Interference alignment-like precoder design in multi-pair two-way relay cognitive radio networks

In this paper, we 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. We 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 our algorithm. Our proposed algorithm can also work in the scenario where the number of antennas at relay node is not large enough and therefore zero-forcing is not possible. The effectiveness of the proposed algorithm is illustrated via simulations and compared with zero-forcing and MSE based designs.