Study On Uplink And Downlink Channel Difference And Non-reciprocity Compensation Correction Model For FSO Massive Linear MIMO Array

Free space optical communications is a means of transmitting laser in the atmosphere.Compared with radio frequency communication,it has many advantages,such as large capacity,no spectrum licensing,quick deployment and convenience.Large scale MIMO technology is an effective measure to use multi antenna arrays of far ultra traditional MIMO technology at the receiving end,and is one of the key technologies of 5G.The use of large scale MIMO technology can effectively control the attenuation of signal intensity caused by atmospheric turbulence in FSO,thus reducing the bit error rate at the receiving end.At the same time,the directivity of the laser beam in FSO reduces the pilot pollution problem in the large-scale MIMO technology.Therefore,the large-scale MIMO-FSO system has a very good development prospect and research significance.In the FSO system,the main influence factors of atmospheric turbulence can be divided into three parts.The first is the atmospheric loss caused by the scattering and absorption of light by water vapor,methane and other particles.The two is the random fluctuation of the intensity and phase of the received signal caused by atmospheric turbulence-the light intensity scintillation;The three is the beam expansion and pointing error caused by the jitter of buildings,atmospheric turbulence,alignment errors and mechanical vibration of optical devices.Under large scale MIMO array,atmospheric loss,light intensity scintillation,beam propagation and pointing error models are no longer simple single antenna form.Instead,the interaction between multiple antennas should be considered under different arrays and the whole model can be built.In the oblique range transmission,the atmospheric turbulence intensity varies with the altitude,which needs to consider the influence of atmospheric turbulence on the signal intensity at different altitudes.Moreover,due to the different atmospheric turbulence effects on laser beams with different beam width and different wavelengths,even if the transmission distance is the same,the influence of up and down is also different,namely the uplink and downlink non reciprocity.At the end of the paper,the uplink and downlink channel models and differences of ground to station short distance slant free space optical communications(FSO)are adopted by large-scale linear MIMO array.Based on the Gauss beam,Kolmogorov turbulence spectrum and the spherical wave in uplink and downlink,the upper and lower channel atmospheric loss,light intensity scintillation,beam propagation and pointing error models respectively,and simulate the differences of air loss,spatial coherence length,receiver beam width and pointing error jitter variance can be constructed.Then,the overall model of the uplink and downlink channel is known.The intensity modulation / direct detection(IM/DD)method can be used to derive the BER and relative mean square error formula of equal gain combining(EGC),and the difference between the mean square error and bit error rate of uplink and downlink can also be analyzed.Then,according to the channel of the whole model,the multi-user massive MIMO-FSO uplink and downlink slant non reciprocal compensation correction model can be constructed.two kinds of channel estimation algorithm named LS and LMMSE and two kinds of modulation methods named OOK and OFDM can verify the effectiveness of the non reciprocal compensation correction model by MATLAB.