The Performance Analysis Of Atmosphere Optical Multiple-input Multiple-output System In Correlated Channel

Atmospheric laser communication is a kind of emerging communication technology combined with microwave communication and optical fiber communication.It integrates the advantages of flexible access of microwave communication and high transmission rate of optical fiber communication.It has wide application prospect in many fields,especially in military field.Laser transmission in the atmospheric channel is suscepti ble to atmospheric turbulence,resulting in signal fading and increased bit error rate,even when there will be serious interruption of communication.Multi-input multiple-output(MIMO)technology can not only resist atmospheric turbulence effectively,but also can increase the system capacity and improve the utilization efficiency of system frequency band greatly without additional system bandwidth and transmit power.Thus it becomes an effective measure to improve system performance and a hot spot in the field of atmospheric laser communication in recent years.However,lots of theoretical researches on the performance of MIMO technology in atmospheric laser communication are based on the assumption that channels are independent.Experiments show that the assumption is too idealistic to achieve.In the actual transmission channel,the large divergence angle and the turbulence effect and all will induce the spatial correlation between the beam s.In addition,the limited system terminal size will make the antenna spacing are too small and spatial correlation increases,further limiting the performance of optical MIMO communication system.In this paper,based on this consideration,the statistical characteristics of the atmospheric turbulence channel are analyzed.On this basis,the influence of spatial correlation on channel capacity and bit error rate of the optical MIMO system are discussed emphatically for intensity modulation / direct detection(IM / DD).(1)The channel fading coefficient matrix in different correlation scenarios are analyzed combining with channel model of optical MIMO system.The channel characteristics of the correlation optical MIMO system under Log-normal distribution are analyzed adopting exponential correlation model,which lay the foundation for the next step.(2)In view of OOK modulation,the expressions of the average channel capacity and the outage channel capacity are deduced for the uniform circular array correlated optical MIMO system.The influences of spatial correlation on the two are simulated and analyzed in the Log-normal channel.For the PPM and MPPM,the expressions of the average channel capacity of the correlated optical MIMO system are deduced under the Poisson mechanism.The results are simplified further.The results show that the spatial correlation will seriously affect the advantages of the optical MIMO technology obtained by increasing the diversity gain.And this effect will increase with the increasing of correlation.(3)The maximum likelihood criterion of PPM modulation with Poisson mechanism is discussed.On this basis,the upper bound of the bit error rate of the atmospheric laser communication correlated MIMO system is deduced and simplified using Wilkinson method.Finally,the bit error rate performance of the correlated optical MIMO system is simulated and analyzed.The results show that the correlation makes the bit error rate of the optical MIMO system increase.The research results of this paper have the significances to instruct the reasonable placement of antennas in practical engineering applications and determine the rational the diversity of optical MIMO technology according to the degree of channel correlation.