High-speed Terahertz Photonic Wireless Communication Assisted By Direct Detection

With the rapid development of ultra-broadband communication services,such as mobile Internet,data traffic in the Beyond-5G and 6G eras will exponentially grow.In order to solve the challenges of upgrading wireless capacity,wireless communication is developing towards higher carrier frequencies.In this context,terahertz(THz)waves with frequencies between 0.1 THz and 10 THz have been considered as one of the ideal carriers to realize ultra-high-speed wireless communications,as it provides the advantages of large bandwidth,good directivity,and strong penetration.Up to date,the scheme of realizing terahertz wireless communication systems can be mainly divided into two categories: photonic scheme and electronic scheme.Comparatively,the terahertz communication systems based on photonics have advantages of large modulation bandwidth,high modulation efficiency and optical fiber transmission transparency,and hence it has become a research hotspot in recent years.According to the detection method of terahertz signals,photonic terahertz communication systems can be further divided into coherent detection and direct detection.Compared to coherent detection systems,a direct detection terahertz photonic system does not need additional elements such as a radio frequency source and a mixer at the receiving end,resulting in simple and cost-effective system structure.However,most of the photonic terahertz direct detection systems employ the NRZ modulation format,which is not sufficient for large-capacity and energy efficient communications.To deal with this situation,utilizing high-order modulation formats and advanced digital signal processing algorithms is an ideal technical solution to increase the communication rate and extend the wireless distance of the system.This thesis proposes to apply two types of high-order modulation formats: pulse amplitude modulation(PAM)and discrete multitone(DMT)to the direct detection terahertz photonic communications,and experimentally demonstrate the high speed terahertz photonic wireless transmission systems.In this thesis,we first introduce the key components in a direct detection terahertz photonic communication system,design the system structure,analyze the main factors affecting the signal-to-noise ratio in the system,as well as estimate the link power budget.Next,the thesis focuses on designing the PAM-4 and DMT digital signal processing flowchart for direct detection systems,and introducing the algorithms for modulation and demodulation.Specifically,the key techniques in PAM-4 signal processing include spline interpolation based clock recovery,Volterra-DFE algorithmbased nonlinear channel equalization,etc.The key techniques in DMT signal processing include PN sequence-based frame synchronization,block pilot-based linear equalization,Volterra time domain filtering algorithm-based nonlinear equalization,etc.Finally,we experimentally implement a direct detection terahertz photonic communication system,and 3 m wireless transmission of 60 Gbit/s PAM-4 and 60Gbit/s 16QAM-DMT signals in the 310 GHz frequency band is successfully achieved.This thesis focuses on the key technologies of realizing direct detection terahertz photonic communications,introduces high-order modulation and advanced signal processing algorithms into direct detection terahertz photonic communication systems,realizes nonlinearity aware signal equalization,breakthrough the bit-distance product of direct detection terahertz photonic communication systems.The relevant results are expected to provide a new dawn and support the development of high-speed terahertz wireless communications.