Research On Adaptive Subcarrier Allocation For Multiuser MIMO-OFDM Systems

The next-generation mobile communication systems are required to provide high data-rate, high reliable transmission, and guarantee end-to-end QoS. MIMO, OFDM, and adaptive transmission have been comprehensively considered as the major candidate techniques for future radio communication systems. In frequency selective channels, adaptive MIMO-OFDM systems are able to support high reliable and high spectral efficiency communication with low complexity, while it can adjust various parameters dynamically so that the system resources adapt to the variations of wireless environment. Adaptive subcarrier allocation technique is employed in multiuser OFDM applications, which assigns the subcarriers among users according to their channel conditions. A large amount of researches have shown that adaptive subcarrier allocation can improve the performance or enhance the capacity significantly by exploiting the multiuser diversity resulting from users'random locations. However, to the best of our knowledge, the researches for its counterpart in MIMO scenarios are scarce yet. This motivates us to investigate the adaptive subcarrier allocation problems for MIMO-OFDM systems. Our work is supported by the National High Technology Research and Development Program of China under Grant No. 2001AA123014 and National Science Foundation of China under Grant No. 603905405. Unlike the subcarrier allocation for SISO-OFDM systems, in MIMO-OFDM scenarios, we should develop corresponding allocation criteria or algorithms for specific transmission strategies and design goals. There are three questions we will keep in my mind through the dissertation, that is, what criteria we should choose, how much gain we can obtain, and how to make them more feasible? In this dissertation, we will provide systematic solutions for multiuser MIMO-OFDM applications, including the allocation strategies for different signaling and objectives. For many problems confronted in real systems, we also give detailed discussion and corresponding way to solve it. To be specific, we have considered multiuser OFDM systems based on three typical MIMO structures, namely, beamforming, space-time coding and spatial multiplexing. With the goal of minimizing the total transmit power, we have proposed the subcarrier assignment criteria for the former two MIMO systems which are designed to provide high reliable transmission. It is shown in analyses and simulations that, compared with fixed allocation schemes, our presented criteria will improve the performance significantly. Although array gain and diversity gain may weaken multiuse diversity by eliminating the fading, adaptive subcarrier allocation may still provide much improvement for beamforming and space-time coding based OFDM systems. Furthermore, we discuss the value of combining bit loading with our adaptive subcarrier allocation. A robust criterion based on partial channel information is derived and an allocation algorithm which can fulfill different users'QoS requirements is given as well. For spatial multiplexing base OFDM systems, we propose the subcarrier allocation criteria aiming at performance improvement and capacity enhancement, respectively. These schemes are suitable to the well-known V-BLAST system, and we can derive corresponding scheme under both ZF and SIC detection. From the analyses and simulation results, we can conclude that adaptive subcarrier allocation is more effective in spatial multiplexing systems than in beamforming or spatial diversity based OFDM systems. By selecting appropriate criterion, adaptive subcarrier allocation is capable of enhancing the capacity of spatial multiplexing base OFDM systems. Two categories of subcarrier assignment criteria are derived, that is, exclusive and shared criteria. Both of them can make efficient use of multiuser diversity to exceed fixed allocation in terms of spectral efficiency remarkably. Shared criteria will undoubtedly bring more gain, while exclusive one seems to be a feasible choice to achieve tradeoff among complexity, overhead, and capacity enhancement. Besides, we discuss some practical methods to reduce the overhead and complexity for adaptive subcarrier allocation, and the algorithm that guarantees fair allocation among users is also addressed. From the dissertation we can find that, if correct subcarrier allocation criteria are chosen for specific transmission strategies, multiuser diversity inherent in environment will be utilized effectively to provide system-level improvement. In different MIMO systems, due to the various functions of multiple antennas, the gain from multiuser diversity will differ. For the allocation strategies obtained under various scenarios, we all give mathematical analyses and study the impacts of user number and antenna number on them.