Impact of misalignment parameter on performance of free-space optical communication system

This paper investigates the error performance of various modulation techniques like on–off keying, binary phase shift keying, differential phase shift keying, and binary pulse position modulation against misalignment parameters. Misalignment parameter is related to the effective beam radius at the receiver plane with respect to the pointing error deviation at the same receiver plane. This manuscript considers a new approach of limiting the deviation of beam centre by using larger beam radius and subsequently considers the reduction of power flux density even when there is less pointing error. The intensity (power flux density) reduction due to misalignment and the losses taking place due to absorption by fog, snow, clouds and many more, are considered in this article. However, the atmospheric absorption losses are modelled using Beers–Lambert Law. In addition to that a simplest form of diversity known as aperture averaging is used to curb the losses due to misalignment. This manuscript unveils the scientific cause of decrease in error performance with increasing pointing error parameter.