Physical layer secure transmission in the coexistence of V2I and V2V with RIS assistance

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Introduction
With the widespread application of 5G communication technology and research on 6G communication technology, future internet of vehicles (IoV) will gradually achieve ultra-efficient and ultra-reliable wireless communication.IoV will bear the burden of transmitting large-scale safety-related and privacy-related data, making the security of IoV terminal communication increasingly important.However, there are currently still issues with low security and unreliable continuity in IoV communication under extreme conditions.The current IoV communication mainly adopts two paradigms: vehicle-toinfrastructure (V2I) and vehicle-to-vehicle (V2V).On one hand, vehicle terminals communicate through base stations using 5G technology; on the other hand, they can also directly communicate with nearby vehicles using device-to-device (D2D) technology.The appropriate paradigm is chosen for communication based on data type and transmission performance requirements.As the scale of IoV expands, eavesdropping terminals may be constantly present, posing challenges to physical layer secure transmission.Therefore, further research is needed to enhance physical layer security spectrum efficiency under the coexistence of V2I and V2V paradigms.
In recent years, the emergence of reconfigurable intelligent surface (RIS) technology has played an important role in promoting mobile communication development, which has brought huge gains in signal enhancement, blind area coverage and sensing.RIS is composed of a large number of electromagnetic units with simple circuits and low power consumption.If regarded as signal reflection function, these units can be flexibly controlled by digital circuits.Through the circuit control of these units, the phase, amplitude or frequency of the incident electromagnetic signal can be adjusted, so that the reflected electromagnetic wave has a directional beamforming.Therefore, RIS is considered as one of the key technologies for future mobile communication due to its advantages of low power consumption, low cost and flexibility.Research on RISassisted mobile communication has also been greatly developed.The channel modeling and channel estimation problem assisted by RIS in [1] and [2], the information and energy synchronous transmission combined with RIS in [3] and [4], the physical layer security transmission assisted by RIS in [5], the multi-antenna beamforming in [6], and the assisted cognitive radio communication in [7] have been more and more widely and deeply studied.
The ability of RIS to reconfigure the wireless environment in a favorable way to improve the quality of service (QoS) of wireless communication makes RIS a promising candidate for enhancing V2X applications, and the research on RIS-assisted vehicle networking communication has also become one of the hot topics.In IoV, the radio propagation is negatively affected by a large number of objects (such as high-rise buildings) and dynamic vehicles, and there are blind areas where the communication link is blocked within the coverage of the base station.RIS can well assist these blind spots to communicate and enhance the wireless link.The authors in [8] and [9] studied the resource allocation problem of RIS-assisted vehicular communication based on slowly varying large-scale fading channel information.By maximizing the total capacity of the V2I link, while ensuring the minimum signal-to-noise ratio (SINR) of the V2V link, the power allocation, IRS reflection coefficient and spectrum allocation are jointly optimized.As the same object function, the authors in [10] studied a multiuser spectrum sharing problem in RIS-assisted vehicular networks, where multiple V2V links reuse the spectrum already occupied by V2I links.To overcome the difficulty of obtaining instantaneous channel state information (CSI) due to the rapidly varying nature of some V2X channels, this study relies on large-scale (slowly varying) CSI to meet the QoS requirements of V2I and V2V communications.The vehicle transmit power, multiuser detection matrix, V2V link spectral reuse and RIS reflection coefficient are jointly optimized.The authors in [11] studied the use of RIS in millimeter wave IoV.The weighted sum-rate maximization problem in the uplink is considered, and in the single-user case, two phase optimization schemes that help reduce the channel estimation overhead are considered.In the multi-user case, the original problem is reformulated into a more convenient form using fractional programming and an alternating optimization algorithm is proposed.Literature [12] further analyzed the possibility of using deep reinforcement learning (DRL) method to solve the above problems, and jointly optimized the base station and RIS beamforming to improve the network performance in millimeter wave V2I communication system.Also using the deep reinforcement learning method, literature [13] formulated the joint optimization problem of roadside units (RSU) resource scheduling and RIS passive beamforming with the objective of maximizing the minimum average transmission rate.A DRL method was used to determine the RSU wireless scheduling, and block coordinate descent (BCD) algorithm was proposed to solve the phase shift matrix.The authors in [14] proposed a three-stage joint resource allocation method for V2I and V2V spectrum sharing.Literature [15] has optimized and solved the spectrum sharing and resource scheduling problem by combining the requirements of low packet delay and high reliable transmission.Literatures [16] and [17] further extended the joint beam optimization and resource allocation problem of MISO IoV assisted by RIS to MIMO system.The above studies demonstrate the gain in system throughput brought by RISassisted IoV on the basis of reasonable resource allocation and active and passive beams.
In addition, the performance analysis of RIS-assisted IoV communication, such as outage probability, average outage time and flat exchange rate of received signal envelope, has also been studied in [18] and [19].In terms of channel estimation, the authors in [20] and [21] gave a low overhead channel estimation protocol in RISassisted IoV, and proposed a compressed sensing channel estimation algorithm based on location information assistance to reduce the additional training overhead of the channel.The performance and joint beam optimization method in the case of imperfect channel estimation were further studied in [22].In order to meet the service requirements of different vehicle communications, the authors in [23] analyzed the sum rate maximization problem under the hybrid OMA/NOMA access scheme, and jointly optimized the transmit power allocation, IRS phase shift and vehicle active beamforming under different QoS levels with the constraints of vehicle reliability and rate requirements.In [24] and [25], the optimal selection method of multiple RIS was designed based on the maximization of ergodic capacity and the minimization of symbol average error probability.In [26], a conformal RIS deployed on the vehicle body was proposed to mitigate the blocking effect by generating artificial reflections and compensate the non-flat shape of the vehicle body with an appropriate phase pattern.A new conformal RIS design with optimized pre-configuration was proposed to demonstrate its advantages in V2V highway scenarios.
In fact, the research on RIS-assisted physical layer secure communication for IoV also needs to be deepened.The authors in [27] studied the influence of RIS deployment location and the number of RIS units on the physical layer secure transmission rate (PLSTR) of the IoV system.Literature [28] also studied and compared the PLSTR of RIS with traditional relay in IoV system.In [29], two cases of V2V communication and vehicle-to-RIS communication were considered and closed-form expressions for the outage probability of PLSTR were derived in presence of one eavesdropper.The potential of using RIS to improve the PLSTR is demonstrated in above studies.
In this paper, we design a new method to maximize the PLSTR of V2I users in the coexistence of V2I and V2V in the case of existing an eavesdropping vehicle user.The main contributions in this paper are as follows: (1) The physical layer secure communication model of the coexistence of V2I and V2V with the help of RIS is established, and the model of the PLSTR maximization problem of V2I users is given under the constraints of the SINR requirement of V2V users, the RIS phase shift range and the limited transmit power.
(2) An alternating optimization algorithm based on generalized Rayleigh entropy and semi-definite relaxation program is proposed to jointly optimize the transmission precoding vector at the V2I base station and RIS reflection coefficient matrix to maximize the PLSTR of the V2I user.
(3) By comparing with the baseline algorithms, the superiority of the proposed algorithm is proved.The simulation results show the gain brought by RIS in the physical layer security transmission of the IoV.The influence of related system parameters on the PLSTR is given.1, where the symbol ℂ  denotes the set of complex column vectors of size , the symbol ℂ × denotes the set of complex matrices of size  × .

Problem formulation and solving
In this paper, we study the PLSTR maximization problem of vehicle C, which requires that the V2V vehicle communication should meet the minimum SINR requirement, the total transmission power of the multi-antenna base station B is limited, and the phase shift range of the RIS unit is limited.In this way, the optimization problem can be formulated as P1.
8) where the base station TPV  and the RIS's RCM  are two variables to be optimized.Constraint (8b) is the SINR demand of the V2V vehicle, and  is the minimum SINR requirement threshold.The constraint (8c) is the transmit power limitation of base station B, and   is the maximum transmit signal power available to B. The constraint (8d) is the RIS phase shift constraint, since the reflection phase shift range is [0, 2π], the modulus of the reflection coefficient   is one.
Problem P1 is a non-convex optimization problem, and the two variables to be optimized are coupled to each other in the objective function (8a) and constraints (8b).This problem is difficult to solve, and it is also difficult to obtain the global optimal solution.In order to decouple, the alternating optimization method can be used to solve the problem iteratively, that is, the RCM  is fixed and the TPV  is solved.Then fix TPV and solve RCM.The two steps are iterated alternately until the problem converges and the optimal  and  are obtained.Each iteration solves a subproblem, and each sub-problem may still not be a convex optimization problem.In order to obtain the global optimal solution of the sub-problem, the non-convex problem is usually approximated by a convex optimization problem by using an appropriate mathematical equivalent transformation.This convex approximation method is also used for processing in this paper.The following two subsections 3.1 and 3.2 present alternate solutions for the two variables, respectively.

Find the optimal TPV
This section find the solution of TPV  under the premise of fixing the RCM .The original problem P1 is then rewritten as problem P2 with only  to be optimized.
When the eavesdropping rate is not lower than the transmission rate of the V2I user, that is, when   −   ≤ 0, the V2I user does not transmit information anymore, and   = 0, the base station transmit power is 0 and   = 0, then no TPV optimization is performed.Therefore, in what follows, we only consider the case when the eavesdropping rate is lower than the transmission rate of the V2I user, that is,   −   > 0. When   −   > 0,   =   −   .In this case, it can be proved that there is a maximum transmission power  ̃ satisfying the constraints of problem P2, such that   is maximum.
In the following, this  ̃ is found by solving the optimization problem P2-1, where the send precoding to be solved in problem P2-1 is denoted by  ̃.Problem P2-3 is convex, which can be directly solved by using convex optimization toolbox CVX and obtain the optimal  ̃ denoted as  ̃ .Then the maximum transmit power that is practically feasible in problem P2 is  ̃ = ‖ ̃ ‖ 2 .Given the practically feasible maximum transmit power  ̃ of the base station, the optimal TPV  can be found by using the method of generalized Rayleigh entropy.
Substitute  into the expressions of   and   , we have This problem conforms to the generalized Rayleigh entropy solution form, and the optimal  ̂ can be obtained as  ̂ =   (    −1 ), where   (    −1 ) is the eigenvector corresponding to the largest eigenvalue of     −1 .Finally, the optimal TPV  can be given by   = √ ̃  ̂ .

Find the optimal RCM
This section find the solution of RCM  under the premise of fixing the TPV .The original problem P1 can be reformulated as P3 with the only variable  to be optimized.

P3:
Problem P3 is still a non-convex optimization problem.Firstly, the object and constraint are simplified and replaced, and then the non-convex problem is transformed into a convex optimization problem by using convex approximation method.
Denote   = diag(   ) and   = diag(   ), then we have and In P3-1, the SINR constraint (11) and RIS phase shift constraint are still non-convex.Let  ̃=   , then the constraint ( 11) is equivalent to and the constraint In order to see the convexity more clearly from (12), another slack variable  ̃ = ( ,1 ,  ,2 ,  ,3 ,  ,4 ) is introduced, and ( 12) can be further given by The constraint (11) can be replaced by (13), the constraint (12) can be replaced by (15) and ( 16), the phase shift constraint can be replaced by (14).However, both the functions (15) and (16) are still non-convex.The first-order Taylor expansion is used again to find the lower bound such that (15) and (16) can be approximated to convex form, as follows Finally, the optimal problem P3-1 can be reformulated as P3-2 P3-2: max We can solve problem P3-2 without considering the constraint rank( ̃) = 1, that is, we can relax P3-2 to an SDP problem.The relaxed SDP problem is a convex optimization problem, which can be solved by using CVX.However, the optimal solution  ̃ for the SDP problem does not necessarily satisfy rank( ̃ ) = 1, so the Gaussian randomization method is used to find the optimal  ̃ satisfying rank( ̃) = 1.The process of Gaussian randomization to determine the optimal  is listed in Algorithm 1.Given the optimal , we can obtain the optimal .Algorithm 1: Gaussian randomization )

The overall algorithm and complexity analysis
Based on the above two sub-problems in Section 3.1 and Section 3.2, the overall algorithm description of problem P1 is given in Algorithm 2. It can be seen from the algorithm that the two sub-problems of optimizing TPV and RCM mainly involve SOC constraints and matrix inversion operations, hence the complexity of the algorithm is ( 3 ) + ( 3 ).In order to verify the effectiveness of the proposed algorithm, a two-lane IoV simulation scenario is set up, and the coordinate settings of each part are shown in Figure 2, in which the positions of B and RIS are fixed in (0, 0) and (0, 8), and the positions of vehicles are random and change with the driving of vehicles.The average speed of vehicles is , and the average interval of vehicles is 2.5.Unless some parameters are specified in simulation, other parameters are set as shown in Table 2.  3. Algorithm convergence Fig. 3 shows the convergence results of the proposed algorithm.When the number of RIS reflection units is different, such as  = 4, 16, 64, the proposed algorithm can obtain the stable optimal solution within about 10 iterations, which verifies that the proposed algorithm in this paper has good convergence.Figure 4. Effect of the number of RIS reflection units on the PLSTR Fig. 4 shows the effect of the number of RIS reflection units on the PLSTR.With the increase of the number of RIS reflection units, the PLSTR gradually increases.For example, when the number of transmitting antennas at base station is 2 and the number of RIS reflection units increases from 16 to 100, the PLSTR can be doubled.With the increasing of number of the RIS reflection units, the reflection shifts can be adjusted more flexibly, so as to achieve stronger reflection beamforming effect and improve the system security transmission rate.Moreover, it can be seen from the three curves in the figure that the increase of the number of transmitting antennas at base station can further improving the level of PLSTR, this is because the multiplexing and diversity gains brought by more multiple antennas are more obvious.
Figure 5.Comparison between different algorithms Fig. 5 shows the performance comparison between the proposed algorithm and other algorithms.The "Rand RIS Phase" algorithm means that the RCM at RIS is randomly given, and the TPV is obtained by the proposed method in this paper.The "Channel Matching Algorithm" means that the TPV adopts the ideal form and is designed by using the singular value decomposition of the channel gain matrix, and the RCM of the RIS is obtained by the optimization method proposed in this paper.The "SOCP+SDP" algorithm means that the solutions of the TPV and the RCM of RIS are obtained by directly transforming the prime optimization problem into a classical second-order cone programming (SOCP) sub-problem and a semidefinite programming (SDP) sub-problem, respectively, instead of using the method of generalized Rayleigh entropy proposed in this paper.Through comparison, it is found that the system performance of the proposed algorithm is similar to "SOCP+SDP" algorithm, and better than "Rand RIS Phase" algorithm and "Channel Matching" algorithm.However, the complexity of the proposed algorithm is ( 3 ) + ( 3 ), which is slightly lower than ( 3.5 ) + ( 3 ) of "SOCP+SDP" algorithm due to the analytical form of generalized Rayleigh entropy.Figure 6.Effect of the maximum transmit power of base station on the PLSTR Fig. 6 further shows the effect of the maximum transmit power of base station and the path loss factor on the PLSTR.The larger the maximum transmission power of base station is, the higher the PLSTR is, because with the increase of the transmission power of the base station, the communication rate of V2I becomes higher, and meanwhile the RIS adjusts the phase to suppress the eavesdropping path, so that the eavesdropping received signal is suppressed and the security transmission signal is enhanced.The trend comparison of the three solid lines in the figure also shows that under the same number of RIS reflection units, the larger the path loss factor  2 between B and E is, the larger the PLSTR of V2I is, because the increase of the path loss factor between B and E will reduce the eavesdropping rate, thereby increasing the security transmission rate.

Conclusion
In this paper, the communication scenario where V2I and V2V share the spectrum at the same time is modeled.By jointly optimizing the base station transmission precoding vector and RIS reflection coefficient matrix, an alternating optimization algorithm based on generalized Rayleigh entropy is proposed, which can improve the V2I physical layer security transmission rate in the coexistence of V2I and V2V network.The proposed algorithm converges faster and has better performance than the classical algorithm and SOCP plus SDP algorithm.In the case of increasing the number of RIS reflection units and the number of base station transmitter antennas, the V2I physical layer security transmission rate can be greatly improved.

Figure 1 .
Figure 1.Downlink transmission of IoV system scenarioIn this paper, we study the downlink physical layer secure transmission aided by RIS in IoV system where V2I and V2V are coexist, as shown in Figure1.There is a V2I vehicle (denoted as C), a base station (denoted as B), a pair of V2V vehicles (denoted as Dt for the message sending vehicle and Dr For the message receiving vehicle), an eavesdropping vehicle (denoted as E), and an RIS reflector plate deployed on the roadside building.The base station is equipped with multiple antennas, which are uniformly and linearly arranged, and the number is denoted as .All other vehicles have single antennas.The number of RIS reflection elements is , and the reflection coefficient of the th element is   =    , where   is the phase shift.The vehicle C communicates with the base station B, considering the downlink transmission, and the eavesdropping vehicle E aims to eavesdrop the transmission information from B to C. RIS is deployed on roadside buildings to enhance the incident signal strength and reduce B's coverage hole.By optimizing the RIS's phases, the PLSTR of V2I is maximized.It is assumed that the V2V vehicles in the system has completed spectrum resource allocation by the base station and always shares the spectrum with the V2I vehicle for information transmission in a D2D manner.The symbol labels of the ̃,, ̃ ̃  1 −  2 −  3 +  4 s.t.(13), { ̃≽ 0, rank( ̃) = 1, diag( ̃) =  +1 }, (15), (15), (15), (16), (16), (16),(17)

Algorithm 2 : 5 :Figure 2 .
Figure 2. The 2D simulation scenario.In order to verify the effectiveness of the proposed algorithm, a two-lane IoV simulation scenario is set up, and the coordinate settings of each part are shown in Figure2, in which the positions of B and RIS are fixed in (0, 0) and (0, 8), and the positions of vehicles are random and change with the driving of vehicles.The average speed of vehicles is , and the average interval of vehicles is 2.5.Unless some parameters are specified in simulation, other parameters are set as shown in Table2.Table 2. Simulation parameters Parameter description Parameter value Noise power spectral density -174 dBm/Hz System bandwidth 10 MHz Total V2I transmit power of base station 1 W V2V total transmit power 0.5 W V2V SINR requirement 0 dB Antennas number of base station 6 RIS units number 36 Road width 8 m The height of vehicle antenna 1.5 m The height of RIS deployed on building 25 m Channel path loss factor {3.75, 2, 3} Rice fading factor 3 Average vehicle speed 70 km/h