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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...