Advanced techniques for MIMO and OFDM systems

When adaptive modulation is employed in orthogonal frequency division multiplexing (OFDM) systems, imperfect channel state information (CSI) at the transmitter could degrade the performance significantly. We analyze the performance of adaptive OFDM when there is noise or delay in CSI. These results show that the degradation is severe for outdoor environments when the Doppler frequency is high. The use of multiple estimates is proposed to mitigate the effect of imperfect CSI.; Multiple-input multiple-output (MIMO) systems provide large capacity in wireless communications. We focus on the capacity of MIMO systems with cochannel interference. Firstly, we prove that for a MIMO system with cochannel interference, it is preferable to have fewer interfering antennas with power unevenly distributed among these antennas. As the interference becomes asymptotically large, a MIMO system with interference can be viewed as a reduced MIMO system with fewer receive antennas. Secondly, we consider the system mutual information of a MIMO interference system, which has multiple users that mutually interfere. Asymptotic performance analysis shows that the system mutual information changes behavior as the interference becomes sufficiently strong. In particular, beamforming is the optimum signaling for all users when the interference is large. We propose several numerical approaches to determine the covariance matrices of the transmitted signals. The global and the iterative gradient projection methods are shown to outperform the other approaches significantly. Thirdly, the optimum signaling problem is studied for MIMO-OFDM interference systems. Both theoretical and simulation results are provided to show that the overall system performance can be improved for cases with strong interference by forcing users to use different subchannels or to decrease the number of streams in some subchannels.; Finally the impact of MIMO on wireless ad hoc networks is investigated. Through the study of ultimate capacity regions, average rate regions and outage capacity regions, we show that the gain from multiple antennas for ad hoc networks is similar as for point-to-point communications. The proposed average rate region also serves as an upper bound on the performance of many existing ad hoc routing protocols.