Author : Isha Thakur and Ravinder Singh

In a free space communication, for transmitting data in the order of mega bits and giga bits, usually radio frequency is used, although as an attractive technique Free Space Optical communication can be used. As the medium for the transmission of the data is air and the light pass through it, some environmental challenges are present like atmospheric turbulence and beam wander. Atmospheric turbulence is caused by random fluctuations in the temperature and the pressure of the atmospheric region through which the Free Space signal has to pass. Compare to large data rates, scintillation process is slow in optical transmission. In order to mitigate this harmful effect of atmospheric turbulence and beam wander, multiple input and multiple output (MIMO) is used. The mitigation of scintillation is achieved through multiple lasers and multiple output apertures, thereby creating a multiple input and multiple outputs (MIMO) channel. This paper investigate the performance of FSO communication systems employing on-off keying (OOK) and Q-ary pulse position modulation (QPPM) in turbulence regime. The performance results are evaluated in terms of bit error rate (BER) employing OOK and QPPM as modulation technique. It is found that the BER performance under the technique Q-ary PPM if we increase the order of Q then the performance will improve and it provide maximum 4dB improvement. In this paper the working of MIMO in Free Space Optics communication is analysed briefly along with the receiver combining techniques.

Keywords: Multiple input/multiple output (MIMO), atmospheric turbulence, free space optics, pulse position modulation (PPM), on-off keying modulation (OOK), and diversity techniques.