In this paper, the bit error rate (BER) performance of a subcarrier intensity-modulated (SIM) free-space optical (FSO) communications system using binary phase shift keying (BPSK) is investigated over a K-distributed turbulence channel. First, the performance is analyzed employing a negative exponential turbulence model, and an exact closed-form expression is derived for the BER. Then, it is shown that the probability density function (PDF) of the K distribution can be approximated accurately by a finite sum of weighted negative exponential PDFs. Based on this interesting result and by using the closed-form expression, which is derived for the case of a negative exponential model, an approximate, closed-form expression for the BER of the BPSK-based SIM FSO over a K channel is derived. Moreover, to improve the BER performance, spatial diversity using selection combining (SC) is considered. It is shown that the PDF of the resulting channel irradiance corresponding to the SC diversity scheme over a K channel can be approximated accurately by a finite linear combination of negative exponential functions. The derived approximate PDF accurately estimates the PDF of the channel irradiance for arbitrary values of diversity order and is valid for a wide range of channel parameters. Then, an approximate, closed-form expression is derived for the average BER of the BPSK-based SIM FSO system employing the SC diversity technique over a K channel. Numerical results presented in this paper show that the derived approximate expressions are very accurate and can be used as efficient tools for performance analysis of the system.
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