Layered Video Transmission on Adaptive OFDM Wireless Systems

EURASIP Journal on Applied Signal Processing 2004:10, 1557–1567 c 2004 Hindawi Publishing Corporation
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: M. G. Martini IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: M. Mazzotti IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: M. Chiani IEIIT-BO/CNR, CNIT, DEIS, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Email: 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. 1. 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 static (e.g., in digital subscribers lines (DSL)) or slowly time varying, the receiver can provide the transmitter with detailed channel state information (CSI) using a robust feedback channel. Based on the CSI, more sophisticated adaptive transmission techniques have the possibility to dynamically modify 1558 the parameters of the modulator in order to improve the performance [3]. Thanks to the characteristic of multicarrier modulations, it is also possible to dynamically change the transmitting power and bit rate of each subchannel according to channel selectivity variations (adaptive bit loading). The first applications of bit loading algorithms appeared in DSL systems [4, 5]. It is a well-known fact that the theoretical channel capacity can be approached by distributing the total transmitted energy according to the water-filling principle [6]. In the realistic case where a finite granularity in constellation size is required, the rounded bit distribution obtained starting from the water-filling solution could still not be the optimum. Some suboptimum algorithms to reduce the complexity have been proposed in the ADSL context [7, 8]. Campello [9] gives the theoretically sufficient conditions for a discrete bit allocation to be optimal. Based on his conditions, a “greedy” algorithm can be used to achieve the optimal discrete bit/power loading distribution. Recently, some studies regarding the application of adaptive bit loading algorithms to wireless channels appeared [10, 11, 12, 13]. In this case, particular attention must be paid to channel estimation and CSI update rate effects on the performance [14, 15, 16]. However, water-filling-based techniques require a large overhead for CSI feedback, making them suitable only for static or very slow time varying channels. Moreover, the modem must be able to continually change the modulation format and power on subcarrier basis (high complexity if high data rates are requested). Hence, simple suboptimal algorithms should be investigated in order to reduce complexity and CSI overhead. The adaptation of the modulation segment also to the source data structure and significance may provide good results by realizing unequal error protection (UEP) in the modulation domain. UEP has proven to provide good performance in the case of transmission of compressed sources, where the bits produced have a different significance. Providing a lower bit error rate for the bits with higher significance and leaving the less significant bits with less protection makes it possible to increase the perceived quality. UEP has been applied for audio transmission [17, 18], for progressive image transmission [19], and for subband coded audio and video transmission, as some kinds of sources lend themselves to be partitioned into differently sensitive groups of bits. Also UEP for block-based video coded sources has been proposed as in [20, 21, 22]. UEP is classically performed at channel coding level, through convolutional and, more recently, turbo codes. Multiresolution constellations allows a nonuniform data protection in the modulation domain [23]. Some recent studies have proposed to perform UEP in the modulation domain, exploiting the characteristics of multicarrier modulations [13, 24]. In this case, the fact that a nonuniform bit and power allocation among the subcarriers is required implies a significant modem complexity and a high CSI signaling overhead between the transmitter and the receiver with respect to the uniform case. This may cause a higher sensitivity to signaling errors. EURASIP Journal on Applied Signal Processing In this paper, a simple bit loading algorithm, where the constellation size and power levels are constrained to be uniform for all used subcarriers, is proposed and extended to the multilayer case to perform UEP of layered video sources at the modulation level. This technique is compared with UEP (...truncated)