performance comparison of harq with chase combining and incremental redundancy for hsdpa

8/9/2019 Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA

http://slidepdf.com/reader/full/performance-comparison-of-harq-with-chase-combining-and-incremental-redundancy 1/5

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



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



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



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



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

performance comparison of harq with chase combining and incremental redundancy for hsdpa

Documents