Scalable algorithms for joint beam and null-forming using distributed antenna arrays

We consider the problem of multicasting a common message signal to a set of designated receivers from a distributed antenna array, while simultaneously forming nulls to another set of null targets. We propose an algorithm under which each transmitter in the distributed array iteratively makes an incremental adjustment to its complex gain (which controls the amplitude and phase of its transmitted RF signal). The cumulative effect of these incremental adjustments is that the individual transmitted RF signals from the transmitters add up at the intended receivers to a desired SNR level, while simultaneously canceling each other perfectly at the null targets. A crucial feature of this algorithm is that it can be implemented in a purely distributed fashion at each transmitter using only an estimate of its own channel gain to each receiver, and an aggregate feedback signal from each of the receivers that is broadcast to all the transmitters. This is an important advantage of our approach and assures its scalability; in contrast any non-iterative approach to the beam/nullforming problem requires knowledge of all channel gains - from all transmitters to all receivers - to be available at every transmitter.