| 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 deal 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.; In the first part of this thesis; several contributions are made to the theory of wide-band 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 auto correlations and baseband spectra of several channel models for some common diversity techniques.; 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.; 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. (Abstract shortened by UMI.)... |
