Replicate Multiple Descriptor Coding for Error Resilience in Wireless ...


Replicate Multiple Descriptor Coding for Error Resilience in Wireless ...

Replicate Multiple Descriptor Coding for Error Resilience in Wireless Video BroadcastingSheau-Ru Tong ( * , Yuan-Tse Yu + , Che-Min Chen * , and You-Chin Lin ** Department of Management Information SystemsNational Pingtung University of Science and Technology+ Department of Software EngineeringNational Kaohsiung Normal University, Taiwan (R.O.C.)ABSTRACTWireless IP-based video broadcasting suffers from dynamicand heavy burst errors exhibited in the wireless links. Thispaper addresses this issue by proposing a transport-layererror-resilience scheme, called the replicate multipledescriptor coding scheme (RMD). In principle, RMDemploys a multiple descriptor/channel transmission strategywith two unique features added. One is to introduce replicatekey frames in distinct channels; the other is to apply a shiftedtransmission schedule. These two features effectively protectthe key frames against corruption even under aheavy-burst-error situation and enhance the overall videoquality. We show that compared with other schemes, RMD isfairly cost-effective in terms of PSNR improved and dataoverhead paid. It is particularly attractive to a wirelessnetwork that exhibits heavy channel error rate (e.g.,15%-35%).RKeywords— mobile video broadcast, multiple descriptorcoding, channel diversity, transport-layer error resilienceI. INTRODUCTIONECENT development of WLAN (e.g., IEEE 802.11 and802.16), WWAN (e.g., MBMS of HSDP/UMTS [10]and BCMCS of CDMA200-1xEV-DO [1][13]) and lowbit-rate video coding schemes (e.g., H.264 [14] andMHEG [8]) promise an all-IP network that ubiquitouslysupports the video broadcast service to mobile hosts (MHs).Unfortunately, due to the movements of MHs, the radiosignals usually exhibits temporal heavy burst errors caused bythe random events, such as the multi-path fading effect andthe base station handoff[6]. Such errors hinderbandwidth-sensitive applications, like video broadcast. Anumber of link-layer error recovery mechanisms wereproposed for enhancing the wireless-channel transmissionquality [16]. For example, the DVB-IPTV has defined twosolutions for real-time services (LMB and CoD) for errorrecovery retransmission (RET) [2] The architecture ofHCDMA utilizes an ARQ scheme for error recovery at thephysical layer, whereas the scheme can be defined asMultilink Stop-and-Wait, because it uses several ARQprotocols with window size of one in parallel[2]. The reliablemulticast mechanisms supporting multimedia QoS in IEEE802.11 or IEEE 802.16 were proposed by [4][7]. However,these schemes are effective for recovering light errors, but areinadequate for heavy errors that are commonly observed in amobile network when a MH moves. Figure 1 shows theprobability of the loss events with different consecutivepacket numbers observed in an experiment where a stream ofUDP packets are multicast at packet rate of 160 packets/swith each packet size of 1024 bytes. Two curves aremeasured when the packet loss rate is about 5% and 10%,respectively. It is clear that loss bursts of several consecutivepackets (≥3) often happen in both cases. If such a long lossburst occurs to a key video frame (like I frames in H.264), theerrors will also be propagated to those frames referred to it,and severely downgrade the video perceptive quality.Fig. 1. Probability of consecutive packet loss events.The multi-channel transmission is a promisingtechnique that carries out diverse transmissions over differentwireless channels (frequencies) to mutually compensate theerror loss randomly appearing in each channel. For instance,MIMO, adopted in IEEE802.11n [12], exploit the frequencydiversity in transmission at the link layer. In the context ofvideos, the multiple descriptor coding scheme (MDC) [9] was

located at frame slot j is also repeated in sub-stream p at frameslot q, where p=(i+x)%k, q=(j+x)%k and x=1, 2, …, (b-1).Next, in the replica shifting phase, we defer the backupframes in sub-stream i with i⋅s frame slots. Normally, b and sare small numbers, and we assume the replica shifting will notcross to the next GOP (i.e., p+b-1≤k⋅s+1). For instance,Figure 3 shows the combo-GOP cycle (i.e., collection of allsub-streams’ GOP cycles) after applying RMD on foursub-streams with GOP(7, 1), b=3 and s=2.Sub-stream 0 …. P 0 P 0 P 0 P 0 P 0 P 0 I 2 I 3 I …. 0Sub-stream 3 …. P 3 P 3 P 3 P 3 P 3 P 3 I 1 I 2 I …. 3Sub-stream 1Sub-stream 2…. P 1 P 1 P 1 P 1 P 1 P 1 I 3 I 0 I …. 1…. P 2 P 2 P 2 P 2 P 2 P 2 I 0 I 1 I …. 2(a) Replica insertion (b=3)I : Primary frameI : Backup frame…. P 0 P 0 P 0 P 0 P 0 P 0 I 2 I 3 I …. 0…. P 1 P 1 P 1 P 1 I 3 I 0 P 1 P 1 I …. 1…. P 2 P 2 I 1 I 0 P 2 P 2 P 2 P 2 I …. 2…. I 1 I 2 P 3 P 3 P 3 P 3 P 3 P 3 I …. 3(b) Replica shifting (s=2)Fig. 3. The illustration of the combo-GOP cycle of four sub-streamsafter applying (a) the replica insertion phase (b=3) and (b) thereplica shifting phase (s=2).We should point out that if certain cross-layer control isavailable for the multicast channels to work on distinct radiobands, the replica insertion phase will be sufficient forsupporting the mutual-channel error recovery. This is becausethe effect of the dynamic channel fading observed in differentradio bands are unlikely to occur at the same time. It meansthe chance of losing the same replicas transmitted in differentchannels is relatively low. Unfortunately, most transport layerimplementations available today have no such control overthe physical layer. As a result, any radio signal impairment islikely to damage data in all multicast channels at the sametime. The replica shifting phase can thus remedy this problemby exploiting time diversity for transmitting key frames sothat they have a better chance to be recovered from corruption.Additionally, the key frames normally contribute the highesttraffic volume. This phase multiplexes key-frames fortransmission at different times and thus smoothes outpotential traffic burst caused by the aggregation ofsub-streams.C. Analysis of RMDFrame intervalFor the stream to be restored from RMD with correctplayback timing at the receiver site, the time taken for acombo-GOP in RMD should be the same as that taken for thesame number of frames (except backup frames) in the originalstream. It means the frame interval (slot) of combo-GOP(after inserting backup frames) should be adjusted as follows.I sub-stream =I stream ⋅p⋅ k / (p+b-1) , (1)where I sub-stream and I stream are the frame interval for thesub-stream and the original stream, respectively.Buffering delayIn the packet-error-recovery module, a corrupted I frameshas to incur additional delay penalty for waiting its backupframes. Such a delay becomes the maximum when theprimary frame of sub-stream (k-b) waits for its last backupframe (received from sub-stream k-1). It gives a bufferingdelay (in frame slots) as follows.s×(k-1)+b+1. (2)For instance, the sub-stream 1 incurs a 9-frame-slot delay inFig. 3. During this delay period all received packets are holdin the buffer. They require an overall buffer space bounded byk×{b×S I +[s×(k-2)+ (b-1)]×S O }, (3)where S I and S O are the maximum frame size of I frames andthe other frames, respectively.Key frame error rateWe assume that for each sub-stream, an I frame is furtherpacketized into g number of I packets. The frame slotassociated with the I frame is evenly divided into g number ofpacket slots, in each of which an I packet is transmitted. In acombo-GOP cycle, we call the first frame slot (or the first gpacket slots) that transmits the primary frame (or packets) theprimary frame slot (or primary packet slots) and the restframe slots (or packet slots) for the backup frames the backupframe slots (or backup packet slots) (in Fig. 4).Fig. 4. The frame- and packet-slot relationships in a combo-GOP.We are interested in finding the probability of observingany unrecoverable I packet loss in a combo-GOP cycle,denoted by P CG_err . Let ε be the channel error rate, i.e., theerror probability of the signal impairments occurring to apacket slot over which presumably the packets transmitted inall sub-streams are lost. For the moment, we assume thats≥b-1. It means backup packets are independently transmitted.The probability of all packets transmitted in a specificprimary packet slot being recoverable is given as follows.( b−1)= (1 − ε ) + ε ⋅ − ε . (4)P pkt _ rcv1( ) kIn Eq. 4, the first term (i.e., (1-ε)) is for the error-free case.The second term is for the error case that all k I packets (one

from each sub-stream) get lost but, fortunately, recoverable ifeach of them can find a correct packet from the correspondentb-1 backup packets in the following backup packet slots. Theprimary frame slot consists of g primary packet slots, so wehavegP = 1−P )CG _ err(pkt _ rcv[ ( ) ]⎞ ⎟⎠g⎛( b−1)k= ⎜1−( 1−ε ) + ε ⋅ 1−ε . (5)⎝Figure 5 shows a chart of P CG_err with k=4 subject to differentb’s. It can be seen that with one backup frame (b=2), P CG_err issignificantly reduced. With two backup frames (b=3), P CG_errbecomes close to zero (even when ε=10%).single descriptor (SD) (i.e., the original stream) casewith/without FEC, a multiple descriptor (MD) casewith/without FEC and the proposed RMD case.A. Aspect of PSNR [5]Figure 6 shows the average peak-signal-to-noise ratio (PSNR)subject to SD, SD/FEC, MD/FEC and RMD (b=2, s=0). Onthe right-hand axis we also label mean opinion score (MOS)for indicating the corresponding picture perceptionquality [15]. It can be seen that without applying any errorrecovery, PSNR of SD and MD soon becomes worse as thechannel error rate (i.e., ε) increases. (Basically, MD performsbetter that SD, because it can provide a better errorconcealment effect through frame interpolation.) For theother schemes with the error recovery, when the channel errorrate is light (

Fig. 9. Average PSNR (MOS) of RMD with respect to various s’eswhen b=3.Fig. 7. The instances of PSNR with respect to various scheme whenchannel error rate = 20%.B. Impact of b and s in RMDAs we mentioned before parameters b and s present certaintradeoffs to the system performance. Fig. 8 and Fig. 9 showthe average PSNR of RMD with different s subject to b=2and b=3, respectively. (MD/FEC is also added forcomparison.) We observe that the PSNR curve is improved(leveraged) as s increases. The improvement seems magnifiedunder a heavy packet loss condition (15%-35%). This isbecause transmissions of key-frames are shifted over a longertime span, which makes it more robust to long burst errors.The improvement of PSNR is also obvious as b increasesfrom 2 to 3, because more key-frame redundancy is provided.When b=3 and s=1 or 2, we can find a MOS value no worsethan “fair” under 15%-35% packet loss. However, suchimprovement is paid by the extra buffering space and delay.According to Eq. 3, the buffer space for b=3 and s=1 is12S I +16S O , and for b=3 and s=2,12S I +24S 0 . Also according toEq. 2, the buffer delay for b=3 and s=1 is 7 frame slots, andfor b=3 and s=2, 10 frame slots. (They are about 719 ms and1026 ms, respectively.) Such delays are incurred at the initialtime which is quite acceptable to a video broadcast scenario.C. Transmission costFinally, we would like compare the transmission cost ofRMD and MD/FEC. Table 1 lists the encoded data sizes andthe corresponding PSNR values subject to different schemesunder a channel error rate of 20%. We see that when b=2,RMD incurs overhead slightly lower than that of MD/FEC,but gains a PSNR about 2.7dB higher. When b increases to 3,in spite that RMD incurs overhead slightly higher than that ofMD/FEC, the improvement of PSNR is about 5 dB. This infact promotes MOS from “fair” to “good”. It means thatcompared with MD/FEC, RMD is quite cost-effective whenthe network condition is poor.Table 1. Encoded data sizes and PSNR for MD/FEC and RMDwhen channel error rate = 20%.Encoded data size Size ratio=MD/FECPSNR(dB)(bytes)encoded/ originalRS (255, 159) 4,229,395 1.5949 25.92RMDb=1 3,168,493 1.19486 24.02b=2 3,786,910 1.42807 28.63b=3 4,405,327 1.66128 30.91IV. CONCLUSIONFig. 8. Average PSNR (MOS) of RMD with respect to various s’eswhen b=2.This paper addressed the issue of long burst errors frequentlyobserved at the transport-layer multicast in a wirelessnetwork environment. Such errors seriously impact theeffectiveness of some error recovery scheme (like FEC), andin turn deteriorates the video broadcasting quality. Forsolving this problem, we proposed a transport-layererror-resilience scheme, called the replicate multipledescriptor coding scheme (RMD). In principle, RMDemploys a multiple descriptor/channel transmission strategywith two unique features added. One is to introduce replicatekey frames in distinct channels; the other is to apply a shifted

transmission schedule. Some analytical results werepresented. Compared with other scheme, the effectiveness ofRMD was also further validated through simulation. Theresults reveal that RMD is fairly cost-effective in terms ofPSNR improved and data overheads paid. It is particularlyattractive to a wireless network that exhibits heavy channelerror rate (e.g., 15%-35%).REFERENCES[1] CDMA2000 High Rate Broadcast-Multicast Packet Data AirInterface Specification, 3GPP2 Std. C.S0054, Rev. 1.0, Mar.2005.[2] Chaehag Yi, Jae Hong Lee, “Hybrid ARQ scheme usinginterleaved Reed-Solomon codes in a powercontrolledDS-CDMA cellular system,” IEEE 44 th Vehicular TechnologyConf., vol. 3, pp. 1402-1406, Jun. 1994.[3] EN 301 790, “Digital Video Broadcasting (DVB); Interactionchannel for satellite distribution systems,” EuropeanTelecommunications Standards Institute (ETSI).[4] Lyakhov, A., Vishnevsky, V., Yakimov, M., “Multicast QoSSupport in IEEE 802.11 WLANs,” IEEE Intl. Conf. on MobileAdhoc and Sensor Systems, pp. 1-3, 8-11 Oct. 2007.[5] MDC Module,[6] Jian-Hao Hu, Yeung, K.L., Siew Chee Kheong, Gang Feng,“Hierarchical cache design for enhancing TCP overheterogeneous networks with wired and wireless links,” IEEEGlobal Telecommunications Conf. (GLOBECOM’00), vol. 1,pp. 338-343, 2000.[7] Jiann-Liang Chen, Kung-Cheng Wang, “Reliable WiMAXMulticast Applications,” IEEE 8th Intl. Conf. on Computer andInformation Technology Workshops, pp. 182-187, 8-11 Jul.2008.[9] Roman, V., Namuduri, K.R., “Combined source-channeldiversity scheme for image transmission over wirelesschannels,” IEEE Conf. on Comm. (ICC 20005), vol. 2, pp.1219-1223, May 2005.[10] S. M. Cheng, W. R. Lai, Phone Lin, K. C. Chen, “KeyManagement for UMTS MBMS,” IEEE Trans. on WirelessComm., vol. 7, No. 9, pp. 3619-3628, Sept. 2008.[11] S. R. Tong, C. W. Hung, “A staggered-channel-clusterapproach to support video multicast handoff in wirelessnetworks,” IEEE Intl. Conf. on Multimedia and Expo(ICME’08), pp. 849-852, 2008.[12] Yi Yong, Jian Haifang, Fang Zhi, Shi Yin, “The FrequencyOffset Estimation and Tracking in MIMO-OFDM BASED802.11n System,” Intl. Conf. on Networks Security, WirelessCommunications and Trusted Computing, vol. 1, pp. 754-757,25-26 Apr. 2009.[13] Yongwoo Cho, Kyungtae Kang, Heonshik Shin, “SeamlessMultimedia Broadcasting Over cdma2000 BCMCS Networks,”IEEE Intl. Conf. on Comm. (ICC’07), pp. 5628-5635, 24-28 Jun.2007.[14] Zhenyu Wei, Kai Lam Tang, Ngan, K.N., “Implementation ofH.264 on Mobile Device,” IEEE Trans. Consumer Electronics,vol. 53, No. 3, pp. 1109-1116, Aug. 2007.[15] Zhizhong Zhe, Hong Ren Wu, Zhenghua Yu, Ferguson, T.,Tan, D., “Performance evaluation of a perceptual ringingdistortion metric for digital video,” Intl. Conf. on Multimediaand Expo, vol. 1, pp. 825-828, 6-9 Jul. 2003.[16] Zheng, H., Boyce, J., “An improved UDP protocol for videotransmission over Internet-to-wireless networks,” IEEE Trans.on Multimedia, vol. 3, no. 3, pp. 356-365, Sept. 2001.[17] Zhengdao W., “Application of Error-Control Coding inSatellite and Wireless Communications,” May. 9, 2000.[8] Kretz, F., Colaitis, F., “Standardizing hypermedia informationobjects,” IEEE Comm. Mag. vol. 30, no. 5, pp. 60-70, May 1992.

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