Receiver-coordinated distributed transmit nullforming with local and unified tracking

A distributed coherent transmission scheme in which a cluster of cooperative transmitters form a beam toward an intended receiver while directing nulls at a number of other “protected” receivers is considered. The receivers coordinate the transmissions by estimating the channels and providing feedback to the transmit cluster to facilitate coherent transmission. Since the effective channels including carrier phase and frequency offsets are time-varying, two tracking schemes are compared: (i) “local tracking” where each receiver independently tracks its own channels from the transmit cluster and (ii) “unified tracking” where one receiver (or transmitter) tracks all of the channels in the system. The results show that, while beamforming performance is effectively unchanged, nullforming performance can be improved with unified tracking, especially over short prediction intervals and for larger networks.