This paper studies the design and performance analysis of relay-assisted satellite freespace optics (FSO) quantum key distribution (QKD) systems for secure vehicular networks. High-altitude platforms (HAPs) equipped with optical amplify-and-forward nodes are used as relay stations. Secrecy performances in terms of quantum bit error rate and ergodic secret-key rate are analytically investigated under the effects of transceiver misalignment, receiver's velocity variation, receiver noises, and atmospheric turbulence conditions. Based on the analyzed results, the design criteria for the legitimate user are determined so that the security of the considered system could be guaranteed. INDEX TERMS Atmospheric turbulence, high-altitude platform (HAP), free-space optics (FSO), pointing error, quantum key distribution (QKD), reciever's velocity variation.
In this paper, we investigate the performance of a high-altitude platform (HAP)-based relaying free-space optical communication (FSO) system, where HAP plays a role as a relay node that employs detect-and-forward relaying scheme to connect between two ground stations (GSs).
$M$-ary pulse-position modulation (PPM), spatial diversity, and heterodyne detection receiver are used to improve the system’s performance. Instead of using multiple-input multiple-output (MIMO), which makes the HAP complex, multiple-input single-output (MISO) is applied to the uplink while single-input multiple-output (SIMO) is utilized for the downlink. Consequently, the HAP only needs a couple of transmit and receive aperture. The expression for the bit error rate (BER) of the proposed FSO system is derived considering the impact of atmospheric attenuation and turbulence. The achievable BER with low values has proved the feasibility of our proposed system. In addition, the advantages of using
$M$-PPM, spatial diversity, and heterodyne detection are demonstrated in terms of the power gain and the geometric distance between two GSs. Other useful information for system design regarding the required transmitted power, the number of transmit/receive apertures, the modulation level, and the local oscillator power is also provided in this paper.
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