Aperture averaging and receiver diversity for FSO downlink in presence of atmospheric turbulence and weather conditions for OOK, M-PPM and M-DPPM schemes

Abstract

We evaluate the error performance of a free space optical (FSO) downlink from a geostationary earth orbit satellite to an earth station in presence of turbulence and different weather conditions such as fog, clouds, etc. Combined channel state probability density function (pdf) is derived by using log-normal model for weak turbulence, gamma–gamma model for moderate to strong turbulence and Beer–Lambert Law for weather effects. Bit error rate (BER) expressions are derived using the combined channel state pdf for on-off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM) schemes. It is observed that the link performance degrades with increase in the strength of ground level turbulence. Presence of weather conditions causes additional degradation with moderate fog causing the worst effect followed by light and thin fog with respect to clear air condition. Dense or thick fog and/or clouds may lead to complete link failure owing to their large attenuation coefficient. Further, enhancement in the link performance by using aperture averaging and receiver diversity techniques is examined and compared for all three schemes. It is seen that performance improves with increase in the receiver diameter and number of multiple independent receivers. Among the three modulation schemes, link with M-PPM scheme gives the best performance in terms of BER followed by M-DPPM and OOK schemes with or without diversity techniques. Thus, an M-PPM based FSO downlink with an array of finite sized receivers i.e., single input multiple output link can be a viable alternative for efficient data transfer in presence of atmospheric turbulence and different weather conditions from a satellite to an earth station.