Laser nonlinearity effects on optical broadband backhaul communication links

In this paper we seek a solution for robust broadband communication using high altitude platforms (HAPs). We address the requirements of the backhaul link and propose a hybrid millimeter-wave/optical solution to reduce weather-induced-outage probability and enhance throughput. We explore the concept of analogue-modulated wireless laser communication based on orthogonal frequency-division multiplexing (OFDM) in order to achieve a compact transparent payload onboard the HAP. Laser nonlinearity effects and the adverse effects of turbulence in the optical communication channel are identified as the major challenges to successful implementation of this solution and are analyzed. The dependence of normalized symbol error rate (SER) on signal power and turbulence strength is demonstrated for two modulation schemes. We show the severe performance degradation due to laser nonlinearity resulting from increasing signal power in sharp contrast to SER improvement with signal power when nonlinear distortion is not present.