Adaptive transmission for two-way relaying optical wireless communications

This paper proposes an adaptive transmission scheme for two-way amplify-and-forward (AF) relaying over optical wireless communications. The two terminals utilize subcarrier phase shift keying (S-PSK) intensity modulation for data transmission, and the atmospheric turbulence impacts the transmitted signals. The proposed transmission scheme exploits the efficient utilization of the optical wireless channel capacity by adjusting the modulation order of S-PSK. While meeting predetermined bit error rate (BER) requirements for each terminal, the selected modulation order of each terminal corresponds to the instantaneous condition of turbulence-induced fading. A new expression is derived for the spectral efficiency of the proposed two-way relaying scheme, and then performance investigations are carried out under various turbulence conditions. Moreover, we derive a closed form expression for the BER performance and analyze the proposed system model from the outage probability point of view. Numerical examples illustrate that the proposed two-way relaying scheme enjoys better spectral efficiency compared with the traditional direct transmission scheme in the high signal-to-noise-ratio regime. The simulation results show that for a fixed turbulence, the TWR saves the electrical signal-to-noise-ratio (SNR) about 2.5 dB compared to the direct transmission scheme when the target spectral efficiency is set to 1.2 bits/s/Hz.