the performance is fairly poor. MD/FEC can improve PSNRa bit, but it is still not adequate for fixing long burst errors.If those burst errors occur to the key frames, the unresolvederrors will be propagated to the entire GOP. On the otherhand, RMD normally gives a higher PSNR. This is becauseRMD transmits backup frames at different timeframes andthus gives a better chance to preserve the entire key framesagainst long burst errors. This also helps decoding of othernon-key frames.Fig. 5. The P kf_err with respect to various b’s when k=4 and g=4.3. SIMULATION AND PERFORMANCE EVALUATIONThe raw video track under test consists of 400 frames with aQCIF format and has a frame rate of 30 fps. We would liketo compare the single descriptor (SD), multiple descriptors(MD) and replicate multiple descriptors (RMD). For MDand RMD, the video stream is split into four descriptors.The video compression scheme used is MPEG-4 with aGOP pattern of IPPPPPP. The packet size is 1024 bytes. ForSD, the same compression parameters are applied, and ineach packet slot at most four packets can be delivered. Wealso consider applying FEC to each descriptor, where databytes are encoded into the Reed Solomon (RS) codingblocks . Each coding block makes up 255 bytes, where159 bytes are regular data, and 96 bytes are the parity codeswhich can fix 96 bytes erasures. We take the bulk-packetlossmodel as our channel loss model. The simulationenvironment is NS2. We simulate several schemes: a singledescriptor (SD) with/without FEC, a multiple descriptor(MD) with/without FEC and the proposed RMD.3.1. Aspect of PSNR [9,10]Figure 6 shows the average peak-signal-to-noise ratio(PSNR) subject to SD, SD/FEC, MD/FEC and RMD (b=2,s=0). Every value is the average of 30 independentexperiments. First of all, we observe that MD shows a betterPSNR compared with SD as the packet loss rate (i.e., )increases. This is because MD disperses errors amongdescriptors and makes itself more immune from heavy bursterrors than SD. Next, when error recovery functions areadded, PSNR is generally improved. When the packet lossrate is small (30dB. But whenthe packet loss rate increases beyond 15%, RMD starts tooutperform than SD/FEC or MD/FEC. Even up to 25%packet loss, RMD can still keep PSNR>25dB. This isbecause the backup frame can more effectively protect thekey frame against long burst errors.To get a closer look of the scheme behavior, Fig. 7shows the instance of PSNR (taken from frame 196 to 290)subject to various schemes in one random experiment withrespect to 20% packet loss rate. As it can been seen, MDonly performs error concealment but no error recovery, soFig.6. Average PSNR under different packet loss rates subject tovarious schemes.Fig. 7. The instances of PSNR with respect to various schemewhen packet loss rate = 20%.3.2. 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 seemsmagnified under a heavy packet loss condition (15%-35%).This is because transmissions of key-frames are shifted overa longer time span, which makes it more robust to long bursterrors. The improvement of PSNR is also obvious as bincreases from 2 to 3, because more key-frame redundancyis provided. When b=3 and s=1 or 2, we can find a PSNR no
worse than 26dB under 15%-35% packet loss. However,such improvement is paid by the extra buffering space anddelay. According to Eq. 3, the buffer space for b=3 and s=1is 12S I +12S O , and for b=3 and s=2,12S I +24S 0 . Alsoaccording to Eq. 2, the buffer delay for b=3 and s=1 is 6frame slots, and for b=3 and s=2, 9 frame slots. (They areabout 622 ms and 933 ms, respectively.) Such initial delaysare quite acceptable to a video broadcast scenario.Table 1. Data sizes, redundancy ratios and PSNR for SD/FEC,MD/FEC and RMD when packet loss rates = 20%.Redundancy ratioSchemes Data size (bytes)()PSNR(dB)SD/FEC 3,009,946 1.00 24.50MD/FEC 4,229,395 1.41 25.92b=1 3,168,493 1.05 24.02RMD b=2 3,786,910 1.25 28.63b=3 4,405,327 1.46 30.914. CONCLUSIONFig. 8. Average PSNR of RMD with respect to various s’es whenb=2.This paper addressed the issue of long burst errorsfrequently observed in the IP multicast in a wireless networkenvironment. Such errors affect the effectiveness of FEC,and in turn deteriorate the video broadcasting quality. Forsolving this problem, we proposed an application-layerapproach, called as the replicate multiple descriptor codingscheme (RMD). RMD exploits the time diversity todisperses burst error over multiple descriptors and mitigatethe damage impact on the decoding. We showed thatcompared with the conventional single descriptor/FEC,RMD can gain a 4.1dB PSNR improvement under a 20%-packet-loss environment with 1.25 times of data size.In terms of the processing cost of RMD, the dominatepart- the encoding/decoding process, is pretty much thesame as that of SD, because the total numbers of framesprocessed are the same for both RMD and SD. Otherprocessing parts (e.g., stream splitting/merging, frameinsertion/ recovery) are fairly straightforward. Thus webelieve RMD can be well handled by most of the today’ssmart mobile devices. Finally, RMD is ideal for combatingexcessive burst errors, particularly protecting key frames.How it can co-work with other adaptive FEC to offer a fullspectrum of protection against various burst errors is worthyof further study.5. REFERENCESFig. 9. Average PSNR of RMD with respect to various s’es whenb=3.3.3. Redundancy costFinally, we would like discuss the redundancy cost. We takeSD/FEC as a reference and define the redundancy ratio of ascheme as the data size employing that scheme over thedata size employing SD/FEC. Table 1 lists the data sizes, theredundancy ratios and the PSNR values subject todifferent schemes under a packet loss rate of 20%. First, wesee that MD/FEC outperforms SD/FEC about 1.4dB andrequires =1.4. Whereas, RMD (b=2) outperforms SD/FECabout 4.1dB but only requires =1.25. If we choose RMD(b=3) (the redundancy ratio is slightly higher than that ofMD/FEC), the gain of PSNR is about 6.4 dB. This in factpromotes PSNR up to 30.9dB, which corresponds to a fairto-goodvideo quality. A. Nafaa, T. Taleb, and L. Murphy, “Forward errorcorrection strategies for media streaming over wirelessnetworks,” IEEE Comm. Magazine, vol. 46, no. 1, pp.72–79, 2008. CDMA2000 High Rate Broadcast-Multicast PacketData Air Interface Specification, 3GPP2 Std. C.S0054,Rev. 1.0, Mar. 2005. D. Gesbert, M. Shafi, D-s. Shiu, P. J. Smith and A.Naguib, “From Theory to Practice: An Overview ofMIMO Space-Time Coded Wireless Systems,” IEEE J.on Select Areas In Communications, vol. 21, pp. 281-302, Apr 2003. D. Gomez-Barquero, D. Gozalvez, N. Cardona,“Application Layer FEC for Mobile TV Delivery in IPDatacast Over DVB-H Systems,” IEEE Trans. onBroadcasting, vol. 55, no. 2, pp. 396-406, 2009.
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