Design and Analysis of Advanced Free Space Optical Communication Systems

Abstract

Free space optical (FSO) communication has emerged as a viable technology for broadband wireless applications. FSO technology offers the potential of high bandwidth capacity over unlicensed optical wavelengths. On long-range FSO links, atmospheric turbulence causes intensity fluctuations, which degrades links performance. The performance of an optical link can be improved by the use of a time delayed diversity technique, which takes advantage of the fact that the atmospheric path from transmitter to receiver is statistically independent for time intervals beyond the coherence time of the intensity fluctuations. Communications performance is improved because the joint probability of error is less than the probability of error from individual channels.

Theoretical analysis and experimental investigation were conducted to assess and characterize the performance of a time delayed diversity FSO system. Two experiments were conducted: inside our laboratory under simulated convective turbulence and inter-building in clear atmospheric turbulence. In both cases, time delayed diversity system is shown to offer a notable performance improvement compared to a non-diversity FSO system, where the signal-to-noise ratio (SNR) performance can gain up to 4.7 dB and the bit error rate (BER) performance is doubled. These experimental studies confirm the effectiveness of a time delayed diversity technique to mitigate turbulence induced fading, and its optimality in a dual diversity scheme. This is the first published report of theoretical and experimental performance characteristics of FSO communication system utilizing time delayed diversity technique.

FSO technology has also emerged as a key technology for the development of rapidly deployable and secure communication and surveillance networks. In networking applications, broadcasting capability is frequently required to establish and maintain inter-node communications. One approach to deal with the broadcasting issue in FSO networking is the use of omnidirectional FSO links, which is based on non-directed line-of-sight (LOS) technique. Prototype omnidirectional FSO transceiver had been constructed and their performance investigated. Although omnidirectional FSO links cannot provide the performance of directional ones, the results suggest that they could be used in sensor networks or as alternative for traditional wireless networks, when the use of radio frequency (RF) technology is prohibited.