Experimental evaluation of the effect of aperture averaging technique on the performance of free space optical communication link for different intensity modulation schemes

Atmospheric turbulence causes severe degradation in the performance of free space optical (FSO) communication links. Among the various techniques used to mitigate the effect of turbulence, aperture averaging is one of the simplest. We experimentally investigate the effect of aperture averaging on the performance of FSO communication links for different intensity modulation (IM) schemes namely on-off keying (OOK), pulse position modulation (64-PPM) and differential pulse position modulation (64-DPPM). Atmospheric turbulence is artificially generated within an optical turbulence generator (OTG) chamber and for the above mentioned IM schemes beam variance measurements are carried out for different temperature induced turbulence conditions. The value of beam variance is used to determine the refractive index structure parameter and hence the Rytov variance. Finally, the bit error rate (BER) in presence of aperture averaging for the above modulation schemes is calculated. It is observed that the PPM scheme is a power efficient modulation scheme with lower error rate in comparison to DPPM and OOK schemes with its highly peaked pulses encountering the effect of turbulence effectively. Also, the link performance improves with aperture averaging for all the modulation schemes with the improvement more pronounced in the case PPM scheme. Thus PPM becomes the preferred modulation scheme in designing a FSO communication link.