Part 1: optical communication over the clear turbulent atmospheric channel using diversity

Optical communication over the clear atmosphere provides a means for high data rate communication (gigabits per second) over relatively short distances (kilometers). However, turbulence in the atmosphere leads to fades of varying depths, some of which may lead to heavy loss of data. We consider spatial diversity at both the transmitter and receiver, as well as time diversity as a means to mitigate the short-term loss of signal strength. Using direct detection receivers and binary pulse position modulation as an example, we derive the outage probability of spatial diversity and time diversity systems that use equal gain combining (EGC), optimal combining, and select-max combining. The power gain of using these diversity systems is found to be substantial, and the performance of equal gain combining is found to be almost as good as optimal combining. Hence, we feel diversity should be considered as one of the system techniques to improve channel performance in clear atmospheric optical channels.

I. Introduction

Atmospheric optical communication over distances on the order of a kilometer or more allows the communication of data at modulation rates as high as tens of gigabits per second. For cloudless days, even space-ground communication is possible. However, the laser beams are susceptible to fading due to refractive index fluctuations induced by airturbulence. As predicted by the Kolmogorov turbulence model, temperature changes in the air on the order of 1 K cause refractive index changes on the order of several parts per million. The Extended Huygens–Fresnel Principle models the fading of a wave due to passage through air turbulence [1]. It is typical for deep fades to last approximately 1–100 milliseconds which may result in the loss of potentially up to consecutive bits at a transmission rate of 10 Gb/s. This motivates the need to consider schemes that minimize the probability that the receiver “sees” a significant fade.