Diversity optical communication over the turbulent atmospheric channel

Optical communication through the atmosphere provides a means for high data rate communication (gigabits per second) over relatively short distances (kilometers). However, the turbulence in the atmosphere leads to fades of varying depths, some of which may lead to heavy loss of data. For example, at a data rate of 2.5 gigabits per second, as many as 250 x 10 consecutive bits can be lost in a single 100 millisecond deep fade. It is feasible to recover the data loss in these fades via error correcting codes but only via substantial hardware complexities and processing delays. Thus, it would be of great benefit if we could reduce the probability of a fade. In this thesis, we examine spatial diversity at 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 several diversity systems: receiver diversity systems that use Equal Gain Combining, Optimal Combining, or Selection Combining, transmitter diversity systems, combined transmitter and receiver diversity systems, and time diversity systems. The outage probabilities for the various diversity systems are compared and the power gain of using these diversity systems is established. It is found that the power gain of diversity systems over non-diversity systems is substantial and that Equal Gain Combining has performance almost equivalent to Optimal Combining. Thesis Supervisor: Vincent W.S. Chan Title: Joan and Irwin M. Jacobs Professor of Electrical Engineering & Computer Science and Aeronautics & Astronautics, Director of Laboratory for Information and Decision Systems