Adaptive modulation and coding can provide robust and spectrally efficient transmission over terrestrial free-space optical channels. Three adaptive modulation schemes are considered in this paper: (i) variable-rate variable-power adaptation, (ii) channel inversion, and (iii) truncated channel inversion schemes. It is shown that a simple channel inversion scheme performs comparable to a variable-rate variable-power adaptation scheme in the weak turbulence regime but faces significant performance degradation in the strong turbulence regime. We further study adaptive coding based on large-girth quasi-cyclic low-density parity-check- (LDPC-) coded modulation. It is shown by simulation that deep fades of the order of and above in the regime of strong turbulence can be tolerated with the proposed scheme. It is demonstrated that communication in the saturation regime is possible with the proposed adaptive LDPC-coded modulation. We also determine the spectral efficiencies for the proposed adaptive modulation and adaptive coding schemes.
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