Near-Optimal Nonlinear Forwarding Strategy for Two-Hop MIMO Relaying

Relaying (1–3) has been considered as a paradigm for improving the quality of service (i.e., bit-error-rate, data rate and coverage) in wireless networks. In this work, we study a two-hop relay channel in which each node can have multiple antennas. It is well-known that utilizing multiple-input multiple-output (MIMO) links can significantly improve the transmission rate (see e.g. (4; 5) and references therein). Thus, one can expect a combination of aMIMOgain and a relaying gain in a MIMO relay link. We focus on one-shot transmission, where the channel is used once for the transmission of one symbol representing a message. This is often referred to as uncoded transmission. The mainmotivation for such a scenario is in considering applications requiring either low-delays or limited processing complexity. The capacity of the MIMO relay channel is studied in (6). The work in (9) establishes the optimal linear relaying scheme when perfect CSI is available at the nodes. The work in (7; 8) investigates linear relay processing for the MIMO relay channel. In this paper, in contrast to (6–9), we study an uncoded system, and we propose a nonlinear relaying scheme which is superior to linear relaying and performs close to the theoretical bound. Our proposed scheme is based on constellation permutation (10; 11) at the relay over different streams obtained by channel orthogonalization. We investigate a two-hop MIMO fading Gaussian relay channel consisting of a source, a relay and a destination. We assume that all three nodes have access to perfect channel state information. We propose a nonlinear relaying scheme that can operate close to the optimal performance. The proposed scheme is constructed using channel orthogonalization by employing the singular value decomposition, and permutation mapping. We also demonstrate that linear relaying can amount to a significant loss in the performance.