Performance analysis of an FSO link in presence of pointing error using multiple PIN photodetectors with equal gain combiner

An analysis is carried out to evaluate the conditional bit error rate conditioned on a given value of pointing error for a Free Space Optical (FSO) link with multiple PIN photodetector receivers using Equal Gain Combining (EGC). The probability density function (pdf) of output signal to noise ratio (SNR) with receive diversity is also derived in presence of pointing error using EGC. The average BER of a SIMO FSO link is analytically evaluated by averaging the conditional BER over the pdf of the output SNR to find out the numerical results. Number of approaches is presented to find the average BER for SIMO FSO link to evaluate the best approach. The BER performance results are evaluated for several values of pointing error with OOK modulation and direct detection. The numerical evaluation show that, the SIMO FSO system suffers significant power penalty due to pointing error and can be reduced by receive diversity. The improvement of receiver sensitivity over a single receiver is about 6 dB and 9 dB when the number of photodetector is 4 and 8 respectively at a BER of 10-10. It is also noticed that, FSO system with receive diversity can tolerate higher values of pointing error at a given BER and transmit power.