Hybrid testbed of a free-space laser communications system for atmospheric turbulence compensation

Abstract

In the last few years free-space laser communications have become a feasible and high-bandwidth wireless alternative to fiber optic cabling and radio frequency (RF) systems due to its extremely robustness against interference and the advantage of working with optical wavelengths in compare to the RF spectral band (flexibility, low loss, smaller size and greater bandwidth). Even though RF systems are nowadays still used in multiple applications the increasing data-rate and bandwidth need, become clear a change to Free Space Communications (FSO). FSO is an optical communication technology where free space acts as a medium between transceivers that should be in line-of-sight (LOS) in order to achieve a successful transmission of optical signal. However, in free-space optical communication links there are also some unavoidable environmental challenges. One of this, atmospheric turbulence causes fluctuations in both the amplitude and the phase of the received light signal and also induces fading to the wave front, impairing link performance. The compensation of transmission impairments can be performed using spatial diversity techniques. In order to mitigate this fluctuations, it is necessary to use a system with a receiver array formed by a number of detectors, where each detector will observe a different section of the optical beam since these fluctuations can cause the signal-to-noise (SNR) that arrives to the receiver is not good enough to retrieve the information. In a previous Treball Final de Grau (TFG) it was proposed a solution for the signal fading caused by atmospheric turbulence by using a hybrid system (optical and RF) formed by an array of four sub-apertures but it was not able to compensate the system on real time. In this project the feasibility of the technique when it comes to compensate for atmospheric turbulence with signals up to 1 GHz bandwidth was demonstrated. However, given the simplicity of hardware it was not possible to perform real-time compensation. This project aims to get the compensation speed required in order to act in real-time on the channel as well as improve some of the results in the previous version of the processing algorithm. For this reason, it has been necessary to include new devices and replace others.