Spatial Modulation Exploited in Non-Reciprocal Two-Way Relay Channels: Efficient Protocols and Capacity Analysis

In this paper, the concept of spatial modulation is exploited in two-way relay channels, where two single-antenna source terminals exchange information through an assisting multi-antenna relay node, and thereby two novel transmission strategies are proposed based on the decode-and-forward protocol, aiming to achieve high throughput. One strategy is to simply engage spatial modulation in the transmissions of the relay node, whereas the other is to develop transmit diversity with spatial modulation for further benefiting from the feature of two-way relaying. Neither of the strategies requires channel state information at the transmitter and, therefore, the assumption of channel reciprocity that is commonly used in two-way relay channels can be released herein. To evaluate the performance of both proposed strategies, their capacity behaviors are analyzed and numerical comparisons are established in the metrics of achievable rate region and sum-rate. Subsequently, the first strategy, simple spatial modulation, is generalized into the two-way relaying with multi-antenna source terminals to improve the flexibility in system design and demonstrate the tractability of spatial modulation. Illustrative discussions on bit error rate not only substantiate the validity of the proposed strategies but also provide useful tools for the two-way relaying layout without the restriction of channel reciprocity.