Use of Markov chain in atmospheric channel modeling of free space laser communications

Free Space Optical (FSO) communications is a promising candidate for broadband applications, achieving bit rates far beyond possible by Radio Frequency (RF) technology. Communications via RF signals are generally reliable and well understood but cannot support emerging data rate needs unless they use a large portion of the precious radio spectrum. FSO communications offer enormous data rates but operate much more at the mercy of the environment. The perennial limitations of FSO communications are manifested in the channel attributes of scintillation (optical turbulence) and path obscurations. Both phenomena reduce the availability of the optical channel to support reliable communications. To design transmission techniques that operate optimally in channel obscurants, a good understanding of the channel behavior is necessary. In most prior works, Monte-Carlo Ray Tracing (MCRT) algorithm has been used to estimate channel parameters. This task is quite numerically intensive. The focus of this paper is on investigating the possibility of simplifying this task by a direct introduction of state transition matrices associated with standard Markov modeling into the MCRT computer simulations programs. We show that by tracing a photon's trajectory in space via a Markov chain model, the angular distribution can be calculated via simple matrix multiplications. We also demonstrate that the new approach produces results that are close to those obtained by MCRT and other known methods. The value of the proposed method lies in its simplicity and numeric efficiency compared to the computationally intensive MCRT.