBINING AT SLER = 0.1
M C S IR gain 2]ld IR gain 3rd IR gain
4
pL
Trans. [dB] Trans. [dB] Trans.
[dB]
I.
0.1 0.2 0.2
0.8
1
o
1
o
r
1.0
1.0
1
o
2.2 2.7 2.7
4.0 4.2
e 4.2
5.4
6.1
3.0
-? 5 10 -5 0
5 I O
Instantaneous I [dB]
5
Fig.
5 .
Achievable through put gain with
IR
compared t o Chase com-
bining versus the momentary I o , . / I oc in
dB.
Results are shown
for channel quality estimation errors (5
of 5: 10:
0. and 100 dB.
gains with IR caii be as high as 70% increased thr oughput .
However for smaller errors in the chaiiiiel quality estimate
t.lie ga.insa.re much smaller. For a
< 5
there is
no
significant
difference between Chase combining aiid IR.
I t is important to note that the results iii Fig.
5
are
obtai ned with perfect channel estimates in tlie receiver. In
iiiaiiy cases it is iiot reasonable to assume tha.t tlie Node-
B have very poor knowledge of tlie chaiiiiel quality while
t.lie receiver has perfect channel knowledge. Even thou gh
t,here are scenarios: e.g. situat, ions nvolving soft handover,
when this assumption might be reasonable, it, is iiiore likely
t,hat. t.he error variance of tlie cliaiinel quality estimate in
t.he transiiiitt.er ailcl the channel estiiiiate in the receiver
are higlily correlat,ed most of tlie times. In Table 111 we
saw that . tlie largest gains with IR conies from the
large
signal coiistellatioiis (i.e. 64
Q A M ) .
With
poor
c l i annr l
estimate s in th e receiver these high coiist,ellatioiis can iiot
be used aiid t.he gains with IR will becoiiie significantly
siiialler tliaii what, we see in Fig.
5.
111 Fig. G we st.ucly
the
sensitivity of channel estiiiiat.ion
erro rs in t,lie receiver. The Ior/Iocequired to obtaili a slot
~ r r o rate
of
10% is slion:n.
versus
the iioriiializrcl chaiinc~l
Io-J Io- 10-2 Io- o0
Normalized Channel EstimationError:
a
Fig. 6. Th e required I,,/I,, in dB to achieve a slot error rate of 10%
versus th e normalized chaiiiiel estimation error
a
=
o, /u ; .
30
10
0'
15
-10
-5 0 5 IO
Instantaneous
Ior I x [dB]
5
Fig.
7.
Achievable throughput gain with
IR
compared
to
Cha se coiii-
biniiig when using only h.ICSI-NCS3. Results are shown versus
the momentary I,,
/ I o c
in
dB
for channel quality estimatiou er-
rors
CT of 5. 10.
20: and
100 d B .
estimatioii error
a = a,'/.,
with
of
defined
as
the er-
ror
variance
of
the cha.niie1estimate
and
g clefiiied as the
variaace of t,lie channe l), for AICS1-MCSG. We see th at
t.lie required accuracy of t,he channel esti mates varies sig-
nificantly from MCS1 to MCSG. Heiice tlie highest MCSs
does not only require
good
climiiiel qualit,y, but. a.lso much
iiioi-e
accurate channel est.iiiiates in t,he receiver.
For users on t.he cell
border:
or uscm with high mobility
it is reasonable to assume that only the lower
NCSs
can
lie used. Since we have seen that t,liere is 110 sigiiificaiit
gain with IR n- l im tlie link ;dap tat, ioii works properly it is
interesting to coinpare t he rrsiilts olitaiiled n-hrii oiilj- S O ~ P
1832
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low MCSs can be selected. In Fig. 7 we show the gain t ha t
can be achieved with IR if we are only allowed to use the
three lowest
,IVICSs,
i.e.
MCS1-MCSS.
We see that the
gains with IR are only about
5
in increased throughput,
even for such large link adaptation errors
g
as 10 dB.
When introducing HSDPA
it
is desirable to reuse as
much as possible of the existing functionality in the
WCDMA system, such as e.g. the turbo encoder. Th e
turbo encoder that is used in WCDNIA is
a
systematic
encoder. This means that the original transmission must
contain all systematic bits and, when IR is used, that the
retransmissions will contain only additional par ity bits. On
a fading channel the channel quality may change from the
time of the first transmission t o the time of the retransmis-
sion. Thus, with IR we can expect some degradation if the
receiver only receives the parity bits in the retransmission
while the systematic bits in the original transmission are
lost. However, Cha.se combining where the retransmissions
are identical t o the original transmission, as well as partial
IR schemes where ail syst ematic bits are included in the re-
transmissions, are expected t o be more robust in this sense.
In Fig. 8 we examine this effect by varying the ratio of re-
ceived energy in th e original transmission E l )and t he re-
ceived energy in the retransmission (&) while keeping the
total received energy (i.e. El + Ez) constant. A positive
value of y 10 logl,
(El/Ez)
hus means that the original
traiisinission contains more energy th an the retransmission.
The curves show for different h/lCSs, the I o r / I o c equired
t.o obtain
a
slot error ra.te of 10% . For Chase combining
(dashed lines) the performance is independent of y a.nd
for IR the best performance is achieved when the received
energy of t he original and t he re-transmissions
are
equal
y
=
0 dB). We see tha t for y =
20
dB: almost all received
energy is put on the original transmission and hence there
are almost no difference between Cha.se combining and IR
in this case. For large nega.tive values of y we can actually
see tha t Chase combining performs better th at I R. The
5
increased throughput that we observed for r = 10 dB in
Fig. 7 assumed that the chaiinel did not change from the
original transmission unt,il th e retransmission. However if
the channel does change (i.e. 0 dB): we see in Fig. 8
that the gains with IR compared to Chase coiiibiniiig will
be even snialler.
IV. DISCUSSION
In this paper we have shown th at although IR gives la.rge
performance gains on t.he link level for high MCSs we may
not be able to see these gains in
a
real system. Apart
from t,he cases we have stud ied in this pa.per we may
add
that since t,he coverage area for the higher
hI CSs
will be
much smaller than for the lower
MCSs
only a relatively
small percenhge of the UEs will be able to benefit from
any event,ual gains with I R .
An alt,rrnat,ive o a full IR scheme is to use a Partial IR
scheine. where each retransmission consists
of
a repetition
of the systeinatic bit,s a i d a new set of parity bits. Part.ia1
20 15 10
-5 5
I O 15 20
dBl
Fig 8
Average I,,/I,, in
dB
tha t IS required t o obtain a slot error
rate of 10% versus
y
in dB y
is
the ratio of th e received energy
in the original transmission and the retransmlssion Results are
shown for Chase combiiiing (dashed lines) and IR (sohd lines)
IR, which was studied in
[4-G] ,
solves the problem with
non self-decodable retransmissions . Th e other drawbacks
that also the full IR scheme suffers reinaiiis however. Fur-
thermore. for high MCSs oiily
a
small set of new parity bits
is included in the retransmission and the difference in link
level performalice for Partial IR and Chase combining is
therefore relatively small.
V.
CONCLUSIONS
Since IR. implies larger memory requirements for the
mobile receivers ancl a larger am oun t of con trol signaling
compared to Chase combining,
it
is important tha t th e in-
creased complexity also results in improved performance.
In this paper we have shown tha t this may not be the case
for HSDPA.
REFERENCES
S.
B.
Wicker, ETTOT control systems for digital communication
and storage, Prentice-Hall, 1995.
3 . Peisa and A l . Meyer. .'Analytical model for T C P file transf ers
over UMTS.
in
Proceedings 3G Wireless, 2001.
S. Parkvall; E. Dahlman. P. Frenger. P. Beming. and R.1. Pers-
son. The evolution of WCDhlA towards higher speed downlink
packet da ta access. i n Proceedings
IEEE
Vehicular Technology
Conference. Rhodes. Greece. N a y 6-9 2001.
Motorola: Performance comparison of hybrid-ARQ schemes.:'
3GPP input paper TSGR1#17(00)139G,
2000.
hlotorola: '.Performalice comparison of hybrid-ARQ schemes:
Additional results.
3CPP
input paper TSGR1#18(01)0044.
2001.
Panasonic. '.Proposal of bitmapping for type-I11 HARQ: 3G PP
input paper TSC+R1#18(01)0031. 2001.
'These docunieiits are available
on
the 3GPP lionir page
I l t t p : / / \ ~ w \ ~ . 3 g ] , ] ~ . ( J r ~ .
1833