Ever increasing demand for more bandwidth has compelled to search in optical communication. But a significant attention has been given in last decades to terrestrial FSO communications. FSO communication is a line of sight technology based on transmitting data laden light through the atmosphere. In addition to providing high bandwidth, FSO system benefits from the license free operation, light weight, low installation cost and immunity to electromagnetic interference. The main challeng in this technology results from its channel. The atmospheric channel is not stationary and changes with time of the day, seasons and weather conditions.The optical beam traversing the atmosphere experiences absorption and scattering results by molecular and aerosol constituent. Unlike RF systems, FSO is less affected by snow and rain but heavily influenced by fog which limits the link range. In the dense fog condition availability of the link is limited.Though the attenuation in clear atmosphere condition is very low the performance of FSO communication systems suffers severely from the turbulence. The turbulence is the results of wind and temperature gradients that create air pockets with varying refractive index. The traversing light beam experiences random fluctuation in its intensity (scintillation) and phase leading to degradation in performance. In order to describe the atmospheric turbulence, some models have been suggested as lognormal and negative exponential which are simulated in this work. The optical carrier can be modulated in its frequency, phase, intensity and state of polarization. The intensity modulation is mostly preferred due to simplicity in implementation and cost. There are many different modulation schemes which are suitable for FSO communication but much attention allocated to OOK modulation for its simplicity. But due to atmospheric turbulence it needs an adaptive threshold which imposes high complexity to receiver. PPM is an orthogonal digital pulse modulationadopted in optical communications due to its high power efficiency but in cost of increasing requirement bandwidth and complexity. To address these challenges the SIM is proed which needs no adaptive threshold and its requiremet bandwidth is less than PPM but its consuming power is high. In this work, two new hybrid PPM-K-SIM and K-SIM-PPM schemes are proposed which combines the advantage of the PPM and K SIM. The theoretical analysis of proposed scheme is carried and is verified using computer simulations. The performance of the 2-PPM-K-SIM is compared with K- SIM and PPM schemes which show good performance compared to K-SIM and PPM in strong turbulence and its improvement in compare ..... Keywords: Free Space optical communication, Pulse Position Modulation, Subcarrier Intensity Modulation
