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In this paper, the potential of hybrid automatic-repeat request (HARQ) algorithm in enhancing the performance of a free- space optical (FSO) communication system, which is impaired by weak turbulence and non-negligible residual pointing error, is explored. In particular, the FSO system is assumed to use orthogonal Hermite-Gaussian (HG) beam patterns to exploit spatial multiplexing of optical radiation. Furthermore, it is assumed that a direct-detection mechanism is utilized at the receiver and that the optical radiation is modulated using pulse-position modulation (PPM). Via analytical results and numerical analysis, it is demonstrated that HARQ not only overcomes the impact of spatial error, but also offers a means of achieving near error free communications provided that the delay associate with HARQ can be tolerated. In general, HARQ offers several orders of magnitude improvement in performance.

Hybrid ARQ communications for severely degraded Hermite-Gaussian FSO link