CN-116707660-B - Terahertz ISAC system design method based on time delay alignment modulation and active RIS

Abstract

The invention provides a terahertz ISAC system design method based on time delay alignment modulation and active RIS, which comprises the steps of firstly, constructing an active RIS-assisted terahertz communication perception Integrated (ISAC) system, calculating signals received by a user and signal to noise ratio based on the terahertz ISAC system, secondly, calculating illumination power at a static target by adopting incoherent accumulation, simultaneously, calculating thermal noise and power consumed by the active RIS based on the static target, and finally, maximizing illumination power of the target by jointly designing reflection coefficient of the active RIS and emission beam forming vector at DFBS, and solving by a semi-definite program relaxation technology. The invention optimizes DFBS emission beam forming vector and reflection coefficient of active RIS, improves perception performance and ensures communication quality, and the terahertz ISAC system provides incomparable benefits in terms of sensing and communication performance by utilizing unique superiority of RIS and DAM.

Inventors

• SUN GANGCAN
• SHI HAO
• HAO WANMING
• XUE QI

Assignees

• 郑州大学
• 嵩山实验室
• 郑州大学产业技术研究院有限公司

Dates

Publication Date

20260512

Application Date

20230413

Claims (1)

1. 1. A terahertz ISAC system design method based on time delay alignment modulation and active RIS is characterized by comprising the following steps: Step one, an active RIS assisted terahertz communication perception Integrated (ISAC) system is built, which comprises a single-antenna user, a static target and a dual-function base station (DFBS), wherein DFBS is assisted by time Delay Alignment Modulation (DAM); calculating a signal and a signal-to-noise ratio of a signal received by a user based on a terahertz ISAC system; The expression of the signal received by the user is: (7) Wherein, the For signals received by the user, the superscript H denotes the conjugate transpose, DFBS to the user Is used for the channel vector of (a), Indicating that the base station transmits a signal to user c, Representing RIS to user Is used for the channel vector of (a), In order to be a reflection coefficient, A channel matrix representing DFBS to RIS, M representing the number of antennas contained in DFBS, N representing the number of reflective elements contained in the active RIS; representing transmit beamforming vectors associated with direct links, Representing a transmit beamforming vector associated with the RIS link; Is Gaussian white noise with variance of ; Is the thermal noise introduced, the variance is ; An identity matrix with a dimension of N; A channel discrete time delay representing DFBS-RIS-User symbol duration; the expression of the signal-to-noise ratio is: (8) Wherein, the Is the signal to noise ratio; Calculating illumination power at a static target by adopting incoherent accumulation, and simultaneously calculating received thermal noise and power consumed by an active RIS at the static target based on the active RIS; The expression of the illumination power is: (9) Wherein, the For the illumination power at the target, In order to transmit the beamforming vector, DFBS to the target Is used for the channel vector of (a), For RIS to target Is a channel vector of (a); The expression of thermal noise received at the static target is: (10) Wherein, the Receiving thermal noise for a static target; The power consumed by the active RIS is: (11) Wherein, the Power consumed for active RIS; step four, constructing an objective function according to the signal received by the user in the step two, the signal-to-noise ratio of the signal, the thermal noise received at the static target in the step three and the power consumed by the active RIS; the objective function is: (12a) (12b) (12c) (12d) (12e) (12f) (12g) Wherein, the Is the maximum transmit power of DFBS a, Is the maximum reflected power of the active RIS, For a maximum thermal noise power of the target, Representing the minimum signal-to-noise ratio of the user, Representing the upper limit of the magnification factor, Represent the first Individual RIS elements; Converting the objective function into two sub-objective functions by using an alternate optimization method, and then respectively solving the two sub-objective functions by using a semi-definite program relaxation technology to obtain an optimal emission beam forming vector and reflection coefficient; In the fifth step, the specific implementation method is as follows: S5.1 fixing Optimizing Given a fixed transmit beamforming vector Objective function Conversion to the first sub-objective function : (13a) (13b) The following symbols are defined: (14) Order the Will be Conversion to Form: (15a) (15b) (15c) (15d) (15e) Wherein, the ; Introducing a new matrix variable And according to Principle of non-convex constraint by SDR Conversion to convex constraint : (16a) (16b) (16c) (16d) (16e) (16f) Relaxing the constraint of (16 f), i.e. Solution using convex optimization toolbox Obtaining By Gaussian randomization method Deriving to obtain optimal values satisfying the formulas (12 b), (12 d), (12 g) and (12 f) and (13 a) According to the optimum Obtaining the optimal value ; S5.2 fixing Optimizing The objective function is set Conversion to the second sub-objective function : (17a) (17b) Defining intermediate variables: (18) Wherein, the 、 All are the intermediate variables of the two-way valve, Representing an identity matrix with dimension M; By using And Obtaining And Thereby ensuring that the result is obtained Satisfying (12 e) while maximizing (17 a), definition And And relaxed using SDR methods Formula (17) can be restated as: (19a) (19b) (19c) The following symbols are defined: (20) Obtained using a convex optimization tool Using gaussian randomization method pairs Deriving to obtain variables satisfying the formulas (12 b), (12 c), (12 d), (12 e) and (17 a) 。

Description

Terahertz ISAC system design method based on time delay alignment modulation and active RIS Technical Field The invention relates to the technical field of terahertz communication, in particular to a terahertz ISAC system design method based on time delay alignment modulation and active RIS. Background Due to the explosive growth of the number of electronic devices, problems such as spectrum congestion may occur in the future. Communication awareness Integration (ISAC) is a technology which is attractive in 6G systems and has a wide prospect. ISACs can achieve efficient sharing of the spectrum of communication and sensing signals and can achieve both communication and sensing functions simultaneously through Dual-function base stations (Dual-Function Base Station, DFBS). Smart reflective surfaces (Reconfigurable Intelligent Surfaces, RIS) also play a critical role in 6G systems. RIS is composed of a large number of elements that can intelligently and instantaneously change the physical properties of a surface. By changing the physical properties, the environment in which the wireless signal propagates can be modified, improving the quality of the wireless signal, and expanding the coverage of the wireless signal by introducing additional paths. However, since RIS is subject to "multiplicative fading," the use of RIS in many wireless environments does not produce significant capacity gain, especially when the direct link signal is strong. To address this problem, documents [Z.Zhang,L.Dai,X.Chen,C.Liu,F.Yang,R.Schober,and H.Vincent Poor,"Active RIS vs.Passive RIS:Which Will Prevail in 6G?,"IEEE Trans.Commun.,pp.1–1,Dec.2022.] and [K.Zhi,C.Pan,H.Ren,K.K.Chai,and M.Elkashlan,"Active RIS Versus Passive RIS:Which is Superior With the Same Power Budget?,"IEEE Commun.Lett.,vol.26,no.5,pp.1150–1154,May.2022.] propose active RIS that can reflect and amplify signals simultaneously. Each reflecting element is equipped with a power amplifier that can be used to compensate for the large path loss of the reflecting link, overcoming the "multiplicative fading". Naturally, RIS-assisted ISAC systems have attracted attention by researchers. In literature [A.A.Salem,M.H.Ismail,and A.S.Ibrahim,"Active Reconfigurable Intelligent Surface-Assisted MISO Integrated Sensing and Communication Systems for Secure Operation,"IEEE Trans.Veh.Technol.,pp.1–13,Dec.2022.], authors have studied active RIS assisted multi-user systems while physical layer security issues are intercepted by Unmanned Aerial Vehicles (UAVs), using DFBS to perceive the position of the UAV while maximizing the privacy rate of the system by proper design. Document [R.P.Sankar and S.P.Chepuri,"Beamforming in Hybrid RIS assisted Integrated Sensing and Communication Systems,"in Proc.of the European Signal Process.Conf.(EUSIPCO),Belgrade,Serbia,Aug.2022.], which also discusses the upper power limit constraint problem of hybrid RIS, explores the hybrid RIS assisted ISAC system to serve multiple users and targets, maximizing minimum target illumination power while ensuring user communication quality by optimizing the reflection coefficients of the hybrid RIS and the transmit beamforming vector of DFBS. However, there is a very important but often neglected problem. The existing research ignores the effect of delay differences, optimistically assumes that multiple RIS links and direct links can achieve perfect time synchronization, and optimizes parameters based thereon to improve system performance. However, in practical applications, due to the difference of path distances, multiple signals cannot reach the user end at the same time. The optimization of the system becomes particularly complex if traditional complex equalization or multi-carrier processing is employed. Fortunately, an emerging technique called Delay Aligned Modulation (DAM) can simply solve this problem. By deliberately introducing delays at the base station and properly designing the transmit beamforming vector, multipath signals can arrive at the receiver simultaneously and can be effectively superimposed to cancel inter-symbol interference (ISI), thereby forming an ISI-free Additive White Gaussian Noise (AWGN) system. The DAM may avoid drawbacks of some OFDM such as lower implementation complexity and Peak-to-Average-Power Ratio (PAPR) while achieving higher spectral efficiency. However, it has superior performance only for multipath sparse channels, which can be achieved by high frequency carriers (e.g., millimeter waves and terahertz). Disclosure of Invention Since RIS is subject to "multiplicative fading," the use of RIS in many wireless environments does not produce significant capacity gains; meanwhile, the influence of time delay difference is ignored, so that the technical problem that the optimization of the system becomes particularly complex is solved; the invention provides a terahertz ISAC system design method based on time delay alignment modulation and active RIS, which is based on a basic