Performance evaluation of satellite-to-earth FSO link in presence of turbulence and weather conditions for different IM schemes

We evaluate the bit error rate (BER) performance of a geostationary earth orbit (GEO) satellite to earth free space optical (FSO) link using intensity modulated/direct detection (IM/DD) system. The analysis is carried out in presence of turbulence and various weather conditions. The combined channel state probability density function (pdf) is obtained by using gamma-gamma distribution to model the turbulence and Beer-Lambert law to incorporate the weather effects. Owing to the fact that IM schemes are simple in implementation and cost effective, we use three IM schemes, viz., on-off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM) for our analysis. The closed from hBERi expressions are derived for the above schemes using the combined channel state pdf. Results show that hBERi increases with increase in the strength of ground level turbulence. Also, the presence of various weather conditions such as moderate, light and thin fog can cause additional degradation. In case of thick or dense fog, the degradation becomes quite high and may lead to link failure. Among the IM schemes, with the same average received power, M-PPM scheme performs the best followed by M-DPPM and OOK schemes in presence of turbulence and different weather conditions.