Research On Channel Capacity Of Reconfigurable Intelligent Surfaces Assisted Communication System Without Direct Link

With the development of the fifth generation(5G)wireless communication system,people’s demand for high security,high quality and ubiquitous wireless services is increasing,which brings great challenges to the existing cellular networks.To adapt to changes,the design goals of the sixth generation(6G)wireless communication system are revolutionary,including data-driven,instantaneous,hyperscale and ubiquitous wireless connectivity and intelligent connectivity,these new applications and services require new transmission technologies to support.Reconfigurable Intelligent Surface(RIS),as one of the key technologies of 6G,can change the reflection phase in real time under the control of software programming,and can reflect the incident signal according to the specified direction,and the noise impact and transmission delay of the transmission signal are very small,so it is widely used in wireless transmission link that actively change signals.In this thesis,the channel capacity of RIS-assisted MIMO communication system and RIS-assisted SISO communication system and the secrecy capacity of RIS-assisted MIMOME and RIS-assisted SISO-SE are studied from the perspective of information theory without considering the direct link and considering the time variability of the reflection phase shift of RIS.In practical communication system,all the inputs are discrete signals.In this thesis,the achievable rate formula and achievable secrecy rate formula under discrete finite base input and continuous Gaussian input are derived,and the upper and lower bounds of secrecy capacity are given.The main research contents of this thesis are as follows:(1)The joint encoding scheme and layered encoding scheme based on the real-time change principle of RIS reflection phase shift are applied to RIS-assisted MIMO system and RIS-assisted SISO system,and the channel capacity of the two systems is derived and analyzed.Firstly,the joint encoding scheme of RIS-assisted MIMO system is studied.The scheme encodes the transmitted codeword and RIS reflected phase shift into a joint codeword,which can be used as the input of the equivalent transmission system.By optimizing the joint probability function of the transmitted code word and RIS reflected phase shift,the maximum transmission rate of the scheme can reach the channel capacity.Secondly,in order to reduce the optimization complexity of the joint encoding scheme,the sub-optimal layered encoding scheme is studied.In this scheme,the transmitted message sequence is divided into two successive layers,which are encoded into RIS reflection phase shift and transmitted codeword respectively.The receiver decodes these two parameters by Successive Cancellation Decoding(SCD)technology.Finally,the joint encoding scheme and layered encoding scheme are applied to RIS-assisted SISO system,and the achievable rate formulas of these two schemes are derived.The simulation results show that the achievable rate of joint encoding is higher than that of layered encoding in RIS-assisted MIMO system and RIS-assisted SISO system,and the encoding scheme combined with RIS reflection phase shift time-varying can effectively improve the transmission rate.(2)In order to solve the problem of physical layer security,the joint secure encoding scheme and layered secure encoding scheme are proposed,and the secrecy capacity of RISassisted MIMO-ME and RIS-assisted SISO-SE is derived.Firstly,the joint secure encoding scheme is obtained by combining the joint encoding with Wyner random packing.The achievable secrecy rate of this scheme is derived,and the upper and lower bounds of the secrecy capacity are given.Secondly,the layered secure encoding scheme is obtained by combining layered encoding with Wyner random packing,and the achievable secrecy rate of this scheme is derived.The simulation results show that the achievable secrecy rate of the joint secure encoding scheme is higher than that of the layered secure encoding scheme in RISassisted MIMO-ME and RIS-assisted SISO-SE,and both schemes may ensure the security of these two systems.