Overview: In this chapter we investigate several performance characteristics of a free space optical (FSO) communication system operating on a terrestrial link for which the index of refraction structure parameter can be treated as essentially constant. Our discussion, however, is limited to receivers operating in a direct detection mode.
In order to quantify the performance characteristics of a FSO communication link operating in free space, it is customary to first introduce the concept of signal-to-noise ratio (SNR) in the receiver system. Noise in the system includes that due to background radiation and/or noise in the receiver system itself (shot noise, circuit and electronic thermal noise, etc.). For a given SNR and threshold level, it is then possible to calculate the probability of detection and probability of false alarm. When the link operates in the open atmosphere, we quantify the performance characteristics in terms of various fade statisticsâthe probability of fade, the expected number of fades per second, and the mean fade time. When the free-space SNR is sufficiently high, the most deleterious atmospheric effect on link performance is that due to scintillation. A reduction in scintillation on the detector, known as aperture averaging, can usually be achieved through the use of a large-aperture receiver. For a digital communication link operating in an atmospheric channel, it is the probability of error or bit error rate (BER) that is usually calculated from the probability of fade and probability of false alarm.
Spatial diversity in a direct detection system offers an alternative to a large-aperture receiver through the use of an array of small receiver apertures. The small apertures should be spatially separated by a sufficient distance that they detect statistically independent signals. In our analysis we compare scintillation reduction (aperture averaging) of a receiver array with that of a single large receiver lens having the same aperture area as the array of receiver lenses. The implied BER performance presented for a single-aperture receiver is likewise extended here for the receiver array system and compared with that for a single-aperture receiver system.