A single receiver wi-fi backscatter communication method and system

Abstract

The application relates to the field of backscatter technology and discloses a single-receiver WiFi backscatter communication method and system, which comprises the following steps: a tag receives OFDM symbols from ambient WiFi signals, a linear phase factor corresponding to a tag-related cyclic shift value is applied to only the middle part of a data load, and the OFDM symbols of a reflection signal are reconstructed based on the middle part of the modulated data load; a receiver extracts the modulated part from the received OFDM symbols of the reflection signal, performs frequency domain inverse shift compensation on each candidate cyclic shift value, obtains a reconstructed data load based on the obtained compensated spectrum, calculates the correlation score between the power spectrum of the reconstructed data load each time and a binary template predefined based on the position of the empty subcarrier, decodes the cyclic shift value, and recovers the corresponding tag data. The application can realize a high data throughput without sacrificing OFDM compatibility.

Description

Technical Field

[0001] This invention relates to the field of backscatter communication technology, specifically to a single-receiver WiFi backscatter communication method and system. Background Technology

[0002] Ambient backscatter communication is a transformative technology in the Internet of Things (IoT) field. It enables passive or ultra-low-power devices to communicate wirelessly by reflecting and modulating existing radio frequency (RF) signals in the environment, freeing them from battery constraints. Among the many available ambient signal sources, Orthogonal Frequency Division Multiplexing (OFDM) WiFi signals are a highly attractive source due to their global ubiquity, abundant spectrum resources, and dominant position in commercial wireless networks. However, utilizing ambient OFDM WiFi signals for backscatter communication faces a fundamental technical challenge: simultaneously satisfying three interdependent objectives in the design—high throughput, operation with a single commercial receiver, and full compatibility with existing WiFi infrastructure. This challenge is known as the OFDM backscattering trilemma.

[0003] Existing technologies, in addressing this problem, typically sacrifice one or more objectives, resulting in two distinct technical approaches. The first approach prioritizes compatibility but sacrifices ease of system deployment. Early systems, such as the HitchHike system and later OFDM-extended systems like FreeRider and MOXcatter, employed techniques like codeword translation. These techniques embed tag data by mapping codewords in received WiFi packets to other valid codewords (usually through phase shifting). While conceptually innovative, these methods suffer from a fatal flaw: the need for a dual-receiver architecture for demodulation. One receiver captures clean, raw ambient signals as a reference, while the other receives and decodes the backscattered modulated signal. This dual-receiver dependency significantly increases system deployment costs, hardware complexity, and energy consumption, contradicting the core principle of low-infrastructure backscattered IoT.

[0004] The second technical approach, aiming to overcome the drawbacks of the dual-receiver architecture, shifts towards a single-receiver solution, but at the cost of compatibility with the OFDM standard. To achieve finer sub-symbol-level modulation on a single receiver, systems such as DFTScatter and PTL-Bscatter have emerged. The PTL-Bscatter system directly modulates the cyclic prefix (CP) of the OFDM symbol, while the DFTScatter system modulates the tail of the symbol.

[0005] These single-receiver schemes face a core problem that remains unresolved to date: they fundamentally violate the "Cyclic Prefix-Tail Consistency Principle" as defined in this invention. In OFDM systems, the cyclic prefix is ​​not merely a simple guard interval; it is identical to the symbol tail it replicates, forming the structural cornerstone of OFDM demodulation. This characteristic allows the linear convolutional effects of the channel to be equivalently converted into cyclic convolutions at the receiver, thus enabling the elimination of multipath effects in the frequency domain through simple single-tap equalization. Systems such as DFTScatter and PTL-Bscatter, by modifying the cyclic prefix or symbol tail, break this consistency, leading to severe inherent distortion. This distortion manifests as extremely high inter-carrier interference (ICI) and spectral leakage, resulting in high bit error rates even in ideal line-of-sight (LOS) channels, and drastically degrading performance in real-world non-line-of-sight (NLOS) multipath environments. This sacrifice of basic compatibility in pursuit of high throughput creates a hidden "compatibility debt," making these systems lack robustness in practical deployments.

[0006] In summary, existing technologies in the OFDM backscatter field face a dilemma: either choose a complex dual-receiver architecture to ensure compatibility, or opt for a seemingly simple single-receiver solution but sacrifice basic operational capabilities in real-world channel environments. Currently, no technology can simultaneously solve the three core challenges of high throughput, single-receiver operation, and complete cyclic prefix compatibility. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a single-receiver WiFi backscatter communication method and system. One objective of this invention is to provide a backscatter system capable of achieving high data throughput at the sub-symbol level. Another objective is to provide a system capable of reliable data demodulation using only a single standard commercial (COTS) receiver (e.g., a commercial WiFi access point or client device) without capturing an additional clean reference signal. A further core objective of this invention is to maintain the structural integrity of OFDM symbols while achieving the above objectives, particularly strictly maintaining the consistency relationship between the cyclic prefix (CP) and the symbol tail, thereby ensuring excellent robustness of the system in challenging multipath channel environments.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a single-receiver WiFi backscatter communication method, comprising:

[0010] The tag receives OFDM symbols from the ambient WiFi signal. These OFDM symbols include a cyclic prefix, a middle portion of the data payload, and a tail region. The cyclic prefix is ​​a copy of the tail region. The tag data to be transmitted is then mapped to cyclic shift values. A linear phase factor corresponding to the cyclic shift value is applied to the middle part of the data payload for modulation, while keeping the cyclic prefix and tail regions unchanged, and the OFDM symbol of the reflected signal is reconstructed based on the modulated middle part of the data payload.

[0011] Demodulation is performed by a single receiver, the modulated part is extracted from the OFDM symbols of the received reflected signal, and multiple candidate cyclic shift values ​​are iterated. Frequency domain inverse shift compensation is performed on each candidate cyclic shift value, and the reconstructed data payload is obtained based on the compensated spectrum. The correlation score between the power spectrum of each reconstructed data payload and a binary template predefined based on the empty subcarrier position is calculated. The candidate cyclic shift value that produces the maximum correlation score is selected as the decoded cyclic shift value, and the corresponding tag data is recovered.

[0012] In one embodiment, the middle portion of the data payload consists of the first NL sampling points of the data payload, and the tail region of the data payload consists of the last L sampling points of the data payload, where N is the total number of sampling points of the data payload and L is the number of sampling points in the tail region of the data payload.

[0013] In one embodiment, the step of applying a linear phase factor corresponding to the cyclic shift value to modulate only the middle portion of the data payload specifically includes:

[0014] ; ;

[0015] This represents the time-domain sampled signal obtained after applying linear phase factor modulation to the middle portion of the data payload. This represents the time-domain sampled signal corresponding to the middle portion of the unmodulated data payload. The imaginary unit, This indicates the index of the time-domain sampling point in the middle part of the data payload.

[0016] In one embodiment, the reconstruction of the OFDM symbol of the reflected signal based on the intermediate portion of the modulated data payload specifically includes:

[0017] By concatenating the original cyclic prefix The middle part of the modulated data payload and the tail region of the original data payload To construct the OFDM symbol ultimately used for the reflected signal. :

[0018] .

[0019] In one embodiment, the step of extracting the modulated portion from the OFDM symbols of the received reflected signal, iterating over multiple candidate cyclic shift values, and performing frequency-domain inverse shift compensation for each candidate cyclic shift value specifically includes:

[0020] Remove the cyclic prefix of the OFDM symbol from the received reflected signal. and data payload tail region Extract the middle part of the modulated data payload ;right Perform a Fast Fourier Transform on each sampling point to obtain the corresponding frequency domain representation. , , k For the frequency domain index of the k-th sampling point, for each candidate cyclic shift value By applying a compensatory cyclic shift to cancel the assumed modulation effect, the compensated spectrum is obtained:

[0021] ;

[0022] in, ; This indicates that the candidate cyclic shift value is used in the assumption that the label is assumed to be... Under the premise of modulation, by... Perform a compensating inverse cyclic shift operation to obtain the compensated spectrum; express The m-th sample value has a length of ; Indicate execution Point Fast Fourier Transform, This is the time-domain index of the m-th sample value. N is the total number of sampling points in the data payload, and L is the number of sampling points in the tail region of the data payload.

[0023] In one embodiment, the reconstructed data payload based on the obtained compensated spectrum specifically includes:

[0024] The compensated spectrum is passed through The inverse fast Fourier transform of the point yields the assumed intermediate part. The assumed middle portion is compared with the tail region of the data payload of the OFDM symbol of the received reflected signal. By splicing together the data, a reconstructed data payload is formed. :

[0025] ;

[0026] These are candidate cyclic shift values.

[0027] In one embodiment, calculating the correlation score between the power spectrum of the reconstructed data payload and a predefined binary template based on the empty subcarrier position for each iteration specifically includes:

[0028] Data load for reconstruction Perform a Fast Fourier Transform to obtain the corresponding spectrum. ;calculate power spectrum ; For frequency domain indexing, Candidate cyclic shift values;

[0029] Introducing binary templates The binary template is '1' at valid subcarrier positions and '0' at empty subcarrier positions:

[0030] The correlation score is obtained by comparing the power spectrum corresponding to the reconstructed data load with the binary template. :

[0031] .

[0032] In one embodiment, selecting the candidate cyclic shift value that produces the highest relevance score as the decoded cyclic shift value and recovering the corresponding label data specifically includes:

[0033] ;

[0034] The decoded cyclic shift value, Candidate cyclic shift values; based on Restore tag data.

[0035] In one embodiment, the data transmission rate of the tag is further improved by using temporal sub-block modulation technology, specifically including:

[0036] The label will contain the middle part of the data payload Divide into K sub-blocks; map the tag data to be sent into a cyclic shift value vector consisting of K cyclic shift values;

[0037] During demodulation, the receiver employs a maximum likelihood estimator to exhaustively search all possible cyclic shift value vectors; for each cyclic shift value vector... The receiver performs a signal reconstruction and correlation scoring process similar to that without time-domain sub-block modulation, assigning the correlation score to the signal. The largest cyclic shift value vector is used as the final decoded cyclic shift value vector. :

[0038] ;

[0039] according to Restore tag data, This represents the Kth candidate cyclic displacement value in the cyclic displacement value vector. This represents the Kth cyclic shift value in the final decoded cyclic shift value vector.

[0040] In a second aspect, the present invention provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the method of any embodiment of the first aspect.

[0041] Compared with the prior art, the beneficial technical effects of the present invention are:

[0042] (1) This invention designs a cyclic shift keying (CCSK) modulation technique compatible with cyclic prefix. The tag achieves cyclic shifting of the corresponding frequency domain signal by applying a linear phase factor to the middle part of the received OFDM symbol time domain signal to carry tag data. This "center region modulation" method achieves sub-symbol level data modulation and improves data transmission efficiency while strictly maintaining the consistency between the cyclic prefix (CP) and the symbol tail, thereby ensuring full compatibility with commercial OFDM receivers and excellent robustness in multipath channels.

[0043] (2) This invention designs a single-receiver demodulation method based on incoherent spectrum correlation. This method uses the inherent, static empty subcarrier in the OFDM standard as a stable frequency domain anchor point, and recovers the tag data through hypothesis testing and signal reconstruction. This method eliminates the dependence of traditional dual-receiver schemes on clean environment reference signals, and can accurately decode based solely on the received reflected signal, which greatly simplifies the system architecture and reduces hardware complexity and deployment costs.

[0044] (3) In the preferred technical solution, the present invention designs a time-domain sub-block modulation method. Based on cyclic shift keying (CCSK) modulation compatible with cyclic prefixes, the modulation state space is expanded exponentially by dividing the middle part into multiple independent sub-blocks and modulating them separately. As a scalable throughput enhancement mechanism, this method enables the system to achieve a high data throughput of several Mbps without sacrificing OFDM compatibility. Attached Figure Description

[0045] Figure 1 This is an architecture diagram of an OFDM WiFi backscattering system;

[0046] Figure 2 This is a schematic diagram of the core principle and frequency domain cyclic shift of the present invention;

[0047] Figure 3 This is a schematic diagram of the time-domain modulation process of the compatible cyclic prefix of the present invention;

[0048] Figure 4 This is a schematic diagram of the single-receiver demodulation process of the present invention;

[0049] Figure 5 This is a flowchart of the method of the present invention. Detailed Implementation

[0050] A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0051] like Figure 5 As shown, a single-receiver WiFi backscatter communication method compatible with cyclic prefixes according to the present invention includes the following steps:

[0052] S1, the tag receives an OFDM symbol from the ambient WiFi signal. The OFDM symbol includes a cyclic prefix, a middle portion of the data payload, and a tail region. The cyclic prefix is ​​a copy of the tail region. The tag data to be transmitted is mapped to a cyclic shift value. A linear phase factor corresponding to the cyclic shift value is applied to the middle part of the data payload for modulation, while keeping the cyclic prefix and tail regions unchanged, and the OFDM symbol of the reflected signal is reconstructed based on the modulated middle part of the data payload.

[0053] S2, demodulation is performed by a single receiver, extracting the modulated part from the OFDM symbols of the received reflected signal, iterating over multiple candidate cyclic shift values, performing frequency domain inverse shift compensation on each candidate cyclic shift value, and obtaining the reconstructed data payload based on the compensated spectrum; calculating the correlation score between the power spectrum of each reconstructed data payload and a binary template predefined based on the empty subcarrier position; selecting the candidate cyclic shift value that produces the maximum correlation score as the decoded cyclic shift value, and recovering the corresponding tag data.

[0054] The OFDM backscattering system architecture designed in this invention is as follows: Figure 1 As shown, it mainly consists of three core components: an environmental OFDM signal source (such as a WiFi transmitter), a backscatter tag, and a standard single receiver. The transmitter emits an excitation signal, and the tag transmits its own data by modulating and reflecting this signal. Finally, the receiver decodes the data without the need for a clean reference signal.

[0055] The primary function of an environmental OFDM signal source is to emit an excitation signal, which can be reflected and modulated by the tag to transmit data. In the system designed in this invention, the signal source can be any device capable of emitting OFDM WiFi signals, such as a WiFi router or any other commercial WiFi device. This method of utilizing existing WiFi signals in the environment as the excitation signal reduces the need for additional hardware.

[0056] The tag is responsible for modulating the received excitation signal so that it can transmit its own data by reflecting the signal. The core innovation of this invention lies in proposing a center-only modulation principle, which stipulates that all modulation operations are strictly limited to a middle portion of the OFDM symbol data payload, while precisely maintaining the cyclic prefix (CP) and the tail region of the data payload completely unmodified. The core principle of this modulation method is as follows: Figure 2 As shown, tag data is encoded into the reflected signal by introducing predictable cyclic shifts in the frequency domain. Based on this principle, this invention designs a CP-Compatible Cyclic Shift Keying (CCSK) modulation technique. This method achieves sub-symbol-level tag data modulation, improves data transmission efficiency, and ensures full compatibility with commercial receivers and robustness in multipath channels by maintaining the integrity of the cyclic prefix.

[0057] Figure 2This diagram visually illustrates the core principle of the modulation method of this invention. The upper part of the diagram represents an original OFDM symbol of an ambient WiFi signal, whose spectrum consists of data / pilot subcarriers and empty subcarriers. The tag encodes its tag data onto this symbol using the "cyclic prefix-compatible cyclic shift keying" technique proposed in this invention. In the frequency domain, this process results in a cyclic shift of the subcarriers of the original OFDM symbol, generating the backscattered OFDM symbol in the lower half. The receiver decodes the tag data by detecting this predictable spectral shift.

[0058] The receiver's task is to decode the signal modulated and reflected by the tag to recover the tag data. This invention presents a meticulously designed single-receiver demodulation method based on incoherent spectrum correlation, which eliminates the dependence of traditional methods on clean environment signals. It utilizes the null subcarriers inherent in the OFDM standard as stable frequency domain anchors, accurately decoding data solely from the received reflected signal, greatly simplifying the system architecture and reducing hardware complexity and cost.

[0059] 1. A cyclic shift keying (CCSK) modulation method compatible with cyclic prefixes.

[0060] The tag data modulation method designed in this invention is based on an important characteristic of the Discrete Fourier Transform: multiplying a block of a signal by a complex linear phase ramp in the time domain is equivalent to introducing a cyclic shift in its corresponding frequency domain representation. The key innovation of this invention lies in the fact that this phase ramp operation is strictly limited to a middle part of the OFDM symbol, thereby achieving data encoding without destroying the integrity of the cyclic prefix structure.

[0061] 1.1 Generation and partitioning of OFDM symbols.

[0062] At the transmitter, a standard time-domain OFDM symbol consists of L cyclic prefix (CP) samples and N data payload samples. The cyclic prefix is ​​an exact copy of the last L samples of the data payload. In this invention, after receiving the OFDM symbol, the tag logically copies the N samples of the data payload. Divided into two parts:

[0063] A central modulation window, also known as the mid-section. It consists of the initial NL sampling points of the data load.

[0064] A tail section It consists of the last L sampling points of the data load.

[0065] 1.2 Tag data modulation.

[0066] (1) Receive OFDM symbol: The tag receives the ambient OFDM WiFi signal.

[0067] (2) Tag data mapping to cyclic shift value: For each OFDM symbol, it is assumed that the tag data bits to be de-dispersed are mapped to a cyclic shift value. ,in .

[0068] (3) Apply a linear phase ramp to the middle section: the label applies only to the middle section. of Each time-domain sample point is multiplied by a linear phase factor, while preserving the cyclic prefix and tail. Unchanged. This operation can be mathematically represented as:

[0069] ; ;

[0070] (4) Reconstructing and reflecting the symbol: The label is constructed by splicing the original cyclic prefix and the modulated middle part. and the original tail area To construct the OFDM symbol ultimately used for the reflected signal. :

[0071] .

[0072] because It has always remained unmodified. An exact copy is obtained, thus the "cyclic prefix-tail consistency principle" is maintained.

[0073] like Figure 3 As shown, this modulation process operates only on the middle portion of the symbol in the time domain, which is equivalent in the frequency domain to introducing a cyclic shift of size d into the spectrum of that portion, thereby embedding the tag data into the signal. Figure 3 As shown in (a), a standard time-domain OFDM symbol is logically divided into a cyclic prefix and an intermediate part. and tail area The core of this invention lies in the fact that the label only applies to the middle portion. A linear phase ramp is applied for data modulation while keeping the cyclic prefix and tail regions completely unchanged. For example... Figure 3 As shown in (b), this precise operation in the middle of the time domain is equivalent in the frequency domain to introducing a cyclic shift of size d into the spectrum of that region, thereby reliably encoding the tag data into the signal and fully maintaining the "cyclic prefix-tail consistency principle".

[0074] 2. A single-receiver demodulation method based on incoherent spectrum correlation.

[0075] To demodulate tag data using only a single receiver, the cyclic shift value d applied by the tag needs to be determined. This invention utilizes the inherent, data-free null subcarriers in OFDM symbols as invariant frequency domain anchors to design a demodulation algorithm based on hypothesis testing and signal reconstruction.

[0076] The core idea of ​​this algorithm is to find an optimal candidate cyclic shift value. When its modulation effect is reversed, the original spectral structure of the signal can be recovered to the greatest extent (i.e., energy is concentrated on the effective data subcarriers, while the energy is lowest in the empty subcarrier regions). This is constructed as a cyclic shift value for all possible candidates. Hypothesis testing problems involving iterative search.

[0077] The demodulation steps are as follows, and the process is as follows: Figure 4 As shown:

[0078] (1) Modulation part extraction: The receiver removes the cyclic prefix from the received OFDM symbol. and tail area Extract the modulated middle part OFDM symbol of the reflected signal emitted by the tag. By cyclic prefix The modulated intermediate part and the original tail area The reflected signal emitted by the tag is affected by channel and noise during transmission to the receiver. Therefore, the receiver sequentially records the cyclic prefix, modulated middle part, and original tail region of the OFDM symbol of the received reflected signal as follows: .

[0079] (2) Cyclic shift compensation: for the extracted modulation portion Perform a one- (NL) point Fast Fourier Transform (FFT) to obtain its frequency domain representation. For each candidate cyclic shift value The receiver applies a compensatory cyclic shift to cancel the assumed modulation effect, resulting in a compensated spectrum:

[0080] ;

[0081] in, Time-domain index Frequency domain index .

[0082] (3) Signal reconstruction: Reconstructing the compensated spectrum pass The inverse fast Fourier transform (IFFT) of the points yields a hypothetical original intermediate portion. Then, this part is combined with the received, unmodified tail region. By splicing the data together, a complete reconstructed data payload with N sampling points is formed:

[0083] ;

[0084] If the assumption is If correct, the reconstructed data payload should be close to a noisy version of the original unmodulated data payload.

[0085] (4) Spectrum Analysis and Scoring: To verify the fidelity of the reconstruction, this invention utilizes template matching to analyze its spectral characteristics. The reconstructed data load... Performing an N-point Fast Fourier Transform (FFT) yields the spectrum. Then its power spectrum was calculated. The core idea is that energy will only be concentrated at the original data subcarrier positions when correctly reconstructed. To this end, this invention introduces a static binary template. The template is '1' at valid subcarrier positions (including data and pilot subcarriers) and '0' at empty subcarrier positions:

[0086] ;

[0087] in, This represents the set of indices for the original data and pilot subcarriers. This represents the set of empty subcarrier indexes.

[0088] The correlation score is obtained by comparing the power spectrum corresponding to the reconstructed data load with the binary template. :

[0089] .

[0090] (5) Decoding decision: For all candidate cyclic shift values Repeat the above process. Use the candidate cyclic shift value corresponding to the maximum relevance score as the decoded cyclic shift value. ,based on Restore tag data.

[0091] .

[0092] The candidate cyclic shift value corresponding to the maximum correlation score can refocus energy on the original effective subcarrier position to the greatest extent.

[0093] 3. Achieve scalable throughput through sub-block modulation.

[0094] To further improve the data transmission rate, this invention incorporates a time-domain sub-block modulation technique in tag modulation. This technique expands the modulation state space exponentially by dividing the middle section into multiple independent sub-blocks and encoding data independently on each sub-block.

[0095] During modulation, the tag will have a single intermediate part Divide into K smaller, independent sub-blocks, each sub-block having a size of K. The tag is for each sub-block. Apply an independent cyclic shift value This transforms the label data into a K-dimensional vector. This method exponentially expands the number of available modulation states from (NL) to... This makes the number of bits carried by each OFDM symbol increase from [number] to [number]. Increase to .

[0096] During demodulation, the receiver employs a maximum likelihood (ML) estimator for all... One possible cyclic shift value vector An exhaustive search is performed. For each hypothesis tuple, the receiver performs a signal reconstruction and correlation scoring process similar to the basic method. The correlation scores are then... The largest cyclic shift value vector is used as the final decoded cyclic shift value vector. :

[0097] .

[0098] This invention enables high data throughput in the Mbps range while maintaining full compatibility with standard OFDM. This innovative encoding and decoding mechanism further enhances the data carrying potential of backscatter communication compatible with cyclic prefixes.

[0099] The specific extension scheme is as follows:

[0100] (1) In the extended scheme, the label first targets the middle part of the data payload. Perform time-domain sub-block partitioning. Let the m-th sample value of the time-domain sampled signal corresponding to the middle part of the data payload be... , ;in, This represents the total number of sampling points in the data payload. This refers to the number of sampling points in the tail region of the data payload. The label divides the middle portion of the data payload into... Non-overlapping temporal sub-blocks, each sub-block having a length of for:

[0101]

[0102] No. Sub-block Defined as:

[0103] , , ;

[0104] The tag maps the tag data to be sent to a data structure composed of... A cyclic shift value vector consisting of cyclic shift values ,in, Indicates the action on the first The cyclic shift value of each sub-block is then applied. Subsequently, the tag applies linear phase modulation associated with the corresponding cyclic shift value only to each sub-block in the middle part of the data payload, without changing the cyclic prefix and the tail region of the data payload.

[0105] Specifically, for the first Sub-block The label is modulated as follows:

[0106] ;

[0107] in, Indicates the first The m-th sample value of the time-domain signal after cyclic shift modulation of the n sub-blocks. This linear phase modulation is equivalent in the frequency domain to the m-th sample value of the time-domain signal after cyclic shift modulation of the n sub-blocks. Each sub-block corresponds to a spectrum that is subjected to a cyclic shift value. The determined cyclic shift operation.

[0108] After completing all After linear phase modulation of each sub-block, the tag reassembles the modulated sub-blocks according to the original time-domain order to obtain the middle part of the modulated data payload:

[0109] .

[0110] Finally, the label will retain the original loop prefix. The middle part of the modulated data payload and the raw data payload tail region The symbols are then spliced ​​together to construct the final OFDM symbol used for reflection:

[0111] .

[0112] Through the above method, the tag achieves OFDM cyclic prefix consistency and subcarrier orthogonality while maintaining the cyclic prefix and data payload tail region unchanged, thus preserving OFDM cyclic prefix consistency and subcarrier orthogonality structure. Sub-block modulation with multiple cyclic shift values ​​enables a single OFDM symbol to carry multiple cyclic shift modulation information, thereby effectively improving the tag data transmission rate of backscatter communication.

[0113] (2) During demodulation, the receiver receives the OFDM symbol formed by the tag reflection and first performs a preprocessing operation consistent with the basic method on the received OFDM symbol, namely, removing the cyclic prefix and the tail region of the data payload, thereby extracting the middle part of the modulated data payload. Let this middle part be represented as... ;

[0114] ;

[0115] in, This represents the m-th sampled value of the time-domain sampling sequence corresponding to the middle part of the data payload, obtained from the reflected signal after the receiver removes the cyclic prefix and tail region.

[0116] Subsequently, the receiver divides the middle section into sections in the same manner as the tag. Non-overlapping temporal sub-blocks, each sub-block having a length of for:

[0117] .

[0118] No. a time-domain sub-block Represented as:

[0119] , ;

[0120] The receiver employs maximum likelihood estimation for all possible cyclic shift value vectors. Perform an exhaustive search.

[0121] For each candidate cyclic shift value vector, the receiver performs frequency domain inverse shift compensation on each sub-block. Specifically, for the... a time-domain sub-block First execute Point Fast Fourier Transform:

[0122] ;

[0123] Then, assuming the first Each sub-block uses candidate cyclic shift values. Under the premise of modulation, a compensatory inverse cyclic shift operation is performed on its spectrum to obtain the compensated spectrum. :

[0124] .

[0125] The receiver then performs the operation on the compensated spectrum. The inverse fast Fourier transform of the point yields the reconstructed first point under the assumed conditions. Time-domain signals of individual blocks :

[0126] .

[0127] After completing all After reconstructing each sub-block, the receiver concatenates the reconstructed sub-blocks in the original time-domain order to form the middle part of the reconstructed data payload corresponding to the hypothetical cyclic shift value vector. :

[0128] .

[0129] Finally, the receiver combines the middle portion of the reconstructed data payload with the tail region of the data payload extracted from the received signal. By concatenating the data, we obtain the reconstructed data payload corresponding to the cyclic shift value vector. :

[0130] .

[0131] (3) After obtaining a candidate cyclic shift value vector After reconstructing the data payload, the receiver further performs frequency domain analysis on the reconstructed data payload to calculate the corresponding correlation score. Specifically, the receiver performs frequency domain analysis on the reconstructed data payload. implement Point Fast Fourier Transform yields its frequency domain representation. :

[0132] , .

[0133] The corresponding power spectrum was then calculated. :

[0134] .

[0135] The receiver pre-constructs a binary template based on the WiFi OFDM physical layer structure. The value is 1 at valid data subcarrier positions and 0 at empty subcarrier positions. Based on this binary template, the receiver calculates the correlation score between the power spectrum and the template. :

[0136] .

[0137] When the candidate cyclic shift vector matches the actual cyclic shift vector used by the tag, the compensation operation effectively cancels the sub-block-level linear phase modulation, allowing the reconstructed data payload to recover the original OFDM subcarrier structure in the frequency domain, thereby generating a higher correlation score at the effective subcarrier location. The receiver ultimately selects the cyclic shift vector that maximizes the correlation score as the decoding result.

[0138] 4. This invention can be implemented by referring to the following steps:

[0139] (1) System setup:

[0140] First, an OFDM WiFi backscattering system is built, consisting of three main parts: a transmitter, tags, and a receiver. The verification environment for this implementation can be built based on a simulation platform or a hardware platform.

[0141] In the simulation, MATLAB R2024a and WLAN Toolbox are used for system construction. The transmitter generates a standard-compliant ambient WiFi signal; the functions of the tags and receiver, as well as the channel model, are implemented through software programming. In the hardware implementation, the transmitter and receiver can be built using commercial WiFi network cards and software-defined radio (SDR). For example, the transmitter can be a laptop equipped with a Qualcomm AR938x network card, and precise data packet transmission can be achieved using software such as CommView. The receiver can be another laptop running the gr-ieee802-11 receiving process based on GNU Radio. The tags can be developed based on field-programmable gate arrays (such as Xilinx Artix-7) to implement the modulation algorithm proposed in this invention.

[0142] (2) Tag modulation implementation:

[0143] In a backscatter communication system, taking the modulation and transmission of 5-bit data "00111" (decimal 7) as an example, the tag needs to modulate and reflect this data. The implementation steps are as follows:

[0144] 1) Symbol Reception and Partitioning: The tag receives a standard 802.11n time-domain OFDM symbol. This OFDM symbol consists of a 16-sample-point cyclic prefix (CP) and a 64-sample-point data payload. Logically, the tag divides the 64-sample-point data payload into a 48-sample-point intermediate portion (…). ) and a tail region with 16 sampling points ( ).

[0145] 2) Data mapping: Map the 5-bit data "00111" to be sent to a cyclic shift value d=7.

[0146] 3) Intermediate cyclic shift modulation: such as Figure 3 As shown, the label only applies to the middle portion of the 48 sampling points. Processing is then performed. A linear phase factor is determined based on the cyclic shift value d=7. The label multiplies the samples in the middle section by this phase factor, thus completing the modulation of the signal in the time domain. During this process, the original cyclic prefix and tail regions... It remains unchanged.

[0147] 4) Signal reflection: After modulation is completed, the tag splices the original cyclic prefix, the modulated middle part and the original tail region into a complete symbol that conforms to the OFDM standard, and reflects it out to transmit data.

[0148] This method enables sub-symbol-level data encoding in tags without compromising the integrity of the cyclic prefix structure, thereby improving transmission efficiency.

[0149] (3) Tag data demodulation implementation.

[0150] The receiving end needs to demodulate the tag data "00111" from the received reflected signal. The following is the specific demodulation process:

[0151] 1) Symbol Sample Extraction: The receiver first performs standard OFDM synchronization and then extracts the middle portion of the 48 modulated sampling points from the received symbols. .

[0152] 2) Hypothesis testing and cyclic shift compensation: The receiver initiates an iterative search process, traversing all possible cyclic shift values. For each assumed value The receiving end processed the 48 extracted samples. Perform a 48-point Fast Fourier Transform (FFT) and compensate for a value of [value] in the frequency domain. Circular shift.

[0153] 3) Cyclic Shift Compensation: In the frequency domain, the receiver attempts to recover the correct position of the subcarrier by compensating for possible cyclic shift values. This process involves performing correlation calculations between the subcarrier and known pilot values ​​to identify possible shift compensations.

[0154] 4) Signal Reconstruction and Inverse Fourier Transform (IFFT): Perform a 48-point Inverse Fast Fourier Transform (IFFT) on the compensated frequency domain signal to convert it back to the time domain, obtaining a reconstructed intermediate portion. Subsequently, this reconstructed intermediate portion is compared with the received tail region. The data is then pieced together to form a complete reconstructed data payload with 64 sampling points.

[0155] 5) Spectrum analysis and scoring: such as Figure 4 As shown, a 64-point Fast Fourier Transform (FFT) is performed on the data payload of the reconstructed 64 samples to obtain its complete power spectrum. Then, the correlation between this power spectrum and a predefined spectral template that identifies the effective subcarrier positions is calculated to obtain a correlation score.

[0156] 6) Label Data Recovery: After iterating through all 48 hypotheses, the receiver will find that the relevance score is... A significant peak is observed when the shift compensation value is 7. This peak indicates that the spectral structure of the reconstructed signal best matches the original OFDM signal when the shift compensation value is 7. Therefore, the receiver successfully demodulated the correct cyclic shift value of 7 and recovered the tag data "00111".

[0157] This demodulation process utilizes Fast Fourier Transform (FFT) and Inverse Fourier Transform (IFFT) to convert between the time and frequency domains, and verifies and recovers the modulated data through energy-based spectral correlation calculations, achieving efficient and robust backscatter communication under single-receiver conditions.

[0158] Through experiments, this invention verifies the high adaptability of the proposed method to different OFDM WiFi signal excitation sources and its superior performance under various channel conditions.

[0159] First, the method proposed in this invention is content-independent, demonstrating high compatibility with existing commercial WiFi. Experimental tests show that regardless of the underlying modulation scheme used by the ambient WiFi source, including BPSK, QPSK, 16-QAM, and 64-QAM, the bit error rate (BER) performance of this invention exhibits extremely high consistency and stability. This is because the demodulation algorithm of this invention relies on the energy spectrum structure of the OFDM signal (the positions of effective subcarriers and empty subcarriers), rather than the specific data content carried on the subcarriers.

[0160] Secondly, this invention demonstrates superior robustness in real, complex channel environments. In challenging non-line-of-sight (NLOS) multipath channel simulations, existing techniques that disrupt the cyclic prefix structure suffer catastrophic performance degradation and persistently high error rates. In contrast, this invention, by fully preserving the functionality of the cyclic prefix, exhibits exceptional robustness, with a smooth performance decline curve, enabling reliable communication.

[0161] Finally, this invention achieves high tag data throughput using only a single receiver. By employing time-domain sub-block modulation technology, the system's maximum throughput reaches 4.5 Mbps, a three-fold improvement compared to the most advanced single-receiver systems currently available. Even under harsh NLOS channel conditions, the system using dual-sub-block modulation still achieves a throughput exceeding 2.0 Mbps, far surpassing other single-receiver solutions. These experimental results verify that the proposed method can utilize various commercial OFDM WiFi signals as excitation, achieving high-throughput backscatter communication while maintaining high compatibility and robustness, demonstrating its enormous application potential in future wireless Internet of Things (IoT) applications.

[0162] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0163] It should be understood that although the steps in the flowcharts of the accompanying drawings are shown sequentially as indicated by the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some of the steps in the flowcharts of the accompanying drawings may include multiple steps or stages, which are not necessarily completed at the same time, but may be executed at different times, and the execution order of these steps or stages is not necessarily sequential, but may be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0164] In one embodiment, a computer system is provided, which may be a server. The computer system includes a processor, memory, and a network interface connected via a system bus. The processor provides computing and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores an operating system, computer programs, and a database. The internal memory provides an environment for the operation of the operating system and computer programs stored in the non-volatile storage media. The database stores data used in the methods described above. The network interface communicates with external terminals via a network connection. The computer program is executed by the processor to implement the methods described above.

[0165] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0166] 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.

[0167] 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 backscatter communication method, characterized in that, include: The tag receives OFDM symbols from ambient WiFi signals. The OFDM symbols include a cyclic prefix, a middle portion of the data payload, and a tail region. The cyclic prefix is ​​a copy of the tail region. Map the tag data to be sent to a cyclic shift value. A linear phase factor corresponding to the cyclic shift value is applied to the middle part of the data payload for modulation, while keeping the cyclic prefix and tail regions unchanged, and the OFDM symbol of the reflected signal is reconstructed based on the modulated middle part of the data payload. Demodulation is performed by a single receiver, the modulated part is extracted from the OFDM symbols of the received reflected signal, multiple candidate cyclic shift values ​​are iterated, frequency domain inverse shift compensation is performed on each candidate cyclic shift value, and the reconstructed data payload is obtained based on the compensated spectrum. Calculate the correlation score between the power spectrum of the reconstructed data payload and a predefined binary template based on the empty subcarrier position for each reconstructed data payload; select the candidate cyclic shift value that produces the maximum correlation score as the decoded cyclic shift value, and recover the corresponding tag data.

2. The single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The middle part of the data payload consists of the first NL sampling points of the data payload, and the tail region of the data payload consists of the last L sampling points of the data payload, where N is the total number of sampling points of the data payload and L is the number of sampling points in the tail region of the data payload.

3. The single-receiver WiFi backscatter communication method according to claim 2, characterized in that, The modulation of the data payload by applying a linear phase factor corresponding to the cyclic shift value only to the middle portion specifically includes: ； ； This represents the m-th sampled value of the time-domain sampled signal obtained after applying linear phase factor modulation to the middle portion of the data payload. This represents the m-th sampled value of the time-domain sampled signal corresponding to the middle portion of the unmodulated data payload. The imaginary unit, This represents the index of the m-th sample value.

4. The single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The OFDM symbols reconstructed from the intermediate portion of the modulated data payload specifically include: By concatenating the original cyclic prefix The middle part of the modulated data payload and the tail region of the original data payload To construct the OFDM symbol ultimately used for the reflected signal. : 。 5. A single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The step of extracting the modulated portion from the OFDM symbols of the received reflected signal, iterating over multiple candidate cyclic shift values, and performing frequency-domain inverse shift compensation for each candidate cyclic shift value specifically includes: Remove the cyclic prefix of the OFDM symbol from the received reflected signal. and data payload tail region Extract the middle part of the modulated data payload ;right Perform a Fast Fourier Transform on each sampling point to obtain the corresponding frequency domain representation. , , For the frequency domain index of the k-th sampling point, for each candidate cyclic shift value By applying a compensatory cyclic shift to cancel the assumed modulation effect, the compensated spectrum is obtained: ； in, ; This indicates that the candidate cyclic shift value is used in the assumption that the label is assumed to be... Under the premise of modulation, by... Perform a compensating inverse cyclic shift operation to obtain the compensated spectrum; express The m-th sample value, with a length of ; Indicate execution Point Fast Fourier Transform, This is the time-domain index of the m-th sample value. N is the total number of sampling points in the data payload, and L is the number of sampling points in the tail region of the data payload.

6. The single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The data payload reconstructed based on the obtained compensated spectrum specifically includes: The compensated spectrum is passed through The inverse fast Fourier transform of the point yields the assumed intermediate part. The assumed middle portion is compared with the tail region of the data payload of the OFDM symbol of the received reflected signal. By splicing together the data, a reconstructed data payload is formed. : ； These are candidate cyclic shift values.

7. The single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The calculation of the correlation score between the power spectrum of the reconstructed data payload and a predefined binary template based on the empty subcarrier position for each iteration specifically includes: Data load for reconstruction Perform a Fast Fourier Transform to obtain the corresponding spectrum. ;calculate power spectrum ; For frequency domain indexing, Candidate cyclic shift values; Introducing binary templates The binary template is '1' at valid subcarrier positions and '0' at empty subcarrier positions: The correlation score is obtained by comparing the power spectrum corresponding to the reconstructed data load with the binary template. : 。 8. A single-receiver WiFi backscatter communication method according to claim 1, characterized in that, The step of selecting the candidate cyclic shift value that produces the highest relevance score as the decoded cyclic shift value and recovering the corresponding label data specifically includes: ； The decoded cyclic shift value, Candidate cyclic shift values; based on Restore tag data.

9. A single-receiver WiFi backscatter communication method according to claim 1, characterized in that, This also includes further improving tag data transmission rates through time-domain sub-block modulation techniques, specifically including: The label will contain the middle part of the data payload Divide into K sub-blocks; map the tag data to be sent into a cyclic shift value vector consisting of K cyclic shift values; During demodulation, the receiver employs a maximum likelihood estimator to exhaustively search all possible cyclic shift value vectors; for each cyclic shift value vector... The receiver performs a signal reconstruction and correlation scoring process similar to that without time-domain sub-block modulation, assigning the correlation score to the signal. The largest cyclic shift value vector is used as the final decoded cyclic shift value vector. : ； according to Restore tag data, This represents the Kth candidate cyclic displacement value in the cyclic displacement value vector. This represents the Kth cyclic shift value in the final decoded cyclic shift value vector.

10. A computer system comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 9.

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