Research On Diversity-Exploiting Technologies In Different Wireless MIMO Communication Systems

After about twenty years of extensive research, multiple-antenna or multiple-input and multiple-output (MIMO) techniques have become one of the most promising tech-nology for many wireless communication systems. It has been adopted by various practical wireless communication systems and standards, for example3G,4G,802.11,802.16, wireless ad hoc network, satellite communications, etc. And it will continu-ously benefit the research and applications in5G and other future wireless systems. MIMO technology provides spatial or polarized degree of freedom1, in addition to the conventional time, frequency or code domain. With this additional degree of freedom, MIMO systems achieve at least two practical and important improvements, diversity gain and multiplexing gain. With the help of diversity, wireless communication system-s can effectively combat the fading effect in wireless channels. Diversity exploitation in MIMO systems, including transmitting diversity, receiving diversity and coopera-tive diversity, has always been one of the most important topic in MIMO research. Although diversity exploiting technologies have already been extensively studied in a-cademia and industry, fast evolution in this area has bring about more and more new challenges, which are still open for further research.In this thesis, after a systematic analysis of the state-of-the-art research results on diversity exploiting technologies, the key practical challenges for diversity exploitation in several typical MIMO applications has been studied based on theoretical analysis, which includes1) low complexity full diversity MIMO detection algorithms1,2) im-pact of nonidealities to transmitter-beamforming based cooperative diversity schemes, and3) system design and performance improvement for orthogonal polarized satellite MIMO systems. Some practical solutions have been proposed and the performance is verified through computer simulations. The main contribution of this work are:1. Considering receiving diversity in MIMO systems, lattice reduction aided (L-RA) detection can achieve near-ML performance with only polynomial complexity, which makes full receiving diversity much more feasible for MIMO, especially when the number of antennas is large. With increasing requirement of higher data rate, the scale of MIMO systems is getting larger and larger, which makes a great leap in com-putational complexity. One of the key focuses in this paper is to exploit full receiving diversity with low complexity based on the improvement of LRA detection. After a detailed study on the well-known Lenstra Lenstra Lovasz (LLL) algorithm, two meth-ods are considered to simplify the lattice reduction process, optimizing the reduction order and approximating the reduction process, i) the original sequential reduction or-der is proved to be non-optimum, which is even worse than a random reduction order. Afterwards, an optimum reduction order criterion is proposed to mitigate the reduction times and achieve same detection performance with lower computational complexi-ty; ii) a new approximated basis vector reordering criterion (ABVR) is also proposed. The full receiving diversity is proved for ABVR aided linear zero forcing (ZF) and minimum mean square error (MMSE) detection and nonlinear successive interference cancellation (SIC) detection through theoretical deduction and computer simulation. The computational complexity of the new lattice reduction algorithms is much lower compared with the original LLL algorithm.2. Considering cooperative transmitting diversity, there are different schemes, for example distributed space time block coding or cooperative beamforming based signal combination. With known channel state information, relays can exploit cooperative transmitting diversity through simple local beamforming, which is quite similar to the signal combination at the receiver. It can avoid complicated problems, such as design of cooperation strategies, which is feasible when nonidealities are not considered. But for real world communications, various nonidealities are probably to destroy the expected cooperative transmitting diversity. The impact of a novel real world nonideality, i.e. the carrier frequency offset (CFO), is analyzed for beamforming based cooperative communications. It is shown through theoretical analysis that even very small CFO can seriously diminish the expected cooperative diversity gain. We also consider a beamforming spatial reuse scheme based on IEEE802.11s-MAC-protocol-compatable station cooperation scheme to improve the capacity for wireless mesh network.3. Recently, MIMO technologies are also explored in mobile satellite commu-nication systems to provide multiplexing or exploit diversity, but the characteristics of the mobile satellite channel render no spatial degree of freedom. Dual orthogonal polarized MIMO seems the only practical scheme for single satellite MIMO communi-cations. After studying the dual orthogonal polarized satellite MIMO channel model, a new polarized modulation scheme is proposed for dual orghogonal polarized satellite MIMO, in which polarization domain is applied as a resource for modulation and cross-polarization interference are mitigated. With same data rate, the proposed scheme are proved through computer simulation to achieve better diversity gain compared with the other schemes.Diversity exploitation technologies are studied for several typical MIMO systems in this thesis. With considerations on practical problems and challenges, it is important to improve the system performance with careful theoretical analysis and research.