Dynamic Coded Cooperative ARQ for Multi-hop Underwater Acoustic Networks

Transcription

1 Dynamc Coded Cooperatve ARQ for Mult-hop Underater Acoustc Netorks Yougan Chen,XaomeXu, Shengl Zhou, Hatao Su, and Lan Zhang MOE Key Laboratory of Underater Acoustc Communcaton and Marne Informaton Technology, Xamen Unversty, Xamen 36005, Chna Dept. of Electrcal and Computer Engneerng, Unversty of Connectcut, Storrs, CT 0669, USA Abstract Dynamc coded cooperaton (DCC) does not need extra transmsson tme scheduled for the relay, hch s appealng to the banddth-lmted hgh-delay underater acoustc (UWA) envronment. In ths paper, e propose dynamc coded cooperatve automatc repeat request (DCC-ARQ) protocol for mult-hop UWA netorks. A transmsson packet th multple blocks s taken as a one-shot unt, here an erasure-correcton code s used for nter-block encodng. Adoptng the DCC scheme n each hop UWA transmsson, the half-duplex cooperatve node stches to cooperaton phase mmedately after t decodes the cooperatve message, hch provdes a more relable cooperatve path for the specfc three-node netork. Further, f the relay (or destnaton n the last hop) node sends a negatve acknoledgement (NACK) to the upstream cooperatve node, the cooperatve node only needs to retransmt parts of the packet under DCC-ARQ mechansm, hence a reduced end-to-end transmsson latency can be acheved. Smulaton results sho that for a one-shot transmsson, the proposed protocol acheves good balance beteen the reduced end-to-end delay and decent outage performance, relatve to exstng protocols. Index Terms Coded cooperaton, OFDM, dynamc coded cooperaton, ARQ, underater acoustc communcatons. I. INTRODUCTION Underater Acoustc (UWA) netorks are experencng a rapd groth, due to ther hgh relevance to commercal and mltary applcatons such as oceanographc data collecton, polluton montorng, offshore exploraton, dsaster preventon, and tactcal survellance [], []. Hoever, the desgn of effcent communcaton protocols for UWA netorks s stll an open research problem due to the unque characterstcs of the UWA communcaton channel such as lmted banddth, hgh and varable propagaton delays, and sgnfcant multpath and scatterng. To establsh relable UWA communcaton, the automatc repeat request (ARQ) protocol that organzes the retransmsson of erroneous packets s requred. Hoever, as the long propagaton delays n UWA channels, varous mproved effcent ARQ schemes [3] [7] have been proposed for UWA netorks. One of the nterestng protocols named cooperatve ARQ scheme [4] s that, the retransmsson happened from the cooperatve node nstead of that from the source node after the destnaton sent the negatve acknoledgement (NACK). Y. Chen, X. Xu, H. Su and L. Zhang ere supported by the Natonal Natural Scence Foundaton of Chna under Grants and , and by the Specalzed Research Fund for the Doctoral Program of Hgher Educaton of Chna under Grant S. Zhou as supported by the NSF Grants ECCS-858 and ECCS As the cooperatve dversty gan, ths cooperatve retransmsson can sgnfcantly ncrease the probablty of successful retransmsson. For the relay-based cooperatve communcatons, multple relay strateges can be appled, e.g., amplfy-and-forard (AF), decode-and-forard (DF) and compresson-and-forard (CF) [8], [9]. Further, coded cooperaton (CC) [0] applyng channel codng to relay cooperaton as studed as a practcal protocol, n hch each user tres to transmt ncremental redundancy for ts partner. Recently, dynamc coded cooperaton (DCC) [] [3] has been proposed by nvestgatng turbo- and lo-densty party-check codes (LDPC)-coded relay cooperaton. We proposed orthogonal-frequency-dvson multplexng (OFDM) modulated DCC scheme for UWA channels n [4], hch s partcularly appealng to UWA netorks here a relay node th abundant resources (e.g., a surface buoy) can enhance communcatons among underater nodes thout changng ther transmsson procedure. Fg. shos the man dfference of conventonal DF-, CC-, and DCC-cooperaton schemes on banddth effcency [5]. We can fnd that there s no extra transmsson tme scheduled for the relay th the DCC scheme, makng t banddth effcent. The source can be even unaare of the exstence of the relay n the DCC scheme. In ths paper, e combne DCC th cooperatve ARQ, termed as the DCC-ARQ protocol, for mult-hop UWA netorks. Smlar to [6], a transmsson packet th multple blocks s taken as a one-shot unt, here an erasure-correcton code s used for nter-block encodng. In per-hop transmsson, e assume that the cooperatve path s pre-determned by the optmal algorthm, formng a specfc three-node netork. Wth or thout retransmsson, the DCC-ARQ protocol can get benefts on both of the to knds of cases. The half-duplex cooperatve node stches to cooperaton phase mmedately after t decodes the cooperatve message, offerng both codng gan and dversty gan, hch provdes a more relable cooperatve path for the specfc three-node netork. When the relay (or the destnaton n the last hop) node receves an erroneous packet, t asks for retransmsson from a cooperatve node, hch s selected n the cooperatve regon. Durng the retransmsson phase, the cooperatve node only needs to retransmt parts of the packet under DCC-ARQ mechansm, hence a reduced end-to-end transmsson latency can be acheved /4/$ IEEE

2 Conventonal DF Cooperaton Cooperatve Transmsson to Destnaton Lstenng Inactve Collaboraton Cooperatve Conventonal Coded Cooperaton Cooperatve Transmsson to Destnaton Lstenng Inactve Collaboraton Next Transmsson Lstenng Cooperatve Relay Destnaton Dynamc Coded Cooperaton Cooperatve Transmsson to Destnaton Lstenng Collaboraton Transmsson to Destnaton Lstenng Collaboraton Fg.. Banddth effcency for dfferent cooperatve schemes [5]. Fg. 3. Cooperatve regon for mult-hop UWA netorks adoptng the DCC scheme. Fg.. s Nh- Nh- Nh- Nh Nh- An example of N h -hop UWA netork adoptng the DCC scheme. The rest of ths paper s organzed as follos. Secton II presents the proposed DCC-ARQ protocol for mult-hop UWA netorks. Secton III contans smulaton results for a one-shot transmsson n a 5-hop UWA netork, and conclusons are contaned n Secton IV. II. THE PROPOSED DCC-ARQ PROTOCOL FOR MULTI-HOP UWA NETWORKS Consder an N h -hop UWA netork, as shon n Fg., here the frst node ndexed by 0 s the source, and the last node ndexed by N h s the destnaton. Assumng that the routng path s pre-determned by the optmal routng algorthm for a gven source-destnaton par. In the -th hop, e select the best node n the cooperatve regon as the cooperatve node the same as n [4], and defne t as the -th cooperatve node helpng the (-)-th relay node transmttng data to the -th relay. Fg. 3 shos a cooperatve regon example th to-hop scenaro. A. Dynamc Coded Cooperatve ARQ Protocol We assume a burst-based transmsson. Each burst conssts of N bl blocks, hch can be an orthogonal-frequency-dvsonmultplexng (OFDM) block as an example. An erasurecorrecton channel code s appled over the N bl blocks for the nter-block encodng. To llustrate the dynamc coded cooperatve ARQ protocol, e ll frst brefly ntroduce the tradtonal ARQ protocol and the cooperatve ARQ protocol n [4], as shon n Fgs. 4 Nh (a) and (b). In each hop transmsson, f the recever can not decode the message correctly, t asks for retransmsson th a NACK sgnal to the transmtter under the tradtonal ARQ protocol, and to the cooperatve node under the cooperatve ARQ protocol. Obvously, the later protocol can provde a more relable retransmsson path snce the cooperatve node les beteen the transmtter and the recever. Addtonally, t also saves the propagaton delay because of the shorter nodeto-node dstance. For the DCC-ARQ protocol, to get a relable transmsson n each hop, t takes the follong steps. Instantaneous transmsson upon successful decodng: Durng the lstenng phase for the cooperatve node, t attempts burst decodng and performs party check hen recevng one more block; mmedately after the nformaton bts thn one burst are successfully recovered, say N l blocks, t regenerates the coded transmsson blocks, say N coop := N bl N l blocks, and stches to cooperatve phase and relays the message to the donstream node. Partal retransmsson upon dynamc decodng: If the retransmsson s asked, the cooperatve node ll only retransmts the frst N l blocks as shon n Fg. 4 (c), nstead of the all N bl blocks as the cooperatve ARQ scheme n [4]. Jont decodng both before and after the retransmsson: Before retransmsson, the recever node ll do jont decodng based on the N bl blocks from the transmtter node th the last N coop blocks supermposed by the relay s transmssons; After the retransmsson, the addtonal N l blocks ll be used together th the prevously avalable N bl blocks for jont decodng. We should hghlght that, th the extremely poor channel lnk qualty, the DCC-ARQ protocol ould not start cooperatve phase,.e., N l = N bl. Then the DCC-ARQ protocol becomes the cooperatve ARQ protocol. Further, the proposed DCC-ARQ protocol s appealng for UWA netorks, but also applcable to terrestral rado communcatons. B. Decodng at the Relay For UWA transmssons, e assume that each hop uses OFDM modulaton [7]. Gven a three-node UWA netork n

3 - C. End-to-End Delay Tpro Ttra Ttradtonal ARQ a Before dscusson on the end-to-end delay for the cooperatve mult-hop UWA netorks, e adopt the follong assumptons for each hop transmsson. - - Nl Ncoop Tcooparatve ARQ Ncoop TDCC ARQ Nl Nl Tdyna NACK b c ACK Fg. 4. Illustraton of several ARQ protocols n per-hop UWA netork. (a) Coded tradtonal ARQ; (b) Coded cooperatve ARQ; (c) Dynamc coded cooperaton (DCC) ARQ. To compare th the proposed DCC-ARQ protocol, both tradtonal ARQ and cooperatve ARQ protocols are equpped th erasure-correcton channel code. The ACK sgnals ll alays be sent to the transmtter, and the NACK sgnals ll be sent to the cooperatve node and transmtter node alternately, hch can be controlled by the response mechansm at the node. For example, f the recever can not decode the packet correctly at the frst tme, the NACK sgnal ll be sent to the cooperatve node and let t do retransmsson. Then the recever node ll decode agan and send ACK or NACK to the transmtter node. Ths s so called a full cycle. Wth ths assumpton, the retransmsson propagaton delay depends on the retransmsson ndex s odd or even. The ACK or NACK sgnal can alays be receved by the cooperatve nodes or the transmtter nodes, and both of them ll alays respond to these feedback sgnals. Hence e ll skp the tme-out cases n ths paper. The end-to-end delay for the N h -hop UWA netork s the -hop, e have nvestgated to cooperaton strateges n [4], named repetton redundancy (RR) and extra redundancy (ER) for the DCC scheme, here the cooperatve node transmts ether dentcal or dfferent OFDM blocks as the ( )- th relay node durng the cooperaton phase. Let N r denote the number of recevng elements at the destnaton. Consder an OFDM modulaton th K subcarrers, and assume a tme-nvarant scenaro. Durng the cooperaton phase, the destnaton (the -th relay node) receves the superposton of the sgnals from the source (the ( )-th relay node) and the cooperatve node. The RR cooperaton strategy can be descrbed brefly as bello. Snce the cooperatve node transmts dentcal OFDM blocks as the source, the nput-output relatonshp of the lth receved OFDM block at the νth recevng element s z ν,l [k] =(H ν,l,sd [k]+h ν,l,cd [k]) s l [k]+n ν,l [k], }{{} :=H ν,l [k] = H ν,l [k]s l [k]+n ν,l [k], () here H ν,l,sd [k] denotes the channel frequency response beteen the source and the destnaton, H ν,l,cd [k] denotes the channel frequency response beteen the cooperatve node and the destnaton, and n ν,l [k] s the ambent nose. Assumng n ν,l [k] CN(0,σν,l ) and all the constellaton symbols are equal-probable, the a posteror probablty (APP) of the kth data symbol n the lth OFDM block durng the demodulaton step s ( ) p s l [k] {z ν,l [k]} Nr ν= { } N r z ν,l [k] H ν,l [k]s l [k] exp. () ν= σ ν,l N h T ee = T,ee. (3) = For the -th hop, the end-to-end delay conssts of the transmsson latency and the propagaton delay T,ee = T,pro + T,tra, (4) here the propagaton delay T,pro s related to the retransmsson tmes, the end-to-end dstance, the sound speed n ater, and dfferent cooperatve schemes. In each hop, let T pro be equal to the end-to-end dstance dvded by the sound speed n ater, N retra be the retransmsson tmes. Then for the tradtonal ARQ protocol as shon n Fg. 4, e have T Trad,,ee =(T pro + T tra )( + N retra ), (5) here T tra s transmsson tme of a burst data. Denote T bl as the block tme-duraton, t can be rtten as T tra = N bl T bl. (6)

4 For the cooperatve ARQ and DCC-ARQ protocols, e have T Coop odd,,ee = (3T pro +T tra ) N retra + T Coop eve,,ee = (3T pro +T tra ) N retra T DCC odd,,ee = N retra =, 3, 5,, (7) +T pro + T tra N retra =0,, 4,, (8) (3T pro + T tra + T dyna ) N retra + N retra =, 3, 5,, (9) T DCC eve,,ee = (3T pro + T tra + T dyna ) N retra +T pro + T tra N retra =0,, 4,, (0) here T dyna s the dynamc transmsson tme for the cooperatve node retransmttng to the recever hen startng ARQ mechansm n the DCC-ARQ protocol, and can be expressed as T dyna = N l T bl, () hch s dynamc and determned by the the channel lnk qualty beteen the transmtter node and the cooperatve node. Hence the start tme for the cooperatve node to send the redundancy blocks s dynamc, hch s so-called dynamc coded cooperaton. For one specfc case, f N retra =, t s the frst full cycle as shon n Fg. 4. III. NUMERICAL RESULTS A. Smulaton Setup We consder a mult-hop UWA netork th N h =5hops, and the dstance beteen each hop s d =km. Assumng that there s only one cooperatve node, and t les n the mddle of the hop. In each burst, there are N bl =0blocks hch are erasure-correcton coded over I bl =0nformaton blocks,.e., 0.5 bt/symbol. The OFDM parameters are chosen as n [7], th the number of OFDM subcarrer K = 04. The system center frequency s f c =0kHz. The multpath channels are randomly generated th 50 taps n the baseband, and are assumed as quas-statc fadng. Durng the relay process, e clam one node successfully recoverng the transmtted message hen ts cumulatve mutual nformaton of receved blocks s no less than the amount of transmtted nformaton bts. In contrast, outage occurs that hen one node fnshes transmsson, ts donstream node can recover the relayed message. The receved sgnal-to-nose rato (SNR) of an UWA system can be expressed by the passve sonar equaton: SNR = SL TL NL + DI, () Overall outage probablty Average end-to-end delay [blocks] Full poer transmt Coded Trandtonal ARQ Coded Cooperatve ARQ DCC-RR ARQ Half poer transmt Full poer transmt P/ (db) (a) Overall outage probablty Coded Trandtonal ARQ Coded Cooperatve ARQ DCC-RR ARQ P/ [db] (b) Average end-to-end delay Fg. 5. Performance comparson of several ARQ protocols for 5-hop UWA netorks. There s only one cooperatve node n each hop. The maxmze retransmsson tme s only once n each hop. Code rate s 0.5. here SL s the source poer level, TL s the transmsson loss, NL s the nose poer level, and DI s the recevng drectvty ndex n decbels. We consder the recevers as non-drectonal hydrophones, and thus the recevng drectvty ndex s zero. Defne σd (f) as the attenuaton of the sgnal at frequency f after transmttng a dstance d, σ d (f) :=dβ α(f) d, (3) here α(f) s the frequency-dependent absorpton coeffcent, and β s the path-loss exponent hch s taken as.5 n a practcal system. Then the transmsson loss TL can be expressed as TL = 0 log σd(f) = β 0 log(d)+d 0 log α(f). (4) The ambent nose n the ocean can be modeled usng turbulence,shppng,aves and thermal nose. The overall poer spectral densty on the ambent nose s gven by [8] NL = 0 log N(f) =0log[N t (f)+n s (f)+n (f)+n th (f)]. (5)

5 Smulated probablty densty P/ =. 8 db P/ = 3. 8 db P/ = 4. 8 db Average dynamc transmsson latency [blocks] Fg. 6. Smulated dstrbuton of average dynamc transmsson latency for each hop for the proposed DCC-RR ARQ protocol. Defne the transmsson SNR as the transmsson poer over the ambent nose varance,.e., γ := P/σ, hch can be obtaned from 0 log γ =SL NL. For smplcty, e set the maxmze retransmsson tmes s only once durng the smulaton,.e.,n retra = {0, }. To compare th the proposed DCC-ARQ protocol, both tradtonal ARQ and cooperatve ARQ protocols are equpped th erasure-correcton channel code, and the code rate s 0.5. Further, as our early ork [4] ndcated, RR cooperaton s more convenent than ER for DCC n UWA system, e adopt DCC-RR ARQ scheme for smulaton n ths paper. B. Smulaton Results For the system operatng at one frequency pont, Fg. 5(a) demonstrates the smulated outage probabltes of the three dfferent protocols. One can see that the proposed DCC- ARQ scheme s slghtly better than the coded cooperatve ARQ scheme n [4], and both of them outperform the coded tradtonal ARQ scheme. Average over channel dynamcs, Fg. 5(b) shos the end-toend delay of the three ARQ schemes. We can fnd that the proposed DCC-ARQ method outperforms the coded cooperatve ARQ scheme va the transmsson latency adaptaton, hle t s close to the coded tradtonal ARQ scheme. Hence, the proposed DCC-ARQ protocol acheves good balance beteen the reduced end-to-end delay and decent outage performance, relatve to exstng protocols. Fg. 6 demonstrates the smulated probablty densty functon of the average dynamc transmsson latency n the proposed scheme. Compared th the coded cooperatve ARQ scheme, a loer transmsson latency T dyna n the proposed DCC-ARQ scheme leads to a reduced end-to-end delay. IV. CONCLUSIONS In ths paper, e nvestgated a dynamc coded cooperatve ARQ scheme for a mult-hop UWA netork. Compared th the tradtonal ARQ and the cooperatve ARQ protocols numercal results shoed that the proposed protocol can acheve good balance beteen the reduced end-to-end delay and decent outage performance through dynamc cooperaton. ACKNOWLEDGEMENT The authors are grateful for the grant from Chna Scholarshp Councl n early support of the research. REFERENCES [] X. Xu, Z.-H. Wang, S. Zhou, and L. Wan, Parameterzng both path ampltude and delay varatons of underater acoustc channels for block decodng of orthogonal frequency dvson multplexng, The Journal of the Acoustcal Socety of Amerca, vol. 3, no. 6, pp , Jun. 0. [] X. Xu, S. Zhou, A. K. Morozov, and J. C. Presg, Per-survvor processng for underater acoustc communcatons th drect-sequence spread spectrum, The Journal of the Acoustcal Socety of Amerca, vol. 33, no. 5, pp , May 03. [3] S. Azad, P. Casar, F. Guerra, and M. 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