Spatial Multiplexing and Antenna Selection in Relay-Assisted Transmissions with Heterogeneous Nodes

This paper considers network information flow in a multiple-input multiple-output (MIMO) wireless network with four users where (i) two primary users equipped with multiple antennas exchange independent messages via a half-duplex relay and (ii) two secondary users with single antenna transmit to and receive data from the relay. This asymmetric traffic flow problem is a specialized version of a multi-pair two-way relay channel where, in the multiple access (MAC) phase, four users send all messages to the relay simultaneously, and, in the broadcast (BC) phase, the relay transmits messages to users also on a single MIMO channel access. When spatially multiplexing two- and one-way data transmissions via a cooperating relay implementing decode-and-forward (DF) processing, the objective is to work with a minimized number of antennas at all nodes in the system. Specifically, this paper presents a design for precoding and nulling vectors in the proposed signal alignment (SA) scheme with zero-forcing (ZF) decoding. In this design, there is a single redundant antenna at the relay during the MAC phase, which is exploited to provide the diversity advantage. The optimum subset of receiving antennas at the relay is selected by a Max-Min criterion with respect to the post-processing SNR. Simulation results show an improvement in terms of BER when optimization is deployed during the antenna selection process.