Wave propagation and slowing the time-fluctuating MIMO channel

It has been reported (Marzetta, T.L., Proc. 37th Annual Allerton Conf. on Commun., Control, and Computing, 1999) that the number of transmitters that can be used beneficially in a MIMO system is limited by the coherence time of the channel due to difficulties with channel estimation at the receiver. These impediments either reduce achievable capacity or impose an effective "speed limit" on the mobile, above which the effective throughput is reduced. We represent the signal incident at the mobile as a sum of plane waves. The channel transfer matrix is found to factor into time-dependent and time-independent parts. The preprocessing method proposed consists of beam forming, followed by Doppler compensation for the signals received on each beam. Beam forming effectively partitions the angular spectrum at the mobile, with each partition suffering a smaller Doppler spread, resulting in the channel fluctuation due to mobile motion being "slowed down". The coherence time of the preprocessed channel is found to increase by a factor of the order of the number of mobile receiving antennas. These results remove constraints imposed by training requirements on the number of transmit antennas and increase the allowed vehicle speeds.