Transmitter precoding to cancel inter-relay interference in AF systems with successive transmissions

This paper presents an inter-relay interference (IRI) cancellation scheme in amplify-and-forward (AF) cellular-type networks with successive downlink forwarding. In these half-duplex alternate relaying systems with two hops, the wireless medium in both hops is always utilized through the simultaneous transmissions of the base station (BS) and one of the relays which in turn causes IRI. In this work, the processing to mitigate IRI is performed exclusively at the BS using channel state information (CSI) between the BS and the multiple relays supporting corresponding receivers. The proposed linear precoding exploits the principles of signal alignment at the BS and considers recursive characteristics of IRI. The scheme performance is affected by the noise accumulation which is controlled in this paper by (i) scheduling relays as to benefit from spatial attenuation of signals and (ii) periodically re-initializing BS transmissions where some of the time slots are not utilized for the BS transmissions. Simulation results show the ability of the new scheme to fully cancel IRI as well as demonstrate the performance trade-offs between the bit error rate (BER) improvements and the loss in throughput efficiency due to flushing.