Performance analysis of a hybrid ARQ system in half duplex transmission at 2400 BPS

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 -