CLPC: Cross-Layer Product Code for Video Multicast over IEEE

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1 CLPC: Cross-Layer Product Code for Vdeo Multcast over IEEE Khwan Cho and Sunghyun Cho School of Electrcal Engneerng and INMC, Seoul Natonal Unversty, Seoul, Korea E-mal: Abstract In the wreless network, both random errors and burst errors do occur due to the channel nose. In order to combat such heterogeneous error patterns, we employ an adaptve error protecton based on product code. The proposed product code-based protecton n network ncludes cross-layer protecton, because each layer experences a dfferent type of channel errors. We mprove the decodng effcency by adoptng a proper protocol stack whch allows upper-layers to explot erroneous packets for the packet reconstructon. In addton, we consder the vdeo multcast over IEEE and formulate an aggregated multcast vdeo-qualty maxmzaton problem under bandwdth constrant. The results show that the proposed protecton scheme mproves the aggregated multcast vdeoqualty thanks to the cross-layer optmzaton. keywords: vdeo multcast over IEEE , Reed-Solomon (RS) code, forward error correcton (FEC), product code, crosslayer optmzaton. I. INTRODUCTION The use of IEEE wreless local area networks (WLANs) as an extenson to the exstng wred IP networks s growng at a rapd pace. The hgh bandwdth provded by WLAN technologes such as IEEE a/g and the upcomng IEEE n wll ultmately lead to ther ncreasng use for multmeda networkng. Recent multcast wreless vdeo, such as dgtal TV streamng and nteractve conference, poses several key requrements whch need to be satsfed n order to provde a relable and effcent transmsson: 1) easy adaptablty to bandwdth varatons of wreless network; 2) relatvely small delay bound; and 3) support for multple recevers. In ths paper, we nvestgate the robust and effcent multcast of vdeo over WLANs. We specfcally consder the hghspeed WLAN standard, IEEE a [2], whch offers hgh transmsson rate up to 54 Mb/s, enablng the transmsson of delay senstve audo/vsual (AV) traffc. Ths paper proposes a novel vertcal system ntegraton that enables the jont optmzaton of the varous protecton strateges exstng n the protocol stack. In the remander of ths paper, we refer to ths vertcal system ntegraton strategy as cross-layer protecton. The error control strateges that can be mplemented at the varous layers, namely, 1) applcaton-layer forward error correcton (FEC), 2) meda access control (MAC)-layer FEC, and 3) physcal-layer (PHY) mode adaptaton wll be nvestgated for the effcent multcast of vdeo over a. In addton to the proposed product code-based protecton strategy, we consder vdeo multcast over WLAN. As the demand on multcast dgtal vdeo streamng grows fast, the hgh-speed low-cost WLANs are consdered the lnk-layer protocol for the wreless multcast. In ths paper, we formulate an aggregated multcast vdeo-qualty maxmzaton problem wth bandwdth constrant, and develop a crosslayer adaptaton problem n order to fnd the optmal protecton strategy consderng the gven channel condtons and multcast polcy. Moreover, we specfcally consder the AP (access pont)-ntated contenton-free transmsson perod whch IEEE e supports by means of TXOP (transmsson opportunty) of HCCA (hybrd coordnaton functon controlled channel access) [3], snce the use of TXOP s an effcent scenaro for real-tme traffc transmsson. Reed-Solomon (RS) product code-based protecton wthout feedback can mprove the error-recovery capacty when both burst and random errors exst. Ths s the reason why product code s one of popular protecton schemes n the dgtal audo/vdeo system of the optcal storage [8]. In general, an error control scheme of the optcal storage s composed of two parts,.e., nner codng and outer codng. The nner decoder s ntended for the correcton of most of the small random byte errors and the detecton of the larger burst errors. The outer decoder s ntended for the correcton of burst errors whch the nner decoder could not correct. In ths paper, we propose a cross-layer product code-based protecton scheme n order to resst random and burst errors n both the WLAN and the wred-ip network. In order to recover from random errors, MAC-layer FEC adopts ntra-packet RS codng. Even f MAC-layer error recovery fals, applcaton-layer FEC based on nter-packet RS codng manages errors n the packets. The reason why we adopt FEC for both the MAC and applcaton layer s clear n that automatc repeat request (ARQ)-based error recovery does not guarantee delay bound whch s crtcal for real-tme transmsson, and that we consder a multcast envronment where multple recevers suffer from dverse channel errors. Today s IEEE do not support relable multcast. It only provdes an unrelable multcast protocol. Wth IEEE , packets wth multcast MAC address can be transmtted to multple recevers, but there s no feedback,.e., acknowledgment (ACK) exchange between the sender and the recevers [1]. Therefore, the relablty for multcast over WLAN cannot be guaranteed. For more robust transmsson, applcaton-layer nter-packet RS codng can be used to reconstruct lost packets. Another beneft s that applcaton-layer FEC does not ntroduce excessve delay whle retransmsson does. Generally speakng, the nteractve-multmeda transmsson s fragle to delay, but can tolerate some loss. However, feedback such as TCP ACK can ntroduce excessve delay snce at least two end-to-end delays are necessary to exchange feedback and retransmtted packets. For the same reason, applcaton-layer FEC s used for multmeda transmsson over WLAN n [6].

2 Moreover, wth ARQ-based recovery, t s a challengng problem to collect feedbacks from multcast recevers for each vdeo packet. However, nter-packet RS codng can explot the dversty of packet error patterns among the multcast recevers [13]. The adaptve cross-layer protecton strateges proposed n ths paper can be appled to any vdeo coded btstreams, such as non-scalable MPEG-4 or H.264 coded btstreams, dataparttoned btstreams, MPEG-2 and MPEG-4 hybrd spatotemporal scalable btstreams, fully scalable wavelet vdeo btstreams, etc. In ths paper, however, we employ MPEG- 4 Fne-Granularty-Scalablty (FGS) for the compresson of the vdeo data, because t can provde an easy adaptaton to bandwdth varatons and devce characterstcs [12]. There exst varous studes on layered protecton n the IEEE and vdeo transmsson. In [9], a transmtter staton can change ts physcal transmsson rate for combatng wreless channel errors. In [4], a MAC-layer FEC protecton s adopted n order to reduce packet losses due to wreless channel errors. The problem of reslent realtme vdeo streamng over IEEE b for both uncast and multcast transmsson s consdered n [11]. For the multcast, progressve vdeo codng based on MPEG-4 FGS s combned wth FEC. Smlarly, the combnaton of MPEG-4 FGS wth a scalable FEC and an unequal protecton strategy are proposed n [12]. The cross-layer consderaton necessary for wreless multmeda transmsson s ntroduced n [6][7]. Our adaptve cross-layer protecton strategy s pursued as follows. We analytcally derve the packet loss probablty of the proposed scheme at varous channel condtons for the gven PHY mode, MAC-layer FEC, and applcatonlayer FEC. An analytcal model s developed to characterze the end-to-end dstorton of multcast vdeo qualty based on the packet loss probablty and bandwdth constrant n WLAN. Based on the end-to-end dstorton model, a cross-layer protecton strategy s developed to adapt such parameters as a) the vdeo bt-rate, b) the applcaton-layer FEC, c) the MAC-layer FEC, and d) PHY-layer modulaton, n order to maxmze the aggregated multcast vdeo qualty across multcast recevers. The rest of the paper s organzed as follows. In Secton II, we present the proposed cross-layer product codebased protecton protocol. We then analyze the packet loss probablty after decodng and the resultant vdeo qualty n Secton III. In Secton IV, we descrbe the cross-layer adaptaton problem of determnng the optmal protecton strategy n order to maxmze the aggregated multcast vdeoqualty. In Secton V, we present the numercal results, and the paper concludes n Secton VI. II. PROTOCOL DESCRIPTION A multcast vdeo streamng network n consderaton s depcted n Fg. 1. The streamng server produces vdeo Fg. 1. Fg. 2. Network topology for multcast vdeo streamng. Protocol stack adopted for the cross-layer product code. and party packets (as shown n Fg. 4) transmtted as IP datagrams n the wred network. The IP datagrams arrve at the AP, whch the multcast recevers are assocated wth. The AP encapsulates the receved MAC servce data unts (MSDUs),.e., IP datagrams, as the MAC protocol data unts (MPDUs or smply MAC frames) [1]. Wth the receved MAC frames from the AP, the recevers reconstruct IP datagrams, and then vdeo packets at the applcaton layer. The protocol stack s depcted n Fg. 2. We adopt UDP-lte for transport-layer protocol n order to enable the applcaton layer to explot erroneous payloads from UDP-lte [14]. Smlar to the UDP-lte, the network-layer protocol,.e., IP, also checks errors only n the IP header part so that the applcaton layer can access erroneous packets. Packets suffer from two dfferent types of errors,.e., packet loss and packet corrupton. Snce congeston n the wred network can cause buffer overflow at routers, packet loss can occur as packets are forwarded through a chan of wred lnks. Whereas, the wreless channel can cause both packet loss and packet corrupton. The errors occurred n the MAC frame header dsable the recever to recognze the mportant nformaton such as the destnaton address so that ths error-patterns result n packet loss. On the other hand, the errors n the payload of the MAC frame lead to the corrupton of the orgnal data. In order to recover from random errors n the wreless network, we adopt ntra-packet RS codng for the MAClayer FEC. The MAC-layer RS encoder at the sender of the wreless network adds redundancy to each orgnal MAC frame. Then, the recever(s) can recover from random errors

3 of receved MAC frames by RS error-correcton decodng. As shown n Fg. 3, one MAC frame protected by MAClayer RS codng conssts of one header codeword and M ndependent payload codewords. The MAC header codeword uses (106, 70) RS code wth the error-correcton capablty of 18 bytes, whereas each payload codeword uses (N M,K M ) RS code wth the error-correcton capablty of t M (= (N M K M )/2 ). All the RS codes n consderaton are defned over GF(256),.e., an RS code symbol s 8 byte long. The message block of the MAC header codeword,.e., 70 bytes, s subdvded by MAC header and hgher-layer headers, and ncludes e MAC header (26 bytes), LLC header (8 bytes), IP header (20 bytes), UDP header (8 bytes), and applcaton-layer header (8 bytes). Therefore, MACheader recovery guarantees that there s no error n the headers of all the assocated protocols. For the recovery from erasures, we adopt nter-packet RS codng for the applcaton-layer FEC. As shown n Fg. 4, across multple vdeo packets, symbols along the same column produce symbols of the party packets by usng (N A,K A ) RS code. Then, the streamng server sends the party packets along wth the orgnal vdeo packets. For the applcaton-layer FEC, erasure-correcton decodng s used so that (N A K A ) erased symbols can be recovered. The decodng mechansm at the recever(s) works as follows: 1) The recever checks errors of the receved MAC frame usng Frame Check Sequence (FCS) based on Cyclc Redundancy Check (CRC)-32 at the tal of the MAC frame. If the CRC fals, the recever tres to reconstruct the header usng a fxed, e.g., (106, 70) MAC header RS code. If both CRC and MAC-header decodng fal, the receved MAC frame s dscarded. 2) If the MAC header decodng of an erroneouslyreceved frame succeeds, each payload codeword s decoded wth (N M,K M ) RS code, where the code nformaton s descrbed n the applcaton-layer header. If a payload codeword decodng fals,.e., due to over t M byte errors, the MAC layer declares erasure for the codeword. 3) The MAC layer reconstructs the MSDU usng both successfully recovered codewords and erasures. The reconstructed MSDUs are forwarded to the applcaton layer through LLC/IP/UDP-lte layers. Because IP and UDP-lte check only the errors n ther headers, even partally erased packets are forwarded onto the applcaton layer. The MAC layer also forwards the erasure nformaton of the partally erased packets onto the applcaton layer. 4) The applcaton layer recovers the orgnal vdeo packets from the partally-erased packets and successfullyreconstructed packets by usng nter-packet RS erasurecorrecton decodng. All the vdeo packets can be reconstructed as long as the number of correctly-receved symbols from the MAC layer s greater than or equal Fg. 3. MAC frame format for MAC-layer RS error-correcton code. Fg. 4. Applcaton-layer RS codng across packets. to K A across all the columns. III. MATHEMATICAL ANALYSIS In ths secton, we derve vdeo qualty wth bottom-up approach. Frst, the bt error probablty of the physcal layer gven a channel condton s derved. Based on the result, the decodng-error probabltes of the MAC header and payload codewords after RS error-correcton decodng are derved. Then, the overall packet loss probablty after applcatonlayer RS erasure-correcton decodng s also derved. Fnally, we descrbe the vdeo qualty based on the packet loss probablty and the wreless bandwdth constrant. A a PHY Bt Error Probablty The analyss of the a PHY error performance here s based on those gven n [9]. In ths analyss, t s assumed that the nose over the wreless medum s addtve Gaussan nose (AWGN) channel. Accordng to the analyss the bt error probablty, P b, after the Vterb decodng s bounded by the value dependng on the selected PHY mode m and the average SNR, γ. Note that a PHY mode bascally represents a specfc transmsson rate of the a. The probablty P PLCP of error n the PLCP header ncludng both SIGNAL (of 24 bts) and SERVICES (wth 7 scrambler ntalzaton bts) felds, when PHY mode m s used for the SERVICE feld of the PPDU (PHY protocol data unt), can be determned by P PLCP =1 (1 P e (1, 24)) (1 P e (m, 7)), where the probablty P e (m, l) of error n a block of length l bts, assumng random bt errors wth P b of PHY mode m, s determned by P e (m, l) =1 (1 P b ) l.

4 More detals of the analyss on the a PHY can be found n [9]. B. Cross-Layer Product Code Performance We here assume that each packet s receved wth the ndependently and dentcally dstrbuted average SNR, γ, where the probablty dstrbuton functon s gven by f(γ). Then, the symbol (of one byte) error probablty, when PHY mode m s used, s gven by P s (m) =P e (m, 8), where P s (m) s the MAC-layer symbol error probablty. In the followng, we use the notaton P s nstead of P s (m) for smplcty as long as there s no confuson. Frst, we derve the probabltes whch MAC-layer RS error-correcton decodng leads to. We assume that MAClayer RS codng for a header codeword uses (N H,K H ) RS code wth the error-correcton capablty t H = (N H K H )/2. Note that N H = 106 and K H =70from Fg. 3. Gven γ and the packet loss probablty, P wred, at the wred network, the success probablty of RS error-correcton decodng for the MAC header codeword can be expressed as t H ( NH ) P s (1 P s ) NH P [H s Γ=γ] = =0 (1 P PLCP ) (1 P wred ), (1) where H s denotes the event of successful RS error-correcton decodng for the MAC header codeword. Smlarly, the success probablty of RS error-correcton decodng for each MAC payload codeword can be expressed as t M ( ) NM P [B s Γ=γ] = Ps (1 P s ) NM, (2) =0 where B s denotes the event of successful MAC-layer RS error-correcton decodng for a gven MAC payload codeword. Then, gven f(γ), P [Bs Γ=γ] P [H s Γ=γ] P [B s H s ]= f(γ)dγ, (3) P [H s ] where P [H s ]= P [H s Γ=γ] f(γ)dγ. (4) Now, we derve the probabltes whch applcaton-layer RS erasure-correcton decodng leads to. As shown n Fg. 4, each byte of vdeo packets s used as the message symbol to produce redundancy bytes n party packets. Gven a packet, we defne a random varable, Q, as the number of the payload codewords n the packet wth unsuccessful MAC-layer RS error-correcton decodng, and another random varable, L, as the number of packets, belongng to the same applcaton-layer RS erasure-correcton decodng group (.e., of N A vdeo plus party packets) wth the gven packet, wth successful MAC-layer decodng for ther header codewords. Hence, the condtonal probabltes for the event, D s, of the successful reconstructon of the gven packet are expressed as P [D { s H s,q= q, L = l] ( ) } q l l = P [B s H s ] (1 P [B s H s ]) l, =K A P [D s Hs,L= c l] { ( ) } M l l = P [B s H s ] (1 P [B s H s ]) l, =K A where H c s s the complement of the event, H s. From the above equatons, the overall packetreconstructon success probablty of the product code can be obtaned by P [D s ]=P[H s,q=0] M N A 1 + P [D s H s,q= q, L = l] q=1 l=k A P [H s,q= q]p [L = l] N A 1 + P [D s Hs,L= c l]p [Hs]P c [L = l], (7) l=k A where ( ) NA 1 P [L = l] = P [H s ] l (1 P [H s ]) NA l 1, (8) l and (5) (6) P [H s,q= q] = P [H s,q= q Γ =γ]f(γ)dγ, (9) P [H s,q= q Γ =γ] ( ) M = P [B s Γ=γ] M q (1 P [B s Γ=γ]) q q P [H s Γ=γ]. (10) Then, the overall packet loss probablty, P [D c s], s obtaned by P [D c s]=1 P [D s ], (11) where D c s s the complement of the event, D s. C. MPEG-4 FGS Vdeo Codng for Wreless Transmsson The adopted model of vdeo qualty n ths secton follows R-D model of FGS vdeo [6]. The peak sgnal-to-nose 255 rato (PSNR = 10 log 2 10 MSE ) s used as a measure of vdeo qualty, where MSE s the dstorton per pxel n mean-squared-error. The PSNR of FGS vdeo at the recever wth tolerable base-layer packet loss ncreases approxmately lnearly wth the effectve receved bt-rate, R EL,d, of the enhancement layer at the recever after channel losses as follows PSNR = θ R EL,d + PSNR BL, (12)

5 where θ s the R-D model parameter, whch depends on the spato-temporal characterstcs of the vdeo sequence [12], and PSNR BL s the vdeo qualty (PSNR) of the base layer. Based on the very smple error concealment method [6], we can determne statstcally the bt-rate of the vdeo data that s receved wthout errors. For the enhancement layer, a sngle packet loss wthn an enhancement-layer vdeo frame causes the remander packets assocated wth that vdeo frame useless. Assumng N f enhancement-layer packets are sent for the current vdeo frame, and the vdeo frame rate of f r (frames/s), the effectve enhancement-layer bt-rate R EL,d at the decoder gven a packet loss probablty, P EL, of the enhancement-layer packets (assumng equal error protecton among enhancement sublayers) s R EL,d = [ Nf (1 P EL ) 1 P EL ( 1) =1 +(1 P EL ) Nf N f ]f r 8K M M. (13) For the packet transmsson, a fxed amount of HCCA TXOP s used by the AP every beacon nterval. If we assume that HCCA TXOP wthout backoff s used, N f can be computed based on bandwdth constrant as TXOPEL + SIFS N f = T EL + SIFS KA N A 1 T B 1, (14) where TXOP EL s the length of HCCA TXOP for the enhancement-layer packet transmsson, SIFS s short nterframe space defned by the , T EL s the transmsson tme for an enhancement-layer packet dependng on PHY mode and (N M,K M ), and T B s the beacon nterval [1]. Because we can explot the adaptaton parameters (.e., N A, K A, N M, K M, and PHY mode, m) for a gven channel condton and HCCA TXOP, our optmzaton problem can be further understood as to maxmze the aggregated PSNR of multcast recevers for a gven bandwdth constrant (.e., HCCA TXOP) on the wreless channel. We delve nto crosslayer optmzaton for vdeo multcast over IEEE n the next secton. IV. CROSS-LAYER OPTIMIZATION In ths secton, we formulate the optmzaton problem under wreless bandwdth constrant consderng each layereddesgn. A. PHY Mode Selecton IEEE WLAN s able to select the approprate physcal-layer (PHY) constellaton and convoluton code rate for a channel condton [9]. When the wreless channel suffers from severe nose, more robust PHY mode can overcome the channel nose and enable to exchange packets successfully. f r B. MAC-layer FEC At the recever, PHY forwards the receved PSDU (PHY servce data unt) to MAC, and then MAC checks errors of the MPDU. Wth the IEEE standard, MAC checks errors usng CRC. If no error occurs, MAC forwards the MSDU to the upper-layer, whle MAC dscards the MSDU f the CRC fals. However, the ntra-packet FEC for IEEE WLAN s benefcal rather than CRC alone [4]. We consder the code adaptaton of MAC-layer RS codng. C. Applcaton-layer FEC and Vdeo Rate Adaptaton Wth a bandwdth constrant for vdeo multcast, the maxmum number of transmtted packets s determned by the gven packet sze and PHY rate. Gven the maxmum number of transmtted packets, the number of vdeo packets and the number of party packets should be determned consderng the gven packet loss probablty n order to maxmze vdeo qualty at the recevers. From Eq. (14), we can also adapt the applcaton-layer RS code, the packet sze, and the vdeo frame rate. D. Bandwdth Constrant In wreless networks, t s typcal that bandwdth constrant on multmeda exsts for co-exstng flows. One of the solutons for the co-exstng problem s the admsson control for each permtted flows consderng the exstng traffc flows. In IEEE e, admsson control s done by assertng TXOP to each flow. In ths context, TXOP lmt of the vdeo multcast can be nterpreted as ts bandwdth constrant. Gven a TXOP lmt, the flow s able to use the wreless channel durng the allowed TXOP lmt. Therefore, for an FGS vdeo flow, the relaton between TXOP for base and enhancement layers s gven by TXOP lmt TXOP BL + TXOP EL, (15) where TXOP BL denotes the TXOP length for the base-layer packet transmssons. Gven base-layer bt-rate, R BL, and protecton strategy, the necessary TXOP for the base-layer transmsson, TXOP BL, can be expressed as TXOP BL = RBL T B 8K M M NA K A (T BL + SIFS), (16) where R BL s the encodng base-layer bt-rate, and T BL s the transmsson tme for a base-layer packet, whch depends on N M, K M, and PHY mode. E. Multcast Optmzaton Multcast servce can vary due to the servce polcy. In ths paper, we exemplfy a multcast servce model explotng the proposed protecton scheme. We consder the multcast recevers wth heterogeneous channel condtons. The multcast polcy of ths paper s that all of the recevers should have a small dstorton for the base-layer vdeo, and the maxmum aggregated vdeo qualty for the enhancement-layer vdeo.

6 The employed vdeo decodng system assumes that f a hgher prorty packet s lost (.e., a base-layer packet or a packet contanng a more sgnfcant enhancement layer btplane), then the lower prorty packets n the same vdeo frame are dscarded. Consequently, the packet loss rate of the base layer should be kept very small. In [10], the performance of non-scalable MPEG-4 base layers has been determned for a varety of channel condtons, and t has been determned that for most sequences, f the base-layer packet-loss rate P BL s lower than 1%, the overall FGS performance remans unaffected. Therefore, gven a partcular channel condton, afxedr BL, and the target P BL, we can determne the error protecton strategy n order to mnmze TXOP BL whle keepng P BL lower than 1%. If X denotes the set of all possble vectors of the adaptaton parameters,.e., (N A,K A,N M,K M,m), and P target denotes the target packet loss probablty of the base-layer packets, then we can defne the adaptaton-parameter selecton problem for the base-layer vdeo transmsson as x BL = arg mn TXOP BL x X subject to TXOP BL TXOP lmt, P BL, P target, (17) where P BL, denotes the base-layer packet loss probablty of recever out of multcast recevers. Thus, gven the obtaned TXOP BL, the optmal soluton of a weghted summaton problem can be constructed as x EL = arg max x X w PSNR subject to TXOP EL TXOP lmt TXOP BL. (18) where PSNR denotes the average PSNR value of staton gven SNR dstrbuton, and w s the gven weght factor for recever dependng on the multcast polcy, where w = 1. In summary, based on the cross-layer product code-based protecton, we can allocate the gven bandwdth n order to mantan tolerable base-layer dstorton across all the multcast recevers as well as to maxmze the aggregated enhancement-layer performance for the multcast recevers. V. NUMERICAL EVALUATION In ths secton, we comparatvely evaluate the proposed cross-layer product code-based protecton scheme va numercal results based on the analyss made n prevous sectons. We assume that all the FGS vdeo packets are transmtted va a PHY wth 8 dfferent PHY modes supportng 6 (mode 1), 9, 12, 18, 24, 36, 48, and 54 (mode 8) Mb/s. The fxed parameters values used for the evaluaton are summarzed n Table I. For smplcty, we assume that all base-layer vdeo packets are correctly receved, and all multcast recevers have an dentcal channel condton. Based on the assumpton and the fxed parameters, we reduce the cross-layer optmzaton problem to fndng the optmal K A n {3,9,15,21,27,33,39,45,51,57,63}, N M Fg. 5. TABLE I PARAMETER SETTING Parameter Value Parameter Value θ 2.49 (db/mb/s) N A 63 PSNR BL (db) K M 200 T B 100 (ms) M 5 TXOP EL 30 (ms) P wred 0 Two-state dscrete tme Markov chan for the wreless channel. n {200,206,212,218,224,230,236,242,248,254}, and m n {1,2,3,4,5,6,7,8} to maxmze vdeo qualty. We consder two channel models,.e., AWGN and tmevaryng wreless channels. Fg. 5 shows the two-state dscrete tme Markov chan modelng the tme varaton of the wreless channel. The wreless channel could be n ether good or bad state. When the wreless channel s n good state, the correspondng SNR at each tme nstant s taken from a unform dstrbuton n the range of 15 to 30 db, and when the wreless channel s n bad state, the SNR value s drawn from the range of 0 to 15 db. The tme spent n the good and bad are take from exponental dstrbutons wth rates 1/μ g and 1/μ b, respectvely. Therefore, the state µ transton probabltes t g,b and t b,g equal to b µ g+µ b and µ g µ g+µ b, respectvely. Dfferent values of t g,b correspond to dfferent wreless channel varaton patterns. For example, f t g,b s close to 0 (1), the wreless channel tends to stay n good(bad) state for most of the tme. We assume that the playout delay bound s large enough to reduce the jtter effects [5]. Large playout delay bound also enables nterleavng between packets, explotng effcent FEC code wth large codeword length, and reducng the effect of burst channel errors. Therefore, we can assume the large and fxed codeword length for applcaton-layer RS code, and random packet error wth average packet loss probablty. As shown n Fg. 6, the cross-layer product code-based protecton (labeled as CLPC ) leads to hgher multcast vdeo qualty than the applcaton-layer FEC protecton wth lnk adaptaton (labeled as AFEC+LA ) under AWGN channel. Here, lnk adaptaton refers to the PHY mode selecton. Ths s because MAC-layer FEC explots RS errorcorrecton decodng for the ntra-packet error recovery, whle applcaton-layer FEC adopts the nter-packet error recovery based on RS erasure-correcton decodng. Gven an dentcal bt error probablty for each packet, the ntra-packet error recovery s more effcent than the nter-packet error recovery. Therefore, the proposed cross-layer protecton does not explot applcaton-layer RS codng under AWGN channel. From Table II, we observe that the optmal K A s 63 for all fve SNR values, and knowng that N A =63, we confrm

7 TABLE II OPTIMAL PARAMETER SELECTION UNDER AWGN CHANNEL SNR (db) K A N M PHY Mode Fg. 6. Comparson under AWGN channel. TABLE III OPTIMAL PARAMETER SELECTION UNDER THE TIME-VARYING CHANNEL t g,b K A N M PHY Mode Fg. 7. Comparson under the tme-varyng channel. that the proposed scheme operates as the MAC-layer FEC protecton wthout the applcaton-layer FEC. Fg. 7 shows the vdeo qualty performance of the crosslayer product code (CLPC), applcaton-layer FEC wth lnk-adaptaton (AFEC+LA), MAC-layer FEC wth lnkadaptaton (MFEC+LA), and lnk-adapton alone (LA) over the tme-varyng wreless channel. We observe that the proposed CLPC consstently outperforms all other schemes across dfferent t g,b values. If the wreless channel s n good state and does not change to bad state, the cross-layer product code-based protecton manly explots the ntra-packet RS codng. However, as the state-transton probablty, t g,b, ncreases, t depends on the nter-packet RS codng rather than the nter-packet RS codng. Therefore, under the tme-varyng wreless channel, the proposed scheme utlzes both MAClayer and applcaton-layer FEC. The adaptaton parameters can be selected consderng the varaton of channel as shown n Table III. VI. CONCLUSION In ths paper, we propose a cross-layer product codedbased protecton whch mproves vdeo qualty under bandwdth constrant. We also nvestgate multcast scenaros of FGS vdeo streamng over WLAN and formulate the aggregated vdeo-qualty maxmzaton problem. The analyss and numercal results show that the proposed protecton scheme s more effcent than the other exstng protecton schemes consderng both AWGN and tme-varyng channels. REFERENCES [1] IEEE Std , Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons, Aug [2] IEEE a, Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons, Hgh-speed Physcal Layer n the 5 GHz Band, Sep [3] IEEE e, Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons: Medum Access Control (MAC) Enhancements for Qualty of Servce (QoS), Nov [4] S. Cho, Y. Cho, and I. Lee, IEEE MAC-Level FEC wth Retransmsson Combnng, IEEE Trans. on Wreless Communcatons, vol. 5, no. 1, pp , Jan [5] J. F. Kurose and K. W. Ross, Computer Networkng: A Top-Down Approach Featurng the Internet, 3rd Ed., Addson Wesley, [6] M. van der Schaar, S. Krshnamachar, S. Cho, and X. Xu, Adaptve Cross-Layer Protecton Strateges for Robust Scalable Vdeo Transmsson over WLANs, IEEE Journal of Selected Areas n Communcatons (JSAC), vol. 21, no. 10, pp , Dec [7] M. van der Schaar, N. S. Shankar, Cross-Layer Wreless Multmeda Transmsson: Challenges, Prcples, and New Paradgm, IEEE Wreless Communcatons Magazne, vol. 12, no. 4, pp , Aug [8] S. Ln and D. J. Costello, Error Control Codng, Fundamentals and Applcatons, 2nd Ed., Englewood Clffs, NJ, Prentce-Hall, [9] D. Qao, S. Cho, and K. G. Shn, Goodput Analyss and Lnk Adaptaton for IEEE a Wreless LANs, IEEE Trnas. on Moble Computng (TMC), vol. 1, no. 4, pp , Oct.-Dec [10] M. van der Schaar and H. Radha, Unequal packet loss reslence for fne-granular-scalablty vdeo, n IEEE Trans. on Multmeda, vol. 3, no. 4, pp , Dec [11] A. Majumdar, D. Sachs, I. Kozntsev, K. Ramchandran, and M. Yeung, Multcast and uncast real-tme vdeo streamng over wreless LANs, n IEEE Trans. on Crcuts Syst. Vdeo Technol., vol. 12, no. 6, pp , June [12] H. Radha, M. van der Schaar, and Y. Chen, The MPEG-4 fne-graned scalable vdeo codng method for multmeda streamng over IP, n IEEE Trans. on Multmeda, vol. 3, no. 1, pp , Mar [13] J. Nonnenmacher, E. Bersack, and D. Towsley, Party-Based Loss Recovery for Relable Multcast Transmsson, n IEEE/ACM Trans. on Networkng (TON), vol. 6, no. 4, pp , Aug [14] L. Larzon, M. Degermark, S. Pnk, L. Jonsson, and G. Farhurst, The Lghtweght User Datagram Protocol (UDP-Lte), RFC 3828, July 2004.