A Closed-Form Approximate BER Expression for Space-Time Block Coding in Free-Space Optical Systems over Negative Exponential Atmospheric Turbulence Channels

In this paper, we derive a novel and effective approximate expression of bit error rate (BER) for space-time block coded (STBC) on-off keying (OOK) in free-space optical (FSO) systems, which is known to improve the BER performance with the increase of peak-to-average optical power ratio (PAOPR), especially over strong atmospheric turbulence channels. By using the derived BER formula, the corresponding asymptotic analysis on the BER in high signal-to-noise ratio (SNR) regime is also explored, the accuracy of which is verified via several numerical results. This further allows us to obtain the explicit insights on the achievable diversity order and coding gain.