| Adaptive transmission technology provides a feasible solution to suppress the impact of various factors in the atmospheric environment on laser signal transmission and improve the communication performance of wireless optical communication systems.It is feasible to apply the wireless laser link adaptation technology in a single input single output system(SISO)to a wireless laser multiple input multiple output(MIMO)link to cope with the spatiotemporal variation of refractive index caused by turbulence.In MIMO links,different spatial transmission strategies can provide varying degrees of multiplexing and diversity performance.A new adaptive dimension has been introduced in complex atmospheric channels,allowing communication systems to switch between different spatial modes to achieve high data rates while maintaining good bit error rate performance.The main research content of this article is as follows:1.Considering the combined effects of Malaga turbulence,pointing error and link loss.the composite probability density function of the light intensity fading in the atmospheric transmission of laser signals is derived,and the atmospheric composite channel model is established.Based on BPSK modulation,the closed-form expressions of average channel capacity and outage probability of atmospheric composite channel are derived,and the effects of turbulence intensity,waist radius,jitter deviation and transmission distance on the system performance are analyzed.2.In the case of good atmospheric channel conditions,the average channel capacity of the system is maximized under the condition of predetermined bit error rate,while ensuring the reliability of adaptive transmission in the whole signal-to-noise ratio(SNR)range.This paper proposes an adaptive transmission scheme for MIMO spatial mode switching based on channel capacity.The approximate closed form expressions of average channel capacity and average bit error rate for spatial multiplexing,spatial diversity and hybrid mode systems are derived respectively.The system average channel capacity and average bit error rate of the three transmission modes under different transmitting and receiving aperture numbers,turbulence intensity,waist radius,jitter deviation and weather conditions are simulated and analyzed.The receiver SNR threshold lookup table for optimal transmission mode switching is established,and the SNR switching threshold is determined.Numerical results show that compared with fixed transmission modes,adaptive transmission with mode switching can significantly improve the channel capacity and reliability of the system in the whole SNR range.Monte Carlo simulation shows that the theoretical analysis results of the proposed adaptive MIMO system performance are consistent with simulation results.3.To achieve adaptive spatial mode switching of irregular constellation M-QAM modulation after transmission through atmospheric channels,the minimum bit error rate is selected for transmission at a given bit rate.This article proposes an adaptive transmission scheme based on minimum Euclidean distance for switching between spatial multiplexing 4QAM and spatial diversity 16QAM at a fixed transmission rate.The closed form expressions for the minimum Euclidean distance and bit error rate of the received signal for spatial multiplexing and spatial diversity schemes are derived,and the Demmel condition number of the instantaneous channel matrix is calculated.This provides a sufficient condition for whether the spatial multiplexing scheme is superior to the spatial diversity scheme in a given channel.Based on the Monte Carlo simulation,the error rate characteristics of using spatial multiplexing and spatial diversity schemes under different pointing error parameters,weather conditions,and turbulence distribution were studied.A SNR threshold lookup table for this adaptive transmission scheme was established based on the error rate curve corresponding to the minimum Euclidean distance.The system can choose the best adaptive transmission scheme in different SNR ranges. |
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