A single-receiver wi-fi backscatter method based on frequency domain cyclic shift

Abstract

The application relates to the technical field of backscatter communication, and discloses a single-receiver WiFi backscatter method based on frequency domain cyclic shift, wherein a sending end modulates data onto N frequency domain subcarriers, converts the N frequency domain subcarriers into N time domain samples through inverse fast Fourier transform, adds a cyclic prefix, and forms a WiFi signal of orthogonal frequency division multiplexing with OFDM symbols as basic units; a tag receives the WiFi signal, and modulates the OFDM symbols in the WiFi signal at a sub-symbol level based on a frequency domain cyclic shift modulation technology; and a receiver determines the cyclic shift value of the frequency domain subcarriers through the frequency domain invariability of the OFDM symbols, demodulates the received reflection signal, and recovers the tag data. The application realizes sub-symbol level tag data modulation, improves the data transmission efficiency, and only needs a single symbol bandwidth, thereby avoiding waste of frequency spectrum resources.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of backscatter communication, and particularly relates to a single-receiver WiFi backscatter method based on frequency domain cyclic shift. BACKGROUND

[0002] In recent years, ambient backscatter has attracted extensive 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) need a special reader to generate an excitation signal, and the tag transmits data by reflecting the excitation signal. In contrast, ambient backscatter uses existing wireless signals in the environment, such as television signals, FM radio, long-range radio (LoRa) or WiFi, as a carrier. This mode greatly expands the source range of the excitation signal and promotes the development of the Internet of Things (IoT).

[0003] WiFi, as the most widely used indoor wireless technology, is an ideal excitation source for backscatter communication. The 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, the tag transmits tag data by reflecting the WiFi signal generated by the ambient WiFi device. This reflection process usually includes modulation of the phase or amplitude of the carrier signal. For example, in the prior art, the FreeRider technology transmits bit '1' by flipping the phase of the OFDM symbol and transmits bit '0' by keeping it unchanged. The MOXcatter technology, RapidRider technology and X-Tandem technology also use similar phase modulation methods.

[0004] However, the prior art has two main limitations:

[0005] 1. Demodulation of tag data depends on ambient data: When decoding tag data, the prior art often needs an additional receiver to obtain ambient 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 needed for demodulation, resulting in low spectrum utilization.

[0006] 2. Symbol level modulation limits the throughput of the system: the modulation granularity of the prior art tag data 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 application aims to break through the limitations of the prior art and achieve finer granularity tag data modulation with only a single receiver, thereby improving the data transmission rate of the uncontrolled orthogonal frequency division multiplexing WiFi backscatter system. SUMMARY

[0008] To solve the above technical problems, the present application provides a single-receiver WiFi backscatter method based on frequency domain cyclic shift, which overcomes the limitations of symbol level modulation and environment-dependent data demodulation in existing WiFi backscatter technologies. The present application designs a modulation scheme based on frequency domain cyclic shift, achieving sub-symbol level modulation and breaking through the limitations of traditional symbol level modulation to improve data transmission rate. In addition, an efficient and accurate decoding scheme is designed to achieve high-precision demodulation with only a single receiver, simplifying the system architecture and reducing the need for additional hardware, thereby reducing the complexity and cost of the system.

[0009] To solve the above technical problems, the present application adopts the following technical solutions:

[0010] A single-receiver WiFi backscatter method based on frequency domain cyclic shift, the backscatter system used includes a sending end, a tag, and a receiver, and the backscatter method includes:

[0011] The sending end uses any device capable of emitting orthogonal frequency division multiplexed WiFi signals; wherein the sending 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 adds a cyclic prefix to form orthogonal frequency division multiplexed WiFi signals with OFDM symbols as the basic unit;

[0012] The tag receives WiFi signals and modulates the OFDM symbols in the WiFi signals at the sub-symbol level based on the modulation technology of frequency domain cyclic shift: according to the bits of the tag data, the cyclic shift value is set, a linear phase factor is applied to the selected time domain samples in the OFDM symbols of the WiFi signals, and the frequency domain subcarriers corresponding to the OFDM symbols are cyclically shifted according to the set cyclic shift value to form a reflection signal carrying tag data;

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

[0014] Further, the sending 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 adds a cyclic prefix to form a WiFi signal of orthogonal frequency division multiplexing taking an OFDM symbol as a basic unit, specifically including:

[0015] The sending end converts the 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 the N time domain samples:

[0016]

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

[0018] Further, the cyclic shift value is set according to the bits of the tag data, a linear phase factor is applied to the selected time domain samples in the OFDM symbol of the WiFi signal, the frequency domain subcarriers corresponding to the OFDM symbol are cyclically shifted according to the set cyclic shift value, and a reflection signal carrying the tag data is formed, specifically including:

[0019] Mapping the tag data to the cyclic shift value: for each OFDM symbol, the tag 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] Applying a linear phase factor: the tag multiplies the N time domain samples after the cyclic prefix in the OFDM symbol by a linear phase factor The index of the time domain sample k1 = 1, 2,..., N, to obtain the reflection signal B(n):

[0021]

[0022] Wherein, j is an 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 a 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 invariability of the OFDM symbol, and specifically includes:

[0024] By traversing all possible cyclic shift values, the correlation of the value of the cyclic shift 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] Wherein, the value of the pilot subcarrier before the cyclic shift adopts the pilot subcarrier estimation value P est :

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

[0027] Wherein, k2 is the index of the pilot subcarrier, P est (k2) is the k2th pilot subcarrier estimation value; I k2 , Q k2 are the real part and the imaginary part of the complex form of P est (k2) respectively, and i is the imaginary unit;

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

[0029] P′ d (k2)=I′ k2 +iQ′ k2 ;

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

[0031] The correlation ρ pilot of the value of the pilot subcarrier before the cyclic shift and the value of the pilot subcarrier after the cyclic shift is calculated using the correlation coefficient matrix:

[0032]

[0033] Wherein, P est (k2)M and P′ d (k2)M are the in-phase and quadrature matrixes of the pilot subcarrier estimation value and the pilot subcarrier value after the cyclic shift respectively, Cov(·) represents calculating the covariance, σ est and σd are P est (k2)M and P′d The standard deviation of (k2)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 means that 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 the different cyclic shift values ​​corresponding to the highest correlation. Then, it performs an inverse fast Fourier transform on the cyclically shifted frequency domain subcarrier to obtain the time-domain symbol of the frequency domain subcarrier. Next, it extracts the n1 time-domain samples at the tail of the obtained time-domain symbol, denoted as the tail time-domain sample S. tail ; Calculate S tail The correlation coefficient ρ between the received OFDM symbol and the cyclic prefix CP CP :

[0037]

[0038] σ tail and σ CP It is S tail The standard deviation of CP.

[0039] Furthermore, it also includes a joint phase-shift keying modulation method; 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, will be the same. tail A phase offset Δθ is introduced; if the label data is "0", no phase offset is introduced.

[0041]

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

[0043] During demodulation, 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 and achieve demodulation.

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

[0045] (1) This invention designs a modulation technique based on frequency domain cyclic shift. The tag carries tag data by applying a linear phase factor to the received OFDM signal to achieve cyclic shift of the subcarrier in the frequency domain. This method realizes sub-symbol level tag data modulation, improves data transmission efficiency, 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 dependence of traditional methods on environmental data. Based solely on the received reflected signal, the data can be accurately decoded, 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 frequency domain cyclic shift modulation, it further utilizes the characteristic that the cyclic prefix (CP) is the same as the tail sample in the time domain. PSK (phase shift keying) modulation is used on the tail sample to transmit more tag data, which greatly improves the backscatter data transmission rate of orthogonal frequency division multiplexing WiFi. Attached Figure Description

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

[0049] Figure 2 This is a schematic diagram of the time-domain OFDM symbol modulation process of the present invention;

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

[0051] Figure 4 This is a schematic diagram illustrating the implementation of the tag modulation process of the present invention. Detailed Implementation

[0052] A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0053] The backscattering system architecture designed in this invention is as follows: Figure 1 As shown, it mainly consists of the following three core components: the transmitter, the tag, and the receiver.

[0054] The primary function of the transmitter is to emit an excitation signal, which can be reflected and modulated by the tag to transmit tag data. In the backscatter system designed in this invention, the transmitter can be any device capable of emitting orthogonal frequency division multiplexing (OFDM) WiFi signals, such as a WiFi router or any commercial WiFi device. This method of utilizing existing WiFi signals in the environment as the excitation signal reduces the need for additional hardware.

[0055] The tag is responsible for modulating the received excitation signal to transmit its own data by reflecting the excitation signal. This invention designs a modulation technique based on frequency-domain cyclic shifting. The tag applies a linear phase factor to the received orthogonal frequency division multiplexing (OFDM) WiFi signal to achieve cyclic shifting of frequency-domain subcarriers to carry tag data. This method achieves sub-symbol-level tag data modulation, improving data transmission efficiency and requiring only a single symbol bandwidth, thus avoiding the waste of spectrum resources.

[0056] The receiver's task is to decode the signal modulated and reflected by the tag to recover the tag data. This invention features a sophisticated decoding method based on a single receiver, eliminating the dependence on environmental data in traditional methods. It can accurately decode the data based solely on the received reflected signal, greatly simplifying the system architecture and reducing hardware complexity and cost.

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

[0058] The tag data modulation method designed in this invention is based on an important characteristic of the Discrete Fourier Transform: multiplying a signal by a linear phase factor in the time domain produces a cyclic shift in the frequency domain. Specifically, in the process of tag signal modulation, the frequency-domain cyclic shift-based modulation method of this invention, for a received OFDM symbol time-domain signal, by appropriately selecting time-domain samples from the OFDM symbol and multiplying them by a designed linear phase factor, can achieve cyclic shifting of the subcarriers of the OFDM symbol frequency-domain signal according to a set shift value. This invention applies different phase shifts 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 transmitting end, data is modulated onto N frequency domain subcarriers. The transmitting end converts the N frequency domain subcarriers into N time domain samples using an inverse fast Fourier transform (IFFT). This process can be represented as IFFT(X... k ), where {X kLet {k = 1, 2, ..., N} be the set of frequency domain subcarriers. To resist multipath interference and ensure symbol spacing separation, a cyclic prefix (CP) is added to the beginning of the time-domain signal. This is achieved by truncating a segment from the end of the time-domain sample and appending it to the beginning. The generated time-domain OFDM symbol is represented as:

[0061]

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

[0063] 2. Tag data modulation:

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

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

[0066] (3) Multiply by a linear phase factor: Multiply the N time-domain samples of the label-to-OFDM symbol by a linear phase factor. Where d is the cyclic shift value in the frequency domain. It's important to note that since the cyclic prefix of the time-domain signal does not contain actual data and is excluded in the FFT calculation at the receiver, the label operation is only performed on the remaining N time-domain samples. This operation can be mathematically represented as:

[0067]

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

[0069] like Figure 2 As shown, for an 802.11n symbol, the last 64 time-domain samples of a typical environmental OFDM symbol are the result of frequency-domain to time-domain conversion, while the first 16 samples are a cyclic prefix copied from the end of the time-domain signal. When a 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 two positions.

[0070] II. Single-receiver demodulation method:

[0071] To demodulate tag data using only a single receiver, this invention requires determining the cyclic shift value of the frequency-domain subcarriers. To achieve this, the invention utilizes information in the OFDM symbol that remains unchanged after tag modulation. This invariant information serves as an anchor point, enabling the invention to calculate the cyclic shift information of the subcarriers.

[0072] 1. Utilization of frequency domain invariance:

[0073] First, this invention analyzes and utilizes frequency-domain invariant information. In Orthogonal Frequency Division Multiplexing (OFDM) WiFi backscattering, known pilot subcarriers are used for channel estimation. Taking an 802.11n OFDM symbol as an example, an OFDM symbol contains 64 frequency-domain subcarriers, of which 4 are pilot subcarriers, whose values ​​and positions are known. After tag modulation, these pilot subcarriers also undergo cyclic shift. 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, this invention uses pilot subcarrier estimates combined with channel estimation. Channel estimation is based on the HT-LTF symbols in the packet preamble, ensuring more accurate correlation calculation and precise determination of the cyclic shift value. The pilot subcarrier estimates are expressed as follows:

[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 for 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 estimated pilot subcarrier values ​​and the cyclically shifted pilot subcarrier values. pilot :

[0079]

[0080] Where P est (k2)M、P′ d (k2)M are the in-phase orthogonal matrices of the estimated pilot subcarrier values ​​and the cyclically shifted pilot subcarrier values, respectively. Cov(·) represents the calculation of the covariance, and σ est and σ d It is Pest (k2)M and P′ d The standard deviation of (k2)M.

[0081] Because the number and value of pilot subcarriers are finite, multiple cyclic shift values ​​may exhibit similar high correlation. To further refine and accurately determine the correct cyclic shift value, this invention requires the use of other invariant information.

[0082] 2. Utilization of time-domain invariance:

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

[0084] The receiver first performs an IFFT on the frequency-domain subcarriers after cyclic shift compensation. Then, it extracts the last 16 samples of the time-domain symbol, denoted as S. tail Next, a correlation coefficient between the signal and the received cyclic prefix (CP) is calculated using a mechanism similar to that in the frequency domain:

[0085]

[0086] By simultaneously utilizing invariant information in both the frequency and time domains, this invention can accurately identify cyclic shift values, thereby ensuring precise demodulation of 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] III. Combined PSK modulation method:

[0088] To further improve the data transmission rate, this invention incorporates a joint PSK modulation method in tag modulation. This method leverages the characteristic that the cyclic prefix (CP) is identical to the tail time-domain sample, and utilizes PSK (phase shift keying) modulation on the tail time-domain sample to transmit more tag data.

[0089] During modulation, the tag modulates more tag data by modifying the phase of the tail time-domain sample. Specifically, when the tag transmits tag data "1", it modulates the phase of the tail time-domain sample S. tail A phase shift Δθ is introduced during the modulation process; no phase shift is introduced when transmitting tag data "0". This modulation process can be represented as:

[0090]

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

[0092] Using this method, the highest-order system implemented in this invention can support joint 64PSK, carrying 12 bits of data in a single symbol. This technology not only improves data transmission efficiency but also fully utilizes the cyclic prefix characteristic in OFDM symbols, overcoming the data transmission limitations imposed by simply relying on subcarrier cyclic shifting. This innovative encoding and decoding mechanism further enhances the data carrying potential of OFDM symbols in backscatter communication.

[0093] Example

[0094] 1. System Setup:

[0095] First, an orthogonal frequency division multiplexing (OFDM) WiFi backscatter system is constructed, consisting of three main parts: a transmitter, tags, and a receiver. The transmitter and receiver are built using commercial WiFi network cards and software-defined radio (SDR). In this embodiment, the commercial equipment uses a Qualcomm AR938x network card, and data transmission and reception are performed using CommView software. The software-defined radio consists of a ZedBoard and an AD9361 daughterboard for signal processing. Matlab is used as the data analysis tool in this embodiment.

[0096] 2. Tag modulation implementation:

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

[0098] 1) Data partitioning: 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 combined PSK method, specifically QPSK (quadratic phase shift keying) modulation in this embodiment, and the last 6 bits are modulated using cyclic shift modulation.

[0099] 2) QPSK (Quadrature Phase Shift Keying) Modulation: The first two "10"s 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 of the OFDM symbol tail for QPSK modulation of the tail.

[0100] 3) Cyclic Shift Modulation: Next, the last 64 samples of the OFDM symbol are cyclically shifted to produce the 6-bit 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 this linear phase factor, thus completing the modulation of the signal in the time domain.

[0101] 4) Signal reflection: After modulation is completed, the tag will reflect the signal to transmit data.

[0102] This method enables tags to transmit data efficiently within limited spectrum resources. QPSK (Quadrilateral Phase Shift Keying) modulation provides a way to apply phase changes at the end of a symbol, while cyclic shift modulation encodes more information by adjusting the subcarrier positions of the symbol samples. Thus, combining these two modulation techniques allows more bits to be transmitted within a single 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 for demodulation:

[0105] 1) Symbol Sample Extraction: The receiver first extracts the last 64 time-domain samples of the symbol. These samples contain the tag data encoded by cyclic shift modulation.

[0106] 2) Fourier Transform (FFT): A Fast Fourier Transform (FFT) is performed on the extracted 64 time-domain samples to convert the time-domain signal into the frequency domain, thereby obtaining information about the frequency-domain subcarriers. This 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 restore the correct position of the frequency domain subcarriers by compensating for possible cyclic shift values. This process involves performing correlation calculations between the frequency domain subcarriers and known pilot subcarrier values ​​to identify possible shift compensations.

[0108] 4) Inverse Fast Fourier Transform (IFFT): Perform an Inverse Fast Fourier Transform (IFFT) on the compensated subcarrier to convert it back to the 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: By using the correct shift value of 2, the last 6 bits of data "000010" are recovered. Combined with the QPSK bit "10" decoded in the previous two steps, the receiver successfully demodulated the complete tag data "10000010".

[0111] This demodulation process utilizes FFT and IFFT to obtain frequency and time domain information, and uses related 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] This invention conducted experimental tests on various WiFi protocols to verify the backscattering method's support for different orthogonal frequency division multiplexing (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 globally, and the backscattering method's support for multiple protocols ensures its proper functioning in various environments.

[0113] In the WiFi 3 protocol, tests were conducted using a typical single-stream orthogonal frequency division multiplexing (OFDM) WiFi signal with a bandwidth of 20 MHz. Each OFDM symbol contains 64 subcarriers. In the basic frequency-domain cyclic shift modulation scheme (excluding PSK, i.e., no phase modulation), each symbol can encode 6 tag bits using only cyclic shift modulation. In the joint PSK modulation method, performance was tested under various joint PSK modulation schemes, ranging from BPSK (Binary Phase Shift Keying) to 64PSK. Specifically, in 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 for different tag modulation schemes is 1.5 Mbps (without PSK), 1.75 Mbps (BPSK), 2 Mbps (QPSK), 2.49 Mbps (16PSK), and 2.94 Mbps (64PSK), respectively.

[0114] Under WiFi 4, the performance of the backscattering method of this invention was verified under multi-band signals with bandwidths of 20MHz and 40MHz. OFDM modulation was set to BPSK (MCS = 0). Under 20MHz conditions, the tag data throughput was similar to that under WiFi 3. Under 40MHz conditions, the maximum tag data throughput increased to 3.21Mbps, representing a 9.85% improvement in average throughput compared to 20MHz. This increase in throughput is primarily due to the increase in the number of subcarriers in the OFDM symbol from 64 to 128, which allows each symbol to modulate 7 bits of tag data via cyclic shifting.

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

[0116] The experimental results above verify that the backscattering method of this invention can utilize various orthogonal frequency division multiplexing (OFDM) WiFi signals as excitation, demonstrating its high compatibility with existing commercial WiFi. Furthermore, the backscattering method of this invention achieves high tag data throughput using only a single access point (AP), showcasing its application potential in future wireless communications.

[0117] It will be apparent 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 invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0118] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A single-receiver WiFi backscattering method based on frequency domain cyclic shift, wherein the backscattering system comprises a transmitter, a tag, and a receiver, characterized in that, Backscattering methods include: The transmitting end can use any device capable of emitting orthogonal frequency division multiplexing (OFDM) 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 WiFi signals and modulates the OFDM symbols in the WiFi signal at the sub-symbol level based on frequency domain cyclic shift modulation technology: the cyclic shift value is set according to the bits 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 to form a reflected signal carrying tag data. The receiver determines the cyclic shift value of the frequency domain subcarrier by using the frequency domain invariance of the OFDM symbol, and demodulates the received reflected signal to recover the tag data; the frequency domain invariance means that the position and value of the pilot subcarrier in the OFDM symbol are known before modulation; It also includes a joint phase-shift keying modulation method; the joint phase-shift keying modulation method specifically includes: The cyclic prefix and tail of OFDM symbols For each time-domain sample that is identical, when the tag modulates the OFDM symbols in the WiFi signal, if the tag data is "1", it will appear at the end... Tail time-domain sample composed of time-domain samples Introducing phase shift If the label data is "0", no phase offset is introduced: ； in, The imaginary unit, Represents the tail time-domain sample index, , This represents the value of the time-domain sample at the tail after tag modulation; During demodulation, 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 and achieve demodulation.

2. The single-receiver WiFi backscattering method based on frequency domain cyclic shift according to claim 1, characterized in that, The 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 adds a cyclic prefix to form an orthogonal frequency division multiplexing (OFDM) WiFi signal with OFDM symbols as the basic unit, specifically including: The transmitter converts N frequency-domain subcarriers into N time-domain samples using an inverse fast Fourier transform. To resist multipath interference and ensure symbol spacing separation, a cyclic prefix is ​​added to the beginning of the time-domain signal composed of the N time-domain samples. ； in, Indicates the index of the time-domain sample; Indicates OFDM symbol; This represents the inverse fast Fourier transform; This represents the index of the k-th frequency domain subcarrier. ; It 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 based on the bits of the tag data, applying a linear phase factor to selected time-domain samples in the OFDM symbol of the WiFi signal, and causing the frequency-domain subcarriers corresponding to the OFDM symbol to cyclically shift according to the set cyclic shift value to form a reflected signal carrying tag data specifically includes: Mapping tag data to cyclic shift values: For each OFDM symbol, map the tag data to a cyclic shift value. The mapping relationship is determined by the number of frequency domain subcarriers of the OFDM symbol; Apply a linear phase factor: The label is applied to the N time-domain samples following the cyclic prefix in the OFDM symbol by a linear phase factor. index of time-domain samples The reflected signal B(n) is obtained: ； in, The imaginary unit, It is the length of the cyclic prefix. express Index of mid-time domain samples, This represents the normalized frequency modulation parameter associated with 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 through the frequency domain invariance of the OFDM symbol, specifically including: By traversing all possible cyclic shift values, the correlation between the value of the cyclic shifted pre-pilot subcarrier and the value of the cyclic shifted post-pilot subcarrier 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 cyclically shifted prepilot subcarrier is the pilot subcarrier estimate combined with channel estimation. : ； in, For the index of the pilot subcarrier, This is the estimated value for the k2th pilot subcarrier; , They are respectively The real and imaginary parts of the complex form, The imaginary unit; For each possible cyclic shift value Calculate the value of the pilot subcarrier after cyclic shift. : ； The value of the k2th pilot subcarrier after cyclic shift; , They are respectively The real and imaginary parts of the complex form; The correlation coefficient matrix is ​​used to calculate the correlation between the values ​​of the cyclically shifted pre-pilot subcarriers and the values ​​of the cyclically shifted post-pilot subcarriers. : ； in , These are the in-phase orthogonal matrices of the estimated pilot subcarrier values ​​and the cyclically shifted pilot subcarrier values, respectively. This indicates the calculation of covariance. and yes and The standard deviation.

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, specifically including: The time-domain invariance means that 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 the different cyclic shift values ​​corresponding to the highest correlation. Then, it performs an inverse fast Fourier transform on the cyclically shifted frequency domain subcarrier to obtain the time-domain symbol of the frequency domain subcarrier. Finally, it extracts the tail portion of the obtained time-domain symbol. These time-domain samples are denoted as the tail time-domain samples. ;calculate Cyclic prefix of received OFDM symbols Correlation coefficient between : ； and yes and The standard deviation.

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