Scintillation bit error rate-reduction for free space optical communications systems

Although relatively unaffected by rain and snow, free space infrared laser optical communication systems are affected by fog and atmospheric turbulence. We have calculated the signal loss from scintillation to predict bit error rates by varying the amount of scintillation, the link distance, and system details. Increasing the number of transmitters and the effective area was predicted to improve scintillation in earlier work in this field. We found that increasing both the number of transmitters and the effective receiver area greatly improved the signal loss caused by scintillation and thus, the bit error rate. For example, under high scintillation (Cn<SUP>2</SUP> equals 10<SUP>-13</SUP> m<SUP>-2/3</SUP>), the four transmitter/receiver system at a link distance of 1 km should experience a bit error rate of better than 10<SUP>-10</SUP> for a 4.0 dB loss. Under the same conditions, an identical system with only one transmitter and receiver should experience a bit error rate of 10<SUP>-1.5</SUP> (0.032 bits per second) for the same 4.0 dB loss. Increasing just the number of transmitters or the number of receivers improved the signal, but increasing both improved the signal even more than expected. In general, single beam systems should be limited to link distances less than about 600 m.