248
PERFORMANCE ANALYSIS OF
A
HYBRID ARQ SYSTEM IN HALF DUPLEX TRANSMISSION AT
2400
BPS
J
Dalxau-Roy0 and
J
Se a -Fe ndndez
Uni e si a Poli ecnica de Ca alunya, Spain
INTRODUCTION
Today HF communica ion sys ems ely on he
concep o adap a ion. In ac , he bene i s
o exploi ing he ime/ equency a ian
capaci y o he HF channel ha e been epo -
ed elsewhe e. Adap i e sys ems can play an
impo an ole in any one o he i s h ee
laye s o a communica ion sys em. This pape
is ocused on he desc ip ion o he bases
o a le el-2 p o ocol aimed o inco po a e
such adap i e capaci y.
Hyb id MQ/FEC p o ocols ha e been p oposed
o p o ide high da a link in eg i ies whils
keeping a he same ime a high mean
h oughpu a e. Ne e heless, hyb id ARQ
s a egies O e a lo o choices and none
o hem can be conside ed he op imum in any
case. Among he di e en aspec s o consi-
de in o de o choose o o design one o
such p o ocols we mus ake in o accoun he
sys em applica ion cons ain s. In his iew
we speci y he ollowing:
The p o ocol is in ended
o
p o ide
le el-2 se ices in a hal duplex
sys em exploi ed wi h wo di e en
wo king equencies.
Long o wa d messages and sho eplies
mus be allowed in o de o minimize
he e ec s o he le el-1 o e heads.
A esidual bi e o a e o abou
can be ole a ed a ough bi a es o
2400 bps.
Al hough we will go insigh h oughou he
pape i is enough o jus i y some o he
app oaches ha we ha e conside ed.
In he ollowing sec ion we p esen he
al e na i es which we e conside ed. Then we
p oceed desc ibing he h ee which appa-
en ly we e he mos p omising candida es.
Finally we discuss he labo a o y es s
esul s wi h emphasis on he channel models
used o he es s.
PROTOCOL
AND
CODING STRATEGIES
The HF channel in oduces a mix u e o
andom and bu s e o s, he
so
called
di use e o s as desc ibed by Da nell and
Tech
[l].
As a consequence he coding s a-
egies should be sui able o comba his
ype o e o s.
The basic di use e o co ec ing echni-
ques a e he ollowing ones:
--
-
E o de ec ion and e ansmission
-
Simple in e lea ing
-
Fi e codes
-
Reed Solomon codes
-
Th eshold decoding echniques
-
Code combining
E o de ec ion and e ansmission is gene-
ally easy o implemen . Mo eo e , i equi-
es less edundancy han o he s o co ec
he same numbe o e o s and i is indepen-
den o he bu s leng h. Howe e , his
echnique equi es a eliable eedback link
and enough bu e s o age and logic a bo h
he ansmi e and he ecei e . In addi-
ion i p esen s low h oughpu a es in
noisy o wa d channels.
Simple in e lea ing comba s he e ec s o
e o bu s s by dis ibu ing hem o e
sepa a e coded da a s eams. I is also easy
o implemen and many good andom e o
co ec ing codes a e known. I s main d aw-
back is ha he e o s co ela ion is no
ully exploi ed in decoding. Mo eo e ,
in e lea ing may s ill be inadequa ed wi h
e y long bu s s.
Fi e codes p esen a e y high e iciency
and a e o simple implemen a ion. Ye , long
Fi e codes a e equi ed o a mode a ed
bu s co ec ion capabili y.
Al hough Reed Solomon codes show a code
e iciency ypically g ea e han
90%,
one
o i s main d awbacks is conce ned wi h
hei igid co ec ion capabili y, as i has
been s a ed by Reed and Hopkinson
121.
Th eshold decoding echniques a e able o
handle la ge bu s s and hey a e also com-
me cially a ailable. Howe e , he code
design is s ill an a and de ices exis o
only a limi ed ange
o
code a es.
Code combining p esen s an a bi a ily low
ou pu e o a e e en unde he mos se e e
channel condi ions. The s a egy is he code
a e adap a ion o he channel condi ions.
Howe e , i p esen s a e y
low
h oughpu
unde se e e condi ions.
Once he basic di use e o co ec ing
echniques ha e been examined, i can be
concluded ha a sui able blend
o
hem
should be he bes solu ion o cope wi h
such e o pa e ns. In his way, a s a egy
based on he ollowing p emises was pu sued:
-
E o de ec ion and co ec ion by
e ansmission
-
Use o simple in e lea ing
-
A ce ain deg ee o code a e adap a-
ion o he p e ailing channel condi-
ions
These conside a ions lend o a ype-I1
hyb id ARQ, as in e e ence
[l].
The design o such coding s a egy mus
conside he ollowing aspec s:
249
-
The selec ion o codes o b ing ou
an op imal p ocess o e o de ec ion
and co ec ion.
-
The in e lea ing pa ame e s.
-
The way o adap he code a e.
-
The e o p o ec ion o he eedback
channel.
Because o i s hal duplex ope a ion, he
selec ion o he e ansmission s a egy
(Go
Back-N, Selec i e Repea , e c.) is clea ly
o e come. In addi ion, he in e lea ing
dep h is cons ained and limi ed by he
ame leng h.
BCH and RS a e known as ones o he mos
e icien codes in e ms o ade o be -
ween e o p o ec ion capabili y and a e.
Because o his, we choosed BCH codes since
he begining al hough wi hou disca ing RS
codes. In o de o p o ide a easonably high
e o p o ec ion whils keeping a he same
ime a sho code block leng h, we picked up
code leng hs o 15 and 31 bi s. Speci ically
we selec ed he BCH(15,10), BCH(15,5) and
BCH(31,ll) in o de o be used as explained
in he ollowing sec ion.
As a as he eedback channel is conce ned
we a e limi ed by he sho leng h o he
eedback ame. Then, codes wi h di use
e o co ec ion capabili y mus be adop ed
he e.
Fo
ins ance, di use con olu ional
codes a e a alid al e na i e. This is
because we selec ed his amily o h eshold
decodable codes o cope wi h he e o
pa e ns o his channel and speci ically a
con olu ional code o leng h
14.
PROPOSED APPROACHES
In he ollowing pa ag aphs we speci y h ee
p o ocol s a egies which a e om now
iden i ied as Sys em 1, Sys em
2
and Sys em
3. The h ee sys ems exchange o wa d a-
mes, con eying he in o ma ion om he
ansmi e o he ecei e , and backwa d
ames only o acknowledgemen pu poses.
The in o ma ion bi s o be ansmi ed a e
segmen ed in blocks, encoded and placed,
o ally
o
pa ially, in o he ansmission
bu e wi h o he blocks in o de o cons i-
u e he o wa d ame. Each block is iden-
i ied by i s posi ion in he ame. I he
ecei e is unable o decode
o
i i de-
ec s e o s in o a gi en block a NACK is
sen back in he posi ion associa ed o his
block in he backwa d ame. A e he
eceip o a NACK he ansmi e may encode
again he block wi h a di e en code
(Sys ems 1 and
2)
o i may send he emai-
ning bi s o he i s encoding p ocess
(Sys em
3)
in o de o allow he ecei e o
do a eliable e o co ec ion p ocess.
The e o e, he h ee sys ems may use only
one
o
mo e o wa d ames o insu e ha
each segmen o in o ma ion bi s is app o-
p ia ely deli e ed a he dis an end.
Sys em 1
Sys em
1
makes use o segmen s o
10
in o -
ma ion bi s. The i s ime ha a segmen
is picked up i is encoded wi h he
BCH(15,lO). In his case he ecei e ex-
ploi s he p ope ies o he BCH(15,lO) o
co ec one e o
o
o de ec wo e o s in
he block. The e o e only when wo e o s
a e de ec ed, a NACK will be sen back and
he ecei e will be wai ing o he e-
ansmission o his segmen . On he o he
cases, he 10 in o ma ion bi s will be
deli e ed o he use a e he app op ia e
e o co ec ion p ocess i necessa y.
When a NACK is ecei ed, he ansmi e
spli s he segmen in wo hal es, encodes
independen ly each one wi h he BCH(15,5)
and places bo h blocks in o he nex o wa d
ame. Now he ecei e will exploi he
capaci y o his code o co ec up o h ee
e o s
o
o de ec i he decoded block is
eally a codewo d. In he e en ha he
decoded block is no a codewo d, a new NACK
would be sen back in o de o o ce he
ansmi e o o wa d again he same code
block.
Summa izing, since he i s e ansmission
each hal segmen is encoded wi h a
BCH(15,5), ansmi ed as many imes as
necessa y and p ocessed independen ly o he
o he hal . The numbe o e ansmissions is
limi ed o a easonable amoun and app o-
p ia e con ol p ocedu es a e ca ied ou in
o de o always iden i y he posi ion o a
gi en hal segmen o in o ma ion bi s
inside he o wa d ame.
The ACK/NACK bi s o each ecei ed block a e
encoded wi h a hal a e con olu ional
di use code o cons i u e he backwa d
ame.
Sys em 2
Sys em 2 exhibi s only a slig h di e ence
wi h espec o Sys em 1 conce ning he way
in which i exploi s he BCH(15,lO) code.
This is, he BCH(15.10) is now used only o
de ec bu no o co ec - e o s in ne
i s ansmission o a segmen . The e o e a
NACK will be sen back o he ansmi e i
1,2,3,5,6,
..
e o s appea inside he 15
bi block. Besides his, Sys em
2
p oceeds
in he same way as Sys em 1.
Sys em 3
Sys em 3 wo ks wi h segmen s o 11 in o ma-
ion bi s encoded wi h he BCH(31,ll). Among
he 31 bi s a he encode ou pu only 16
a e selec ed and s o ed in he ansmission
bu e o be included in he nex o wa d
ame. These a e he 11 bi s o he segmen
and 5 edundancy bi s specially selec ed in
o de o maximize he minimum dis ance
be ween he 16 bi codewo ds. By compu e
sea ch we ealized ha he minimum ee
dis ance is 3. This means ha he ecei e
will be able o de ec up o
2
e o s in
each
16
bi block.
When some e o is de ec ed a NACK will be
sen back. Then, a e he eceip o a NACK
he ansmi e will load he emaining 15
edundancy bi s plus a s u ing bi in he
ansmission bu e in o de o be o wa ded
in he nex ame. The e o e he ecei e
will be enabled o decode he
31
bi block
o he BCH(31.11) ansmi ed in wo conse-
cu i e o wa d ames. The amewo k o his
p ocedu e is explained by Kallel [31 and i
can also be seen as a modi ied memo y
ARQ
as
desc ibed by Se inken [4].
In case ha he i s ansmi ed block
we e conside ed e o ee, he ecei e
would deli e 11 in o ma ion bi s o he
use and an ACK would be issued. Then he
ansmi e would dele e he abo e men ioned
--
250
15
edundancy bi s and i would use his
posi ion in he bu e o send a new encoded
segmen .
The backwa d ame is buil up om he
ACK/NACK
bi s o each ecei ed block as in
Sys ems
1
and
2.
LABORATORY TEST BED
In o de o assess he pe o mance o he
h ee sys ems se e al es s we e ca ied ou
based on he a angemen shown in Figu e
1.
In his igu e
PC1
emula es a ansmi ing
s a ion and
PC2
he ecei ing s a ion. The
link be ween bo h s a ions is a h ee wi e
line plugged on he asynch onous se ial po
o each
PC.
The p ocessing in ol ed in each o he h ee
sys ems is ca ied ou by an ad-hoc so wa e
de eloped o each
PC.
On he o he hand, as
no modem
o
channel simula o a e used, he
e ec s o he HF channel need o be simula-
ed a one o bo h
PCs.
In o de o do i a
o al
o
nine channel models comming om
he same numbe o eal HF links we e consi-
de ed. Because o he in ol emen o he
esul s and hei signi icance wi h he used
models we de o a pa o he p esen sec-
ion o deal wi h his opic.
Figu e
2
shows he unc ional block diag am
o he so wa e unning on he ansmi e
(PC1).
On op o i s laye ed s uc u e we
ha e he module de o ed o he use in e a-
ce which is in ended o allow him he p o-
g amming and ini ializa ion be o e i s
execu ion. Among he ini ializa ion pa ame-
e s he use mus speci y he name o he
ile o be ansmi ed, he ype
o
p o ocol
and he HF simula ion model.
A
he bo om
we ha e he communica ion module which o
hese es s was designed o ansmi / ecei e
cha ac e s h ough he asynch onous po o
he
PC.
The in e ace be ween he communica-
ion module and he uppe laye s has been
ca e ully speci ied in o de o allow o he
communica ion links in he u u e. The
o he laye s cons i u e he co e o he
ARQ
p o ocols desc ibed in he p e ious sec ion
in o de o p ocess he backwa d ames and
o gene a e he o wa d ames. Conce ning
he so wa e o
PC2
i
looks
like Figu e
2
bu now he in e medium le els a e in ended
o p ocess he o wa d ame and o gene a e
he backwa d ame.
An
special ea u e o
he use in e ace a
PC2
is ha i also
gene a es a epo ile con aining he mos
ele an da a o he ansmission/ ecep ion
p ocess like he numbe o esidual e o s,
mean h oughpu a e, numbe o blocks
ansmi ed one ime wo imes, e c.
All
he
esul s p esen ed in he nex sec ion ha e
been ob ained om he epo iles w i en
a he end o each un.
HF channel models
The
HF
channel is based on a disc e e model
de i ed om wo se s o expe imen s:
-
HF
Ionosphe ic communica ions a
2400
bps wi h
PSK-2
modems epo ed by
Spun icchia and Ca oggio 151.
-
HF
g ound wa e mobile communica ions
a
600
bps wi h
FSK-2
modems epo ed
by Dalmau
161.
The second se o expe imen s co esponds o
da a ob ained in he me opoli an a ea o
Ba celona. Due o he sho leng h o he
links (be ween 5 and
30
Km)
he main p opa-
ga ion mode is g ound wa e. The egis e ed
e o pa e ns exhibi di use e o s like
hose encoun e ed in sky wa e p opaga ion.
The e o e, he esul s go om es s using
hese channels may be also ex ensi e o any
HF channel.
A
F i chman's
171
pa i ioned h ee s a e
model was selec ed because i gi es he bes
ade o be ween accu acy and complexi y.
TEST RESULTS
In Figu e
3
o 5 we show a summa y o he
esul s ob ained a e se e al uns. Each
un was in ended o ansmi a ile
o
60
Kbi s om
PC1
o
PC2
in o de o e alua e
he use 's bi e o a e and he mean
h oughpu a e.
The use 's BER is compu ed di iding he
e idual e o s in he ecei ed ile by
6
loB.
I can be a gued ha his p ocedu e
does no gi e a eliable es ima e mainly i
he e o coun is low. This is because we
w i e in he app op ia e igu es he absolu-
e e o coun .
As
a as he mean h oughpu is conce ned
i mus be said ha i is compu ed di iding
he
6
lo4
in o ma ion bi s by he o al
numbe o bi s ansmi ed h ough he
channel in bo h di ec ions bu excluding he
s a and s op bi s o each cha ac e in he
asynch onous link. Then i gi es an es ima e
o he maximum a ainable mean h oughpu
a e o a hypo he ical synch onous sys em
using he same ough bi a e
o
bo h
modems.
Figu e
3
plo s use 's BER e sus channel BER
o he nine channels and he h ee sys ems
conside ed. The numbe o blocks pe ame
is
always
160
and he backwa d ame is
encoded by he
1/2
a e di use con olu io-
nal code.
As
i can be seen he h ee
sys ems show he same beh iou up o chan-
nel BERs o abou
4
IO-'.
A
his poin
Sys em
1
begins o lose pe o mance wi h
espec o he o he s. On he o he hand,
Sys em
3
alls be ween
1
and
2
excep o
he wo s channel whe e i deli e s up
o
11
e o s. The same beha iou was also obse ed
in an ideal backwa d channel.
This esul s a e consis en . In ac , Sys em
1
loses e o de ec ion capabili y compa ed
wi h Sys em
2
because he BCH(15,lO) is also
used o co ec e o s.
Also
Sys em
3
is
less powe ul as a as e o de ec ion is
conce ned because he minimum dis ance
o
i s e o de ec ing code is
3
ins ead
o
4.
Figu e
4
shows he mean h oughpu a e o
he h ee sys ems e sus channel BER. Again
he numbe o blocks in he o wa d ame is
160
and he backwa d ame is encoded by he
same con olu ional code. Sys em
3
clea ly
achie es he highes h oughpu because o
i s pa icula phyloso y as a modi ied
memo y
ARQ
sys em, hus ma ching be e he
code o he channel.
No
signi ica i e di e-
ences we e obse ed conside ing an ideal
backwa d channel.
Finally, igu e 5 is a plo o he use 's
e o s e sus he ame leng h o Sys em
3
wo king in he wo s channel (BER
=
The numbe o e o s depends on he ame
leng h as i could be expec ed because his
25
1
one is a ec ing he e o pa e n dis ibu-
ion. Excluding he sho es ame case, he
mean numbe o e o s is
3
o e he
60000
in o ma ion bi s, a good enough igu e in
espec o he design objec i es.
No
di e-
ences we e obse ed as a as h oughpu
no conside ing an ideal backwa d channel.
Sys em
2
clea ly ou pe o ms Sys em
1
consi-
de ing use 's e o s and i shows a low
h oughpu
loss
wi h espec o i .
Sys em
3
achie es he highes h oughpu o
all he channels conside ed. In ac , he
di e ence wi h espec
o
Sys ems
1
and
2
is conside able.
As
as as use 's e o s is
conce ned, al hough i is ou pe o med by
Sys em
2
o channel BERs highe han
6.5
10-
,
i seems o be he bes choice assu-
ming a long enough o wa d ame. On he
o he hand, i s implemen a ion complexi y is
also lowe han he o he sys ems.
1.
2.
3.
4.
5.
6.
7.
The p o ec ion o he backwa d channel is
PC
1
PC
2
enough o cope wi h he e o s in oduced by
all he channels conside ed. Ac ually, he
ac o conside ing he back channel ideal
o
no does no a ec he esul s.
REFERENCES
Da nell. M. 1985, "P oblems o Mobile
HF
Comunica ion and Techniques o
Pe o mance Imp o emen ".
~~~
IEE P oceedings,
5
,-
Augus .
Reed, A.P.C. and Hopkinson,
J.N.,
1990,
"Adap i e Da a Link P o ocols: Design
and Pe o mance o e Au oma ed HF
Skywa e Links". Plessey,
U.K.
Kallel,
S.,
1990,
"Analysis o Type I1
Hyb id
ARQ
Scheme wi h Code Combining".
IEEE T ans. on Comms.,
38,
0,
Augus .
Se inken, N.M.,
1988,
"HF Da a Message
and Facsimile Te minal Sys em". 3 h IEE
-,
London.
Spun icchia and Ca oggio,
1986,
"Un
P ocedimen o di Misu a e di Analisi
degli E o i pe Sis emi di T ansmisio-
ni Da i". XVII Conc esso In . pe
1'Ele onica".
Dalmau, J.L.,
1991,
"A Con ibu ion o
he Analysis o Coding Schemes o
Mobile Da a Communica ions o e HF
Channels". Doc o a e Thesis, Uni e si-
a Poli ecnica de Ca alunya, Spain.
F i chman, B.D.,
1967,
"A Bina y Cha-
ac e iza ion using Pa i ioned Ma ko
Chains". IEEE T ans. on In . Theo ,
IT-13,
2,
Ap il.
FOR
I
FIGURE
1:
LABORATORY TEST
OED
I
I
USER AND DISK INTERFACE
I
ENCODER
1
TRANSYISION
INTERLEYER
HF CHANNEL
BACK
DECODER
COYYUNICATIONS
FIGURE
2:
LAYERED STRUCTURE OF THE OOFTIURE
RUNNINQ IN PC
1
8-
e-
4-
__
-~
2-
0
-
c
".
..
..
c
0.1
channel be (x1E-2)
-
Se ies
1
+
Se iem
2
*
Se ies
9
sys em
1
sys em 2 sys em
3
igu e
3:
use 's e o s
s
channel be
h oughpu
0.8,
0.56
L:--
.
0.4
'
I
0.1
channel be (XI€-2)
1
sys em
1
sys em
2
sys em
3
Se ies
1
+
Se ies
2
-+-
Se ies
3
igu e
4:
h oughpu
s
channel be
252
use 's e o s
I'
I
0'
I
Mo
I80
200
220
240
280
280
300
320
numbe o blocks pe ame
So i08
1
igu e
5:
use 's e o s
s
numbe
o
blocks pe ame
I
-