Slowing The Time-Fluctuating Mimo Channel

It has been reported [1], [2] that the number of transmitters that can be used in a MIMO system is limited by the coherence time of the channel. Furthermore, rapid channel fluctuations degrade the performance of feedback schemes that would otherwise increase the information throughput that may be achieved. 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 in terms of a sum of plane waves. The channel transfer matrix is found to factor into time-dependent and time-independent parts. The result is found to be suggestive of a processing method whereby the received signals are first pre-processed so as to produce signals that fluctuate on a much slower time scale. The pre-processing consists of beam- forming, followed by Doppler compensation for signals received on each beam. Both operations are non-singular and do not alter the capacity of the MIMO channel. This is shown to effectively "slow down" the 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. In the limit of an infinite number of mobile receive antennas, the MIMO channel is shown to become static. These results remove constraints imposed by training requirements on the number of transmit antennas and increase allowed vehicle speeds. The "slowed down" channel characteristic may also be fed back to the transmitter, allowing an increase in information throughput. .