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Variable bit rate video transmission for code-division multiple-access systems in wideband fading channels Iskander, Cyril-Daniel

Abstract

Efficient real-time transmission of video data over bandwidth-constrained wireless channels is challenging in several ways: in particular, due to the underlying compression algorithms, the source rate can vary in bursts, which complicates the resource allocation problem, isolated channel errors can totally corrupt a video frame if sensitive information is affected, and errors in earlier frames can cause damage to later frames due to error propagation. This thesis will dear in particular with the effect of source rate variability on current and future cellular systems which employ code-division as the multiple-access strategy, such as IS-95B and IS-2000 systems. The problem will be approached from a physical-layer perspective: hence issues relating to the channel- and cellular-level performances will be addressed in detail, and then integrated into the system-level performance. This nonconventional cross-layer approach allows us to obtain additional insights over studies which tackle the issue mainly or exclusively at the higher system layers. In the first part of this thesis, several contributions are made to the theory of wideband fading channels, which will be considered as the physical channel model throughout the thesis. We derive the analytical level-crossing rates, average fade durations, envelope autocorrelations and baseband spectra of several channel models for some common diversity techniques. Based on some of the previously derived properties we design a fast wideband Nakagami channel simulator. We then derive the exact analytical error probabilities of several linear modulation schemes with diversity in correlated Nakagami channels, and validate them through simulation. In a second part, we derive accurate analytical or semi-analytical error probability expressions for the multicode and multirate configurations used in the physical layers of both the uplink and downlink of IS-95B and IS-2000 systems, in the presence of wideband fading. It is demonstrated that the effect of the multicode interference must be precisely taken into account to obtain reliable error statistics in wideband channels, especially for cellular systems with a low number of users. To this end, the fading dependence across multiple codes of a given user must be taken into account in the analysis, whereas for single-code systems this situation didn't occur. We consider systems which employ either maximal-ratio or equal-gain combining. The proposed methodology places no restrictions on the type of fading distribution, and examples are given for the cases of Rayleigh, Rice, Nakagami and lognormal fading, for both independent and correlated diversity branches. For the IS-95B uplink, the analysis is extended to deal with closed-loop power control using the inverse update algorithm, successive interference cancellation, and multicell systems. All analytical results are thoroughly validated through numerous entire system simulations, for different values of several transceiver and channel parameters. In the final part of this thesis, we demonstrate the benefits of employing rate smoothing for variable bit rate video applications in DS/CDMA cellular systems, and present and evaluate practical algorithms to achieve these gains. To support our exposition, a generic rate smoothing algorithm is developed, whose goal is to minimize the degradation caused by source bursts in such systems. Its performance in terms of decoded video quality is compared to that of a popular algorithm which was developed in the context of wireline communications, and which serves as a benchmark. It is shown that for systems subject to certain practical constraints, in particular concerning the granularity of the transmission rates, the proposed algorithm can offer an improved decoded video quality with respect to the benchmark algorithm. The influence of smoothing-related parameters such as the startup buffering delay and sliding window length are quantified. In addition, the effects of some transceiver and channel parameters on the decoded video quality are presented. To carry out these performance evaluations, a flexible software platform has been developed which emulates the transmission of video data at the physical/link layers in IS-95B and IS-2000 cellular systems with wideband fading, and allows the user to objectively measure the decoded video quality directly at the application layer.

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