Relay-aided interference alignment for bidirectional communications in multi-pair multi-relay networks

In this paper, bi-directional communication between K node pairs is considered. Each node has N antennas and wants to transmit d data streams to its communication partner. Q relays with R antennas each aid in their communication. Two-way relaying is assumed. The nodes do not have transmit channel state information and the relays do not have enough antennas to spatially separate the data streams. Taking these two constraints into account, the relays and the receive filters are designed based on two different objectives. In the first case, the relays cooperate with each other to design their filters such that interferences are aligned at the receivers. The receive filters are simple zero forcing filters that nullify the interferences. In the second case, the relays and the receive filters jointly minimize the mean square error (MMSE) at the receivers. Through simulation results it is shown that both schemes achieve the same number of degrees of freedom in the system. However, the MMSE based scheme has better sum rate performance than the interference alignment based scheme.