CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is based upon and claims priority to Chinese Patent Application No. 202010162635.7, filed on Mar. 10, 2020, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention belongs to the technical field of high-frequency wireless communication, and more particularly relates to a design method for an intelligent reflecting surface (IRS) aided terahertz secure communication system.

BACKGROUND

With the rapid expansion of wireless communication technology, academia and industry pay more attention to the exploitation and utilization of high-frequency spectrum resources. These resources include terahertz frequency band. However, terahertz communication has challenges in practical application scenarios. First, due to the severe path attenuation and molecular absorption loss, terahertz communication is limited to short-range indoor scenarios. Second, because of the strong directivity and poor diffraction of terahertz waves, the terahertz wave is more likely to be blocked by indoor obstacles, like furniture and walls.

To address these problems, an intelligent reflecting surface (IRS) is proposed, which can effectively improve the coverage range and spectral efficiency of a terahertz communication system. Specifically, the IRS is able to change the direction of the terahertz wave by adjusting phase shifts for each reflecting element. And thus, the secrecy rate of the communication system can be improved. One of the simplest schemes to control the phase shifts of the IRS is the exhaustive search method, which can obtain the maximal secrecy rate by comparing all the possible phase-shift combinations for all the reflecting elements. The calculation of the exhaustive search method, however, is extremely complicated.

SUMMARY

The objective of the present invention is to optimize the secrecy rate for the IRS aided terahertz multi-input single-output (MISO) secure communication system, where a cross-entropy based method for phase search is disclosed to reduce the phase search complexity of IRS elements and to improve the secrecy rate of the system simultaneously. In the present invention, the estimated phase shift matrix of the IRS is successively updated according to the principle of maximizing the secrecy rate during the iterative process, so as to decrease the optimal phase search complexity of the IRS and to achieve the basically consistent secrecy rate performance with the optimal exhaustive search method. In addition, the secrecy rate performance of the present invention is superior to the terahertz MISO secure communication system without the IRS.

The technical solution of the present invention is a design method for an IRS aided terahertz secure communication system, which can improve the data transmission rate of the downlink terahertz MISO system by designing the phase shifts of all the reflecting elements in the downlink terahertz MISO system. In this system, a base station (BS) equipped with NBSantennas communicates to a single-antenna mobile authorized user, while a single-antenna eavesdropper exists. To assist this communication, an IRS with NIRSreflecting elements is installed in the communication process to enhance the signals of the authorized user and to suppress the signals of the eavesdropper. In the downlink terahertz MISO secure communication, the transmission signals can be sent to the relay with the IRS by active beamforming at the BS, and the IRS reflects the signals to the mobile authorized user side by adjusting the phase shift of each reflecting element. In this way, the received signals of the eavesdropper can be greatly restricted and thus the secure communication performance can be enhanced accordingly. One may note that the recently disclosed IRS and the traditional relays are different in that the IRS composed of passive components just passively reflects the received electromagnetic waves and cannot realize the signal processing, which greatly reduces the power consumption of the system. For the IRS aided terahertz system, assuming that a working frequency of the system is f, an optional discrete phase set of each reflecting element of the IRS is F, a channel between the BS and the IRS is Ht, a channel between the IRS and the authorized user is hru, a channel between the BS and the authorized user is hdu, a channel between the IRS and the eavesdropper is hre, a channel between the BS and the eavesdropper is hde, an average receiving power is ρ and a channel noise power is δ2.

The phase shift design method of the IRS includes:

maximizing the downlink secrecy rate Rsec=[RU−RE]+, where RUis the data transmission rate of the authorized user and REis the transmission data rate of the eavesdropper, RUand REare expressed as follows:

RU=log2⁡[1+ρ⁢(hruH⁢Θ⁢⁢HtH+hduH)⁢F2δu2],⁢RE=log2⁡[1+ρ⁢(hreH⁢Θ⁢⁢HtH+hdeH)⁢F2δe2],

where F is a precoding matrix at the BS, and Θ is the phase shift matrix at the IRS. Further, the equation Rsec=[RU−RE]+means that when Rsec=[RU−RE]>0, Rsec=RU−RE, otherwise Rsec=0. Since the optimal value of the optimization problem must be more than 0, the equation Rsec=RU−REis always satisfied.

By jointly optimizing two matrix variables F and Θ to maximize the secrecy rate, establishing an objective function as follows:

(Θopt,Fopt)=arg⁢⁢max⁢⁢Rsec,⁢s.t.⁢θn∈F,∀n=1,…⁢,NIRS,⁢Θ=diag⁡([ej⁢⁢θ1,ej⁢⁢θ2,…⁢,ej⁢⁢θNIRS]T),⁢FF2=1;

where the first constraint comes from the discretization of phase shifts of the reflecting element of the IRS, the second constraint comes from the communication model of the IRS, and the third constraint comes from the normalized transmission power. The optimal precoding matrix Foptand the phase shift matrix Θoptare obtained by solving the objective function.

The method for solving the objective function includes:

S1, performing an initialization, wherein

an optional phase set of the IRS is

F={0,2⁢π2b,L,2⁢π2b⁢(2b-1)},
where b is a bit quantization number; an initial probability phase shift matrix of the IRS is

p(O)=12b×NIRS2b,
where

12b×NIRS
is an all-ones matrix with size 2b×NIRS; the total number of iterations of the algorithm is I1, and the current iteration of the algorithm is i1=1; the total number of the iteration of the sub-algorithm is I2, and the current iteration of the sub-algorithm is i2=1; the number of samples for each iteration is S, and the number of optimal phase shift matrix samples for each iteration is Selite;

S2, when i1≤I1, cyclically performing S3-S11;

S3, computing the optimal precoding matrix Fopt=umax(A,B)/∥umax(A,B)∥2, where umax(A,B) is a generalized eigenvector based on the largest generalized eigenvalue of matrix B−1A, and the matrix A and the matrix B are expressed as follows:

A=INBS+ρδu2⁢(hruH⁢Θ⁢⁢HtH+hduH)H⁢(hruH⁢Θ⁢⁢HtH+hduH),⁢B=INBS+ρδe2⁢(hreH⁢Θ⁢⁢HtH+hdeH)H⁢(hreH⁢Θ⁢⁢HtH+hdeH);

where INBSis an identity matrix with size NBS×NBS;

S4, when i2≤I2, cyclically performing S5-S10

S5, randomly generating S phase shift matrices {Θs}s=1Sbased on the current probability phase shift matrix p(i)of the phase shifts of the IRS;

S6, calculating the objective

γ=1+ρ⁢(hruH⁢Θ⁢⁢HtH+hduH)⁢F2⁢/⁢δu21+ρ⁢(hreH⁢Θ⁢⁢HtH+hdeH)⁢F2⁢/⁢δe2;

S7, sorting {γ(Θs)}s=1Sin a descending order as γ(Θ(1))≥γ(Θ(2))≥ . . . ≥γ(Θ(S));

S8, selecting the first Selitevalues of the objectives γ(1), γ(2), . . . , γ(Selite), where the corresponding phase shift matrix is {Θs}s=1Selite;

S9, updating the probability phase shift matrix p(t+1)based on {Θs}s=1Selite;

S10, updating the current iteration to be i2=i2+1;

S11, updating the current iteration to be i1=i1+1;

S12, obtaining the optimal phase shift matrix Θopt=Θ(1), and the secrecy rate of the IRS aided terahertz MISO system R=log2γ(Θ(1)).

In the IRS aided terahertz MISO secure system of the present invention, since the traditional exhaustive search method must calculate all the phase shift combinations of the IRS elements, the possible combinations of the superabundant reflecting elements of the IRS increase exponentially. This significantly increases the computational complexity of the exhaustive search method. To overcome the tremendous calculation burden, the invention includes a cross-entropy based phase shift search method, which effectively reduces the computational complexity of the phase shift search of the IRS. Specifically, the cross-entropy based method updates the phase probability of each reflection element during the iterative processes, thereby causing the estimated phase shift matrix to gradually approach the optimal value. Therefore, the disclosed method of the present invention can improve the secrecy rate achievable for the system by optimizing the phase shift matrix and the precoding matrix with minimal computational complexity.

The advantages of the present invention are as follows:

Compared with the terahertz MISO secure communication system without IRS, the cross-entropy based method for the IRS-aided secure system is able to significantly improve the secrecy of the communication.

Besides, the disclosed cross-entropy based method in the present invention is easily achieved by hardware and applied in communication due to minimal computational complexity.

