Optimal Relay Node Placement For Free Space Optical Communication System

Free-space optical communications(FSO)technology transmits information in the atmosphere by modulating the laser light.Its transmission rate can be as high as several G bps.However,the weather-dependent channel loss and turbulence fading would limit the FSO link distance.Relay technology as a common solution uses the relay nodes to serve a long link by dividing it into several short links to improve the end-to-end system performance.Therefore,optimal relay node placement becomes the main problem to be solved.In this thesis,we study optimal relay node placement techniques in FSO relay systems.The main contributions can be summarized as follows:(1)Based on the proposed multi-hop FSO broadcasting system model,a relay node placement algorithm without any placement constraints is proposed.In order to achieve the lowest outage probabilities of users,the relay placement problem is formulated as a unconstrained multi-objective optimization problem.The relay node position and the outage probability of the user nodes are described as the variables and the objective function,respectively.Then,the multi-objective optimization problem is transformed into a single-objective problem using the linear weight parameter method,according to each different communication service requirement.At last,the optimal position of each relay node can be obtained by the single-objective optimization model.(2)Several theorems for optimal relay node placement based on Lognormal and Gamma-Gamma channel models are proposed without the obstacle and feasible region conditions.It is proved that when the transmit power is sufficiently large,equal-distance placement is the optimal solution regardless the number of users for each relay and the relay weights.Simulation results have verified the accuracy of the proposed theorems.(3)Under the obstacle and feasible region condition,a relay node placement algorithm is proposed.The round obstacles,elliptical obstacles and line-segment obstacles are described as the constraint function.The round region,half-plane region and polygonal region are described as the feasible region constraint function.The problem of relay node placement can be formulated as multi-objective optimization with constraint function.Based on the simulation results in Lognormal channel,some useful conclusions for practical applications are drawn.