Performance of rectangular QAM/FSO systems using APD receiver over atmospheric turbulence channels

We theoretically analyze the performance of free-space optical (FSO) communication systems using rectangular quadrature amplitude modulation (QAM) and avalanche photodiode (APD) over atmospheric turbulence channels. The log-normal fading channel is used in the analysis. We derive the system´s average symbol error rate (SER) taking into account the APD shot noise and thermal noise, turbulence strengths, channel distances and various system parameters.We find that using APD can greatly benefit the performance of the system in comparison with that of using PIN photodiode. However the selection of APD gain is critical to the system performance. In addition, the optimal value of APD gain also significantly depends on various conditions of the atmospheric turbulence channels.