| Free-space optical(FSO)communication has attracted much attention because of its large bandwidth,low fabrication costs,resistance of electromagnetic interference license-free.With the development of the science and technology,higher data transmission capacity is desired for future communication.Optical vortex with orbital angular momentum(OAM)and multi input multi output(MIMO)are two kinds well-established technology in spatial degrees of freedom which could exhibit much spectral efficiency and transmission capacity efficiently.Nevertheless,the OAM mode will be seriously impacted by atmospheric turbulence and the existence of atmospheric turbulence has been one of the main impairments of the OAM-based MIMO FSO system due to the intensity fluctuation and beam wandering of OAM beams.These conditions further cause the power spreading of OAM modes.Therefore,the orthogonality among OAM modes is no longer maintained,and crosstalk is induced between FSO links.Therefore,it is significant to explore and study how to mitigate the impact of atmospheric turbulence and exploit the multipath interference of this OAM-FSO system.Theoretical and experimental investigations indicate that space-time coding(STC)is also an excellent technique which has been studied in FSO systems to obtain diversity gain over atmospheric turbulence,uniform circular array(UCA)flexibility in radiating OAM beams with different charge numbers simultaneously.In addition,low-density parity-check(LDPC)codes are very promising error-correcting codes,which are normally used to enhance the system performance due to their achievements of near capacity limit and high coding gain.On the basis of modified Van Karman optical spectrum,channel estimator carry out LS algorithm is utilized to obtain channel state information(CSI).STC,UCA,and LDPC are introduced to this OAM-FSO system and the performances of the system with different schemes are studied in detail.The main contents are as follows:(1)Considering the modified Von Karman turbulence,the average bit error rate(ABER)performance of OAM multiplexing-based MIMO FSO system has been investigated with space-time coding.On the basis of STC code criterion of wireless communication,the OAM multiplexing-based vertical bell labs layered space-time(V-Blast)and space-time block codes(STBC)scheme are designed.With the help of LS channel estimator,the analytical expressions of the received signals carried by the k-th OAM mode of the n-th receive antenna for the V-Blast and STBC are derived,respectively.The ABER performance of this OAM-FSO system with and without STC are presented under different turbulence strengths firstly.The impact of different types of space-time coding,modulation orders,turbulence strengths,receive antenna numbers on the transmission performance of this OAM-FSO system is also taken into account.The results show that both V-Blast and STBC schemes can significantly improve the system performance by mitigating the distortions of atmospheric turbulence as well as additive white Gaussian noise(AWGN).The ABER performances of both space-time coding schemes can be further enhanced by increasing the number of receive antennas for the diversity gain and STBC outperforms V-Blast in this system for data recovery.(2)Changed the MIMO antenna configuration to a circular array,an LDPC coded OAM-FSO system exploring equalization over the Von Karman atmospheric turbulence was studied in detail.Then,with the help of the large number theorem,the ABER expressions with M_PPSK modulation were derived by ensemble average combined with the CSI obtained by LS channel estimator.Subsequently,LDPC codes are adopted to enhance the ABER performance.The ABER performances of this system are analyzed with different turbulence strengths,modulation orders,and the receive antenna numbers,respectively.Results show that this OAM-FSO system with MMSE equalization performs better than ZF equalization.And the ABER performance is significantly enhanced with an increase in the number of receive antennas for considerable diversity gain.Furthermore,a substantial coding gain can be attained by LDPC codes,especially under strong turbulence condition.This work will benefit the design and development of OAM-FSO system. |
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