On the Performance of Alamouti Scheme in Gamma-Gamma Fading FSO Links With Pointing Errors

The error performance of the free space optical (FSO) link suffers from the atmospheric turbulence. By employing an additional transmit aperture at the transmitter along with Alamouti code, the error rate of the FSO communication system can be significantly improved. However, the pointing errors, generated because of the building sway, has the potential to eradicate the benefits of the Alamouti scheme. Therefore, for a general and realistic study of the FSO based Alamouti scheme, the effect of pointing errors in the Gamma-Gamma fading atmospheric fluctuations is considered in this paper. Specifically, we derive the symbol error rate and the average capacity of the Alamouti scheme over the Gamma-Gamma fading FSO links with pointing errors. The moment generating function (MGF) of the scheme is derived in terms of the Meijer-G function. An MGF based approach is used to derive the ergodic capacity of the scheme. The effect of pointing errors over the performance of the scheme is analyzed under different scenarios and it is observed that the pointing errors significantly degrade the diversity of the considered FSO system.