An analytical approach is presented to evaluate the impact of timing jitter on the bit error rate performance of optical M-ary pulse position modulation (M-PPM) free space optical (FSO) link under the influence of atmospheric turbulence modelled by Log-normal distribution. The expression of the conditional bit error rate (BER) for a given timing jitter is developed and the average BER is found by averaging the conditional BER over the probability density function of the timing jitter which is considered to be zero mean Gaussian. The results are evaluated at a bit rate of 2.4 Gbps over weak turbulence with Avalanche Photodiode (APD) receiver for different order of M-PPM modulation. It is found that there is significant deterioration in BER performance with increase in timing jitter variance and results in BER floor at higher value of jitter variance for a given M-PPM order and APD gain. Further, it is noticed that higher order M-PPM system suffer less than lower order PPM system at a given system BER. The analysis can also be useful for evaluating the impact of slot timing jitter for M-PPM FSO system with strong turbulence.

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