Precoder design for asymmetric two-way AF shared relay

Two-way relaying (TWR) reduces the loss in spectral efficiency caused in a conventional half-duplex relay. TWR is possible when two nodes exchange data simultaneously through a relay. In the case of cellular systems, data exchange between base station (BS) and users is usually not symmetric, e.g., a user might have uplink data to transmit during multiple access (MAC) phase, but might not have downlink data to receive during broadcast (BC) phase. This asymmetry in data exchange will reduce the gains of TWR. With infrastructure relays, where multiple users communicate through a relay, the BC phase following the MAC phase of a transmitting user (UE1) can be used by the relay to transmit downlink data to a second user (UE2). This will result in the receiving user UE2 not being able to cancel the back-propagating interference in the usual way. Precoders are designed in [1] to mitigate the back-propagating interference at UE2 for an amplify-and-forward (AF) relay. The present work studies the asymmetric data-flow problem for a shared AF relay, wherein multiple BS and users communicate using a common relay with multiple antennas. In this case, UE2 will observe inter-user interference (IUI) in addition to the back-propagating interference. Also, BS will now observe the IUI. We propose a precoder to jointly mitigate the back-propagating interference for UE2 and IUI for BS and UE2. It is shown that the sum-rate performance is better for the proposed precoder than the conventional zero-forcing precoder.