Near-field holographic MIMO system and communication method based on digital coding metasurface

Abstract

The invention discloses a near-field holographic MIMO system based on a digital coding metasurface and a communication method. A transmitting end of the near-field holographic MIMO system mainly comprises the digital coding metasurface, a feed source antenna and a digital control module. The receiving end is an array antenna and demodulation equipment; a feed source antenna transmits a carrier signal to a metasurface, and according to an orthogonal basis function obtained by singular value decomposition of a free space radiation operator in a Hilbert space, the spatial distribution condition of a reflected signal in a near-field region is jointly controlled, so that a signal on a receiving antenna array has orthogonality in a spatial domain, and multi-channel spatial multiplexing communication is realized; different space coding vectors are switched in real time through a digital control module, and multiple paths of data streams are independently transmitted to a receiving end; and the receiving end multiplies the spatial demodulation matrix and the receiving vector to obtain a final receiving signal. The transmitting end has the functions of a spatial mixer and antenna radiation, radio frequency modules such as digital-to-analog conversion, the mixer and a power amplifier are omitted, and the architecture of a transmitter in communication is simplified.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication and novel artificial electromagnetic materials, and in particular relates to a multi-user wireless communication system and method based on digital coding metasurface. Background Art

[0002] Holographic MIMO communication systems have broad application prospects in the field of wireless communications in the future. The radiation structure in traditional MIMO and Massive-MIMO systems is usually composed of array antennas with unit spacing slightly less than half a wavelength. However, the concept of holographic MIMO requires that the transmitter can generate a continuously adjustable aperture electric field, and realizing holographic MIMO by reducing the unit size of the array antenna will bring strong coupling and sharply increased costs, which greatly limits the application of traditional array antenna technology in holographic MIMO systems.

[0003] On the other hand, the new artificial electromagnetic surface, referred to as metasurface, is developed from metamaterials with three-dimensional structures. Because metasurfaces have longitudinal dimensions that are negligible compared to the wavelength, they are thin, light, low-loss, low-cost, easy to process, and easy to integrate, and have attracted extensive research from domestic and foreign scholars in recent years. By designing the characteristics of the units and arranging them in space, they can be used to control the amplitude, phase, frequency, polarization, orbital angular momentum, waveform and other parameters of electromagnetic waves, and have been applied in scientific research and engineering applications. In 2014, the research group of Professor Cui Tiejun of Southeast University proposed the concept of "digital coding and programmable metamaterials / metasurfaces" [Reference [1]: TJ Cui, MQQi, X. Wan, J. Zhao, Q. Cheng, Coding metamaterials, digital metamaterials and programmable metamaterials. Light Science & Applications 3, e218 (2014)]. Combined with adjustable devices, discrete binary coding is used to achieve real-time dynamic control of electromagnetic waves, and a bridge is also established between information science and digital signal processing. Therefore, as a new technology that has both low cost and subwavelength electromagnetic wave control capabilities, digitally coded metasurface is very suitable for realizing holographic MIMO communication systems. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a low-cost, low-complexity near-field holographic MIMO system and communication method based on a digitally coded metasurface, which can realize the information transmission function of multiple spatial channels. The communication architecture uses a digitally coded metasurface as a transmitter and a traditional array antenna as a receiver. In addition, in order to make full use of the spatial degrees of freedom brought by holographic MIMO, the singular value decomposition in the Hilbert space is applied to the free-space Green's function to obtain the optimal spatial coding vector and demodulation matrix.

[0005] Technical solution: To achieve the above purpose, a near-field holographic MIMO system based on a digitally coded metasurface of the present invention adopts the following technical solution:

[0006] The system includes a transmitting end and a receiving end. The transmitting end includes a feed antenna, a digital coding metasurface and a digital control module interconnected by metal wires; the receiving end is an antenna array and a receiving demodulation device; the feed antenna is a linearly polarized horn antenna that can excite an electric field with similar amplitude on the digital coding metasurface; the antenna unit of the receiving end antenna array adopts a probe antenna or a rubber stick antenna; the receiving demodulation device includes a mixer and a software radio receiver. The mixer down-converts the received RF signal to baseband, and then completes demodulation and symbol judgment through the software radio receiver to finally restore the information stream.

[0007] The digital control module includes a baseband signal processor capable of performing information mapping and a voltage control circuit capable of controlling the voltage of an adjustable device on a digitally encoded metasurface.

[0008] The digital coding metasurface is composed of time-varying programmable units arranged periodically in space to form an M×N array. The side length of each programmable unit is less than half a wavelength, and the unit contains a number of adjustable devices.

[0009] The wireless communication method of the near-field holographic MIMO system based on the digital coding metasurface of the present invention comprises the following steps:

[0010] Step 1: By performing singular value decomposition on the free space radiation operator in Hilbert space, we can obtain the left and right singular value functions of the operator and select the first N ch The left and right singular value functions of the group are used to construct the orthogonal spatial channel discretization to obtain the spatial coding vector, N ch The number satisfies the following formula:

[0011]

[0012] Among them, A t and A r are the aperture sizes of the receiving end and the transmitting end, d is the distance between the transmitting end and the receiving end, λ is the wavelength of the electromagnetic wave used for communication, and the number of available channels Nch It is not a constant value, but a quantity related to the geometric parameters of the transmitter and receiver.

[0013] Step 2: The baseband signal processor in the digital control module translates and splits the original data information to be sent into several binary bit streams of 0 and 1; the baseband signal processor maps the bit stream information into modulation symbols, modulates the information symbols using the PSK method, and multiplies these symbols with the selected right singular value function and then accumulates them to obtain the spatial coding vector of the digital coding metasurface;

[0014] Step 3: discretely concatenate the corresponding left singular value functions to obtain the spatial demodulation matrix;

[0015] Step 4: The voltage control circuit in the digital control module controls the adjustable device on the metasurface according to the corresponding spatial coding vector in the baseband signal processor, and the feed antenna transmits a single-tone signal of the carrier frequency to the metasurface; the digital control module changes different coding matrices in real time to control the metasurface reflected signal to transmit different information symbols in each spatial channel;

[0016] Step 5: The user uses an antenna array and receiving and demodulating equipment to receive the modulated signal emitted by the metasurface. The signal is down-converted to the baseband processing module through the RF module in the receiving and demodulating equipment. Then, the receiving signal vector of the antenna array is multiplied by the spatial demodulation matrix. Finally, the received PSK signal is symbol detected and judged, and the original bit stream information of the transmitter is restored.

[0017] In step 1, the radiation operator of the free space is a linear operator calculated according to the location of the target user and the dyadic Green's function, and then the left and right singular value functions are obtained by decomposing the linear operator in the Hilbert space.

[0018] In step 4, the voltage control circuit outputs different voltages according to the spatial coding vector to control the state of the adjustable device in each unit, thereby realizing the modulation of the reflected signal in the near-field space of the metasurface.

[0019] In step 5, since the holographic MIMO system uses spatial diversity to achieve multi-channel communication, the receiving end must be an antenna array of a multi-antenna system, with the number of antennas being no less than 2, and the specific number needs to be greater than or equal to N. ch The value of the received signal of each antenna is a linear combination of multiple data streams. It is necessary to multiply the received vector and the spatial demodulation matrix to obtain each separate information stream.

[0020] Beneficial effects: Compared with the prior art, the advantages of the present invention are:

[0021] 1. The near-field holographic MIMO system based on digital coding metasurface proposed by the present invention replaces the traditional RF link and antenna. It also has the functions of spatial mixer and antenna, directly modulates the carrier signal, and eliminates RF modules such as digital-to-analog conversion, mixer and power amplifier, simplifies the architecture of the transmitter in communication, reduces the design difficulty, and reduces the system cost.

[0022] 2. In the digitally coded metasurface of the present invention, the side length of each programmable unit is less than 1 / 2 wavelength, which can approximately continuously control the electric field distribution on the emitting aperture surface to construct orthogonal spatial channels; the digital control module outputs different voltages according to the spatial coding vector to control the state of the adjustable device in each unit, thereby controlling the metasurface to transmit the bit stream through each spatial channel.

[0023] 3. In the communication system proposed by the present invention, the digital coding metasurface at the transmitting end has the characteristic of location confidentiality, and users in other areas cannot demodulate the information. Moreover, the transmitted information is evenly distributed on the entire receiving aperture. If the electric field on the receiving aperture cannot be fully obtained, the information cannot be correctly demodulated. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the structure of the near-field holographic MIMO system based on digitally coded metasurface proposed in the present invention;

[0025] Figure 2 The coding vector V1 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0026] Figure 3 The coding vector V2 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0027] Figure 4 The coding vector V3 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0028] Figure 5 The coding vector V4 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0029] Figure 6 The coding vector V5 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0030] Figure 7The coding vector V6 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0031] Figure 8 The coding vector V7 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0032] Fig. 9 The coding vector V8 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0033] Fig.10 The coding vector V9 and its corresponding receiving aperture electric field distribution, the received signal after spatial demodulation and the information coding method used in the present invention;

[0034] Fig.11 is the encoding vector V used in the present invention 10 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0035] Fig.12 is the encoding vector V used in the present invention 11 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0036] Fig.13 is the encoding vector V used in the present invention 12 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0037] Fig.14 is the encoding vector V used in the present invention 13 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0038] Fig.15 is the encoding vector V used in the present invention 14 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0039] Fig.16 is the encoding vector V used in the present invention 15 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0040] Fig.17 is the encoding vector V used in the present invention 16 and its corresponding receiving aperture electric field distribution, received signal after spatial demodulation and information encoding method;

[0041] Fig.18 is the spatial demodulation matrix U used in the present invention;

[0042] Fig.19 Direct modulation flow chart for generating PSK signals using baseband data for digitally coded metasurfaces;

[0043] Fig. 20 This is a block diagram of the near-field holographic MIMO communication system of the present invention, which mainly includes a receiving and demodulating module. DETAILED DESCRIPTION

[0044] The near-field holographic MIMO system based on the digitally encoded metasurface in the present invention directly loads information on the near-field spatial distribution characteristics of the metasurface reflection signal through a digital control module, and simultaneously and independently transmits multiple different information streams to the same user through multiple spatial channels.

[0045] The present invention is further described below through a specific embodiment of a dual-channel holographic MIMO system in conjunction with the accompanying drawings.

[0046] The present invention proposes a near-field holographic MIMO system based on a digital coding metasurface, which is mainly composed of a transmitting end and a receiving end. Figure 1 As shown in the figure, the transmitting end mainly includes a digital coding metasurface, a horn antenna, and a digital control module (a development circuit board based on a field programmable gate array FPGA is used here, and the baseband signal processor and voltage control circuit in the digital control module are implemented based on the FPGA development circuit board). The receiving end mainly includes a binary probe antenna array and two traditional demodulation modules. The operating frequency of the entire system is 5GHz, and the corresponding electromagnetic wave wavelength is 60mm. The digital coding metasurface is composed of 24×16 programmable units with a side length of 15mm (0.25λ). Each programmable unit is loaded with 2 PIN diodes to achieve phase quantization with 2-bit accuracy, that is, the reflection phase has four conditions of 0 degrees, 90 degrees, 180 degrees and 270 degrees, which are marked as codes "0", "1", "2" and "3" respectively. The aperture size of the transmitting end is 360mm (6λ). The two probe antennas at the receiving end are 150mm (2.5λ) apart to achieve uniform sampling of the electric field on the receiving aperture of 300mm (5λ). The distance between the transmitting aperture and the receiving aperture is 800mm (13.3λ). According to the formula, the number of spatial channels N that the system can support can be calculated. ch =2.15, which is 2 after discarding the decimal part.

[0047] According to the relative position between the transmitter and the receiver and the dyadic Green's function, the linear radiation operator in free space is calculated. Then, the linear operator is decomposed into singular values ​​in the Hilbert space, sorted according to the size of the singular values, and the singular value functions corresponding to the first two singular values ​​are selected to construct the orthogonal channel. Here, the case of two channels transmitting QPSK information streams is taken as an example. The two QPSK symbols are multiplied by the calculated right singular value function and then accumulated to obtain the ideal electric field distribution on the transmitting aperture. Then, it is quantized according to the number of phase quantization bits of the programmable unit to obtain 16 groups of spatial coding vectors V1, V2, ... and V 16 .

[0048] Figures 2 to 17 The 16 groups of spatial coding vectors V1, V2, ... and V2 for realizing dual-channel QPSK information stream transmission are given. 16 As well as their corresponding receiving aperture electric field distribution, receiving constellation diagram and information encoding method. Each programmable unit has its own independent spatial coding sequence, and different color blocks represent different codes. Here the programmable unit uses 2-bit reflection phase coding, corresponding to phases of 0 degrees, 90 degrees, 180 degrees and 270 degrees, marked as codes "0", "1", "2" and "3" respectively. By switching these 16 spatial coding vectors, two groups of two-bit numbers "00", "01", "10" and "11" can be transmitted simultaneously through channels #1 and #2 independently.

[0049] Fig.18 The spatial demodulation matrix U at the receiving end is given. In the present invention, the signals received by antennas #1 and #2 are not the final received signals of the two channels. A spatial demodulation step is required between the two: the received signal vector of the antenna is multiplied by Fig.18 The demodulation matrix U given in can be used to obtain the final received signals of channels #1 and #2.

[0050] Fig.19 The direct modulation flow chart of the digital coding metasurface using baseband data to generate QPSK signals is given. Fig.18 A block diagram of a near-field MIMO communication system is given, which mainly includes a receiving and demodulation module.

[0051] The transmission and reception steps of the entire communication system are as follows:

[0052] 1) The baseband signal processor in the digital control module translates the original data images sent by the two users into binary bit stream information of 0 and 1. Here, the transmission of different images through two channels is taken as an example. The data stream of a picture can also be split and transmitted simultaneously through multiple channels;

[0053] 2) The baseband signal processor groups the bit stream information into QPSK symbols in pairs and then maps them into corresponding spatial coding vectors V1, V2, ... and V 16 , each set of spatial coding vectors corresponds to the modulation coding sequence of the metasurface in the spatial domain;

[0054] 3) The voltage control circuit in the digital control module controls the adjustable devices on the metasurface according to the corresponding spatial coding matrix; the digital control module switches different spatial coding matrices in real time to control the energy distribution of the metasurface reflected signal in the spatial domain, where the QPSK method is used to encode information;

[0055] 4) The user uses a binary probe antenna array and traditional receiving and demodulating equipment to receive the modulated signal emitted by the metasurface, down-converts it to the baseband processing module through the RF module, and then multiplies the received signal vector with the spatial demodulation matrix U. The QPSK symbol detection and judgment are performed on the signals in the two demodulated channels, and the original bit stream information of the transmitter is restored to obtain the corresponding pictures.

[0056] The above example shows that images are transmitted to the same user independently through two channels. It should be noted that by designing a metasurface and receiving antenna array with a suitable array size, more channels and multiple users can be simultaneously and independently transmitted. Figure 2 to Figure 17 It can be seen from the coding scheme that the system of the present invention has the characteristics of position modulation and can realize confidential communication. It also acts as a spatial mixer and antenna radiation, and also eliminates the need for radio frequency modules such as digital-to-analog conversion, mixer and power amplifier, thus simplifying the architecture of the transmitter in communication.

Claims

1. A near-field holographic MIMO system based on digitally coded metasurface, characterized in that: The system includes a transmitting end and a receiving end. The transmitting end includes a feed antenna, a digital coding metasurface and a digital control module interconnected by metal wires; the receiving end is an antenna array and a receiving demodulation device; the feed antenna is a linearly polarized horn antenna that can excite an electric field with similar amplitude on the digital coding metasurface; the antenna unit of the receiving end antenna array adopts a probe antenna or a rubber stick antenna; the receiving demodulation device includes a mixer and a software radio receiver. The mixer down-converts the received RF signal to baseband, and then completes demodulation and symbol judgment through the software radio receiver to finally restore the information stream.

2. The near-field holographic MIMO system based on digitally coded metasurface according to claim 1, characterized in that: The digital control module includes a baseband signal processor capable of performing information mapping and a voltage control circuit capable of controlling the voltage of an adjustable device on a digitally encoded metasurface.

3. The near-field holographic MIMO system based on digitally coded metasurface according to claim 1, characterized in that: The digital coding metasurface is composed of time-varying programmable units arranged periodically in space to form an M×N array. The side length of each programmable unit is less than half a wavelength, and the unit contains a number of adjustable devices.

4. A wireless communication method for a near-field holographic MIMO system based on a digitally coded metasurface as claimed in claim 1, characterized in that: The following steps are involved: Step 1: By performing singular value decomposition on the free space radiation operator in Hilbert space, we can obtain the left and right singular value functions of the operator and select the first N ch The left and right singular value functions of the group are used to construct the orthogonal spatial channel discretization to obtain the spatial coding vector, N ch The number satisfies the following formula: Among them, A t and A r are the aperture sizes of the receiving end and the transmitting end, d is the distance between the transmitting end and the receiving end, λ is the wavelength of the electromagnetic wave used for communication, and the number of available channels N ch It is not a constant value, but a quantity related to the geometric parameters of the transmitter and receiver. Step 2: The baseband signal processor in the digital control module translates and splits the original data information to be sent into several binary bit streams of 0 and 1; the baseband signal processor maps the bit stream information into modulation symbols, modulates the information symbols using the PSK method, and multiplies these symbols with the selected right singular value function and then accumulates them to obtain the spatial coding vector of the digital coding metasurface; Step 3: discretely concatenate the corresponding left singular value functions to obtain the spatial demodulation matrix; Step 4: The voltage control circuit in the digital control module controls the adjustable device on the metasurface according to the corresponding spatial coding vector in the baseband signal processor, and the feed antenna transmits a single-tone signal of the carrier frequency to the metasurface; the digital control module changes different coding matrices in real time to control the metasurface reflected signal to transmit different information symbols in each spatial channel; Step 5: The user uses an antenna array and receiving and demodulating equipment to receive the modulated signal emitted by the metasurface. The signal is down-converted to the baseband processing module through the RF module in the receiving and demodulating equipment. Then, the receiving signal vector of the antenna array is multiplied by the spatial demodulation matrix. Finally, the received PSK signal is symbol detected and judged, and the original bit stream information of the transmitter is restored.

5. The wireless communication method of the near-field holographic MIMO system based on digitally coded metasurface according to claim 4, characterized in that: In step 1, the radiation operator of the free space is a linear operator calculated according to the location of the target user and the dyadic Green's function, and then the left and right singular value functions are obtained by decomposing the linear operator in the Hilbert space.

6. The wireless communication method of the near-field holographic MIMO system based on digitally coded metasurface according to claim 4, characterized in that: In step 4, the voltage control circuit outputs different voltages according to the spatial coding vector to control the state of the adjustable device in each unit, thereby realizing the modulation of the reflected signal in the near-field space of the metasurface.

7. The wireless communication method of the near-field holographic MIMO system based on digitally coded metasurface according to claim 4, characterized in that: In step 5, since the holographic MIMO system uses spatial diversity to achieve multi-channel communication, the receiving end must be an antenna array of a multi-antenna system, with the number of antennas being no less than 2, and the specific number needs to be greater than or equal to N. ch The value of the received signal of each antenna is a linear combination of multiple data streams. It is necessary to multiply the received vector and the spatial demodulation matrix to obtain each separate information stream.

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