Research On IMC Over Gamma-gamma Channel In WLC

With the development of lasers and optoelectronic devices,wireless laser communication(WLC)has become a new research hotspot.With the characters of high speed,high bandwidth and good confidentiality,WLC can provide various services.At present,atmospheric turbulence is a vital factor that restricts the development of WLC.In order to improve the stability and reliability of WLC system and weaken the influence of atmospheric turbulence,it is very important to study the suitable modulation and coding technologies.In this paper,we first introduce the WLC system model and atmospheric channel characteristics,and analyze the influence of atmospheric attenuation and atmospheric turbulence on WLC system.Then,the OOK and PPM modulations which are commonly used are introduced.And we compare the average transmit power and average bandwidth requirement between OOK and PPM.We point out that more efficient modulation and coding technologies for high speed WLC system are needed.Therefore,a new modulation coding technique,the inverter Manchester code(IMC),is studied,and its performance under Gamma-Gamma channel model in WLC system is researched.IMC can be divided into two branches according to its code element.The branch loading the amplitude information is called amplitude branch;the branch loading the phase information is called phase branch.In a code,the amplitude branch and phase branch can transmit 1bit and 2bits information respectively,which improves the bit rate of the WLC system.For the demodulation of IMC,this paper gives a detailed solution.In the Gamma-Gamma channel,the amplitude branch of IMC is direct detection(DD)mode,and the error rate of the amplitude of the Gamma-Gamma channel is deduced according to the influence of the APD on the received optical signal.For the phase branch,heterodyne detection and equal gain combining(EGC)diversity reception technique are employed.In addition,the coherent detection demodulation is used,and the relevant formula is derived.At the same time,we analyze the factors affecting the transmission performance of IMC's amplitude and phase branch under Gamma-Gamma channel,and put forward the relevant optimization schemes,which improve the performance of IMC in WLC system.Through the data analysis and simulation,the IMC can effectively adapt to the change of the atmospheric turbulence,which shows that IMC is a suitable modulation and coding method in the Gamma-Gamma channel,and that it can effectively improve the laser signal's tolerance for atmospheric turbulence.So,the IMC can improve the reliability and stability of WLC system.