Performance of free space optical links in presence of turbulence, pointing errors and adverse weather conditions

The ergodic capacity, bit error rate (BER) and outage probability of a free space optical communication link is analyzed in the presence of gamma–gamma distributed turbulence and pointing errors in presence of various weather conditions viz., very clear air, drizzle, haze and fog. It is seen that the capacity decreases, BER and outage probability increase as the pointing error jitter increases. The worst degradation of the above performance parameters occurs in fog conditions while very clear air conditions give the least degradation. Further, the effect of varying beam divergence angle/beam waist is studied. It is observed that the beam divergence angle/beam waist can be optimized to attain maximum capacity, minimum BER and outage probability. The optimum value depends on the normalized pointing error jitter and increases with increasing jitter. However, for a fixed value of normalized jitter, it is independent of the weather conditions. Also, the system performance is more susceptible to channel impairments for beam divergence angles lower than the optimum value as compared to larger angles.

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