Research On Adaptive Subcarrier Allocation For Multiuser MIMO-OFDM Systems And Performance Of Wireless Cooperative Communication Systems

The next-generation mobile communication systems are required toprovide high data-rate, high reliable transmission and guarantee end-to-endQoS. MIMO and OFDM techniques have been comprehensively consideredas the major candidate for next-generation mobile communication systems. Infrequency selective fading channels, adaptive MIMO-OFDM systems are ableto support high reliable and high spectral efficiency communications withlower complexity, and the system resources can adapt to the variations ofwireless channels by adjusting various parameters dynamically. Adaptivesubcarrier allocation technique is employed in multiuser OFDM applications,which assigns the subcarriers among users according to their channelconditions. A large amount of researches have shown that, adaptive subcarrierallocation technique can enhance the capacity or improve the performancesignificantly by exploiting multiuser diversity resulting from users’ randomlocations. Based on multiuser MIMO-OFDM systems, the research ofadaptive subcarrier allocation has been carried out.Unlike the subcarrier allocation for SISO-OFDM systems, inMIMO-OFDM scenarios, corresponding allocation schemes or algorithms forspecific transmission strategies and design goals should be developed. Thedissertation provides systematic solutions for multiuser MIMO-OFDMapplications, including the allocation criteria and algorithms for differentstrategies and objectives.Chapter3and4of the dissertation have considered two typical structuresof multiuser MIMO-OFDM systems, namely, capacity maximization and beamforming. Aiming at capacity enhancement, chapter3proposes differentsubcarrier allocation criteria under different SNR levels. Analyses andsimulation results show that, adaptive subcarrier allocation is capable ofimproving the capacity of the systems. Besides, algorithm that givesconsideration to fair subcarrier allocation among users is also addressed. Inchapter4, it is shown that compared with fixed allocation mode, the presentedscheme based on beamforming can improve the performance significantly.Although space diversity may weaken multiuser diversity by eliminating thefading, adaptive subcarrier allocation may still provide improvement of thesystems. Furthermore, a robust criterion based on partial channel informationis also derived in this chapter.In wireless communication systems，the signal in wireless channels isimpaired by the random fluctuations across space, time and frequency, whilediversity is a powerful technique to combat the fading. Space diversitytechnique is particularly attractive as it can be readily combined with otherforms of diversity, e.g., time and frequency, and still offers dramaticperformance gains under these bases. One of the traditional space diversitytechniques is antenna array, e.g., MIMO technique. However, due to the size,energy and cost limitations, the deployment of antenna array is difficult torealize at users. Recently, a brand-new solution scheme is put forward basedon user cooperation, relay cooperation or antenna cooperation, which is calledcooperation diversity technique. The essence of the technique is that users in agroup sharing their antennas and resource through distributed transmition andprocess to form a virtual array, i.e., a virtual MIMO system. The dissertationdevelops the performance research according to different relay schemes forcooperation diversity.Chapter5of the dissertation has proposed an AF-DF mixed relaycooperation scheme: the source broadcasts the signal to all the relays, relayswhich can decode the signal adopt DF scheme to retransmit the signal, whilethe rest adopt AF scheme for retransmission. Therefore, the high SNR at thedestination of DF scheme is utilized, while the relays which cannot decode the signal are not wasted, which means the availability of the relays isimproved. The performance of DF cooperation systems in the presence ofdifferent co-channel interference is also investigated in this chapter.The outage probability of multi-antenna relay in DF cooperation systemsis developed in chapter6, along with that of minimum routes MRC schemebased on practical QoS. Finally, an adaptive grouping algorithm of relayantennas is presented to improve the system performance.