Link dimensioning of hybrid FSO/fiber networks resilient to adverse weather conditions

摘要

The presented paper deals with wireless networks composed of FSO (free space optics) links supported by terrestrial optical fiber connections in order to increase resilience to adverse weather conditions. An FSO link realizes, at low cost, a broadband optical transmission system established by means of two parallel light beams connecting a pair of (remote) transceivers placed in the line of sight. However, a major disadvantage of FSO links (with respect to fiber links) is their sensitivity to weather conditions: bad weather affects optical channel, resulting in substantial degradation of the transmission power at the receivers, which in general leads to capacity degradation on multiple FSO links. When the transmission power received at the end node of an FSO link is decreased, the signal modulation and coding scheme (MCS) at the transmitter should be adjusted accordingly; in effect, an FSO link can be kept operational, but with decreased capacity, even if the channel is affected. Yet, since in severe weather conditions some FSO links may not be able to realize any reliable transmission at all (whichever MSC is used), the network may become disconnected and a portion of traffic lost. Therefore, it is reasonable to consider using fibers instead of free space light beams since weather insensitive (and high capacity) fibers installed on the key links will ensure network connectivity in all weather states. Certainly, the so obtained connectivity does not come for free because of the high cost of the optical fiber connections as compared to the FSO systems and hence the number of fibers should be kept at minimum. Thus, for designing hybrid FSO/fiber networks we need an optimization model for finding cheapest configurations of links (and their capacities) that will be able to carry the demanded traffic on acceptable level in all weather states foreseen in network operation. This is not a simple task as it requires not only robust network optimization methods but also a tractable way of characterizing possible weather states (which are not known in advance) and their influence on FSO link capacity. In the paper we present an approach to the so described task and illustrate its effectiveness by means of a numerical study. (C) 2019 Published by Elsevier B.V.