Experimental investigation of transmission diversity and reception aperture averaging for terrestrial near-field long-distance free-space optical turbulence residence

Atmospheric turbulence is a critical factor affecting the performance of free-space optical communications, as it causes fading and sudden errors, significantly degrading the bit error rate and overall communication performance of On-Off keying systems. Many approaches have been proposed by researchers to suppress atmospheric turbulence effects, among which large aperture reception and transmitting spatial diversity are considered the simplest and effective ways. In this paper, we experimentally investigate the aperture averaging effect with a 250-mm receiver and the transmitting spatial diversity with 12 transmitters in a terrestrial near-field, long-distance environment. In both cases, the scintillation index is reduced from 0.3916 to 0.0107 in a 7-km experiment and from 0.4431 to 0.0328 in a 13-km experiment, which can theoretically yield a signal-to-noise ratio gain of about 7 dB for On-Off keying signals with a bit-error rate of 1 × 10⁻³. Moreover, we discuss the transmitting diversity gain loss, considering both channel correlation and averaged receiving power. Additionally, for the calculation of $C_n^2$ in moderate-to-strong turbulence, we propose a method based on gamma-gamma model using experimental results.