Single-receiver WiFi backscattering method based on frequency domain cyclic displacement

Abstract

The invention relates to the technical field of backscatter communication, and discloses a single-receiver WiFi backscatter method based on frequency domain cyclic shift. The method comprises the following steps: a transmitting end modulates data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform, and forms an orthogonal frequency division multiplexing WiFi signal taking an OFDM symbol as a basic unit after adding a cyclic prefix; the label receives the WiFi signal, and performs sub-symbol-level modulation on an OFDM symbol in the WiFi signal based on a modulation technology of frequency domain cyclic shift; and the receiver determines the cyclic shift value of the frequency domain subcarrier through the frequency domain invariance of the OFDM symbol, and demodulates the received reflected signal to recover the label data. According to the invention, sub-symbol-level label data modulation is realized, the data transmission efficiency is improved, only a single symbol bandwidth is needed, and the waste of spectrum resources is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of backscatter communication, and in particular to a single-receiver WiFi backscatter method based on frequency domain cyclic shift. Background Art

[0002] In recent years, ambient backscatter has attracted widespread attention due to its great potential in ultra-low power data transmission. The basic principle of backscatter communication is to transmit data by reflecting existing signals. Traditional backscatter systems (such as radio frequency identification, RFID) require dedicated readers to generate excitation signals, and tags transmit data by reflecting these excitation signals. In contrast, ambient backscatter uses existing wireless signals in the environment, such as TV signals, FM radio, long-range radio (LoRa) or WiFi, as carriers. This mode greatly expands the range of sources of excitation signals and promotes the development of passive Internet of Things (IoT).

[0003] As the most widely used indoor wireless technology, WiFi is an ideal stimulus for backscatter communications. Current mainstream WiFi protocols, including WiFi 3 / 4 / 5 / 6 (also known as 802.11g / n / ac / ax), use orthogonal frequency division multiplexing (OFDM) modulation to enhance throughput and resist interference. In a typical OFDM backscatter system, tags transmit tag data by reflecting WiFi signals generated by ambient WiFi devices. This reflection process usually includes modulation of the phase or amplitude of the carrier signal. For example, in the prior art, FreeRider technology transmits bit '1' by flipping the phase of the OFDM symbol and transmits bit '0' by keeping it unchanged. MOXcatter technology, RapidRider technology, and X-Tandem technology also use similar phase modulation methods.

[0004] However, existing technologies have two major limitations:

[0005] 1. The demodulation of tag data depends on environmental data: When decoding tag data, existing technologies often require additional receivers to obtain environmental data from the environment as a reference, which limits the flexibility and ease of use of the system. Not only does it increase hardware costs, but it also requires precise synchronization between the two receivers. In addition, two independent frequency bands are required for demodulation, resulting in low spectrum utilization.

[0006] 2. Symbol-level modulation limits the system throughput: The modulation granularity of tag data in existing technologies is often limited to the OFDM symbol level. For example, FreeRider requires 4 OFDM symbols to modulate 1 bit of tag data, and RapidRider requires 1 OFDM symbol to modulate 1 bit. Therefore, the data transmission rate is limited by the symbol rate, which is particularly insufficient in scenarios with high data rate requirements.

[0007] The present invention aims to break through the limitations of the prior art, achieve finer-grained tag data modulation using only a single receiver, and improve the data transmission rate of an uncontrolled orthogonal frequency division multiplexing WiFi backscatter system. Summary of the invention

[0008] In order to solve the above technical problems, the present invention provides a single-receiver WiFi backscattering method based on frequency domain cyclic shift. Aiming at the limitations of symbol-level modulation and environmental data-dependent demodulation of existing WiFi backscattering technology, the present invention designs a modulation scheme based on frequency domain cyclic shift to achieve sub-symbol level modulation, break through the limitations of traditional symbol-level modulation, and improve the data transmission rate. In addition, an efficient and accurate decoding scheme is designed, and high-precision demodulation is performed only through a single receiver, which simplifies the system architecture and reduces the need for additional hardware, thereby reducing the complexity and cost of the system.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] A single-receiver WiFi backscattering method based on frequency domain cyclic shift, the backscattering system adopted includes a transmitter, a tag and a receiver, and the backscattering method includes:

[0011] The transmitting end uses any device that can transmit orthogonal frequency division multiplexing WiFi signals; wherein, the transmitting end modulates the data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform, and adds a cyclic prefix to form an orthogonal frequency division multiplexing WiFi signal with OFDM symbols as the basic unit;

[0012] The tag receives the WiFi signal and performs sub-symbol-level modulation on the OFDM symbol in the WiFi signal based on the modulation technology of frequency domain cyclic shift: the cyclic shift value is set according to the bit of the tag data, and a linear phase factor is applied to the selected time domain sample in the OFDM symbol of the WiFi signal, so that the frequency domain subcarrier corresponding to the OFDM symbol is cyclically shifted according to the set cyclic shift value, thereby forming a reflection signal carrying the tag data;

[0013] The receiver determines the cyclic shift value of the frequency domain subcarrier through the frequency domain invariance of the OFDM symbol, and demodulates the received reflected signal to recover the label data; the frequency domain invariance is that the position and value of the pilot subcarrier in the OFDM symbol before modulation are known.

[0014] Furthermore, the transmitting end modulates the data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples by inverse fast Fourier transform, and adds a cyclic prefix to form an orthogonal frequency division multiplexing WiFi signal with OFDM symbols as basic units, specifically including:

[0015] The transmitter converts N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform; in order to resist multipath interference and ensure symbol interval separation, a cyclic prefix is ​​added to the beginning of the time domain signal composed of N time domain samples:

[0016]

[0017] Where n is the index of the time domain sample; S(n) is the OFDM symbol; IFFT(·) is the inverse fast Fourier transform; X k represents the kth frequency domain subcarrier, the frequency domain subcarrier index k = 1, 2, ..., N; L CP is the length of the cyclic prefix.

[0018] Further, the cyclic shift value is set according to the bit of the tag data, and a linear phase factor is applied to the time domain sample selected in the OFDM symbol of the WiFi signal, so that the frequency domain subcarrier corresponding to the OFDM symbol is cyclically shifted according to the set cyclic shift value to form a reflection signal carrying the tag data, which specifically includes:

[0019] Mapping label data to cyclic shift values: For each OFDM symbol, the label data is mapped to a cyclic shift value d, and the mapping relationship is determined by the number of frequency domain subcarriers of the OFDM symbol;

[0020] Apply a linear phase factor: The label multiplies the N time domain samples after the cyclic prefix in the OFDM symbol by the linear phase factor The index of the time domain sample k1=1, 2, ..., N, the reflected signal B(n) is obtained:

[0021]

[0022] Where j is the imaginary unit, L CP is the length of the cyclic prefix, n represents the index of the time domain sample in B(n), and θ represents the normalized frequency modulation parameter related to the linear phase factor.

[0023] Further, the receiver determines the cyclic shift value of the frequency domain subcarrier through the frequency domain invariance of the OFDM symbol, specifically including:

[0024] By traversing all possible cyclic shift values, the correlation between the value of the pilot subcarrier before the cyclic shift and the value of the pilot subcarrier after the cyclic shift is calculated, and the cyclic shift value corresponding to the highest correlation is the final cyclic shift value of the frequency domain subcarrier;

[0025] The value of the pilot subcarrier before the cyclic shift adopts the pilot subcarrier estimation value P combined with the channel estimation. est :

[0026] P est (k2) = I k2 +iQ k2 ;

[0027] Where k2 is the index of the pilot subcarrier, P est (k2) is the estimated value of the k2th pilot subcarrier; k2 , Q k2 P est The real and imaginary parts of the complex form of (k2), where i is the imaginary unit;

[0028] For each possible cyclic shift value d, calculate the value P′ of the pilot subcarrier after cyclic shift d :

[0029] P′ d (k2) = I′ k2 +iQ′ k2 ;

[0030] P′ d (k2) the value of the k2th pilot subcarrier after cyclic shift; I′ k2 , Q′ k2 P′ d The real and imaginary parts of the complex form of (k2);

[0031] The correlation coefficient matrix is ​​used to calculate the correlation ρ between the value of the pilot subcarrier before the cyclic shift and the value of the pilot subcarrier after the cyclic shift pilot :

[0032]

[0033] Where P est (k2)M, P′ d (k2)M are the in-phase orthogonal matrices of the pilot subcarrier estimation value and the pilot subcarrier value after cyclic shift, Cov(·) represents the calculated covariance, σ est and σd YesP est (k2)M and P′d (k2)The standard deviation of M.

[0034] Furthermore, when there are multiple cyclic shift values ​​corresponding to the highest correlation, the receiver determines the cyclic shift value of the frequency domain subcarrier through the time domain invariance of the OFDM symbol, specifically including:

[0035] The time domain invariance is: the value of the cyclic prefix in the OFDM symbol remains unchanged before and after modulation;

[0036] The receiver first performs cyclic shift compensation on the frequency domain subcarrier based on different cyclic shift values ​​corresponding to the highest correlation, performs inverse fast Fourier transform on the cyclic shift compensated frequency domain subcarrier, and obtains the time domain symbol of the frequency domain subcarrier; then, extracts the n1 time domain samples at the tail of the obtained time domain symbol, which are recorded as the tail time domain samples S tail ; Calculate S tail The correlation coefficient ρ between the cyclic prefix CP of the received OFDM symbol CP :

[0037]

[0038] σ tail and σ CP YesS tail and the standard deviation of CP.

[0039] Furthermore, a joint phase shift keying modulation method is also included; the joint phase shift keying modulation method specifically includes:

[0040] The cyclic prefix of the OFDM symbol is the same as the n1 time domain samples at the end. When the tag modulates the OFDM symbol in the WiFi signal, if the tag data is "1", the tail time domain sample S composed of the n1 time domain samples at the end tail A phase offset Δθ is introduced; if the tag data is "0", no phase offset is introduced:

[0041]

[0042] Where j is an imaginary unit, n′ represents the tail time domain sample S tail The index of B, 1≤n′≤n1, tail (n′) represents the value of the tail time domain sample after tag modulation;

[0043] During the demodulation process, the receiver needs to compensate for all possible phase offsets introduced by the tag and perform correlation calculations with the cyclic prefix to find the correct phase offset for demodulation.

[0044] Compared with the prior art, the beneficial technical effects of the present invention are:

[0045] (1) The present invention designs a modulation technology based on frequency domain cyclic shift. The tag implements cyclic shift of subcarriers in the frequency domain to carry tag data by applying a linear phase factor to the received OFDM signal. This method implements sub-symbol level tag data modulation, improves the efficiency of data transmission, and only requires a single symbol bandwidth, avoiding the waste of spectrum resources.

[0046] (2) The present invention designs a demodulation method based on a single receiver, which uses the invariant information of the reflected signal in the frequency domain and time domain for demodulation, thus getting rid of the traditional method's reliance on environmental data. Data can be accurately decoded based only on the received reflected signal, greatly simplifying the system architecture and reducing hardware complexity and cost.

[0047] (3) The present invention designs a joint phase shift keying (PSK) modulation method. Based on the frequency domain cyclic shift modulation, it further utilizes the same characteristics of the cyclic prefix (CP) as the tail sample in the time domain, and uses PSK (phase shift keying) modulation on the tail sample to transmit more tag data, thereby greatly improving the orthogonal frequency division multiplexing WiFi backscatter data transmission rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a diagram of the backscatter system architecture used in the present invention;

[0049] Figure 2 A schematic diagram of the process of modulating time-domain OFDM symbols according to the present invention;

[0050] Figure 3 A schematic diagram of cyclic shifting of frequency domain subcarriers of OFDM symbols according to the present invention;

[0051] Figure 4 It is a schematic diagram of the implementation of the label modulation process of the present invention. DETAILED DESCRIPTION

[0052] A preferred embodiment of the present invention is described in detail below with reference to the accompanying drawings.

[0053] The backscatter system architecture designed by the present invention is as follows: Figure 1 As shown in the figure, it mainly consists of the following three core components: sender, tag and receiver.

[0054] The main function of the transmitter is to send out an excitation signal, which can be reflected by the tag and modulated to transmit the tag data. In the backscattering system designed by the present invention, the transmitter can be any device that can send out an orthogonal frequency division multiplexing WiFi signal, such as a WiFi router or any commercial WiFi device. This method of using the existing WiFi signal in the environment as an excitation signal reduces the need for additional hardware.

[0055] The tag is responsible for modulating the received excitation signal so as to transmit its own data by reflecting the excitation signal. The present invention designs a modulation technology based on frequency domain cyclic shift. The tag implements cyclic shift of frequency domain subcarriers to carry tag data by applying a linear phase factor to the received orthogonal frequency division multiplexing WiFi signal. This method realizes sub-symbol level tag data modulation, improves the efficiency of data transmission, and only requires a single symbol bandwidth, avoiding the waste of spectrum resources.

[0056] The task of the receiver is to decode the signal modulated and reflected by the tag to recover the tag data. The present invention carefully designs a decoding method based on a single receiver, which gets rid of the dependence of the traditional method on environmental data. It can accurately decode the data only based on the received reflected signal, greatly simplifying the system architecture and reducing hardware complexity and cost.

[0057] 1. Modulation method based on frequency domain cyclic shift:

[0058] The tag data modulation method designed by the present invention is based on the observation and utilization of an important characteristic of discrete Fourier transform: multiplying a signal by a linear phase factor in the time domain will produce a cyclic shift in the frequency domain. Specifically, in the process of tag modulation signal, the modulation method based on frequency domain cyclic shift of the present invention can realize the cyclic shift of the subcarriers of the OFDM symbol frequency domain signal according to the set shift value by appropriately selecting the time domain samples in the OFDM symbol and multiplying them by the designed linear phase factor for a received OFDM symbol time domain signal. The present invention applies different phase offsets according to different bits of the tag, thereby realizing cyclic shift modulation of the OFDM symbol.

[0059] 1. Generation of OFDM symbols:

[0060] At the transmitter, the data is modulated onto N frequency domain subcarriers. The transmitter converts the N frequency domain subcarriers into N time domain samples through the inverse fast Fourier transform (IFFT). This process can be expressed as IFFT (X k ), where {X k, k = 1, 2, ..., N} is a set of frequency domain subcarriers. In order to resist multipath interference and ensure symbol interval separation, a cyclic prefix (CP) is added to the beginning of the time domain signal, which is achieved by cutting a segment from the end of the time domain sample and appending it to the beginning. The resulting time domain OFDM symbol is represented as:

[0061]

[0062] Among them, L CP is the length of the cyclic prefix.

[0063] 2. Tag data modulation:

[0064] (1) Receiving OFDM symbols: After receiving the OFDM WiFi signal, the tag detects the data packet and modulates the tag data onto the payload symbol.

[0065] (2) Mapping of label data to cyclic shift value: For each OFDM symbol, it is assumed that the label data to be backscattered is mapped to a cyclic shift value d. The mapping relationship is determined by the number of frequency domain subcarriers in the OFDM symbol. For N frequency domain subcarriers, there are N types of cyclic shifts. Each cyclic shift value can be uniquely mapped to its corresponding binary data, i.e., the label data bit. For example, when N = 64, the label data corresponding to the cyclic shift value 2 is "000010".

[0066] (3) Multiply by a linear phase factor: The label multiplies the N time domain samples of the OFDM symbol by a linear phase factor Where d is the cyclic shift value in the frequency domain. It should be noted that since the cyclic prefix of the time domain signal does not contain actual data and is excluded in the FFT calculation at the receiving end, the tag only operates on the remaining N time domain samples. This operation can be mathematically expressed as:

[0067]

[0068] Where B(n) is the reflected signal after tag modulation.

[0069] like Figure 2 As shown in Figure 1, for an 802.11n symbol, the last 64 time domain samples of a typical environment OFDM symbol are the result of frequency domain to time domain conversion, while the first 16 samples are the cyclic prefix copied from the end of the time domain signal. When the tag receives this time domain OFDM symbol, it processes the last 64 samples (i.e., N = 64). Figure 3 As shown, the tag backscatters the bit sequence "000010" by cyclically shifting the frequency domain subcarrier by 2 positions.

[0070] 2. Single receiver demodulation method:

[0071] In order to demodulate the tag data using only one receiver, the present invention needs to determine the cyclic shift value of the frequency domain subcarrier. To achieve this goal, the present invention uses the information in the OFDM symbol that remains unchanged after the tag modulation. This unchanged information can be used as an anchor point, enabling the present invention to calculate the cyclic shift information of the subcarrier.

[0072] 1. Utilization of frequency domain invariance:

[0073] First, the present invention analyzes and utilizes the invariant information in the frequency domain. In orthogonal frequency division multiplexing WiFi backscatter, known pilot subcarriers are used for channel estimation. Taking the 802.11n OFDM symbol as an example, the OFDM symbol contains 64 frequency domain subcarriers, of which there are 4 pilot subcarriers, whose values ​​and positions are known. After tag modulation, these pilot subcarriers will also undergo cyclic shifts. To determine the cyclic shift value, it is necessary to traverse all possible cyclic shift values ​​and calculate the correlation between the shifted pilot subcarrier and its known value. The position with the highest correlation is the cyclic shift value of the subcarrier.

[0074] To improve the accuracy of correlation calculation, the present invention uses pilot subcarrier estimation combined with channel estimation. Channel estimation is based on the HT-LTF symbol in the data packet preamble, ensuring more accurate correlation calculation and accurate determination of cyclic shift value. The pilot subcarrier estimation is expressed as:

[0075] P est (k2) = I k2 +iQ k2 .

[0076] Where k2 is the index of the pilot subcarrier. For each possible cyclic shift value d, calculate the value P' of the pilot subcarrier after compensating this shift. d :

[0077] P′ d (k2) = I′ k2 +iQ′ k2 .

[0078] Finally, the correlation coefficient matrix is ​​used to calculate the correlation ρ between the pilot subcarrier estimate and the cyclically shifted pilot subcarrier value pilot :

[0079]

[0080] Where P est (k2)M, P′ d (k2)M are the in-phase orthogonal matrices of the pilot subcarrier estimation value and the pilot subcarrier value after cyclic shift, Cov(·) represents the calculated covariance, σ est and σ d YesPest (k2)M and P′ d (k2)The standard deviation of M.

[0081] Since the number of pilot subcarriers is limited and the values ​​are fixed, multiple cyclic shift values ​​may show similar high correlation. In order to further refine and accurately determine the correct cyclic shift value, the present invention needs to utilize other invariant information.

[0082] 2. Utilization of time domain invariance:

[0083] Furthermore, the present invention analyzes and utilizes the invariant information in the time domain. Note that in the time domain, an OFDM symbol contains a cyclic prefix (CP), which is a segment of samples copied from the end of the symbol. During the tag modulation process, the value of the cyclic prefix remains unchanged. Therefore, the receiver can use this information to further confirm the cyclic shift value. Specifically, the receiver calculates the time domain value of the symbol using IFFT after compensating for the shift in the frequency domain. Then, the tail of these calculated time domain samples is extracted and correlated with the received time domain symbol cyclic prefix value, thereby accurately determining the cyclic shift value applied by the tag.

[0084] The receiver first performs IFFT on the frequency-domain subcarriers that have undergone cyclic shift compensation. Then, the last 16 samples of the time-domain symbol are extracted and recorded as S tail Next, the correlation coefficient with the received cyclic prefix CP is calculated using a correlation mechanism similar to that in the frequency domain:

[0085]

[0086] By utilizing the invariant information in both the frequency domain and the time domain, the present invention can accurately identify the cyclic shift value, thereby ensuring accurate demodulation of the tag data. This method not only improves the robustness of the system, but also provides a solid foundation for achieving more efficient backscatter communication.

[0087] 3. Joint PSK modulation method:

[0088] In order to further improve the data transmission rate, the present invention further designs a joint PSK modulation method in tag modulation. The designed method utilizes the same characteristics of the cyclic prefix (CP) in the time domain as the tail time domain sample, and uses PSK (Phase Shift Keying) modulation on the tail time domain sample to transmit more tag data.

[0089] During the modulation process, the tag modulates more tag data by modifying the phase of the tail time domain sample. Specifically, when the tag transmits the tag data "1", the tail time domain sample S tail When transmitting the tag data "0", no phase offset is introduced. This modulation process can be expressed as:

[0090]

[0091] During the demodulation process, the receiver needs to compensate for all possible phase offsets introduced by the tag and perform correlation calculations with the cyclic prefix to find the correct phase change.

[0092] Through this method, the highest order of the system currently implemented by the present invention can support joint 64PSK, and can carry 12 bits of data in a single symbol. This technology not only improves the efficiency of data transmission, but also makes full use of the cyclic prefix characteristics in OFDM symbols, breaking through the data transmission limitations brought by relying solely on subcarrier cyclic shift. This innovative encoding and decoding mechanism further enhances the data carrying potential of OFDM symbols in backscatter communications.

[0093] Example

[0094] 1. System construction:

[0095] First, an orthogonal frequency division multiplexing WiFi backscatter system is built, which consists of three main parts: a transmitter, a tag, and a receiver. The transmitter and receiver are built using commercial WiFi network cards and software defined radio (SDR). In this embodiment, the commercial device uses a Qualcomm AR938x network card, and CommView software is used to send and receive data. The software defined radio consists of a ZedBoard and an AD9361 daughter board for signal processing. This embodiment uses Matlab as a data analysis tool.

[0096] 2. Tag modulation implementation:

[0097] like Figure 4 As shown, in the backscatter system, taking the modulation and transmission of "10000010" as an example, the tag needs to modulate and reflect the data. The implementation steps are as follows:

[0098] 1) Data division: First, the 8-bit binary data is divided into two parts: the first 2 bits and the last 6 bits. The first 2 bits are modulated using the joint PSK method, specifically QPSK (quadrature phase shift keying) modulation in this embodiment, and the last 6 bits are modulated using cyclic shift modulation.

[0099] 2) QPSK (Quaternary Phase Shift Keying) modulation: The first two bits "10" are modulated using QPSK. According to the QPSK modulation scheme, the phase rotation corresponding to "10" is 1 / 2π. Specifically, this phase rotation is applied to the 16 samples at the tail of the OFDM symbol for QPSK modulation of the tail.

[0100] 3) Cyclic shift modulation: Next, the last 64 samples of the OFDM symbol are cyclically shifted to modulate the last 6 bits of data "000010". The cyclic shift value corresponding to this bit sequence is 2. Based on this shift value, a linear phase factor is determined. The tag multiplies the last 64 samples of the symbol by the linear phase factor to complete the modulation on the time domain signal.

[0101] 4) Signal reflection: After completing the modulation, the tag reflects the signal and transmits data.

[0102] In this way, the tag can effectively transmit data within limited spectrum resources. QPSK (Quadrature Phase Shift Keying) modulation provides a way to apply phase changes at the end of the symbol, while cyclic shift modulation encodes more information by adjusting the subcarrier position of the symbol sample. In this way, combining the two modulation techniques can transmit more bits in one symbol, thereby improving data transmission efficiency.

[0103] 3. Tag data demodulation implementation:

[0104] The receiving end needs to demodulate the tag data from the received reflected signal. The following is the specific process of implementing demodulation:

[0105] 1) Extract symbol samples: The receiver first extracts the last 64 time domain samples of the symbol. These samples contain the label data encoded by cyclic shift modulation.

[0106] 2) Fourier Transform (FFT): Perform a fast Fourier transform (FFT) on the extracted 64 time domain samples to convert the time domain signal into the frequency domain to obtain the frequency domain subcarrier information. The frequency domain information helps determine the relative position of each frequency domain subcarrier.

[0107] 3) Cyclic shift compensation: In the frequency domain, the receiver attempts to recover the correct position of the frequency domain subcarriers by compensating for possible cyclic shift values. This process involves correlating the frequency domain subcarriers with known pilot subcarrier values ​​to identify possible shift compensations.

[0108] 4) Inverse Fourier Transform (IFFT): Perform an inverse fast Fourier transform (IFFT) on the compensated subcarrier to convert it back to a time domain signal. If the shift compensation is correct, the tail of the converted time domain signal will match the tail of the original modulated signal.

[0109] 5) QPSK phase compensation: Next, the possible QPSK modulation is compensated and correlated with the cyclic prefix (CP) to identify the correct linear phase factor. In this example, the highest correlation value is obtained when the linear phase factor is 1 / 2π, indicating that the recovered bit is "10".

[0110] 6) Tag data recovery: The last 6 bits of data "000010" are recovered through the correct shift value 2. Combined with the QPSK bit "10" decoded in the first two steps, the receiver successfully demodulates the complete tag data "10000010".

[0111] The demodulation process uses FFT and IFFT to obtain frequency and time domain information, and uses correlation calculations to verify and recover the modulated data. By compensating for possible cyclic shifts and phase adjustments, the receiver can accurately identify and extract tag data, achieving efficient backscatter communication.

[0112] The present invention has conducted experimental tests on a variety of WiFi protocols to verify the support of the backscattering method of the present invention for different OFDM WiFi signal excitation sources, demonstrating its high adaptability to different WiFi standards and good performance under various OFDM WiFi signals, such as 802.11g (WiFi 3), 802.11n (WiFi 4), 802.11ac (WiFi 5) and 802.11ax (WiFi 6). These protocols are widely used around the world, and the backscattering method of the present invention supports multiple protocols to ensure that it can work properly in various environments.

[0113] In the WiFi 3 protocol, a typical single-stream OFDM WiFi signal with a bandwidth of 20MHz is used for testing. Each OFDM symbol contains 64 subcarriers. In the basic frequency domain cyclic shift modulation scheme (excluding PSK, that is, without phase modulation), 6 tag bits can be encoded per symbol by using only cyclic shift modulation. In the joint PSK modulation method, the performance of multiple joint PSK modulation schemes from BPSK (binary phase shift keying) to 64PSK is tested. In particular, under the joint 64PSK modulation method, each symbol can carry 12 bits of tag data. The results show that the demodulation accuracy of the tag data is very high. The maximum throughput of different tag modulation schemes is 1.5Mbps (no PSK), 1.75Mbps (BPSK), 2Mbps (QPSK), 2.49Mbps (16PSK) and 2.94Mbps (64PSK).

[0114] Under WiFi 4, the performance of the backscattering method of the present invention under multi-band signals with 20MHz and 40MHz bandwidths was verified. Among them, the OFDM modulation was set to BPSK (MCS=0). Under 20MHz conditions, the throughput of tag data was similar to the results of WiFi 3. Under 40MHz conditions, the maximum tag data throughput increased to 3.21Mbps, and the average throughput increased by 9.85% compared with the 20MHz condition. This increase in throughput is mainly due to the increase in the number of subcarriers in the OFDM symbol from 64 to 128, which enables each symbol to modulate 7 bits of tag data through cyclic shift.

[0115] In WiFi 5, a 40MHz bandwidth was used, and then the bandwidth was increased to 80MHz, with OFDM modulation set to 64QAM (MCS=7). The throughput of tag data at 40MHz remained similar compared to WiFi 4. At 80MHz, the maximum tag data throughput increased to 3.47Mbps. In WiFi 6, the same bandwidth settings of 40MHz and 80MHz were used. The results show that the maximum observed tag data throughput is 1.15Mbps, which is lower than the other WiFi stimulus throughputs, mainly because each WiFi 6 symbol has a longer transmission time of 13.6 microseconds.

[0116] The above experimental results verify that the backscattering method of the present invention can use various OFDM WiFi signals as excitation, showing its high compatibility with existing commercial WiFi. In addition, the backscattering method of the present invention achieves high tag data throughput when using only a single access point (AP), demonstrating its application potential in future wireless communications.

[0117] It is obvious to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention, and any reference numerals in the claims should not be regarded as limiting the claims involved.

[0118] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A single-receiver WiFi backscattering method based on frequency domain cyclic shift, the backscattering system adopted includes a transmitter, a tag and a receiver, characterized in that: Backscatter methods include: The transmitting end uses any device that can transmit orthogonal frequency division multiplexing WiFi signals; wherein, the transmitting end modulates the data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform, and adds a cyclic prefix to form an orthogonal frequency division multiplexing WiFi signal with OFDM symbols as the basic unit; The tag receives the WiFi signal and performs sub-symbol-level modulation on the OFDM symbol in the WiFi signal based on the modulation technology of frequency domain cyclic shift: the cyclic shift value is set according to the bit of the tag data, and a linear phase factor is applied to the selected time domain sample in the OFDM symbol of the WiFi signal, so that the frequency domain subcarrier corresponding to the OFDM symbol is cyclically shifted according to the set cyclic shift value, forming a reflection signal carrying the tag data; The receiver determines the cyclic shift value of the frequency domain subcarrier through the frequency domain invariance of the OFDM symbol, and demodulates the received reflected signal to recover the label data; the frequency domain invariance is that the position and value of the pilot subcarrier in the OFDM symbol before modulation are known.

2. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 1, characterized in that: The transmitting end modulates the data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples by inverse fast Fourier transform, and adds a cyclic prefix to form an orthogonal frequency division multiplexing WiFi signal with OFDM symbols as the basic unit, specifically including: The transmitter converts N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform; in order to resist multipath interference and ensure symbol interval separation, a cyclic prefix is ​​added to the beginning of the time domain signal composed of N time domain samples: Where n is the index of the time domain sample; S(n) is the OFDM symbol; IFFT(·) is the inverse fast Fourier transform; X k represents the kth frequency domain subcarrier, the frequency domain subcarrier index k=1,2,…,N; L CP is the length of the cyclic prefix.

3. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 1, characterized in that: The step of setting a cyclic shift value according to the bit of the tag data, applying a linear phase factor to a time domain sample selected in an OFDM symbol of a WiFi signal, so that a frequency domain subcarrier corresponding to the OFDM symbol is cyclically shifted according to the set cyclic shift value to form a reflection signal carrying the tag data, specifically includes: Mapping label data to cyclic shift values: For each OFDM symbol, the label data is mapped to a cyclic shift value d, and the mapping relationship is determined by the number of frequency domain subcarriers of the OFDM symbol; Apply a linear phase factor: The label multiplies the N time domain samples after the cyclic prefix in the OFDM symbol by the linear phase factor The index of the time domain sample is k1=1,2,…,N, and the reflected signal B(n) is obtained: Where j is the imaginary unit, L CP is the length of the cyclic prefix, n represents the index of the time domain sample in B(n), and θ represents the normalized frequency modulation parameter related to the linear phase factor.

4. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 1, characterized in that: The receiver determines the cyclic shift value of the frequency domain subcarrier by the frequency domain invariance of the OFDM symbol, specifically including: By traversing all possible cyclic shift values, the correlation between the value of the pilot subcarrier before the cyclic shift and the value of the pilot subcarrier after the cyclic shift is calculated, and the cyclic shift value corresponding to the highest correlation is the final cyclic shift value of the frequency domain subcarrier; The value of the pilot subcarrier before the cyclic shift adopts the pilot subcarrier estimation value P combined with the channel estimation. est : P est (k2)=I k2 +iQ k2 ; Where k2 is the index of the pilot subcarrier, P est (k2) is the estimated value of the k2th pilot subcarrier; k2 , Q k2 P est The real and imaginary parts of the complex form of (k2), where i is the imaginary unit; For each possible cyclic shift value d, calculate the value P′ of the pilot subcarrier after cyclic shift d : P′ d (k2)=I′ k2 +iQ′ k2 ; P′ d (k2) the value of the k2th pilot subcarrier after cyclic shift; I′ k2 , Q′ k2 P′ d The real and imaginary parts of the complex form of (k2); The correlation coefficient matrix is ​​used to calculate the correlation ρ between the value of the pilot subcarrier before the cyclic shift and the value of the pilot subcarrier after the cyclic shift pilot : Where P est (k2)M, P′ d (k2)M are the in-phase orthogonal matrices of the pilot subcarrier estimation value and the pilot subcarrier value after cyclic shift, Cov(·) represents the calculated covariance, σ est and σ d YesP est (k2)M and P′ d (k2)The standard deviation of M.

5. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 4, characterized in that: When there are multiple cyclic shift values ​​corresponding to the highest correlation, the receiver determines the cyclic shift value of the frequency domain subcarrier through the time domain invariance of the OFDM symbol, including: The time domain invariance is: the value of the cyclic prefix in the OFDM symbol remains unchanged before and after modulation; The receiver first performs cyclic shift compensation on the frequency domain subcarrier based on different cyclic shift values ​​corresponding to the highest correlation, performs inverse fast Fourier transform on the cyclic shift compensated frequency domain subcarrier, and obtains the time domain symbol of the frequency domain subcarrier; then, extracts the n1 time domain samples at the tail of the obtained time domain symbol, which are recorded as the tail time domain samples S tail ; Calculate S tail The correlation coefficient ρ between the cyclic prefix CP of the received OFDM symbol CP : σ tail and σ CP YesS tail and the standard deviation of CP.

6. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 1, characterized in that: Also included is a joint phase shift keying modulation method; The joint phase shift keying modulation method specifically includes: The cyclic prefix of the OFDM symbol is the same as the n1 time domain samples at the end. When the tag modulates the OFDM symbol in the WiFi signal, if the tag data is "1", the tail time domain sample S composed of the n1 time domain samples at the end tail A phase shift Δθ is introduced; if the tag data is "0", no phase shift is introduced: Where j is an imaginary unit, n′ represents the tail time domain sample S tail The index of B, 1≤n′≤n1, tail (n′) represents the value of the tail time domain sample after tag modulation; During the demodulation process, the receiver needs to compensate for all possible phase offsets introduced by the tag and perform correlation calculations with the cyclic prefix to find the correct phase offset for demodulation.

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