Analysis of Free Space Optical Communication with Different Modulation Schemes

Free Space Optical data links can provide high bandwidth and secure data transmission. However the optical signal travelling through free space atmospheric channel suffers due to scattering, absorption of signal by atmospheric turbulence conditions. This cause in poor performance and huge noise intrusion. This project experiments the performance of free-space optical communication (FSO) system based on the various modulation techniques such as, PAM, PPM, and ASK separately. The test signal has been transmitted through turbulence conditions by employing various modulation schemes. Phase Jitter and NdB are the parameters experimentally measured. Initially a laser based free space optical communication system; an artificial controlled turbulence chamber was constituted. The modulated laser beam containing data was passed through the free space inside the chamber, provided there is no turbulence present inside. The results were noted for no turbulence condition. Further in the next step an artificial turbulence condition was built-up inside the chamber; simultaneously a modulated optical signal is set to pass through the chamber. Each modulation and corresponding demodulation scheme was separately employed and the signal was experimented for phase jitter and NdB. The results under no turbulence condition and with turbulence condition were compared and results were analyzed. It is concluded from our experimental results that in FSO ASK and PPM are the modulation techniques that provide very reliable and robust performance under turbulence conditions. These modulation techniques provide a high withstand ability to turbulence than any other modulation technique. These modulation techniques can be deployed in free space optical communication systems.