Uplink-Downlink Duality of Interference Alignment in Cellular Relay Networks

This paper focuses on signal space interference alignment for the uplink and downlink transmissions in a cellular relay network with multiple cells where a single base station serves multiple mobile stations in each cell and several amplifyand-forward relays are deployed. We show that the interference alignment problems in the uplink and downlink transmissions are a pair of formally dual problems. Exploiting the reciprocity of the channels, a two-step procedure first nullifying the intercell interferences following the idea of relay-aided interference alignment and then designing a zero-forcing filter for each base station to suppress the intra-cell interferences is proposed to obtain the dual interference alignment solutions. Furthermore, the dual solutions also achieve the same sum rate in both the uplink and the reciprocal downlink transmissions with a sum transmit power constraint even if the power consumed by the relays and the relay retransmitted noises are considered.