Bull, DR

Doufexi, A

Nix, AR

Redmill, DW

English

At present, wireless local area networks (WLANs) supporting broadband multimedia communication are being developed and standardized around the world in the 5 GHz band. Systems such as IEEE802.11a and HIPERLAN/2 will provide data rates up to 54 Mbps. Highly compressed data such as video are extremely sensitive to the errors that are commonly encountered in wireless channels. This paper addresses the problems of transmitting MPEG-2 video over WLANs, and in particular HIPERLAN/2. Error resilient video transcoding is proposed as a means of handling channel errors

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                          Doufexi, A., Redmill, D. W., Nix, A. R., & Bull, D. R. (2001). MPEG-2video transmission using the HIPERLAN/2 WLAN standard. IEEETransactions on Consumer Electronics, 47(3), 354 - 363. 10.1109/30.964121Link to published version (if available):10.1109/30.964121Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.TUPM 6.1 % Rate Scrambling + ~ ~ ~ ~ ~ l ~ t i ~ ~ ~  Puncturing + Interleaving Mapping OFDM PDU from DLC code MPEG-2 VIDEO TRANSMISSION USING THE HIPERLAN/2 WLAN STANDARD . Angela Doufexi, David Redmill, David Bull, Andrew Nix Centre for Communications Research, University of Bristol, BS8 lUB, U.K. E-mail: A.Doufexi@bristol.ac.uk ABSTRACT At present, Wireless Local Area Networks ( WLANs) supporting broadband multimedia communication are being developed and standardised. These will provide data rates up to 54 Mbps in the 5GHz band. Highly compressed data such as video are extremely sensitive to the errors that are commonly encountered in wireless channels. This paper addresses the problems of transmitting MPEG-2 video over WLANs, and in particular HIPERLANn, and describes error resilient video transcoding as a means of handling channel errors. INTRODUCTION Recently, there has been an increasing interest in multimedia communications such as the transmission of video, images, and data over wireless networks. HIPERLAN/2 [l] is a wireless LAN standard that will provide data rates up to 54 Mbps. Highly compressed data are extremely sensitive to bit or burst errors that are commonly encountered in wireless channels. ARQ protocols, as recommended in HIPERLANl2, may be unsuitable for real time video due to latency constraints. HIPERLAN/2 also supports an unacknowledged (unreliable, low latency transmission) and a repetition mode that will be employed in broadcast and multicast scenarios. Unicast data can be sent using either acknowledged or unacknowledged mode. In order to handle channel errors, one altemative to ARQ protocols is the use of error resilient video coding techniques [2]. This paper investigates the transmission of MPEG-2 video over HIPERLAN/2 and proposes video transcoding as a means of handling channel errors. The strategy employed in the video transcoder is to partition the MPEG-2 video stream into low and high priority packetized bit streams. Error resilience is achieved by employing EREC (Error Resilient Entropy Coding) [3] in the low priority data while the high priority bit stream is adequately protected by means of ARQ. HIPERLAN/2 WLAN STANDARD HIPERLAN12 ,will support multiple transmission 'modes', providing data rates up to 54 Mbps. The physical layer (Figurel) of the standard is based on the use of Orthogonal Frequency Division Multiplexing (OFDM). The physical layer modes (Tab1el)with different coding and modulation schemes are selected by a, link adaptation scheme. The physical layer of IEEE 802.1 l a  is very c:imilar [4]. Table 1: PHY Layer Mlodes 1 Mode 1 Modulation I Codine I Bit rate I QPSK 3/4 16QAM 9/16 27 314 16QAM I 7 1  64QAM I The channel model used to perform the simulations, is channel model A specified by ETSI [5]. The channel is wideband multipath, Rayleigh, with an rms delay spread of 50ns. The MPEG-2 video coding standard [6] is being extensively used for the provision of video services worldwide. It is designed to provide distribution of quality television (normally using Main Level, Main Profile) at a bit rate between 4 and 9 MbiVs depending on scene content. The MPEG standard relies on two basic techniques: an intraframe discrete cosine transform (DCT) for the reduction of the spatial redundancy, and interframe motion compensation for the reduction of temporal redundancy. VIDEO TRANSCODING In our scheme the data is partitioned into low and high priority bit streams so that most of the headers, motion vectors and low frequency image information is given a high priority while the remaining high frequency information is placed in the low priority partition. The high priority bit stream is adequately protected by means of ARQ protocols while the low priority bit stream does not employ ARQ in order to avoid netwlxk congestion and jitter. Error resilience is also achieved by applying EREC [3,7] to the low priority data. One of the most serious forms of error propagation within an image codec is caused by the loss of block synchronisation (horizontal strips in Figure 3a). The EREC proclzss rearranges the variable length blocks of data, so that these can be packed into a set of fixed-length slots. MPEG-2 BASICS Figure 1: HIPERLAN/2 Transmitter 52 0-7803-6622-0101 $10.00 0 2001 IEEE RESULTS A fully compliant HIPERLAN/2 modem has been developed [4]. Figure 2 presents the BER performance of HIPERLANj2 for channel model A. l.E+00 1 E-01 l.E-02 L 1 E-03 1.E-04 ?.O R o o  .O (dB) 20 1 E-05 0 5 10 15 20 25 30 35 40 1 3 4  W) Figure 2: BER Performance of HIPERLAN/2 MPEG-2 data is read from a standard main level main profile (ML@MP) video file containing 4.0MbitIsec data. The MPEG-2 video test sequence with and without transcoding is processed through the modem model. Figure 3 shows example frames at a bit error rate of (mode 5 in channel A). The standard MPEG-2 system is seen to suffer from a large amount of error propagation both spatially and temporally. It can be seen that the system with data partitioning and EREC gives much better subjective quality. Figure 4 shows the measured PSNR values against BER in channel A. Note that the PSNR in this case measures distortion due to channel errors and not due to compression (i.e. the received video frames are compared with the MPEG-2 frames before transmission). 6 MPEDL " O m  Erron  8- MPED2 WZ Em" f -1. Pmnlllonmg rrlth EREC W2 Er- BER 'Ihrrrhold -- ------------- ii-- Figure 3: Example frames (BER=104), frame number 115. a) MPEG-2, b) data partitioning with EREC. 53 

MPEG-2 video transmission using the HIPERLAN/2 WLAN standard

MPEG-2 Video Transmission Using the Hiperlan/2 WLAN Standard

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MPEG-2 video transmission using the HIPERLAN/2 WLAN standard

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