A Study on Mimo Beamforming for Wireless Communication Systems in Frequency-selective Fading Channels

In recent years, multiple-input multiple-output (MIMO) antennas systems have quickly become an inevitable wireless technology not only for WLANs but also for mobile networks. Based on the benefits of using multiple transmit antennas and multiple receive antennas, MIMO systems can provide high quality and high speed services. However, for high speed transmission, frequency-selective fading (FSF) is a factor that will degrade the system performance and may cause significant link failure in a broadband wireless communications environment. Two architectures have been investigated for MIMO system to mitigate the effect of FSF. The first architecture is transmission of multiple data streams through spatial multiplexing such as MIMO OFDM (Orthogonal Frequency Division Multiplexing), MIMO SDM (Spatial Division Multiplexing). The second architecture is transmission of a single data stream such as MIMO system based on the transmit diversity technique or using Tapped Delay Line (TDL) structure. Unfortunately, those proposed schemes for both single and multiple data streams in FSF are still a lot of complexities concerning the configuration. In principle, they are not yet to show how many delayed channels should be cancelled by a MIMO system. This dissertation gives insight into the weights determination scheme for a simple MIMO beamforming configuration with assumption of perfect channel state information at both the transmitter and receiver. Based on our proposed iterative update algorithm, the optimal transmit and receive weight vectors are determined to apply to MIMO frequency-selective fading channels that will maximize the output SINR (Signal to Interference and Noise Ratio) and show maximum number of delayed channels cancellation. Firstly, the dissertation presents an issue problem of a single data stream transmission using MIMO beamforming scheme. An iterative update algorithm for the transmit and receive weight vectors is determinated in a single user case. Using this method, the degree of freedom (DOF) of both the transmit and the receive antennas are joined together to suppress the delayed channels and maximize the output SINR. The maximum number of delayed channels cancellation is analyzed based on the DOF and confirmed by the computer simulation results. Next, an iterative update algorithm for transmit and receive weight vectors for multiuser system has been studied to apply to MIMO frequency-selective fading channels. Based on the proposed iterative algorithm, improvement of output SINR and maximum number of delayed channels cancellation are ascertained by our analysis and results of computer simulation. Finally, based on assumption of the delayed channels are known as well as the desired channel, a spatial-temporal adaptive MIMO beamforming scheme has proposed to improve the system performance by utilizing multiple delayed versions of the transmitted signal. Numerical results demonstrate in comparison with simulation results to validate the analysis.