Metasurface-based direction modulation method and device

Abstract

The invention discloses a direction modulation method based on a metasurface, and relates to the technical field of wireless communication and artificial electromagnetic new materials, and the method comprises the steps: mapping a source bit stream into a control signal, and loading the control signal to periodically arranged electromagnetic units of a programmable metasurface; carrier signals are incident to the metasurface, and the control signals are dynamically switched, so that direction modulation is realized. The invention further discloses a direction modulation device based on the metasurface. The direction modulation device comprises the radio frequency carrier source, the digital baseband processing link and the programmable metasurface. Direction modulation is realized by using the angle sensitive characteristic of the metasurface, namely the electromagnetic characteristic of the electromagnetic unit is related to the receiving angle. The direction modulation transmitter is constructed based on the angle sensitive characteristic of the programmable metasurface, and has the advantages of being simple in system structure, low in hardware cost and small in power consumption.

Description

Technical Field

[0001] The present invention relates to the technical field of wireless communication and artificial electromagnetic new materials, and in particular to a directional modulation method and device based on a metasurface. Background Art

[0002] Directional modulation technology is one of the key technologies in the field of wireless secure communications. Traditional directional modulation technology generally adopts the principle of phased array, and directional modulation is achieved by deploying multiple phase shifters at the antenna array at the transmitting end. This technology can achieve that while the main lobe of the antenna pattern transmits valid information, the side lobe carries erroneous information, increasing the bit error rate in the non-desired communication direction, thereby achieving the purpose of secure communication. However, one of the bottlenecks in the current implementation of directional modulation technology is that when a directional modulation transmitter is built using the phased array principle, a phase shifter needs to be deployed for each array antenna. As the number of antenna arrays increases, the hardware design complexity and cost of the transmitter will also increase significantly. Therefore, the existing devices for implementing directional modulation have the problems of complex system structure and high hardware cost. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a directional modulation method and device based on a metasurface. The present invention can implement a directional modulation method and device, which has the advantages of simple hardware architecture, low cost and low power consumption.

[0004] The present invention adopts the following technical solutions to solve the above technical problems:

[0005] According to a directional modulation method based on a metasurface proposed by the present invention,

[0006] Mapping the source bit stream into a control signal and loading it onto the periodically arranged electromagnetic units of the programmable metasurface;

[0007] The carrier signal is injected into the metasurface, and directional modulation is achieved by dynamically switching the control signal.

[0008] As a further optimization scheme of the directional modulation method based on the metasurface described in the present invention, the correspondence between the electromagnetic characteristics of the programmable metasurface and different control signals under various receiving angles is measured to achieve modulation in the desired direction; under the condition that the receiving angle remains unchanged, different control signals are loaded on the programmable metasurface, and each control signal value makes the programmable metasurface exhibit corresponding and fixed electromagnetic characteristics, producing a fixed modulation on the amplitude and phase of the incident carrier signal. By selecting different control signal values, a digital modulation scheme is implemented in the desired communication direction to generate a constellation diagram with a standard distribution.

[0009] As a further optimization scheme of the directional modulation method based on the metasurface described in the present invention,

[0010] After confirming the desired communication direction, the source bit stream to be transmitted is mapped to the corresponding control signal according to the known correspondence between the electromagnetic characteristics of the programmable metasurface and different control signals; the process of obtaining the control signal is as follows: first, according to the digital modulation method required by the modulation scheme, the normalized amplitude and phase sequence of the modulation wave signal corresponding to the source bit stream to be transmitted is calculated, and the normalized amplitudes and phases of different modulation wave signals correspond to different electromagnetic characteristics of the electromagnetic unit; then, according to the correspondence between the control signal loaded on the programmable metasurface and the electromagnetic characteristics of the electromagnetic unit, the control signal corresponding to the source bit stream to be transmitted is calculated.

[0011] As a further optimization scheme of the directional modulation method based on the metasurface described in the present invention,

[0012] The control signal is loaded onto the electromagnetic unit of the programmable metasurface through the feeding network to control its electromagnetic characteristics and realize directional modulation. Specifically, at a specified frequency point and in the desired communication direction, the baseband data can be directly modulated onto a carrier of a certain frequency through a pre-calculated control signal, and the baseband data can be correctly demodulated at the receiving end. At the same time, in the non-desired direction, the electromagnetic characteristics of the metasurface will change with the change of the receiving angle, thereby generating a modulated wave signal with distorted amplitude and phase, causing obvious distortion of the constellation diagram.

[0013] A directional modulation device based on a metasurface includes a digital baseband processing link, a programmable metasurface and a radio frequency carrier source, wherein the programmable metasurface includes a plurality of periodically arranged electromagnetic units; wherein:

[0014] A digital baseband processing link for mapping the source bit stream into control signals to be loaded onto the electromagnetic units of the programmable metasurface;

[0015] The RF carrier source is used to provide a carrier signal and inject it into the metasurface, thereby achieving directional modulation by dynamically switching the control signal.

[0016] As a further optimization scheme of the directional modulation device based on the metasurface described in the present invention, while transmitting the correct signal in the desired direction, a distorted signal is transmitted in the undesired direction; in the desired communication direction, the correspondence between different control signal values ​​in this direction and the electromagnetic characteristics of the metasurface is measured to generate a standard constellation diagram.

[0017] As a further optimization scheme of a directional modulation device based on a metasurface described in the present invention, the electromagnetic units are all the same.

[0018] As a further optimization scheme of the directional modulation device based on the metasurface described in the present invention, the electromagnetic unit is a PIN diode or a varactor diode.

[0019] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0020] (1) Compared with the traditional directional modulation transmitter architecture, the present invention utilizes the angular sensitivity of the metasurface itself to achieve communication in the desired direction while naturally distorting the information in the undesired direction. This allows the original data stream to be normally received and demodulated in the desired direction while increasing the bit error rate in the undesired direction, thereby improving communication security.

[0021] (2) The device can achieve directional modulation without deploying an additional phase shifter for each antenna, which reduces the design difficulty and manufacturing cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the system principle of the directional modulation device of the present invention.

[0023] Figure 2 Schematic diagram of a programmable metasurface used in one embodiment of the present invention.

[0024] Figure 3 The metasurface used in one embodiment of the present invention has an acceptance angle θ r =0°, and the reflection phase measurement results at f=4.25GHz under different control voltages.

[0025] Figure 4a It is a constellation diagram result after receiver demodulation of a conventional wireless communication system under the QPSK directional modulation scheme when the receiving angle is 0° in one embodiment of the present invention.

[0026] Figure 4b It is a constellation diagram result after the receiver demodulates the directional modulation device under the QPSK directional modulation scheme when the receiving angle is 0° in one embodiment of the present invention.

[0027] Figure 4c It is a constellation diagram result after the receiver demodulates the directional modulation device under the QPSK directional modulation scheme when the receiving angle is 25° in one embodiment of the present invention.

[0028] Figure 4d It is a constellation diagram result after the receiver demodulates the directional modulation device under the QPSK directional modulation scheme when the receiving angle is 45° in one embodiment of the present invention.

[0029] Figure 4e It is a constellation diagram result after the receiver demodulates the directional modulation device under the QPSK directional modulation scheme when the receiving angle is 70° in one embodiment of the present invention.

[0030] Figure 5a In one embodiment of the present invention, in the desired communication direction θ r =0°, the result of the bit error rate of the traditional wireless communication system and the system using directional modulation changing with the signal-to-noise ratio.

[0031] Figure 5b In one embodiment of the present invention, in the undesired communication direction θ r =45°, the result of the bit error rate of the traditional wireless communication system and the system using directional modulation changing with the signal-to-noise ratio. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Metasurface is a new type of artificial electromagnetic material. By designing its size, electromagnetic micro-unit structure and spatial arrangement, real-time control of electromagnetic waves can be achieved. By controlling the tunable elements in the electromagnetic micro-units, the electromagnetic properties of the metasurface can be changed, thereby changing the amplitude, phase, beam and other parameters of the reflected electromagnetic waves. Therefore, the metasurface has the ability to control electromagnetic waves to achieve modulation. In addition, the metasurface has an angle-sensitive characteristic. The reflection coefficient of the metasurface will change with the change of the receiving angle. The specific correspondence is related to the design and hardware implementation of the electromagnetic unit. Therefore, the metasurface can be used to build a directional modulation transmitter for wireless communication. Because the angular sensitivity of the metasurface is used to achieve directional modulation, there is no need to deploy an additional phase shifter for each unit, and the modulation is directly implemented on the metasurface, so the RF link of the traditional transmitter is not required. Therefore, the present invention is a new idea for the design of a directional modulation transmitter for wireless communication and has broad application prospects in the future.

[0034] The directional modulation device proposed in the present invention is mainly realized based on a programmable metasurface. The metasurface is composed of a series of identical electromagnetic units arranged periodically in space, and the electromagnetic characteristics of each unit are related to the unit size, material, tunable elements, etc. Therefore, the electromagnetic characteristics of the unit can be changed in real time by applying a control signal to the tunable element, thereby modulating the amplitude and phase of the signal incident on the metasurface; in addition, since the metasurface has an angle-sensitive characteristic, specifically, the electromagnetic characteristics of the metasurface are also related to the receiving angle θ r Under the same control signal, the electromagnetic characteristics of the metasurface in different communication directions are different. By utilizing the angle-sensitive characteristic of the metasurface, it is possible to transmit the correct signal in the desired direction while transmitting the distorted signal in the undesired direction, that is, to achieve directional modulation.

[0035] The present invention utilizes the angle-sensitive characteristics of a programmable metasurface to achieve directional modulation and builds a directional modulation device based on a metasurface. Figure 1 The figure shows a schematic diagram of a directional modulation system based on a metasurface, including a digital baseband processing link, a programmable metasurface, and a radio frequency carrier source. In the desired direction, the digital baseband processing link maps the source bit stream into a control signal and loads it on the electromagnetic unit of the metasurface, changing the electromagnetic properties of the metasurface in real time; the carrier signal provided by the radio frequency carrier source is incident on the metasurface, and the standard digital modulation scheme can be realized by dynamically switching the control signal at high speed, and it can be correctly demodulated at the receiving end; in the undesired direction, the electromagnetic properties of the metasurface are affected by the receiving angle, and the constellation diagram at the receiving end will be distorted, resulting in a significant increase in the bit error rate.

[0036] The programmable metasurface of the present invention has an angle-sensitive characteristic, that is, the electromagnetic characteristics of the electromagnetic unit are not only affected by the control signal, but also by the receiving angle θ. r The programmable metasurface is a sub-wavelength artificial layered material composed of periodically arranged electromagnetic units, where the electromagnetic units are designed with tunable elements such as PIN diodes and varactor diodes. The control signal is generated by the digital baseband processing link and loaded to the two ends of the tunable element of the electromagnetic unit through the feeding network, which can change the electromagnetic characteristics of the electromagnetic unit. In addition, the electromagnetic characteristics of the metasurface are also related to the receiving angle θ r In the method and device, in the desired communication direction, by selecting a set of control signals and loading them onto the electromagnetic unit of the metasurface, the metasurface presents electromagnetic characteristics corresponding to the set of control signals, thereby realizing a standard digital modulation scheme; at the same time, in the undesired communication direction, due to the angle-sensitive characteristics of the metasurface, the set of control signals causes the metasurface to present another electromagnetic characteristic different from that in the desired communication direction, resulting in a significant increase in the bit error rate in the undesired communication direction, thereby improving communication security.

[0037] The correspondence between the electromagnetic characteristics of the metasurface and different control signals under various receiving angles is measured to achieve modulation in the desired direction. Although the electromagnetic characteristics of the metasurface will change with the change of the control signal, under the condition that the receiving angle remains unchanged, different control signals are loaded on the metasurface, and each control signal value will cause the metasurface to present corresponding and fixed electromagnetic characteristics, which will also produce fixed modulation on the amplitude and phase of the incident carrier signal. By selecting different control signal values, the digital modulation scheme is implemented in the desired communication direction, and a constellation diagram with a standard distribution is generated.

[0038] After confirming the desired communication direction, the source bit stream to be transmitted is mapped to the corresponding control signal according to the known correspondence between the electromagnetic characteristics of the metasurface and different control signals. The process of obtaining the control signal is as follows: first, according to the digital modulation method required by the modulation scheme, the normalized amplitude and phase sequence of the modulated wave signal corresponding to the source bit stream to be transmitted is calculated. The normalized amplitude and phase of different modulated wave signals correspond to different electromagnetic characteristics of the electromagnetic unit; then, according to the correspondence between the control signal loaded on the metasurface and the electromagnetic characteristics of the electromagnetic unit, the control signal corresponding to the digital baseband signal to be transmitted can be calculated.

[0039] The control signal is loaded onto the electromagnetic unit of the metasurface through the feeding network to control its electromagnetic characteristics and realize directional modulation. Specifically, at a specified frequency point and in the desired communication direction, the baseband data can be directly modulated onto a carrier of a certain frequency through a pre-calculated control signal, and the data can be correctly demodulated at the receiving end; at the same time, in the undesired direction, the electromagnetic characteristics of the metasurface will change with the change of the receiving angle, thereby generating a modulated wave signal with distorted amplitude and phase, causing obvious distortion of the constellation diagram, which greatly increases the bit error rate during demodulation.

[0040] In order to make the experiment more clearly present the effect of directional modulation, the following specific supplementary explanations are given to the embodiment:

[0041] This embodiment adopts the QPSK directional modulation scheme;

[0042] The transmitting antenna is always facing the metasurface, that is, the incident angle θ i =0°;

[0043] The metasurface is a reflective metasurface, where the signal is modulated on the metasurface and reflected by the metasurface into free space;

[0044] Select θ r =0° is the desired communication direction, and the corresponding relationship between different control signal values ​​and the electromagnetic properties of the metasurface in this direction is measured to generate a standard constellation diagram.

[0045] These designs are all an embodiment of QPSK directional modulation using the directional modulation method and device based on the metasurface of the present invention. The actual modulation scheme includes but is not limited to QPSK, PSK, and QAM digital modulation. The incident angle θ of the carrier signal i and the receiving angle θ in the desired communication direction r They are not limited to 0°, they can be any angle; the metasurface can be reflective / transmissive, and correspondingly, the modulated wave signal can be the reflected wave / transmitted wave of the metasurface.

[0046] Figure 2The figure shows a physical image of a metasurface used in an embodiment of the directional modulation device of the present invention, which comprises 128 electromagnetic units in total. Each column of electromagnetic units is subjected to the same voltage signal u i Control, can be regarded as a macro unit. Therefore, the metasurface used in the embodiment can be modeled as a horizontally placed uniform linear array with 16 reflective macro units at an interval of d = 12 mm. For ease of understanding, all 16 macro units of the metasurface in the embodiment are controlled by one voltage signal, demonstrating the use of the method proposed in the present invention to achieve single data stream wireless communication with QPSK directional modulation. It is worth noting that the metasurface itself can be controlled by unit, that is, each electromagnetic unit is controlled by a different independent voltage signal to achieve richer and more complex functions, such as indoor positioning, arbitrary beamforming, and simultaneous transmission of multiple data streams. This embodiment only uses a single signal to control all the metasurface electromagnetic units in order to more clearly demonstrate the method of the present invention.

[0047] Figure 3 is the receiving angle θ of the metasurface r =0°, under different control voltages, the reflected phase measurement results at the operating frequency f=4.25GHz show that as the control voltage changes from 0V to 20V, the reflected wave phase has an adjustable range of nearly 450°. In the embodiment, since the QPSK directional modulation method is adopted, the constellation diagram has four modulation symbols of "00", "01", "11", and "10", and the corresponding normalized amplitudes of the modulation waves are all 1, and the corresponding normalized phases of the modulation waves are respectively By selecting Figure 3 The four voltages V1, V2, V3 and V4 marked in FIG. 1 can realize the QPSK modulation scheme in the desired direction.

[0048] Directional modulation is achieved by using the device of the present invention. The received signal at the receiver can be expressed as

[0049]

[0050] Where P is the transmit power, is the wireless channel from the transmitting antenna to the metasurface, is the wireless channel from the receiving antenna to the metasurface, is the reflection coefficient matrix of the metasurface, x is the transmission symbol, is additive complex Gaussian white noise.

[0051] In the embodiment of implementing directional modulation, x represents an unmodulated carrier signal, so x can be considered to be a constant value of 1, and the received signal can be further expressed as

[0052]

[0053] The reflection coefficient matrix can be expressed as

[0054]

[0055] where α i (·,·) and φ i (·,·) represent the amplitude and phase of the reflection coefficient of the i-th macro unit respectively. Since the metasurface has the angle-sensitive characteristic, α i (·,·) and φ i (·,·) are all about the receiving angle θ r It should be noted that the difference in the design and implementation of the metasurface hardware will also lead to different angular sensitivity characteristics. The present invention only uses this characteristic of the metasurface to achieve directional modulation, rather than modeling the function in detail.

[0056] For the sake of demonstration, it is assumed in the embodiment that all macro units use the same control signal u i =u, therefore, all macro units have the same reflection coefficient, and the reflection coefficient matrix can be simplified to

[0057]

[0058] in, It is a unit array.

[0059] The incident angle θ in the embodiment i =0°, under far-field conditions, the incident carrier signal is irradiated onto the metasurface in the form of a plane wave, so h1 can be simplified to the following formula:

[0060] h1=[1,1,1,…,1] T

[0061] For the directional modulation device in the embodiment, the communication between the receiving antenna and the RIS is line-of-sight communication, so h2 can be expressed as

[0062]

[0063] in is the phase difference between two wireless signals reflected from adjacent macro units to the receiver, and λ is the wavelength of the wireless signal.

[0064] Substituting h1, h2 and Φ into the formula of the received signal y, we can get the final expression of the received signal y:

[0065]

[0066] Analyzing the expression of the received signal y, it can be found that it is mainly composed of three parts. First, the reflection coefficient The control signal and the receiving angle θ r Together, each control signal value corresponds to a modulation symbol, for example, by selecting Figure 3 The four voltages in can realize QPSK modulation. It is worth noting that due to the angle sensitivity of RIS, different θ r It will bring unexpected changes to the reflection coefficient, resulting in distortion of the constellation diagram in the undesired direction, which naturally increases the bit error rate in these undesired directions. This is the key principle of the present invention to achieve directional modulation; then, the array factor shows that in θ r =0° position has the largest received power. r As the received power increases, the bit error rate will gradually increase; finally, the phase shift constant will introduce the same phase shift to all macro units, which will not have a negative impact on the demodulation process at the receiver.

[0067] The following is an experimental result of an embodiment of the directional modulation method and device of the present invention. The constellation diagram at the receiving end is as follows: Figure 4a-4e As shown, through Figure 4a and Figure 4b It can be intuitively felt that in the desired communication direction, the system using metasurface to achieve directional modulation has similar modulation and demodulation effects as the traditional wireless communication system; Figure 4c , Figure 4d , Figure 4e It can be found that as the receiving angle changes, the constellation diagram of the receiving end of the directional modulation device is obviously distorted, and the bit error rate during demodulation will naturally increase. Furthermore, the embodiment also measures the bit error rate performance of the traditional wireless communication system and the system using the metasurface to achieve directional modulation under different signal-to-noise ratios. The results are as follows: Figure 5a , Figure 5b shown. Figure 5a It is shown that in the desired direction, the directional modulation device proposed by the present invention can achieve a bit error rate performance that is almost exactly the same as that of the traditional wireless communication system; Figure 5b It shows that in the non-desired direction, under the condition of the same signal-to-noise ratio, the bit error rate of the directional modulation device proposed in the present invention is significantly higher than that of the traditional wireless communication system, and the purpose of ensuring communication security through directional modulation is well achieved.

[0068] The above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a technician familiar with the technical field within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A directional modulation method based on a metasurface, characterized in that: Mapping the source bit stream into a control signal and loading it onto the periodically arranged electromagnetic units of the programmable metasurface; The carrier signal is injected into the metasurface, and directional modulation is achieved by dynamically switching the control signal.

2. A directional modulation method based on a metasurface according to claim 1, characterized in that: The correspondence between the electromagnetic characteristics of the programmable metasurface and different control signals at each receiving angle is measured to achieve modulation in the desired direction. Under the condition that the receiving angle remains unchanged, different control signals are loaded on the programmable metasurface. Each control signal value makes the programmable metasurface exhibit corresponding and fixed electromagnetic characteristics, producing a fixed modulation on the amplitude and phase of the incident carrier signal. By selecting different control signal values, the digital modulation scheme is implemented in the desired communication direction, generating a constellation diagram with a standard distribution.

3. The directional modulation method based on a metasurface according to claim 1, characterized in that: After confirming the desired communication direction, the source bit stream to be transmitted is mapped to the corresponding control signal according to the known correspondence between the electromagnetic characteristics of the programmable metasurface and different control signals; the process of obtaining the control signal is as follows: first, according to the digital modulation method required by the modulation scheme, the normalized amplitude and phase sequence of the modulation wave signal corresponding to the source bit stream to be transmitted is calculated, and the normalized amplitudes and phases of different modulation wave signals correspond to different electromagnetic characteristics of the electromagnetic unit; then, according to the correspondence between the control signal loaded on the programmable metasurface and the electromagnetic characteristics of the electromagnetic unit, the control signal corresponding to the source bit stream to be transmitted is calculated.

4. The directional modulation method based on a metasurface according to claim 1, characterized in that: The control signal is loaded onto the electromagnetic unit of the programmable metasurface through the feeding network to control its electromagnetic characteristics and realize directional modulation. Specifically, at a specified frequency point and in the desired communication direction, the baseband data can be directly modulated onto a carrier of a certain frequency through a pre-calculated control signal, and the baseband data can be correctly demodulated at the receiving end. At the same time, in the non-desired direction, the electromagnetic characteristics of the metasurface will change with the change of the receiving angle, thereby generating a modulated wave signal with distorted amplitude and phase, causing obvious distortion of the constellation diagram.

5. A directional modulation device based on a metasurface, characterized in that: It includes a digital baseband processing link, a programmable metasurface and a radio frequency carrier source, wherein the programmable metasurface includes a plurality of periodically arranged electromagnetic units; wherein, A digital baseband processing link for mapping the source bit stream into control signals to be loaded onto the electromagnetic units of the programmable metasurface; The RF carrier source is used to provide a carrier signal and inject it into the metasurface, thereby achieving directional modulation by dynamically switching the control signal.

6. A directional modulation device based on a metasurface according to claim 5, characterized in that: While transmitting the correct signal in the desired direction, a distorted signal is transmitted in the undesired direction; in the desired communication direction, the correspondence between different control signal values ​​and the electromagnetic properties of the metasurface is measured to generate a standard constellation diagram.

7. The directional modulation device based on a metasurface according to claim 5, characterized in that: The electromagnetic units are all identical.

8. The directional modulation device based on a metasurface according to claim 5, characterized in that: The electromagnetic unit is a PIN diode or a varactor diode.

Images