| With the rapid growth of the terminal users and data services,not only the backbone network is put forward to higher requirements,but also the access network technology is faced with the challenges of high-rate data access,large capacity and information security.Wireless optical communication,a new access network technology,has received extensive attention due to its advantages of ultra-high transmission rate at the level of Gbps,flexible installation,low cost,etc.However,the reliability of wireless optical communication system is seriously affected by atmospheric attenuation,atmospheric turbulence and pointing error.Optical multiple-input multiple-output technology can improve the channel capacity and anti-fading ability without increasing spectrum resources and transmit power,effectively solving the problems of wireless optical communications.Unfortunately,wide actual application of the optical multiple-input multiple-output system is restricted by the problems of inter-channel interference,spatial correlation and inter-antenna synchronization.As a novel optical multiple-input multiple-output technology,optical spatial modulation improves the transmission rate by selecting laser index and digital modulation symbol to carry information.Only one laser is activated at each time interval of optical spatial modulation,it overcomes the inter-channel interference and inter-antenna synchronization.But the transmission rate of optical spatial modulation is limited.Then,optical generalized spatial modulation is proposed to improve the transmission rate,which the less of lasers are activated simultaneously.But the transmission rate of the optical generalized spatial modulation is improved at the cost of the lost bit error rate performance.Therefore,the target of this thesis is to improve the transmission rate and bit error rate performance of wireless optical communication system.According to the influence of atmospheric attenuation,atmospheric turbulence,pointing error and other factors,the channel model and bit error rate performance of optical multiple-input multiple-output technology,optical spatial modulation technology,optical generalized spatial modulation technology and signal detection are investigated in this thesis.The main work and innovation are as follows.1.The correlated channel model of optical multiple-input multiple-output system under the combined effects of atmospheric attenuation,atmospheric turbulence,pointing error and spatial correlation is established.Ergodic channel capacity and bit error rate of optical multiple-input multiple-output system are derived by using pulse position modulation and multiple pulse position modulation,respectively.Then,the expressions are simplified by approximating the sum of the channel fading coefficients as another variable and calculating the probability density function of the variable.The upper bound with only one integration is given,and the effects of various factors on the performance of optical multiple-input multiple-output system is discussed.The simulation results demonstrate that the combined effects seriously affect the performance of optical multiple-input multiple-output system.The pointing error is the most prominent influence factor over weak correlated channel.The performance degradation caused by strong channel correlation coefficient is more than that of pointing error in strong correlated channel.Therefore,the research results provide a theoretical basis for the parameter design in the actual communication scenario,and lay the foundation for the successive research of optical spatial modulation and optical generalized spatial modulation.2.For the sake of the improvement of bit error rate performance in optical spatial modulation scheme,an optical space time shift keying scheme is proposed based on the combination of linear dispersion code and optical space shift keying.In this scheme,the combinations of activated laser indices are mapped onto orthogonal space time dispersion matrices.Meanwhile,the channel with the best channel state is selected to transmit signal through channel state information and laser selection technology,enhancing the bit error rate performance.The bit error rate of the proposed scheme based on maximum likelihood sequence detection is derived by using the union bound technique.The feasibility of the scheme are verified by Monte Carlo simulation and Field Programmable Gate Array hardware model.Finally,since the transmission signals of the proposed schemes are sparse,the compressed sensing theory is utilized to decode at the receiver.Furthermore,the low complexity decoding algorithm near optimal detector,the improved orthogonal matching pursuit algorithm based on threshold judgment,is proposed,and its computational complexity is about70% lower than that of the maximum likelihood sequence detection.3.In order to further improve the transmission rate of the optical space time shift keying scheme,the optical space time pulse position modulation scheme is proposed by introducing signal constellation.Its spatial constellation is the same as optical space time shift keying scheme,while the information is transmited by mapping different pulse position modulation symbols on each of the activated lasers of orthogonal space time dispersion matrix in signal constellation.The sparsity of the transmitted signal is stronger due to the expansion of the time slot.Similarly,an improved orthogonal matching pursuit algorithm based on threshold judgment is adopted at the receiver to reduce the decoding complexity.Then,the bit error rate of the proposed scheme is derived by using the union bound technique,and the analytical expressions are verified via a Monte Carlo simulation.The advantages of the proposed scheme in terms of transmission rate and bit error rate performance also are proved.4.In order to improve the transmission rate of optical generalized spatial modulation,the enhanced optical spatial modulation scheme with a variable number of activated lasers is proposed.In this scheme,the spatial constellations are increased by activating different index combinations of three types,where only one laser is activated,two lasers are activated simultaneously and two lasers are activated repeatedly.The orthogonality and grouped mapping of pulse position modulation are used to distinguish different mapping types,so as to increase the number of constellations in spatial domain.Moreover,the bit error rate performance is improved by taking advantage of bit repetition mapping method that the part of spatial constellation is repeatedly mapped in signal domain.The simulation results show that the proposed scheme can reach a compromise among computational complexity,bit error rate performance and spectrum efficiency with higher transmission rate.5.To improve the bit error rate performance of optical generalized spatial modulation,the optical generalized space time pulse position modulation scheme is proposed.In this scheme,the combinations of activated laser indices are mapped to the generalized space time dispersion matrix through spatial domain mapping twice,improving the space resources utilization.In the process of signal domain mapping,the pulse position modulation symbols mapped on different time interval satisfy orthogonality,which can improve the adjudged accuracy of the pulse position.In addition,the sphere decoding algorithm is used at the receiver,and it can reduce the decoding complexity while ensuring the better decoding performance. |
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