A time-modulated backscatter-based secure communication system and method

Abstract

The application discloses a kind of security communication systems and methods based on time modulation backscattering.The system includes radio frequency irradiation end, time modulation backscattering array node and receiving processing end.Radio frequency irradiation end irradiates continuous carrier wave to node;Node includes control unit, radio frequency switch, matching load and antenna array, control unit encapsulates the information to be transmitted as the communication frame containing synchronization, training and data section, and is mapped as the time modulation control sequence of each switch, so that the antenna switches between absorption state and reflection state, makes target harmonic component simultaneously carry modulation information and phase compensation, forms the coherent enhanced directional scattering echo in target direction, and due to spatial time delay decoupling characteristics, residual phase difference in non-target direction causes constellation diffusion, realizes physical layer security enhancement;Receiving processing end acquires echo and restores information by double-branch weighted decision and demodulation.The application has the advantages of simple structure, low power consumption, reconfigurable high-order modulation and high security.

Description

Technical Field

[0001] This invention belongs to the field of passive Internet of Things communication and array signal processing technology, and in particular, it is a secure communication system and method based on time-modulated backscattering. Background Technology

[0002] Backscatter communication uses an external radio frequency source or ambient electromagnetic waves as the incident carrier. By changing the equivalent load state of the antenna port, the scattered echo is modulated, thereby achieving low-power data transmission. This communication method does not require nodes to actively generate high-power radio frequency carriers, making it suitable for scenarios such as unmanned field monitoring, low-power sensing nodes, lightweight data backhaul for border defense, and industrial classified sensing.

[0003] Existing backscatter communication schemes have at least the following problems: First, when using a single two-state RF switch, only low-order amplitude modulation can usually be achieved. To achieve high-order modulation, a multi-stage impedance network or a complex RF front end is required. Second, the backscatter echo of a single antenna is approximately omnidirectional, and receivers in non-target azimuth directions may also intercept and demodulate the information. Third, existing schemes mostly use random symbol transmission as the verification object, lacking synchronization, training, verification, and complete parsing mechanisms for real text or command data. Summary of the Invention

[0004] The purpose of this invention is to address the problems existing in the prior art by providing a secure communication system and method based on time-modulated backscattering, so as to achieve reconfigurable high-order modulation, target direction enhancement, non-target direction suppression, and reliable text data backhaul under the conditions of no active radio frequency transmission link and no multi-level impedance network.

[0005] The technical solution to achieve the purpose of this invention is as follows: On the one hand, a secure communication system based on time-modulated backscattering is provided, the system including a radio frequency illumination end, a time-modulated backscattering array node and a receiving and processing end;

[0006] The radio frequency irradiation end is used to generate and radiate a continuous radio frequency carrier signal to the time-modulated backscatter array node;

[0007] The time-modulated backscatter array node includes a control unit, an antenna array, an impedance network, and a switch array; the impedance network is connected between the antenna array and the switch array; the control unit is used to generate multiple independent time-modulation control sequences according to the information to be transmitted and output them to the switch array, driving the switch array to switch the port impedance state of each element in the antenna array, so as to modulate the spatial characteristics and amplitude-phase characteristics of the radio frequency continuous carrier signal without changing the radio frequency hardware structure, forming a harmonic backscatter echo that carries the information to be transmitted and is radiated directionally in a specific target direction;

[0008] The receiving and processing end is used to collect the harmonic scattered echo in the specific target direction, and recover the information to be transmitted by harmonic feature extraction and demodulation.

[0009] The time modulation control sequence includes a reference time parameter for controlling the switching of the port impedance state and an array element additional delay parameter for beam phase compensation.

[0010] Furthermore, the antenna array is a linear array comprising N array elements, where N≥2; the switch array comprises N switch channels, each switch channel being independently connected to the port impedance of the corresponding array element, and each switch channel being driven by an independent control sequence output by the control unit.

[0011] Furthermore, the switching array controls the ports of each array element to switch between a first impedance state and a second impedance state within each symbol period, so that the corresponding time-varying reflection coefficient alternates between the absorption state and the reflection state. By using the two-state switching, a reconfigurable constellation point with high-order amplitude-phase modulation characteristics is constructed on the preset target order harmonic components.

[0012] Furthermore, the reference time parameters include at least the switching duration of the corresponding target constellation point amplitude and the switching start time of the corresponding target constellation point phase; the control unit has a pre-stored discrete time parameter mapping table, and the control unit queries the discrete time parameter mapping table according to the currently selected modulation method and the symbol index to be transmitted to obtain the corresponding discrete switch start count and discrete switch width count.

[0013] Furthermore, the discrete-time parameter mapping table is optimized by minimizing the Euclidean distance deviation between the theoretical target constellation point and the target order harmonic complex coefficients corresponding to the discrete-time parameters, and a minimum pulse width constraint based on the hardware response boundary is introduced in the optimization process.

[0014] Furthermore, the control unit calculates the target harmonic phase compensation amount required for each array element according to the spatial beam pointing requirement of the specific target direction, and converts the target harmonic phase compensation amount into the corresponding array element additional time delay parameter, so as to superimpose it on the reference switching start time of each array element, thereby realizing the synchronous and decoupled control of high-order symbol modulation and array beamforming in the time domain.

[0015] Furthermore, the control unit encapsulates the information to be transmitted into a communication frame structure that includes a synchronization segment, a training segment, and a data segment;

[0016] The data segment includes a frame header indicating frame characteristics and modulation parameters, a payload carrying valid service data, and a cyclic redundancy check field for integrity verification.

[0017] Furthermore, the receiving processing end includes:

[0018] The synchronization module performs relevant operations using the synchronization segment to capture the frame start point and performs symbol slicing according to the symbol period;

[0019] The extraction module performs a single-frequency discrete Fourier transform on each symbol slice to extract the complex features of the target order harmonics.

[0020] The calibration decision module estimates the training reference center for each constellation category using known full constellation symbol samples from the training segment, and performs symbol decision on the data segment based on the two-branch weighted nearest neighbor criterion to recover the symbol sequence.

[0021] Furthermore, during the demodulation and recovery of the information to be transmitted, the positive first harmonic branch features and the negative first harmonic branch features are extracted and utilized, and the conjugate redundancy relationship of the positive and negative first harmonics is used to perform dual-branch joint decision.

[0022] On the other hand, a secure communication method based on time-modulated backscattering is provided, implemented using the time-modulated backscattering secure communication system as described above. The method includes the following steps:

[0023] The radio frequency irradiation end radiates a continuous radio frequency carrier signal to the time-modulated backscatter array node.

[0024] The control unit receives the information to be transmitted and encapsulates it into a communication frame containing synchronization, training, and data fields;

[0025] The control unit determines the reference time parameters corresponding to each symbol to be transmitted based on the current modulation constellation mapping relationship, and determines the array element additional delay parameters of each array element based on the preset specific target direction. The two are superimposed to generate multiple independent time modulation control sequences.

[0026] The switching array drives each element of the antenna array to switch port impedance states according to the time modulation control sequence, so that the scattered echo carries higher-order modulation information on the target harmonic and is coherently enhanced in the specific target direction, while generating residual phase difference in the spatial region deviating from this direction, resulting in constellation diffusion.

[0027] The receiving processing end captures the scattered echo in the direction of the specific target, and recovers the information to be transmitted through frame synchronization, single-frequency harmonic feature extraction, channel calibration based on training segments, and decision decoding.

[0028] On the other hand, a computer device is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the time-modulated backscatter-based secure communication method.

[0029] On the other hand, a computer-readable storage medium is provided on which a computer program is stored, which, when executed by a processor, implements the time-modulated backscatter-based secure communication method.

[0030] Compared with the prior art, the significant advantages of this invention are:

[0031] (1) The system does not require a complex active RF transmission link or a multi-level impedance network. It can operate under a very simple two-state (absorption state and reflection state) RF switch hardware structure. The hardware structure is simple and very suitable for passive or low-power IoT nodes.

[0032] (2) By simply changing the time parameters (switching start time and duration) by the control unit, high-order amplitude and phase modulation such as 4ASK, 8PSK and 16QAM can be freely reconfigured and implemented on the same set of two-state hardware, breaking through the limitation that traditional single switches can only perform low-order modulation.

[0033] (3) By using the array element to add time delay, spatial beamforming and high-order symbol mapping are realized synchronously in the time domain, which makes the signal coherent in the target direction more enhanced and the symbol error rate extremely low; while when deviating from the target direction, residual phase difference will be generated, causing constellation diffusion, thereby effectively suppressing interception and demodulation in non-target directions.

[0034] (4) Unlike conventional verification schemes that can only transmit random symbols, this invention designs a complete communication frame structure that includes a Barker synchronization segment, a full constellation training segment, and a Data segment with CRC16 check. It has strong robustness against DC, slow drift and weak scattering, and can support 100% correct parsing and transmission of actual short text or instruction data.

[0035] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0036] Figure 1 This is a complete system flow diagram in one embodiment.

[0037] Figure 2 This is a block diagram of the overall system structure in one embodiment.

[0038] Figure 3 This is a schematic diagram of the beam pointing of a four-element backscattering array in one embodiment.

[0039] Figure 4 This is a schematic diagram of the time modulation switch control waveform in one embodiment.

[0040] Figure 5 This is a schematic diagram of the FPGA transmission control structure in one embodiment.

[0041] Figure 6 This is a schematic diagram of the communication frame structure in one embodiment.

[0042] Figure 7 This is a schematic diagram of the text data segment structure in one embodiment.

[0043] Figure 8 This is a graph showing the symbol error rate results for 4ASK at different receiving angles in one embodiment.

[0044] Figure 9 This is a graph showing the symbol error rate results for 16QAM at different receiving angles in one embodiment.

[0045] Figure 10 This is a comparison chart of symbol error rates for different modulation schemes in one embodiment.

[0046] Figure 11 This is a flowchart of text data return in one embodiment.

[0047] Figure 12 This is a diagram showing the text data parsing results in one embodiment, where... Figure 12 In the figure, (a) represents the frame-by-frame verification pass rate. Figure 12 (b) shows the frame-by-frame CRC pass rate. Figure 12 (c) in the figure represents the amplitude distribution of the training center. Figure 12 (d) in the figure represents the text recovery verification result.

[0048] Figure 13 This is a block diagram of a secure communication system based on time-modulated backscattering in one embodiment. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0050] It should be noted that if the embodiments of the present invention involve descriptions such as "first" and "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" and "second" may explicitly or implicitly include at least one of those features. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0051] In one embodiment, a secure communication system based on time-modulated backscattering is provided, the system comprising a radio frequency illumination end, a time-modulated backscattering array node, and a receiving and processing end;

[0052] The radio frequency irradiation end is used to generate and radiate a continuous radio frequency carrier signal to the time-modulated backscatter array node;

[0053] The time-modulated backscatter array node includes a control unit, an antenna array, an impedance network, and a switch array; the impedance network is connected between the antenna array and the switch array; the control unit is used to generate multiple independent time-modulation control sequences according to the information to be transmitted and output them to the switch array, driving the switch array to switch the port impedance state of each element in the antenna array, so as to modulate the spatial characteristics and amplitude-phase characteristics of the radio frequency continuous carrier signal without changing the radio frequency hardware structure, forming a harmonic backscatter echo that carries the information to be transmitted and is radiated directionally in a specific target direction;

[0054] The receiving and processing end is used to collect the harmonic scattered echo in the specific target direction, and recover the information to be transmitted by harmonic feature extraction and demodulation.

[0055] The time modulation control sequence includes a reference time parameter for controlling the switching of the port impedance state and an array element additional delay parameter for beam phase compensation.

[0056] Preferably, in some embodiments, the switch array includes multiple radio frequency (RF) switches. The control unit is used to generate a time modulation control sequence for each RF switch according to the information to be transmitted, so as to control each RF switch to switch the load state of the corresponding antenna array element, so that the antenna array switches between an absorption state and a reflection state, so as to simultaneously carry modulation information and beam phase compensation on the target harmonic component, thereby forming a directional scattering echo.

[0057] Furthermore, in one embodiment, the control unit generates the time modulation control sequence based on the switching start time, duration, and array element additional delay of the symbol to be transmitted in the information to be transmitted within the symbol period.

[0058] Furthermore, in one embodiment, the antenna array is a linear array comprising N array elements, where N≥2; the switch array comprises N switch channels, each switch channel being independently connected to the port impedance of the corresponding array element, and each switch channel being driven by an independent control sequence output by the control unit.

[0059] Furthermore, in one embodiment, the switching array controls the ports of each array element to switch between a first impedance state and a second impedance state within each symbol period, so that the corresponding time-varying reflection coefficient alternates between the absorption state and the reflection state. By using the two-state switching, a reconfigurable constellation point with high-order amplitude-phase modulation characteristics is constructed on the preset target order harmonic components.

[0060] Specifically, in some embodiments, the RF switch connects the antenna port to a matched load in a first state to form an absorption state, and in a second state, places the antenna port in a mismatched state to form a reflection state; the reflection coefficients corresponding to the absorption state and the reflection state are respectively... and And satisfy:

[0061]

[0062] In the formula, Indicates matching load impedance. Indicates the impedance of the mismatched load. Indicates the antenna input impedance. It represents the conjugate of the antenna input impedance.

[0063] Specifically, in some embodiments, the first The array element in the first Time-varying reflection coefficient within one symbol period satisfy:

[0064]

[0065] In the formula, This is a binary time modulation control function.

[0066] Preferably, in some embodiments, for Time of the first one symbol period The binary time modulation control function satisfy:

[0067]

[0068] In the formula, For the first Individual Element The start time of the switch for each symbol, For duration.

[0069] Preferably, in some embodiments, the binary time modulation control function The Fourier coefficients satisfy:

[0070]

[0071] in, The value is a non-zero integer; the control unit constructs a mapping relationship between the complex symbols to be transmitted and the time parameters based on the Fourier coefficients.

[0072] Here, when selecting When the first harmonic of 1 is used as the target information carrier component, the complex code to be transmitted The normalized amplitude and phase satisfy:

[0073]

[0074]

[0075] In the formula, The maximum normalized code in the constellation set. Indicates the first The amplitude of each complex code element to be transmitted. Indicates the first The phase of each complex code element to be transmitted. It is an integer, representing the phase equivalent integer compensation term.

[0076] Preferably, in some embodiments, the above-mentioned generation of time modulation control sequence includes:

[0077] Determine the reference time parameters based on the symbols to be transmitted. and ;

[0078] According to the target direction Calculate the additional delay of array elements ;

[0079] Will , and The RF switch control sequences of each array element are superimposed to form the RF switch control sequence.

[0080] Furthermore, in one embodiment, the reference time parameter includes at least the switching duration of the corresponding target constellation point amplitude and the switching start time of the corresponding target constellation point phase; the control unit pre-stores a discrete time parameter mapping table, and the control unit queries the discrete time parameter mapping table according to the currently selected modulation method and the symbol index to be transmitted to obtain the corresponding discrete switch start count and discrete switch width count.

[0081] Here, the control unit calculates the time parameter set based on 4ASK, 8PSK, or 16QAM constellation points. Furthermore, by switching the time parameter mapping table, different modulation methods can be implemented under the same two-state RF switch hardware structure.

[0082] Preferably, in some embodiments, the discrete-time parameter mapping table is optimized by minimizing the Euclidean distance deviation between the theoretical target constellation point and the target order harmonic complex coefficients corresponding to the discrete-time parameters, and a minimum pulse width constraint based on the hardware response boundary is introduced in the optimization process.

[0083] Specifically, in some embodiments, the time parameter mapping table minimizes the target constellation point. With actual harmonic coefficient The error between them is obtained, and its objective function is:

[0084]

[0085] In the formula, This refers to the link amplitude and phase normalization coefficient. The control unit can reconstruct and implement 4ASK, 8PSK, or 16QAM modulation under the same hardware structure by switching the time parameter mapping table.

[0086] Specifically, in some embodiments, the control unit (FPGA) superimposes the array element additional time at the reference switching start time of each array element. , making the first The first harmonic complex coefficients of each array element satisfy:

[0087]

[0088] In the formula, For the first The position of each array element This is the operating wavelength.

[0089] Preferably, the array element has an added time delay. satisfy:

[0090]

[0091] In the formula, For the target direction, This is not the target direction.

[0092] Here, the target direction Non-target direction Spatial isolation ratio between satisfy:

[0093]

[0094] The receiving processing end or control unit according to Evaluate the physical layer isolation capability in non-target directions;

[0095] In the formula, Indicates non-target direction Spatial isolation ratio; Indicates the direction of the target Upper The received field strength of the first harmonic of each symbol; Indicates non-target direction Upper The first harmonic received field strength of each symbol; the receiving processing end or control unit according to... Evaluate the physical layer isolation capability in non-target directions.

[0096] Here, the antenna array is in the direction of observation. Target harmonic array factor satisfy:

[0097]

[0098] in, For the first The position of each array element This is the operating wavelength.

[0099] Preferably, in some embodiments, the control unit discretizes each symbol period into Each sub-time unit satisfies:

[0100]

[0101]

[0102]

[0103]

[0104] In the formula, For discrete clock periods, , and All are non-negative integers.

[0105] Furthermore, in one embodiment, the control unit calculates the target harmonic phase compensation amount required for each array element according to the spatial beam pointing requirement of the specific target direction, and converts the target harmonic phase compensation amount into the corresponding array element additional time delay parameter, so as to superimpose it on the reference switching start time of each array element, thereby realizing the synchronous and decoupled control of high-order symbol modulation and array beamforming in the time domain.

[0106] Furthermore, in one embodiment, the control unit encapsulates the information to be transmitted into a communication frame structure that includes a synchronization segment, a training segment, and a data segment;

[0107] The data segment includes a frame header indicating frame characteristics and modulation parameters, a payload carrying valid service data, and a cyclic redundancy check field for integrity verification.

[0108] Specifically, the control unit encapsulates the information to be transmitted into a communication frame including a Barker synchronization segment, a Training segment, and a Data segment; wherein the Data segment includes a Header field, a Payload field, a CRC16 integrity check field, and a Padding word for symbol length padding.

[0109] The Barker synchronization segment uses the Barker synchronization sequence, the Training segment includes full constellation cyclic symbols under the current modulation mode, and the CRC16 field is used to perform integrity verification on the Header field and Payload field.

[0110] Furthermore, in one embodiment, while demodulating and recovering the information to be transmitted, the positive first harmonic branch features and the negative first harmonic branch features are extracted and utilized, and the conjugate redundancy relationship of the positive and negative first harmonics is used to perform dual-branch joint decision.

[0111] Furthermore, in one embodiment, the receiving processing end includes:

[0112] The synchronization module performs relevant operations using the synchronization segment to capture the frame start point and performs symbol slicing according to the symbol period;

[0113] The extraction module performs a single-frequency discrete Fourier transform on each symbol slice to extract the complex features of the target order harmonics.

[0114] The calibration decision module estimates the training reference center for each constellation category using known full constellation symbol samples from the training segment, and performs symbol decision on the data segment based on the two-branch weighted nearest neighbor criterion to recover the symbol sequence.

[0115] Preferably, in some embodiments, the receiving processing end is used to perform DC removal, slow drift removal, resampling, Barker correlation synchronization, single-frequency DFT harmonic extraction, training calibration, symbol decision, byte reassembly, and CRC check on the collected IQ data to recover the information to be transmitted.

[0116] Preferably, in some embodiments, the receiving processing end... Within a slice of symbols sampling points Perform a single-frequency point DFT to obtain the first First harmonic characteristics And satisfy:

[0117]

[0118] Preferably, in some embodiments, the receiving processing end simultaneously extracts the positive first harmonic branch features from Q sampling points within each symbol slice. and negative first harmonic branch characteristics :

[0119]

[0120]

[0121] The positive and negative first-order harmonic training centers for each constellation category are estimated using the aforementioned Training segment. and For the symbols to be decided in the Data segment The symbol class is recovered using the two-branch weighted nearest neighbor criterion. :

[0122]

[0123] In the formula, and These are the weights for the positive and negative first harmonic branches, respectively.

[0124] Preferably, the set of samples belonging to the m-th class of symbols in the Training segment is denoted as . Then the training center of this type of symbol satisfies:

[0125]

[0126]

[0127] In one embodiment, a secure communication method based on time-modulated backscattering is provided, implemented using the secure communication system based on time-modulated backscattering as described above. The method includes the following steps:

[0128] The radio frequency irradiation end radiates a continuous radio frequency carrier signal to the time-modulated backscatter array node.

[0129] The control unit receives the information to be transmitted and encapsulates it into a communication frame containing synchronization, training, and data fields;

[0130] The control unit determines the reference time parameters corresponding to each symbol to be transmitted based on the current modulation constellation mapping relationship, and determines the array element additional delay parameters of each array element based on the preset specific target direction. The two are superimposed to generate multiple independent time modulation control sequences.

[0131] The switching array drives each element of the antenna array to switch port impedance states according to the time modulation control sequence, so that the scattered echo carries higher-order modulation information on the target harmonic and is coherently enhanced in the specific target direction, while generating residual phase difference in the spatial region deviating from this direction, resulting in constellation diffusion.

[0132] The receiving processing end captures the scattered echo in the direction of the specific target, and recovers the information to be transmitted through frame synchronization, single-frequency harmonic feature extraction, channel calibration based on training segments, and decision decoding.

[0133] Based on the aforementioned secure communication system, the method specifically includes:

[0134] Step 1: Illuminate the time-modulated backscatter array node with a continuous carrier wave from the radio frequency illumination end;

[0135] Step 2: The control unit receives the information to be transmitted and encapsulates the information to be transmitted into a communication frame including a synchronization segment, a training segment, and a data segment;

[0136] Step 3: The control unit maps each symbol to be transmitted to the start time and duration of the switch within the symbol period according to the current modulation method.

[0137] Step 4: The control unit adds time delay to different antenna array elements according to the set target receiving direction to generate a multi-channel radio frequency switch control sequence.

[0138] Step 5: Multiple RF switches synchronously switch the load state of the corresponding antenna array elements according to the multi-channel RF switch control sequence, so that the scattered echo carries modulation information on the target harmonics and is coherently enhanced in the target receiving direction to form a directional scattered echo.

[0139] Step 6: The receiving and processing end collects the sampling data of the directional scattered echo, and completes harmonic extraction, training calibration, symbol decision and data verification and parsing to output the recovered information to be transmitted.

[0140] Furthermore, in one embodiment, the step of adding additional delay to different antenna array elements according to the set target receiving direction includes:

[0141] Determine the element positions, operating wavelength, and target receiving direction of the antenna array;

[0142] Calculate the target harmonic phase compensation amount required for each array element based on the target receiving direction, and convert the phase compensation amount into the array element additional time delay;

[0143] The additional delay of the array element is superimposed on the reference switching start time of each array element to form the actual switching start time of each RF switch.

[0144] Here, the target receiving direction is a predetermined target receiving direction. When the receiving angle deviates from the target receiving direction, the residual phase difference between array elements disperses the receiving constellation and increases the symbol error rate.

[0145] Furthermore, in one embodiment, step 3, which maps each symbol to be transmitted to a switching start time and duration within a symbol period according to the modulation scheme, specifically includes:

[0146] Step 3-1: Determine the constellation set corresponding to the current modulation mode. ,in The modulation order;

[0147] Step 3-2: Select the first harmonic as the target information carrier component, and determine the duration based on the amplitude of the constellation points. The start time of the switch is determined based on the phase of the constellation points. ;

[0148] Step 3-3, convert the continuous time parameters and Quantized into FPGA-implementable integer count values and And write it into the modulation parameter mapping table;

[0149] Steps 3-4: During the transmission process, the modulation parameter mapping table is queried according to the symbol index to be transmitted to generate the corresponding two-state radio frequency switch control waveform.

[0150] Furthermore, in one embodiment, step 4, which involves adding additional time delays to different array elements based on the target receiving direction, specifically includes:

[0151] Step 4-1: Determine the positions of the antenna array elements. Operating wavelength and target receiving direction ;

[0152] Step 4-2, according to the target direction Calculate the target harmonic phase compensation required for each array element;

[0153] Step 4-3: Convert the phase compensation amount into array element additional delay. and will Quantization to FPGA clock count ;

[0154] Step 4-4: Add time delay to the array elements Superimposed to the start time of the reference switch This forms the actual switching start time of each RF switch. .

[0155] Furthermore, in one embodiment, step 5, in which the RF switch switches the antenna load state according to the control sequence, specifically includes:

[0156] Step 5-1: When the control signal is in the first state, connect the antenna port to the matched load and form an absorption state;

[0157] Step 5-2: When the control signal is in the second state, make the antenna port in a mismatched state and form a reflection state;

[0158] Step 5-3, within one symbol period according to and By controlling the duration of the absorption and reflection states, the scattered echo can form corresponding amplitude and phase on the target harmonic.

[0159] Step 5-4 involves coherently superimposing the target harmonic phases of multiple array elements in the target receiving direction, generating a residual phase difference in the non-target direction, thereby achieving directional secure transmission.

[0160] Furthermore, in one embodiment, step 6, in which the receiving and processing end acquires IQ data and completes harmonic extraction, training calibration, symbol decision, data parsing, and CRC verification, specifically includes:

[0161] Step 6-1: Perform DC removal, slow drift removal, and resampling processing on the IQ data;

[0162] Step 6-2: Use the Barker synchronization segment to perform relevant calculations to determine the frame start position;

[0163] Step 6-3: Perform a single-frequency point DFT on each symbol slice to extract the target harmonic complex features;

[0164] Step 6-4: Use the Training segment to estimate the reference center for each constellation category;

[0165] Step 6-5: Perform nearest neighbor or bi-branch weighted decision on the Data segment to restore the symbol sequence;

[0166] Step 6-6: Reassemble the symbol sequence into a byte stream, parse the Header, Payload, CRC16 and Padding fields, and output valid data based on the CRC16 check result.

[0167] Furthermore, the reflection coefficient of the backscattering node is determined by the antenna input impedance and the load impedance, and its expression is:

[0168]

[0169] In the formula, Indicates the antenna port reflection coefficient. Indicates the load impedance. Indicates the antenna input impedance. This represents the conjugate of the antenna input impedance. When and During conjugate matching, nodes are primarily in the absorbing state; when and When mismatch occurs, the node is primarily in a reflective state.

[0170] Furthermore, the incident carrier generated at the radio frequency irradiation end can be expressed as:

[0171]

[0172] In the formula, Indicates carrier amplitude. Indicates the carrier frequency. Indicates the initial phase. The reflection coefficient of the backscattering node varies with time. The incident carrier is modulated, and the observed signal at the receiving end can be expressed as:

[0173]

[0174] In the formula, This represents the direct link coefficient from the RF irradiation end to the receiving end. This represents the equivalent two-way link coefficient via the backscattering node. This indicates noise and residual interference.

[0175] Furthermore, in the one symbol period Internally, the two-state time-modulated control waveform output by the control unit. From the start time of the switch and duration Confirmation can be written as:

[0176]

[0177] After performing a Fourier expansion on the periodic time-modulated waveform, the first... The first harmonic complex coefficients are:

[0178]

[0179] In the formula, It is a non-zero integer. The above formula shows that the duration The main factor determining the target harmonic amplitude is the switching start time. The primary factor determining the target harmonic phase is the complex symbol to be transmitted. and By establishing a mapping relationship, high-order amplitude and phase modulation can be achieved on a two-state RF switch.

[0180] In a preferred embodiment, either a positive first harmonic or a negative first harmonic is selected as the information-carrying component. Let... =1, then the first The first-order harmonic complex coefficients corresponding to each symbol can be written as:

[0181]

[0182] If the maximum amplitude code element is used As a normalization benchmark, the complex symbols to be transmitted The following conditions must be met with the time parameter:

[0183]

[0184]

[0185] In the formula, Let be any integer. The formula represents the duration. Used to determine the radius of constellation points and the starting time. Used to determine constellation point angles; therefore, the control unit can map different amplitude layers of 4ASK, different phase states of 8PSK, and the amplitude-phase joint constellation of 16QAM to the same set of two-state RF switch waveforms by generating time parameter tables offline or querying them online.

[0186] Furthermore, in FPGA discrete implementation, the symbol period Classified as Each sub-time unit, let Then the above equation transforms into:

[0187]

[0188] For a given target constellation set The control unit or host computer can generate a modulation parameter mapping table by minimizing the deviation between the theoretical constellation and the discrete harmonic coefficients. The optimization objective is:

[0189]

[0190] In the formula, The link amplitude-phase normalization coefficient, These are the first-order harmonic complex coefficients corresponding to the discrete-time parameters. To ensure FPGA implementability, and satisfy:

[0191]

[0192]

[0193] When it is necessary to avoid excessively narrow switching edges or insufficient hardware response, a minimum pulse width constraint can be further added:

[0194]

[0195]

[0196] In the formula, The minimum achievable count width is determined by the rising and falling edges of the RF switch and the FPGA clock resolution.

[0197] To suppress non-target harmonic leakage, the receiving processing unit or control unit can also define a target harmonic efficiency. This is used to measure the proportion of first-order harmonic energy to all visible harmonic energy.

[0198]

[0199] In the formula, This refers to the set of harmonic orders that the receiver is interested in. The larger the value, the higher the proportion of first-order harmonic energy selected as the information carrier, and the higher the effective signal-to-noise ratio that the receiver can obtain.

[0200] Furthermore, for those with A time-modulated backscattering array with n elements, the first The control waveform of each array element is superimposed with an additional time delay relative to the reference control waveform. Then, its target harmonic equivalent complex coefficients are:

[0201]

[0202] Array in the direction of observation The target harmonic array factor can be expressed as:

[0203]

[0204] In the formula, Indicates the first The position of each array element Indicates the operating wavelength. To ensure the target direction... The phases of the array elements are coherently superimposed, with added time delay. satisfy:

[0205]

[0206] For FPGA discrete control Quantifiable as And determined in the following way:

[0207]

[0208] in This indicates taking the nearest integer. If joint optimization between target direction gain and non-target direction suppression is required, an array delay optimization function can be constructed:

[0209]

[0210] In the formula, This is the set of non-target directions. and where represents the weighting coefficients. This optimization function maintains coherent superposition in the target direction while suppressing the target harmonic energy in non-target directions.

[0211] Furthermore, by substituting the equation, the directional scattered echo can be decomposed into a modulation term and a spatial term:

[0212]

[0213] In the formula, For carrying symbols The modulation term and the summation term are spatial beamforming terms. This decomposition illustrates that the higher-order symbol mapping and array beamforming of this invention can be synchronously implemented in the time domain, while also being mathematically decoupled from each other.

[0214] Furthermore, if the receiving end is located in the target direction Then we have the following formula:

[0215]

[0216] If the receiving end is located in a direction other than the target direction Then the received field strength is:

[0217]

[0218] The spatial isolation ratio can be defined from the above two equations:

[0219]

[0220] The larger the value, the more significant the difference in demodulated signals between the target direction and non-target directions. Unlike traditional encryption algorithms, this spatial isolation ratio is determined by array geometry, target direction, and time delay, and is a physical layer security enhancement mechanism.

[0221] Furthermore, the receiving processing end processes the IQ sampling sequence after each symbol slice. Perform a single-frequency point DFT to extract positive or negative first-order harmonic features:

[0222]

[0223]

[0224] Training segment belongs to the first The sample set of class symbols is denoted as Then the training centers for the positive and negative first harmonics of this type of symbol are respectively:

[0225]

[0226]

[0227] For the symbols to be decided in the Data segment The two-branch weighted nearest neighbor criterion is adopted:

[0228]

[0229] Furthermore, weight and It can be estimated from the training data, for example:

[0230]

[0231]

[0232] In the formula, and These represent the intra-class variances of the positive and negative first harmonic branches in the training segment, respectively.

[0233] In the formula, and These are the weights for the positive and negative first harmonic branches, which can be estimated based on the branch energy or noise variance of the training segment. This decision method can utilize the conjugate redundancy relationship of the positive and negative first harmonics to improve the robustness of symbol recovery under weak scattering and hardware non-ideal conditions.

[0234] In one embodiment, a computer device is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the computer program to implement the time-modulated backscatter-based secure communication method.

[0235] In one embodiment, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the time-modulated backscatter-based secure communication method.

[0236] As a specific example, in one embodiment, the invention is illustrated with reference to the accompanying drawings.

[0237] like Figure 1 and Figure 13 As shown, the RF illumination end consists of an RF signal source and a transmitting antenna, used to provide a continuous carrier to the time-modulated backscatter array node. The backscatter array node consists of a Cyclone IV FPGA, four MASW-007107 RF switches, a quaternary linear antenna array, and a 50Ω matched load. The receiving and processing end consists of a receiving antenna, a spectrum acquisition device, and a host computer processing program. The continuous carrier output by the RF illumination end can be represented as:

[0238]

[0239] In the formula, This indicates the signal emitted by the radio frequency irradiation end. Indicates carrier amplitude. Indicates the carrier frequency. This represents the initial phase. In the FPGA implementation, let the discrete clock period be... Each symbol period contains If there are discrete clock units, then the symbol period is :

[0240]

[0241] No. The activation start count value and activation width count value corresponding to each symbol are respectively: and Then its actual time parameter is:

[0242]

[0243]

[0244] No. The target direction additional delay count value of each array element is The corresponding additional delay is:

[0245]

[0246] Therefore, the first The array element in the first The actual switching start time within one symbol period can be expressed as:

[0247]

[0248] like Figure 3 As shown, each element of the four-element backscatter array carries the same modulation symbol, but different target harmonic phases are formed by adding time delay. The elements in the target direction are coherently superimposed, while the phases of the elements in the non-target direction are inconsistent, thus causing the constellation distribution in the non-target direction to spread, increasing the demodulation difficulty at the receiver in the non-target direction.

[0249] like Figure 4 As shown, the time modulation control waveform is determined by the start position and duration within each symbol period, corresponding to the target harmonic phase and amplitude, respectively. For FPGA discrete implementation, the modulation method, the index of the symbol to be transmitted, and the beam number can be used together as the lookup table address. The lookup table outputs the switch control parameters.

[0250]

[0251] In the formula, Indicates the modulation method. Indicates the first Constellation index of the symbols to be sent. This indicates the beam parameter number. The lookup table output is:

[0252]

[0253] In the formula, This represents the number of array elements. In this embodiment... Using the lookup table structure described above, the FPGA can switch between different modulation schemes such as 4ASK, 8PSK, and 16QAM without changing the RF hardware structure.

[0254] like Figure 5 As shown, the FPGA transmission control structure includes text buffering, frame field generation, CRC calculation, data segment padding, training segment concatenation, time modulation mapping, and multi-channel backscatter switch driving. Let the number of bytes of the host computer input payload be... Each byte is 8 bits, and the modulation order is [number missing]. The number of symbols required for the Payload portion of the Data field is:

[0255]

[0256] In the formula, This indicates rounding up. If the total number of bits in the Header and CRC16 fields is... Then the total number of symbols in the Data segment can be expressed as:

[0257]

[0258] When the last symbol is insufficient to complete the modulation bits, the FPGA pads with zeros in the Padding field to ensure that the number of symbols in the Data field meets the integer mapping requirements.

[0259] like Figure 6 and Figure 7 As shown, the communication frame includes a Barker synchronization segment, a Training segment, and a Data segment. The Data segment includes a Header, Payload, CRC16, and Padding. The Header identifies the frame type, modulation scheme, and Payload length. The Payload carries valid text or instructions. CRC16 is used for integrity verification, and Padding is used to pad the symbol length. For CRC16 verification, this embodiment can be described using the following recursive form:

[0260]

[0261] In the formula, , The first The, the CRC register state before processing each input bit For input bits, Generate the binary representation of the polynomial for CRC16. This indicates an XOR operation. The receiving end recalculates the CRC16 of the Header and Payload fields and compares it with the CRC16 field in the received frame; if they match, the frame is deemed to have passed the integrity check.

[0262] The FPGA transmission control flow includes serial port reception, data buffering, frame format encapsulation, CRC generation, training concatenation, modulation LUT mapping, and beam delay superposition. For the "HELLO" test string, ASCII bytes can be observed internally within the FPGA: The payload length field being 5 indicates that serial port reception and data buffering are correct. For the test string "ABC123", the valid payload length field is 6, and the receiving end can determine the subsequent payload parsing length based on the header field.

[0263] The receiving and processing end synchronizes the acquired IQ sequence. Let the discrete complex baseband sequence at the receiving end be... This embodiment uses a Barker sequence of length 13. If relevant synchronization is performed, the relevant detection quantity can be expressed as:

[0264]

[0265] In the formula, Indicates the first The sampling points represent the Barker correlation peak values ​​at the starting position. Represents the discrete complex baseband sequence at the receiving end. Middle Each sample value, Represents the local Barker sequence The conjugate value of the m-th chip.

[0266] when When a peak value is obtained and exceeds a preset threshold, that position is determined as the frame synchronization start point. :

[0267]

[0268] After synchronization is complete, the receiving processing end slices the IQ data according to the symbol period. The sampling intervals corresponding to each symbol are:

[0269]

[0270] In the formula, The number of sampling points corresponds to each symbol period. Subsequently, the receiver performs single-frequency harmonic extraction, training calibration, and data decision for each symbol slice.

[0271] In a specific test embodiment, the single-antenna baseline test frequency was 1.95 GHz, the IQ sampling rate was 12.5 MHz, and the single sampling duration was approximately 20 ms; the multi-antenna array test frequency was 1.5 GHz, and 4ASK and 16QAM data were collected at receiving angles of 15°, 30°, 45°, -35°, and -45° around a 15° target direction. Symbol error rate Statistics can be compiled in the following manner:

[0272]

[0273] In the formula, The number of symbols that indicate an error. This represents the total number of symbols included in the statistics.

[0274] Test results are as follows Figures 8 to 10As shown in Table 1 below, both 4ASK and 16QAM maintain low symbol error rates near the target direction; as the receiving angle deviates from the target direction, the symbol error rate increases significantly, indicating that the time modulation array of the present invention can achieve spatial domain isolation. This effect does not rely on traditional key encryption, but is determined by the target harmonics, array phase, and receiving direction, and can achieve demodulation suppression at the physical layer for receivers in non-target directions.

[0275] Table 1. Performance Comparison of 4ASK and 16QAM at Different Reception Angles

[0276] 15 0.0045 0.0056 The main direction of reception is the best. 30 0.0134 0.0091 The nearest direction still maintains good recovery ability. 45 0.0680 0.0894 Both degenerate at the edges -35 0.2359 0.2705 The deviation from the direction error increased significantly. -45 0.2282 0.2394 When the direction of travel is far from the main direction, the recovery performance decreases significantly.

[0277] In the text data return implementation example, such as Figure 11 and Figure 12 As shown, the transmitting end inputs "HELLO" and "ABC123", and the FPGA transmits using 4ASK modulation and beam001 beam parameters. The receiving end completes Barker synchronization, training calibration, data decision, header parsing, and CRC16 verification. Let the payload byte sequence recovered by the receiving end be... The original payload byte sequence at the sending end is The condition for correctly restoring the text is:

[0278]

[0279] In the formula, For the CRC field carried in the received frame, This is the CRC value recalculated by the receiving end based on the recovered Header and Payload fields.

[0280] Both sets of text were correctly recovered, and the CRC16 check passed 100%, proving that the present invention can not only transmit random symbols, but also support actual short text or instruction-type business data.

[0281] In summary, this invention achieves 4ASK, 8PSK, and 16QAM reconfigurable modulation, directional backscatter transmission, Barker synchronization, training calibration, and CRC16 integrity verification without adding an active RF transmit link or a multi-level impedance network.

[0282] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention without departing from its spirit and scope should be included within the protection scope of the present invention.

Claims

1. A secure communication system based on time-modulated backscattering, characterized in that, The system includes a radio frequency irradiation end, a time-modulated backscatter array node, and a receiving and processing end; The radio frequency irradiation end is used to generate and radiate a continuous radio frequency carrier signal to the time-modulated backscatter array node; The time-modulated backscatter array node includes a control unit, an antenna array, an impedance network, and a switch array; the impedance network is connected between the antenna array and the switch array; the control unit is used to generate multiple independent time-modulation control sequences according to the information to be transmitted and output them to the switch array, driving the switch array to switch the port impedance state of each element in the antenna array, so as to modulate the spatial characteristics and amplitude-phase characteristics of the radio frequency continuous carrier signal without changing the radio frequency hardware structure, forming a harmonic backscatter echo that carries the information to be transmitted and is radiated directionally in a specific target direction; The receiving and processing end is used to collect the harmonic scattered echo in the specific target direction, and recover the information to be transmitted by harmonic feature extraction and demodulation. The time modulation control sequence includes a reference time parameter for controlling the switching of the port impedance state and an array element additional delay parameter for beam phase compensation.

2. The secure communication system based on time-modulated backscattering according to claim 1, characterized in that, The antenna array is a linear array comprising N array elements, where N≥2; the switch array comprises N switch channels, each switch channel being independently connected to the port impedance of the corresponding array element, and each switch channel being driven by an independent control sequence output by the control unit. The switching array controls the ports of each array element to switch between a first impedance state and a second impedance state within each symbol period, so that the corresponding time-varying reflection coefficient alternates between the absorption state and the reflection state. By using the two-state switching, a reconfigurable constellation point with high-order amplitude and phase modulation characteristics is constructed on the preset target order harmonic components.

3. The secure communication system based on time-modulated backscattering according to claim 2, characterized in that, The reference time parameters include at least the switching duration of the corresponding target constellation point amplitude and the switching start time of the corresponding target constellation point phase; the control unit has a pre-stored discrete time parameter mapping table, and the control unit queries the discrete time parameter mapping table according to the currently selected modulation method and the symbol index to be transmitted to obtain the corresponding discrete switch start count and discrete switch width count. The discrete-time parameter mapping table is generated by minimizing the Euclidean distance deviation between the theoretical target constellation point and the target order harmonic complex coefficients corresponding to the discrete-time parameters. In the optimization process, a minimum pulse width constraint based on the hardware response boundary is introduced.

4. The secure communication system based on time-modulated backscattering according to claim 1, characterized in that, The control unit calculates the target harmonic phase compensation amount required for each array element according to the spatial beam pointing requirement of the specific target direction, and converts the target harmonic phase compensation amount into the corresponding array element additional time delay parameter, so as to superimpose it on the reference switching start time of each array element, thereby realizing the synchronous and decoupled control of high-order symbol modulation and array beamforming in the time domain.

5. The secure communication system based on time-modulated backscattering according to claim 1, characterized in that, The control unit encapsulates the information to be transmitted into a communication frame structure that includes a synchronization segment, a training segment, and a data segment; The data segment includes a frame header indicating frame characteristics and modulation parameters, a payload carrying valid service data, and a cyclic redundancy check field for integrity verification.

6. The secure communication system based on time-modulated backscattering according to claim 5, characterized in that, The receiving and processing end includes: The synchronization module performs relevant operations using the synchronization segment to capture the frame start point and performs symbol slicing according to the symbol period; The extraction module performs a single-frequency discrete Fourier transform on each symbol slice to extract the complex features of the target order harmonics. The calibration decision module estimates the training reference center for each constellation category using known full constellation symbol samples from the training segment, and performs symbol decision on the data segment based on the two-branch weighted nearest neighbor criterion to recover the symbol sequence.

7. The secure communication system based on time-modulated backscattering according to claim 6, characterized in that, During the demodulation and recovery of the information to be transmitted, the positive first harmonic branch features and the negative first harmonic branch features are extracted and utilized, and the conjugate redundancy relationship of the positive and negative first harmonics is used to perform dual-branch joint decision.

8. A secure communication method based on time-modulated backscattering, implemented using the system described in any one of claims 1-7, characterized in that, The method includes the following steps: The radio frequency irradiation end radiates a continuous radio frequency carrier signal to the time-modulated backscatter array node. The control unit receives the information to be transmitted and encapsulates it into a communication frame containing synchronization, training, and data fields; The control unit determines the reference time parameters corresponding to each symbol to be transmitted based on the current modulation constellation mapping relationship, and determines the array element additional delay parameters of each array element based on the preset specific target direction. The two are superimposed to generate multiple independent time modulation control sequences. The switching array drives each element of the antenna array to switch port impedance states according to the time modulation control sequence, so that the scattered echo carries higher-order modulation information on the target harmonic and is coherently enhanced in the specific target direction, while generating residual phase difference in the spatial region deviating from this direction, resulting in constellation diffusion. The receiving processing end captures the scattered echo in the direction of the specific target, and recovers the information to be transmitted through frame synchronization, single-frequency harmonic feature extraction, channel calibration based on training segments, and decision decoding.

9. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the secure communication method based on time-modulated backscattering as described in claim 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the secure communication method based on time-modulated backscattering as described in claim 8.

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