Research On The Circle Polarization Shift Keying And Multi-level Modulation In Free Space Optical Communication

Recently, there has been a notably resurgent research interest in free-space optical(FSO) communications for its advantages of larger bandwidth, higher rate data transmission,more compact equipment, faster link installation, lower power consumption and better security over radio frequency(RF) communications. However, the most imperative problem is that FSO links suffer from random change of refractive index caused by the variation of air temperature and pressure, which will directly lead to intensity scintillation and beam drift. This impairment measured by scintillation index(SI) and phase noise will severely decline the beam quality, thus resulting in high error rate, high probability of occurring communication interruption and low channel capacity, and thus seriously affect the stability and reliability of the FSO systems and largely restrict the further development of FSO technology.Analyzing the impacts of atmospheric turbulence to received performances of FSO systems and increasing the total power of the received optical signals from the perspective of improving the beam quality are key factors to improve the stability and reliability of FSO systems.Atmospheric channel has small changes to polarization state of optical light with a rotationally symmetrical characteristic, which makes the platforms of relative motions with no need to be in alignment between transmitter and receiver. Besides, the light intensity distribution of a circular polarization beam will be more even through the atmospheric channel after particle scattering.Among these counts, the polarization modulation technique has the potential widespread applications. Therefore, this paper propose a theoretical model of the circle polarization modulation(CPol SK) based on the polarization properties. A further study is conducted of CPol SK system for beam collimation system and modulation system in the transmitter, and demodulation system in the receiver. Besides, a FSO system model of CPol SK/DPSK dual modulations has also been proposed from the perspective of immediate channel transmission capacity. More details are as follows:Firstly, we demonstrate the impacts of atmospheric turbulence to optical communication links, analyze the advantages of circular optical polarization modulation technology, and propose a FSO system model of circular polarization shift keying(CPol SK) modulation. A beam collimation system of direct-coupled polymer straight fiber cone with plano-convex lens in front-end has been proposed from the perspective of improving the quality of the transmission beam, the light transmitted theory and simulations are specifically given for the optimized designof cone angle and fiber length. Further more, we attach importances to the specific implementation of CPol SK system with consideration of the circular polarization control error caused by system designer, and intensity fluctuation and phase noise caused by the atmospheric turbulence.Secondly, the Gamma-Gamma distribution channel model that is suitable for the description of all turbulent scenarios from weak to strong is chosen to study the effects of both intensity scintillation and phase fluctuation caused by atmospheric turbulence to the proposed CPol SK system with special consideration for circular polarization control error caused by system design.Moreover, the closed form expressions of BER, outage probability and average channel capacity are derived for evaluating the BER performance, communication interruption and channel capacity. Performance advatages of the proposed CPol SK scheme are obtained through comprehensive analysis.Finally, a novel FSO system model of dual modulation based on CPol SK and DPSK was proposed to increase the instant transmission capacity. The specific theoretical model of dual modulation was derived. Simulations demonstrated that the proposed CPol SK/DPSK system could significantly overcome the effects of intensity scintillation and phase fluctuation caused by atmospheric turbulence, and entirely eliminated the modulation crosstalk. It is noted that performance advantages of CPol SK/DPSK system were highlighted by comparing with CPol SK system and DPSK system.