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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.