Research On Photonics-Aided Terahertz Multi-Subcarrier Communication Technologies Towards 6G

With the increasing demand for data communication traffic,it is difficult to further improve the capacity of traditional low-frequency communication system due to the limitation of bandwidth.In the Beyond-5G and 6G eras,it is an inevitable trend for the carrier frequency to develop to a higher frequency and gradually enter the terahertz band.Terahertz(THz)wave is an electromagnetic wave with a frequency ranging from 0.1 to 10 THz,which features large bandwidth,good directivity and strong penetration,and is one of the ideal bands for ultra-high rate wireless communication.However,it is difficult to transmit ultra-high rate THz signal generated by all-electric methods for bandwidth limitation of electron device,so the photonicsaided generation of THz signal is generally adopted.Most of the current photonics-aided THz wireless communication systems are based on single-carrier(SC)modulationthat are greatly affected by dispersion and nonlinear effects of fiber channel and multipath effects of wireless channel,so they have limited transmission rate.In addition,for the photonics-aided THz communication system based on vector modulation,the cost is high and the receiver digital signal processing(DSP)is complex.Therefore,we conduct the following research on the photonics-aided THz wireless communication system.Firstly,a scheme of photonics-aided THz wireless communication system is proposed based on subcarrier multiplexing(SCM).The overall system structure is designed and models of constituent units are introduced and derived.We also describe the functions and algorithms of each module of DSP at the transmitter and receiver in detail.Then,the key problems are analyzed for SCM-based photonics-aided THz wireless communication system,including limited bandwidth,excessive peak to average power ratio(PAPR)and phase noise,and the corresponding solutions are proposed.We apply power preemphasis at the transmitter to offset the power fading of high-frequency subcarriers caused by limited bandwidth.Hard-limiting is used to reduce PAPR by nonlinear signal processing,and a cascade algorithm is adopted to accurately estimate and compensate phase noise of high order modulation scheme.Next,digital demodulation algorithms are designed for multi-subcarrier photonics-aided THz wireless communication system based on vector modulation and heterodyne coherent detection.The feasibility of algorithms is proved by simulation in case of quadrature phase shift keying(QPSK)and hexadecimal orthogonal amplitude modulation(16-QAM).We respectively demonstrate the transmission of 46 Gbit/s QPSK and 92 Gbit/s 16-QAM signals with 4subcarriers in the 300 GHz frequency band over 20 km fiber link and back to back(B2B)wireless link.Then,the error performance and hardware complexity of two used phase recovery algorithms are compared and analyzed.Finally,simplified DSP algorithms are designed for multi-subcarrier photonics-aided THz wireless communication system based on intensity modulation and heterodyne coherent detection.The feasibility of transmission system is verified by simulation in case of phase insensitive and phase sensitive four-level pulse amplitude modulation(PAM-4)signals.25 km fiber transmission and B2 B wireless transmission of 46 Gbit/s phase insensitive PAM-4 and 46Gbit/s phase sensitive PAM-4 signals with 4 subcarriers at 100 GHz is successfully achieved.