In last few years, the demand for bandwidth has been increasing exponentially. Installation of high bandwidth media like optical fiber has, of course, provided a good backbone, but still the projects like fiber-to-the-home (FTTH) find the bottleneck problem at the user ends. Free-space optical (FSO) communication can be a potential solution to this problem. FSO has received considerable attention in the recent few years. Although FSO has the -capability to provide extremely high speed communication, but it also has some problems associated with it. In this paper, performance of FSO system, working under weak and strong turbulence scenarios, has been compared in terms of their bit error rates (BERs). The modulation schemes considered are binary pulse position modulation (BPPM) and M -ary pulse position modulation ( M -ary PPM). The photodetector used is an avalanche -photodiode (APD). The comparison of performance in weak and strong turbulence for either system clearly brings the requirement of additional signal photons as the turbulence level increases. The results obtained in the paper also show the suitability of BPPM over M -ary PPM in strong turbulent environment. Further, optimum gain of APD to achieve minimum probability of error in the turbulent environment is given. © 2008 by the IETE.