High speed communications over broadband Multi-input Multi-output (MIMO) OFDM system is widely recognized as a leading technology for high speed wireless applications. In a broadband environment, the combination of orthogonal frequency division multiplexing (OFDM) and space time processing is an effective method of combating fading and achieving high transmission rates. Multi-carrier space time processing allows multi-transmit and multi-receive communication through multiple parallel sub-channels at high data rate that can provide diversity gain and spatial multiplexing gain. In many wireless standards, OFDM is considered as a suitable modulation scheme for broadband networks due its ability to combat multipath fading. One of the main advantages of OFDM is that it provides low computational complexity system by transforming the broadband fading channel into multiple parallel narrow band flat fading subchannels. In this dissertation we develop an adaptive MIMO channel estimation algorithm for space-time block coded OFDM systems. The presented algorithm is based on Expectation Maximization (EM) technique by decomposing the superimposed received signals, into their signal components and estimating the channel parameters of each signal component separately. We also study and compare our proposed EM-based algorithm with a previously introduced recursive-least-squares based algorithm for MIMO OFDM systems.

At each iteration the EM algorithm decomposes the problem of multi-channel estimation into channel estimation for each transmit-receive link. In slow time varying environment, the EM algorithm can be used for both estimation and tracking. In a rapidly changing environment, the rapid changes of the channel response due to Doppler spread results in inter-carrier interference (ICI) between the OFDM subcarriers. The amount of ICI is directly related to the spacing of the subcarriers. The closer the subcarriers, the more susceptible is the OFDM system to Doppler spread, resulting in degradation in the system performance. In this dissertation we study the Doppler spread tolerance of our proposed algorithm in a fast fading environment, and investigate how it affects the system BER performance.