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Efficient video transmission over correlated nakagami fading channels for IS-95 CDMA systems Chan, Norman

Abstract

This thesis deals with the problem of efficient transmission of video signals over generalized fading channels in Direct Sequence-Spread Spectrum (DS-SS) Code Division Multiple Access (CDMA) systems. The video codec is based upon the ITU H.263 compression algorithm which targets at providing low bit-rate video telephony services suitable for wireless transmission. In order to reduce the overall impact of errors due to mobile channels on the video sequence, a modified version of the original H.263 codec is proposed incorporating a selective Forward Error Correction (FEC) coding scheme combined with a forced INTRA frame update mechanism. This modified version of H.263 codec provides improvement in both average video and frame-to-frame performance. We further consider a coherent DS - CDMA system based upon the IS-95 standard for the forward link (base-to-mobile) in both single-cell and multiple-cell environment. We provide performance evaluation results by both analysis, employing the Gaussian approximation, and computer simulations, using Monte Carlo error counting techniques. The proposed CDMA system uses concatenated Walsh/maximal-length coding scheme for spectrum spreading. The resulting spread codes maintain orthogonality while reducing inhomogenous cross-correlations among Walsh sequences. The frequency-selective fading channel is modeled by a tapped delay line model with channel coefficients of each path following an independent Nakagami-m distribution. We have implemented in software a correlated Nakagami fading simulator based upon the principle of superposition of complex partial waves with a (possibly strong) component resulting from the direct waves. The time correlation is generated by assigning each partial wave a Doppler shift as a function of time. This approach is an exact replica of the actual physical situation as it reproduces the wave propagation process, as opposed to the Doppler spectrum approximation approach used in other simulators. The received signal is demodulated coherently using a RAKE receiver with variable resolving fingers, where multipath components are maximal-ratio combined'. For our analysis, we assume perfect knowledge of the channel, which could be accomplished either by the usage of pilot tone or some type of channel-parameter estimation circuits. However, for the computer simulation, such perfect knowledge of the channel is not necessary. In terms of performance evaluation results, we first present the improved performance of the modified H.263 codec as a function of Peak Signal-to-Noise Ratio (PSNR) transmitted in additive white Gaussian noise (AWGN) environment. Then, the analytical and computer simulated results for the bit error rate (BER) performance of CDMA forward link in Nakagami fading channels for both single-cell and multiple-cell environment are presented. Further, we present the PSNR performance results for the video transmission featuring the modified H.263 coding scheme over the proposed CDMA systems. Finally, a variety of performance evaluation results, both in single-cell and multiple-cell environment, are presented for different number of resolving paths, signal propagation characteristics, cell user capacity, as well as for the presence of channel estimation errors. In all cases, heuristic explanations and interpretations of the trend of the obtained results are also given.

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