HAP-based FSO system using all-optical detect-amplify-and-forward relaying and coherent detection receiver

In this paper, we propose a high-altitude platform (HAP)-based free space optical communication (FSO) system using all-optical relaying to connect two ground stations (GSs) via a HAP. Optical detect-amplify-and-forward is carried out at the HAP thanks to the use of an optical hard-limiter and an optical amplifier. In addition, coherent detection is applied at the GS's receiver to improved the receiver sensitivity. The closed-form expression for the bit error rate (BER) of the proposed FSO system is deduced assuming that the atmospheric channel is modeled as Gamma-Gamma distribution. The achievable BER with low values has proved the feasibility of our proposed system. In addition, the advantage of using coherent detection compared to the direct detection is demonstrated in term of power gain. Other useful information for system design regarding the required transmitted power, the local oscillator power, and the optimum threshold power is also provided in this paper.