Optical wireless communication through fog in the presence of pointing errors.

Terrestrial optical wireless communication (OWC) is emerging as a promising technology, which makes connectivity possible between high-rise buildings and metropolitan and intercity communication infrastructures. A light beam carries the information, which facilitates extremely high data rates. However, strict alignment between the transmitter and the receiver must be maintained at all times, and a pointing error can result in a total severance of the communication link. In addition, the presence of fog and haze in the propagation channel hampers OWC as the small water droplets scatter the propagating light. This causes attenuation due to the resultant spatial, angular, and temporal spread of the light signal. Furthermore, the ensuing low visibility may impede the operation of the tracking and pointing system so that pointing errors occur. We develop a model of light transmission through fogs of different optical densities and types using Monte Carlo simulations. Based on this model, the performance of OWC in fogs is evaluated at different wavelengths. The handicap of a transceiver pointing error is added to the model, and the paradoxically advantageous aspects of the transmission medium are exposed. The concept of a variable field of view receiver for narrow-beam OWC is studied, and the possibility of thus enhancing communication system performance through fog in an inexpensive and simple way is indicated.