Abstract
According to its distinctive feature of spatial orthogonality, multiple-input multiple-output (MIMO) transmitter beamforming can improve spatial multiplexing gain, spatial diversity gain, or signal-to-noise ratio (SNR)/signal-to-interference-plus-noise ratio (SINR) without requiring extra channel bandwidth and transmission power comparing with single-antenna based scenarios. In the literature, MIMO transmitter beamforming has been quite well investigated. However, it is realized that the existing schemes can achieve the requirements only when the transmitter-side channel state information is assumed, or, with the limited channel feedback, the number of users in the cell should be much larger than the number of transmit antennas, where the multiuser diversity can be well exploited. Otherwise, significant performance loss will be observed. Furthermore, in dense cellular networks, multi-cell coordinated beamforming plays a prominent role for interference pre-cancellation. However, most of coordinated beamforming schemes can only handle the optimization problems with single constraints, and the optimization process is significantly complicated if the objective functions involve the increased number of constraints. Motivated by these, the major contributions of this thesis are in two folds: A joint space-frequency user scheduling approach to enhance opportunistic beam-forming with a small number of users in the cell is proposed. With the limited channel feedback to be assumed, two complexity reduced joint space-frequency user scheduling algorithms are proposed in order to maximize the system throughput and the number of active users. Simulation results show that the proposed SINR-based algorithm can largely improve the system throughput of conventional opportunistic beamforming up to 80 more, when the individual user’s rate constraint is set up to 1 bit/s/Hz. Moreover, the performances of proposed SNR-based algorithm is even better with the pay of relatively increased computational complexity and signaling overhead. A novel coordinated beamforming approach for multi-cell MIMO downlink communication networks is proposed with the aim to maximize the system sum-rate subject to per transmitter power and individual user’s rate constraints. The idea is basically a two-step iterative optimization procedure. Numerical results show that the proposed approach outperforms the existing schemes up to 3. 2 bits/s/Hz per transmitter in terms of spectral efficiency. Moreover, by comparing with the optimal solution for finding the initial state at the first step, the proposed solution can exhibit close performance with faster convergence and lower computational complexity.