Wave propagation and slowing the time-fluctuating MIMO channel

It has been reported [1] 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. In this paper the signal incident at the mobile is represented as a sum of plane waves. The channel transfer matrix is found to factor into time-dependent and time-independent parts. The preprocessing method is proposed, consisting of beam forming, followed by Doppler compensation for 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 a "slowed down" channel fluctuation due, to mobile motion. The coherence time of the pre-processed channel is found to increase by a factor on the order of the number of mobile receive antennas. These results remove constraints imposed by training requirements on the number of transmit antennas and increase allowed vehicle speeds.