Layered Video Transmission on Adaptive OFDM Wireless Systems
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Layered Video Transmission on Adaptive OFDM Wireless Systems D. Dardari IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: [email protected]
M. G. Martini IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: [email protected]
M. Mazzotti IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: [email protected]
M. Chiani IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: [email protected] Received 28 February 2003; Revised 26 January 2004 Future wireless video transmission systems will consider orthogonal frequency division multiplexing (OFDM) as the basic modulation technique due to its robustness and low complexity implementation in the presence of frequency-selective channels. Recently, adaptive bit loading techniques have been applied to OFDM showing good performance gains in cable transmission systems. In this paper a multilayer bit loading technique, based on the so called “ordered subcarrier selection algorithm,” is proposed and applied to a Hiperlan2-like wireless system at 5 GHz for efficient layered multimedia transmission. Different schemes realizing unequal error protection both at coding and modulation levels are compared. The strong impact of this technique in terms of video quality is evaluated for MPEG-4 video transmission. Keywords and phrases: OFDM, adaptive modulation, bit loading, UEP, MPEG-4.
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INTRODUCTION
One of the main goals in the near future of communication systems is the development of multimedia efficient data coding, compression, and transmission techniques that permit real-time mobile communications. In this context, the major challenge is the integration of different categories of networks and wireless local area networks (WLAN). Systems have to be adaptive, that is, they have to react to changing quality conditions, like varying channel capacity. In high-speed wireless data applications, the orthogonal frequency division multiplexing (OFDM) modulation scheme has been considered due to its relatively simple receiver structure compared to single-carrier transmission in frequency-selective fading channels. OFDM modulation is adopted by IEEE for the extension of the 802.11 wireless LAN standard to the 5 GHz band (IEEE802.11a), providing data
rates up to 54 Mbps [1]. ETSI adopted the OFDM scheme for the high performance LAN physical layer standard (Hiperlan2) As well [2]. Conventional OFDM modems use fixed constellation size and power level allocation of all subchannels. In more recent standards (i.e., IEEE802.11a), the adaptation of the constellation size (the same for all subchannels) according to the global channel-state time-variation is admitted. Due to multipath fading, some subchannels could experience severe degradation in the signal-to-noise ratio (SNR), resulting in high overall bit error rates. Channel coding is a common technique to mitigate this effect. If the channel is st
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