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Urban optical wireless communication systems are considered a "last mile" technology. An optical wireless communication system uses the atmosphere as a propagation medium. In order to provide line-of-sight (LOS), the transceivers are placed on high-rise buildings. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways require the designer to increase the transmitter beam divergence angle so as to maintain LOS between the transmitter and the receiver. It is clear that an overly wide divergence angle increases the required laser power, and, as a result, terminal cost and complexity increase. On the other hand, an overly narrow beam divergence angle may result in cutoff in communication when there is building sway. In this paper, we derive a mathematical model to minimize transmitter power and optimize transmitter gain (divergence angle) as a function of the building-sway statistics, the communication system parameters, and the required bit-error probability (BEP). Reduction in laser power could improve overall system performances and cost. For example, for BEP of 10-9, we can attain at least a 4-dB reduction of the required transmitter power in comparison to a system with both half and twice the optimum beam divergence angle.
Date of Publication: July 2003