Virtual-MIMO systems with compress-and-forward cooperation

TLDR: This thesis investigates a virtual-MIMO wireless syste m using the receiver-side cooperation with the compress-and-forward (CF) protocol, and proves that the smallest singular value of the cooperative channel matrix determine s th system error performance.

Abstract: Multiple-input multiple-output (MIMO) systems have recen tly emerged as one of the most significant wireless techniques, as they can greatly improv e the channel capacity and link reliability of wireless communications. These benefits have e ncouraged extensive research on a virtual MIMO system where the transmitter has multiple ante n as and each of the receivers has a single antenna. Single-antenna receivers can work togeth er to form a virtual antenna array and reap some performance benefits of MIMO systems. The idea of re ceiv r-side local cooperation is attractive for wireless networks since a wireless receiv er may not have multiple antennas due to size and cost limitations. In this thesis we investigate a virtual-MIMO wireless syste m using the receiver-side cooperation with the compress-and-forward (CF) protocol. Firstly, to p erform CF at the relay, we propose to use standard source coding techniques, based on the analy sis of its expected rate bound and the tightness of the bound. We state upper bounds on the syste m error probabilities over block fading channels. With sufficient source coding rates, the co operation of the receivers enables the virtual-MIMO system to achieve almost ideal MIMO perfor mance. A comparison of ideal and non-ideal conference links within the receiver group is also investigated. Considering the short-range communication and using a channel-aware adapt ive CF scheme, the impact of the non-ideal cooperation link is too slight to impair the syste m performance significantly. It is also evident that the practicality of CF cooperation wi ll be greatly enhanced if a efficient source coding technique can be used at the relay. It is even mo re desirable that CF cooperation should not be unduly sensitive to carrier frequency off sets (CFOs). Thus this thesis then presents a practical study of these two issues. Codebook des igns of the Voronoi VQ and the tree-structure vector quantization (TSVQ) to enable CF coo peration at the relay are firstly described. A comparison in terms of the codebook design comple xity and encoding complexity is presented. It is shown that the TSVQ is much simpler to desi gn and operate, and can achieve a favourable performance-complexity tradeoff. We then dem onstrate that CFO can lead to significant performance degradation for the virtual MIMO syste m. To overcome it, it is proposed to maintain clock synchronization and jointly estimate the CFO between the relay and the destination. This approach is shown to provide a significant per formance improvement. Finally, we extend the study to the minimum mean square error (MMSE) detection, as it has a lower complexity compared to maximum likelihood (ML) dete ction. A closed-form upper bound for the system error probability is derived, based on w hich we prove that the smallest singular value of the cooperative channel matrix determine s th system error performance. Accordingly, an adaptive modulation and cooperation scheme i s proposed, which uses the smallest singular value as the threshold strategy. Depending on the i nstantaneous channel conditions, the system could therefore adapt to choose a suitable modula tion type for transmission and an appropriate quantization rate to perform CF cooperation. T he adaptive modulation and cooperation scheme not only enables the system to achieve comparab le performance to the case with fixed quantization rates, but also eliminates unnecessary c omplexity for quantization operations and conference link communication. Declaration of originality I hereby declare that the research recorded in this thesis an d the thesis itself was composed and originated entirely by myself in the School of Engineering a t The University of Edinburgh.