Research On Key Technologies Of Coverage Enhanced Wireless Transmission Based On Programmable Metasurface

2019 is the first year of 5G commercialization.With the commercial deployment of5 G,research on the next-generation mobile communication system has begun.The research of 6G urgently needs breakthrough innovative technologies to meet new challenges raised in many aspects,such as hardware cost,system energy consumption and spectrum efficiency.As a new type of artificial electromagnetic surface,programmable metasurface has attracted widespread attention of scientists due to its unique electromagnetic physical properties.With the development of metamaterials and micro electromechanical systems,programmable metasurface can control electromagnetic parameters such as amplitude and phase in real time through external signal control.At the same time,programmable metasurface can carry information through internal coding control sequence,which provides new ideas and possibilities for 6G mobile communication system architecture.Based on the above background,we take advantage of the programmable metasurface's directional control capability of electromagnetic waves to study the key technologies of coverage enhancement based on programmable metasurface in wireless communication systems.First,we review the development history of five generations of mobile communication systems,and briefly introduce the key technologies and standards of each generation's mobile communication systems.We next introduce three typical application scenarios and three core key technologies of 5G in detail.With the further evolution of 5G key technologies to 6G,we present the 6G vision and three key candidate technologies,and point out the challenges in 6G research and the possibility of using programmable metasurface to solve problems.Subsequently,the structure,classification and development history of programmable metasurface are introduced in detail,and two hot research directions of programmable metasurface are summarized.Among them,regarding the coverage enhancement problem concerned in this dissertation,we summarize the advantages of programmable metasurface-based relays and the disadvantages of traditional relays.Next,we study the coverage enhancement technology based on centralized programmable metasurface wireless relay system.The main components of a centralized programmable metasurface wireless relay system architecture are introduced,and the system channels and received signals are modeled.Under the condition that the perfect CSI is known at the base station,we derive the upper bound of the system uplink ergodic achievable rate based on Jensen inequality.Monte-carlo simulation results show that the upper bound has good tightness;meanwhile,a closed-form expression of the optimal phase design of the programmable metasurface is theoretically derived based on the result of the upper bound.Under the condition that the base station cannot acquire perfect CSI,a cascaded channel estimation method based on segmented pilots is proposed.The phase design of programmable metasurface is attained through theoretical derivation and iterative algorithm.The MSE curve obtained from the data simulation shows that when the signal-to-noise ratio is 14 d B,the MSE approximates-10 d B,which can achieve good signal transmission,and we also gives the suboptimal phase value of the programmable metasurface based on the iterative algorithm.Afterwards,we focus on the coverage enhancement technology based on distributed programmable metasurface wireless relay system.After modeling the system channels and received signals,based on the Jensen inequality,the upper bound of the system downlink ergodic achievable rate is derived.Monte-carlo simulation results showcase that under high SNR region and different Rician K-factors,this upper bound can be viewed as a tight approximation.In order to simplify the difficulty of phase design in distributed systems,considering the scenario of large-scale antennas equipped at the base station,the theoretical optimal phase expression in Chapter 2 can be applicable as a adjacent optimal solution in distributed systems.considering the two criteria of maximizing user achievable rate and scheduling fairness,we propose a distributed programmable metarsurface scheduling algorithm based on greedy algorithm.With the exhaustive method,greedy algorithm,and random selection as reference algorithms,simulation results show that our proposed algorithm can achieve the same performance as the greedy algorithm with the number of scheduled programmable metasurface,and can balance the scheduling fairness and efficiency.Finally,based on programmable metasurface,we conducts hardware implementation and verifies its performance as a wireless relay.A simple prototype system based on programmable metasurface as a relay is set up,and the system architecture and hardware equipment are introduced in detail.We explain the principle of controlling the direction of the electromagnetic wave and the mapping between the control voltage and the phase of the programmable metasurface.The relay capability of the programmable metasurface is tested in a microwave far-field darkroom.the received electromagnetic wave pattern shows that the programmable metasurface can control the propagation direction of the electromagnetic waves and matches well with theoretical simulation results.At the same time,the relationship between the phase setting gradient of different columns and the reflection angle of the electromagnetic waves is analyzed,thereby providing measured support for introducing the programmable metasurface as a relay in wireless communication systems.