Evaluation of free space optical link availability in metropolitan Tokyo area

摘要

The link availability of a free space communication system is affected by weather conditions such as rain, snow, and most notably fog. For carrier class services, the link availability of 99.999% is highly desired. In this paper, we will report the evaluation of atmospheric attenuation variation depending on the weather condition in metropolitan Tokyo area to obtain some network design parameters for desired link availability. For the evaluation we constructed an experimental communication network with a free space communication system. The network consists of nine free space communication devices installed at locations deferent in height. The link distance between these devices are not identical. We will show that we can estimate the attenuation of atmosphere of local links by using those communication devices. We also installed two present weather detectors (PWD) to measure attenuation in terms of visibility at two locations we installed free space communication devices. These PWD units can detect the amount and type of a precipitation allowing us to compare the attenuation caused by various weather factors. We have observed that attenuation value is considerably high starting from 150m (mean sea level) resulting in higher link outage rate compare to other low-height links.