Full-Duplex Mixed RF/FSO using Multiple Relays with Self-Interference

In this paper, two transmission strategies for multiple relay-based full-duplex (FD) mixed radio-frequency/free space optical (RF/FSO) system are proposed and analyzed while considering the effect of self-interference at the relay nodes and RF source node. Unlike existing relay selection (RS)-based techniques, the proposed strategies do not require relay selection phase, as all relays are always active. Therefore, the proposed strategies preserve the time required to select the best relay, leading to improved throughput of the systems. The first strategy transmits the information from all relays to the RF source node with uniform power and is therefore termed the uniform transmission (UT) strategy. On the other hand, to examine the absolute limit of FD mixed RF/FSO system, the second strategy utilizes transmit beamforming (TB) instead of UT strategy. The analytical expression for outage probability (OP) is derived for both strategies considering Rayleigh channel for RF links and Gamma-Gamma channel with pointing error for FSO links. Furthermore, the upper bound-achievable sum-rate (UB-ASR) is examined after self-interference cancellation. Numerical results show that the proposed strategies outperform existing RS strategies in terms of the OP and UB-ASR.