Perceptually Enhanced ARQ for Live Video Streaming over Congested 802.11e Networks P. BUCCIOL 1 , E. MASALA 1 , E. FILIPPI 3 , J. C. DE MARTIN 2 1 Dipartimento di Automatica e Informatica / 2 IEIIT-CNR Politecnico di Torino, Corso Duca degli Abruzzi, 24 — I-10129 Torino 3 Advanced System Technologies, STMicroelectronics S.r.l. — Cornaredo (Milano) ITALY Abstract: - In this paper we present a perceptual ARQ algorithm specifically designed for real-time 802.11 video com- munications. A fundamental characteristic of the algorithm is its ability to take into account the perceptual and temporal importance of each packet at the same time. A priority value is associated to each packet to determine which one to retrans- mit at each retransmission opportunity. Compared to the standard 802.11 MAC-layer ARQ scheme, the proposed technique delivers higher perceptual quality because only the most perceptually important packets are retransmitted. Simulations of H.264 live video streaming in a realistic 802.11e infrastructured scenario show that the proposed method consistently out- performs the standard link-layer 802.11 retransmission scheme, delivering gains up to 10 dB of PSNR as well as very low transmission delays. Key-Words: - Perceptual ARQ, H.264 live video streaming, 802.11e wireless LAN’s 1 Introduction The number of devices equipped with an 802.11 wireless network interface [1] is dramatically increasing. Consumer electronics devices, especially the ones with multimedia capabilities, are expected to integrate such interfaces in a very short time. Hence it is very important for the 802.11 standard to efficiently transport different kinds of traffic, and multimedia in particular. In 802.11, radio link noise and MAC-level collisions are addressed by an automatic link-layer retransmission scheme. While data-agnostic, link-layer ARQ is both fast and simple to implement, for the specific —and increasingly important— case of mul- timedia traffic, more advanced ARQ techniques could use network resources more efficiently as well as deliver higher perceptual quality. Two of the most important characteristics of multime- dia streams are their highly non-uniform perceptual impor- tance and their strong time sensitivity. One or both char- acteristics are usually considered by most ARQ techniques designed for multimedia communications. The Soft ARQ proposal [2], for instance, avoids retransmitting late data that would not be useful at the decoder, thus saving band- width. Variants of the Soft ARQ technique have been de- veloped for layered coding [2]. Compressed multimedia bitstreams are composed of syntax elements of varying perceptual importance. Some techniques exploit this feature by assigning different prior- ities to the syntax elements. In [3] video packets are pro- tected by error correcting codes whose amount depends on the kind of frame to which the video packets belong. Chan- nel adaptation is achieved by an additional ARQ scheme that privileges the most important classes of data. Schedul- ing of video frames according to the priority given by their position inside the Group of Pictures (GOP) in presented in [4], coupled with the assignment of different priorities to the various kinds of data (i.e. motion and texture infor- mation) contained in each packet. Further improvements are possible optimizing the trans- mission policy for each single packet, rather than rely- ing on a priori determination of the average importance of the elements of the compressed bitstream [5]. For in- stance, packets could be retransmitted or not depending on whether the distortion caused by their loss is above a given threshold, as in the low-delay wireless video trans- mission system presented in [6]. However, it is not clear how to optimally determine such threshold. Given a way to associate distortion values to each packet, rate-distortion optimization of the transmission policies has also been pro- posed [7] [8]. In this paper, we focus on the specific case of real- time video transmission over 802.11e networks. Unlike the 802.11 MAC-level ARQ which retransmits all pack- ets regardless of their importance, we propose a perceptual ARQ scheme, implemented at the application level, which exploits information about the perceptual and the tempo- ral importance of each packet. In our proposal, a set of re- transmission opportunities is determined at the beginning Proceedings of the 5th WSEAS Int. Conf. on APPLIED INFORMATICS and COMMUNICATIONS, Malta, September 15-17, 2005 (pp404-410)