Nested Lattice Codes for Gaussian Two-Way Relay Channels

Abstract: In this paper, we consider a Gaussian two-way relay channel (GTRC), where two sources exchange messages with each other through a relay. We assume that there is no direct link between sources, and all nodes operate in full-duplex mode. By utilizing nested lattice codes for the uplink (i.e., MAC phase), and structured binning for the downlink (i.e., broadcast phase), we propose two achievable schemes. Scheme 1 is based on compute and forward scheme of [1] while scheme 2 utilizes two different lattices for source nodes based on a three-stage lattice partition chain. We show that scheme 2 can achieve capacity region at the high signal-to-noise ratio (SNR). Regardless all channel parameters, the achievable rate of scheme 2 is within 0.2654 bit from the cut-set outer bound for user 1. For user 2, the proposed scheme achieves within 0.167 bit from the outer bound if channel coefficient is larger than one, and achieves within 0.2658 bit from the outer bound if channel coefficient is smaller than one. Moreover, sum rate of the proposed scheme is within 0.334 bits from the sum capacity. These gaps for GTRC are the best gap-to-capacity results to date.