Date Added: Nov 2012
In this paper, the authors design relay precoder in a MIMO cognitive radio network where two-way transmission of multiple secondary user pairs occurs concurrently with primary network's transmission. They propose an interference alignment like precoder design which jointly aligns the direction of interference and the desired signal while interference to primary network is completely canceled. When the secondary transmitters and receivers are equipped with multiple antennas, sources, relay and receivers can all participate in aligning the interference and signal directions. It is shown that zero-forcing relay beamforming in which inter-pair interference is aligned to the null space of desired signal space is a special case of their algorithm.