An Efficient Adaptive Channel Estimation Algorithm for MIMO OFDM Systems—Study of Doppler Spread Tolerance Haideh M. Karkhanechi & Bernard C. Levy Received: 26 March 2008 / Accepted: 23 April 2008 / Published online: 4 June 2008 # 2008 Springer Science + Business Media, LLC. Manufactured in The United States Abstract In this paper 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 compo- nents, 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 this paper we also study the Doppler spread tolerance of our proposed algorithm in a fast fading environment, and investigate how it affects the system BER performance. Keywords OFDM . MIMO OFDM . Multi user . EM . Doppler . Channel estimation . Adaptive 1 Introduction Estimating a frequency selective fading channel in wire- less broadband high speed networks is a challenging prob- lem. In MIMO OFDM systems, the channel state information (CSI) is required at the receiver for efficient decoding and accurate signal detection. Channel estimation or identification in MIMO OFDM systems is a topic being recently pursued by many researchers. It is important to provide high signal quality and design a low complexity channel estimation algorithm, considering limitations and impairments of broadband wireless channels such as time- varying, noise, interference, and delay spread. Channel estimation is usually performed by periodically transmitting training sequences (e.g. at the start of each frame). In slowly varying environment, the channel parameters, which are slow varying, maybe estimated using different estimation techniques (e.g., [2–5,7,12,14–16]). Adaptive iterative channel estimation algorithms can effectively estimate and track the channel, with higher bandwidth efficiency and lower system overhead com- pared to conventional methods, where the channel estimation is done by inserting training sequences in each data block. Adaptivity feature in a channel estimation algorithm usually results in computational gain due to tracking capability. In adaptive techniques the channel is initialized using first few training blocks and has the advantage of tracking channel variations by using the previous estimate for initializing the current estimate. Therefore adaptive techniques have the ability to follow changes in the channel impulse response. In this paper we develop and design a low complexity adaptive Expectation Maximization (EM) based channel estimation algorithm specifically for space time block coded MIMO OFDM systems, based on decomposing the observed signals at the receiver into its signal components and estimating the channel parameters of each signal component separately. This technique exploits the structure of Alamouti STBC codes [1] to reduce complexity at the receiver. At each iteration the EM algorithm decomposes the problem of multi-channel estimation into channel estimation for each transmit–receive link [7]. J Sign Process Syst (2009) 56:261–271 DOI 10.1007/s11265-008-0228-0 H. M. Karkhanechi (*) : B. C. Levy Department of Electrical and Computer Engineering, University of California, Davis, Davis, CA, USA e-mail: hmkarkha@ece.ucdavis.edu B. C. Levy e-mail: levy@ece.ucdavis.edu