Spectral efficiency analysis of multi-pair two-way full-duplex relay systems with direct link

In this paper, we consider a multi-pair two-way full-duplex relaying system where each pair of users exchange information with the aid of an amplify-and-forward relay with massive number of antennas. In this work, we assume the direct link between a pair of users is non-negligible, and propose a transmission scheme that captures the joint benefits of relay and direct links. First, we derive the communication model and the sum spectral efficiency of the proposed system. Then, as a benchmark, two transmission modes are considered: i) the mode where the direct link is considered as interference (DLI mode) and ii) the device-to-device (D2D) mode where the relay is silent. Finally, using analytical evaluation, we compare the proposed transmission mode with the DLI and D2D modes, and study the impact of the transmit power on the system spectral efficiency. Analytical results show that the proposed transmission mode, the DLI mode, and the D2D mode can outperform each other depending on the direct link quality and the packet length; which is confirmed by the presented numerical results.