Orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation

Abstract

The invention relates to the technical field of backscatter communication, and discloses an orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation, which specifically comprises: converting signals from a time domain to a frequency domain by fast Fourier transform, processing the frequency domain signals by channel estimation and equalization, and removing extra phase differences; converting the signals with phase differences removed from the frequency domain to the time domain by inverse fast Fourier transform, and then demodulating label data; when converting the signals from the time domain to the frequency domain by fast Fourier transform, retaining all subcarriers modulated by the label, including data subcarriers, pilot subcarriers and null subcarriers, so as to retain all label data; the invention proposes a new channel equalization framework based on analyzing the characteristics of the backscattering system, successfully removes the extra phase difference introduced by three independent channels in the backscattering system, realizes single sampling level phase modulation, and effectively improves the transmission rate.

Description

Technical Field

[0001] The present invention relates to the technical field of backscatter communication, and in particular to an orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation. Background Art

[0002] Backscatter systems use ambient signals as carrier waves, carrying tag data for transmission. Wi-Fi (Wireless Fidelity) is widely used in our daily lives, providing abundant carrier resources for backscatter communications. Starting with Wi-Fi 3, Orthogonal Frequency Division Multiplexing (OFDM) modulation has been incorporated into the Wi-Fi protocol and has become its foundational technology. Consequently, backscatter systems based on OFDM Wi-Fi signals have become a research focus in recent years.

[0003] Transmission capacity is a very important indicator of communication systems. Over the past decade, researchers have been committed to exploring the transmission rate potential of OFDM WiFi backscatter systems. Early systems provided packet-level solutions, which determined the on / off state of the tag by measuring changes in channel state information (CSI). Each data packet could only carry one bit of information, and the throughput of such systems was only about 1 kbps. Subsequently, researchers increased the granularity of modulation to the symbol level. The FreeRider backscatter system uses four OFDM symbols to modulate one tag bit. The RapidRider backscatter system achieves the modulation of one tag bit using a single OFDM symbol. However, they are all limited to symbol-level units, with a maximum throughput of only about 250 kbps, which can only support low-data-rate applications such as audio transmission. Recently, some technical solutions have attempted to break the limitations of symbol units and further improve the modulation accuracy to increase the transmission rate: the Subscatter-o backscatter system uses 8 different frequency shift modes to modulate 3 tag bits within one OFDM symbol. However, the system using frequency shift modulation is not compatible with existing commercial equipment and is complex to implement, making it difficult to deploy and use in real life. In addition, the basic modulation unit of this system is multiple subcarriers, which still has redundancy. The Tscatter backscatter system uses phase modulation and uses a 40-bit pseudo-noise sequence (PN) to encode 4 tag bits within one OFDM symbol. Although the system performance has improved to a certain extent, it uses multi-sampling point modulation and the redundancy is still high. The maximum throughput of this type of system is only 1Mbps, which is difficult to meet the real-time transmission requirements of high-speed video streams in the 5G era.

[0004] Therefore, existing OFDM WiFi backscatter systems all rely to varying degrees on redundant coding, which limits the system's transmission rate. It is necessary to further increase the granularity of the coding to improve system performance. In recent years, related work has also been conducted to increase the data rate of WiFi backscatter systems by adding additional restrictions. For example, the Backfi backscatter system requires the control of the WiFi carrier's data information, which affects the system's normal communication function; the Subscatter-b backscatter system requires that the outgoing signal must be based on complementary code keying (CCK) modulation. Only 802.11b signals use this modulation method, but this protocol has been gradually phased out and is no longer supported in current WiFi devices. The present invention chooses to use today's mainstream OFDM WiFi signals as carriers, and does not require control of carrier information, so it will not interfere with existing communication systems. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides an orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] An orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation, the backscattering system adopted includes a transmitter, a tag, and a channel equalization framework with a first receiving end Rx1 and a second receiving end Rx2; the transmitter sends an ambient WiFi signal And send it to the first receiving end and the tag. The tag uses phase modulation to carry the tag data on the received signal, sends a backscattered signal, and sends it to the second receiving end. The signal received by the first receiving end is recorded as , the signal received by the second receiving end is recorded as The orthogonal frequency division multiplexing WiFi backscattering method specifically includes:

[0008] By fast Fourier transform, and Convert from time domain to frequency domain to obtain the first frequency domain signal and the second frequency domain signal ; Through channel estimation and equalization, and Processing is performed to remove the extra phase difference and obtain the first channel equalized signal and the second channel equalized signal ; Through the inverse fast Fourier transform, and Convert from frequency domain to time domain to obtain the first time domain signal and the second time domain signal ;based on and Demodulate tag data to achieve backscattering;

[0009] Among them, the ambient WiFi signal divides the data stream into multiple subcarriers for transmission; and When converting from the time domain to the frequency domain, all subcarriers modulated by the tag are retained, including data subcarriers, pilot subcarriers and null subcarriers, so as to retain all tag data; and When converting from the frequency domain to the time domain, the data subcarriers, as well as the pilot subcarriers and null subcarriers retained during the fast Fourier transform are used instead of the pilot subcarriers and null subcarriers set in the standard WiFi protocol, so that all tag data is retained during the conversion between the time domain and the frequency domain.

[0010] Furthermore, the channel estimation and equalization are respectively and Processing is performed to remove the extra phase difference and obtain the first channel equalized signal and the second channel equalized signal , specifically including:

[0011] Channel from the sender to the first receiver The channel condition is recorded as the first channel condition , the channel from the sender to the tag The channel condition is recorded as the second channel condition , the channel from the label to the second receiving end The channel condition is recorded as the third channel condition ;

[0012] Let the first receiving end Rx1 do backtracking channel equalization to remove the first channel condition Then add the second channel condition The first channel equalization signal is obtained :

[0013] ;

[0014] Remove the third channel condition for the second receiving end Rx2 Impact:

[0015] ;

[0016] Indicates the circular convolution of the signal. represents the product of the signals, To perform inverse circular convolution on the signal, Indicates label modulation.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are:

[0018] To address the problems of high modulation redundancy and low data transmission rate in existing OFDM WiFi backscatter systems, the present invention proposes a new channel equalization framework based on the analysis of the characteristics of the backscatter system. This framework successfully removes the additional phase difference introduced by the three independent channels in the backscatter system, realizes single-sampling-level phase modulation, and thus effectively improves the system's transmission rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the architecture diagram of the classic backscatter system;

[0020] Figure 2 is the constellation diagram of binary phase shift keying modulation;

[0021] Figure 3 is the constellation diagram affected by channel interference;

[0022] Figure 4 Schematic diagram of the channel equalization framework of the present invention;

[0023] Figure 5 is the constellation diagram after channel equalization;

[0024] Figure 6 Schematic diagram of the principle of the method for mutual conversion between time-frequency and frequency domain of the present invention;

[0025] Figure 7 A schematic diagram of a backtracking-based channel estimation and equalization method according to the present invention;

[0026] Figure 8 A comparison chart of signal recovery error rates under different time-frequency and frequency domain conversion methods;

[0027] Figure 9 A comparison chart of label decoding error rates under different time-frequency and frequency domain conversion methods;

[0028] Figure 10 Comparison of label decoding error rates under different channel estimation and equalization methods;

[0029] Figure 11 This is a comparison chart of data transmission rates under different channel estimation and equalization methods;

[0030] Figure 12 This is an overall flow chart of the backscattering system of the present invention. DETAILED DESCRIPTION

[0031] A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

[0032] Orthogonal frequency division multiplexing (OFDM) technology is widely used in WiFi communications. It divides data streams into multiple subcarriers for transmission. Each subcarrier transmits data at a different frequency. Due to the orthogonality between subcarriers, they can be transmitted simultaneously in the same frequency band without interfering with each other, thus achieving efficient spectrum utilization. The OFDM symbol is the basic unit of information transmitted on these subcarriers. Each OFDM symbol consists of a set of orthogonal subcarriers, each carrying independent information. These subcarriers are orthogonal in the frequency domain, meaning that their spectra are independent and do not interfere with each other. The OFDM symbol is a unit of information represented in the time domain, containing data transmitted simultaneously on multiple subcarriers. Each OFDM symbol lasts for a certain period of time, and during this time, all subcarriers transmit data simultaneously.

[0033] An OFDM symbol consists of multiple time-domain samples. The number of samples per OFDM symbol depends on system parameters, particularly the Fast Fourier Transform (FFT) size. The FFT size determines the number of subcarriers in the OFDM symbol, and each subcarrier contributes one sample to the OFDM symbol.

[0034] In Wi-Fi, OFDM symbol generation typically involves subjecting the bit stream to a series of processing steps, including modulation, serial-to-parallel conversion, and IFFT (Inverse Fast Fourier Transform). The modulation process maps the bit stream onto a corresponding constellation, forming complex symbols. These complex symbols are then assigned to different subcarriers and transformed into the time domain using IFFT, generating multiple OFDM symbols consisting of these time-domain samples, which are then transmitted as electromagnetic waves. Subcarriers include data subcarriers, pilot subcarriers, and null subcarriers.

[0035] To address the problems of high modulation redundancy and low transmission rate in existing OFDM WiFi backscatter systems, this paper, based on a thorough analysis of the limitations of existing systems, proposes a high-rate solution based on single-sampling-level modulation. This aims to increase the data rate of OFDM WiFi backscatter systems, thereby supporting the application requirements of high-speed data transmission in the 5G era and promoting the widespread application of backscatter technology in real life.

[0036] The following first introduces the fundamental reason for limiting single-sample level modulation, and then introduces the new backscattering system and implementation method designed by the present invention based on the observation results.

[0037] 1. Observation and cause analysis of the failure of existing backscatter systems:

[0038] The architecture of the classic backscatter system is as follows Figure 1 As shown in the figure, the ambient WiFi signal emitted by the transmitter Tx is used as the carrier, and the tag Tag uses phase modulation to carry the tag data at each sampling point of the carrier. Two receiving ends, the first receiving end Rx1 and the second receiving end Rx2, are used to receive the ambient WiFi signal and the backscattered signal after the tag modulation respectively.

[0039] The two receiving ends calculate the tag data based on the phase difference of the received I / Q sequence. There are three independent channels in the existing backscatter system, namely the channel from the transmitting end to the first receiving end , the channel from the sender to the tag , and the channel from the tag to the second receiving end .

[0040] Phase modulation is used, that is, label data is encoded by applying a certain phase to each sampling point in the time domain.

[0041] Intuitively, in the absence of other interference, the tag data can be demodulated at the receiving end by checking the phase difference of each sampling point. The initial phase of the ambient WiFi signal is defined as: The tag is modulated using binary phase shift keying (BPSK). The phase shift introduced by the tag modulation is , the corresponding constellation diagram is as follows Figure 2 As shown, its phase is centered between 0 and Assuming that the signal is not interfered by other factors during transmission, the phase of the signal received by the receiving end Rx1 is , the phase of the signal received by the receiving end Rx2 , the phase difference of the signals received by the two receiving ends It should be the phase offset introduced by tag modulation, as shown in formula (1):

[0042] Formula (1): .

[0043] However, in actual transmission, the wireless channel will introduce additional phase offset. Figure 1 As shown, there are three channels in the backscatter system, namely the channel from the transmitter to the receiver Rx1, the channel from the transmitter to the tag, and the channel from the tag to the receiver Rx2. The phase offsets generated by the three channels are recorded as 、 、 In this case, the phase of the signal received by the first receiving end Rx1 is and the phase of the signal received by the second receiving end Rx2 , the phase difference of the signals received by the two receiving ends As shown in formula (2):

[0044] Formula (2): .

[0045] Comparing Equation (1) and Equation (2), it can be seen that the wireless channel introduces an additional phase difference: The three channels are independent of each other and the phase differences they introduce cannot cancel each other out, so the phase differences of the received signals are randomly distributed (e.g. Figure 3 As shown in ), it is impossible to extract the label data from it. Figure 3 As shown in the figure, due to the introduction of the channel and the additional phase offset, the original focus on 0 and The phase difference becomes randomly distributed between them, which is difficult to distinguish and the tag data cannot be demodulated from it.

[0046] The existing system cannot solve the above problems, and therefore cannot achieve single sampling level modulation.

[0047] 2. Design of OFDM WiFi backscatter system with single sampling level modulation:

[0048] In order to achieve single sampling level modulation, it is necessary to eliminate the additional phase difference caused by the wireless channel. Based on this, the present invention designs a new channel equalization framework, such as Figure 4 First, the present invention uses fast Fourier transform to transform the signal received by the first receiving end into and the signal received by the second receiving end Convert from time domain to frequency domain; then, the present invention estimates the channel conditions and performs channel equalization to remove the additional phase difference introduced by the wireless channel; then, since the single sampling level demodulation is based on a single sampling point in the time domain, the present invention converts the signal back to the time domain through the inverse fast Fourier transform to obtain the first time domain signal and the second time domain signal Finally, the present invention can extract a phase difference with sufficient discrimination from each sample pair, such as Figure 5 As shown in Figure 2, the tag data is demodulated. A sample refers to each sampling point in the time domain, and a sample pair refers to a pair of sample points at the same location (time) when sampling two signals (the original path and the backscattered path) simultaneously. The tag data can be demodulated by calculating the phase difference between these sample pairs.

[0049] like Figure 5 As shown, the phase difference of the constellation diagram after the channel equalization framework of the present invention is basically distributed between 0 and Nearby, the tag data can be decoded.

[0050] Figure 2 、 Figure 3 、 Figure 5 The horizontal axis I of the constellation diagram represents in-phase, and the vertical axis Q represents quadrature.

[0051] In this channel equalization framework, there are two core steps: one is the information loss-free conversion between the time domain and the frequency domain, and the other is the channel estimation and equalization based on backtracking. The following will elaborate on these two core technologies in detail.

[0052] 2.1, Conversion between time domain and frequency domain without information loss:

[0053] The present invention uses Fast Fourier Transform and Inverse Fast Fourier Transform to convert signals between frequency domain and time domain. In the standard WiFi protocol, only subcarriers are retained after demodulation (Fast Fourier Transform), and pilot subcarriers and null subcarriers are discarded because they do not carry data information. However, in a single-sample backscatter system, the tag The up-modulation phase information is , then the modulated time domain signal (i.e. backscattered signal) can be expressed as: [ ,in, Indicates the time domain sampling points, represents a complex number, and 64 represents the number of sampling points modulated in one OFDM symbol of the present invention. After fast Fourier transform, the corresponding subcarrier can be obtained , as shown in formula (3):

[0054] Formula (3): ;

[0055] in, Indicates the subcarrier index value, k=[1,2,…,64].

[0056] Problem Analysis: From formula (3), we can see that each subcarrier The sum of all sampling points is calculated, which means that the label data on each sample will be extended to all subcarriers. Therefore, all subcarriers will change after label modulation. If the modulated pilot subcarriers (Pilot) and null subcarriers (Null) are discarded, such as Figure 6 As shown, some label data will be lost and the signal cannot be correctly restored. Therefore, the traditional time domain and frequency domain conversion method is not applicable to the present invention.

[0057] Method introduction: Based on this, the present invention proposes a method for mutual conversion between time-frequency and frequency domains without information loss. The present invention proposes to use all subcarriers after label modulation during domain conversion to ensure that all label data is retained. The specific operating steps are as follows: When using fast Fourier transform to convert from time domain to frequency domain, only all modulated subcarriers need to be retained. When using inverse fast Fourier transform to convert from frequency domain to time domain, the present invention uses the pilot subcarriers and empty subcarriers retained during the fast Fourier transform instead of the pilot subcarriers and empty subcarriers predetermined in the standard WiFi protocol. In this way, the present invention can completely retain all label data during the mutual conversion between time-frequency and frequency domains. The method of the present invention does not add additional computational burden to the system because the present invention only replaces the objects used in the calculation.

[0058] 2.2, Channel Estimation and Equalization Based on Backtracking:

[0059] Channel estimation and equalization are important steps in wireless communication. In this paper, the ambient WiFi signal is recorded as , the channel from the sender to the first receiver The channel condition is recorded as the first channel condition , the channel from the sender to the tag The channel condition is recorded as the second channel condition , label the channel to the second receiving end The channel condition is recorded as the third channel condition The channel acts in the frequency domain, and its effect on the signal is equivalent to circular convolution of the time domain signal. The corresponding symbol is " ", the label is based on the phase modulation effect in the time domain, which is equivalent to multiplying the signal. The corresponding symbol is " ”.

[0060] The signal received by the first receiving end Rx1 can be expressed as: .

[0061] The signal received by the second receiving end Rx2 can be expressed as: ;in Indicates label modulation.

[0062] In the present invention, the goal of channel equalization is to remove the additional phase difference caused by the channel and completely retain the label modulation information. That is, after performing channel equalization, Rx1 and Rx2 should respectively obtain: and .

[0063] Problem Analysis: In the standard WiFi protocol, the receiver considers the entire transmission process as a whole and performs end-to-end channel estimation. The channel conditions evaluated at Rx1 and Rx2 are: and For the first receiving end Rx1, the original signal can be correctly restored using the traditional method, as shown in Equation (4). This is because the tag does not affect the transmission of the original data:

[0064] Formula (4): ;

[0065] in, represents inverse recurrent convolution.

[0066] For the tag-modulated backscatter transmission path, the signal recovered using the traditional method is shown in Equation (5):

[0067] Formula (5): .

[0068] Since the label is modulated between the two signals, only when the circular convolution operation ( ) and product operations ( ) are exchangeable, the influence of the channel can be correctly removed.

[0069] However, the present invention can prove that these two operations are not interchangeable. The present invention constructs two scenarios to prove this. In the first scenario, the signal first passes through the channel and then undergoes label modulation. In the second scenario, the signal first undergoes label modulation and then passes through the channel. The present invention denotes the input signal, channel, and label modulation as 、 and , the output signals of the above two scenarios and They can be calculated by formula (6) and formula (7) respectively:

[0070] Formula (6): ;

[0071] Formula (7): ;

[0072] Where N is the number of modulation sampling points in an OFDM symbol, represents the mth sampling point in the time domain, and Indicates the nth bit data and the mth bit data of the modulated tag data, Indicates the serial number is The channel condition of the channel, m and n represent indexes.

[0073] From the above formula, we can see that the output signal in the first scenario is calculated as When , the sum operation is performed first, and then the product operation is performed to calculate the output signal in the second scenario When , the product operation is performed first, and then the sum operation is performed. From a mathematical analysis, only when the channel and label modulation When is a fixed constant, Will and However, in actual communication systems, this situation does not exist because the channel is nonlinear and often accompanied by the influence of multipath. The tag modulation of the present invention is at the sampling level, and the data modulated at each sampling point in a symbol is different. Therefore, in the backscattering system of the present invention, the channel response and tag modulation are not interchangeable.

[0074] Based on this, the present invention proposes a new channel estimation and equalization method for backscatter systems: channel estimation and equalization based on backtracking. Although channel operation and tag modulation are not interchangeable, these channels can be synthesized. Therefore, the present invention allows the first receiving end Rx1 to perform backtracking channel equalization, first removing the first channel condition Then add the second channel condition The two channel conditions can be compounded, such as Figure 7 As shown, the signal after processing is shown in formula (8):

[0075] Formula (8): ;

[0076] The second receiving end Rx2 only needs to remove the third channel condition The influence of is shown in formula (9):

[0077] Formula (9): .

[0078] Combining equations (8) and (9), it can be found that after the channel estimation and equalization of the present invention, the first channel equalized signal is obtained and the second channel equalized signal Only the phase difference is the tag modulation. Therefore, after the two signals are channel equalized, the extra phase can be eliminated, and then they are converted to the time domain. Finally, by calculating the direct phase difference of each pair of time domain sampling points, the corresponding tag data can be decoded.

[0079] The present invention first evaluates the overall performance of the backscatter system and compares the transmission rate of the backscatter system of the present invention with that of existing backscatter systems, as shown in Table 1. It can be seen that the transmission rate of the backscatter system of the present invention (Soscatter) is much higher than that of other backscatter systems, 263 times higher than that of the FreeRider backscatter system, 63 times higher than that of the RapidRider backscatter system, 21 times higher than that of the Subcatter-o backscatter system, and 16 times higher than that of the Tscatter backscatter system.

[0080] Table 1, comparison of transmission rates between the backscatter system of the present invention and the existing backscatter system:

[0081]

[0082] Next, the backscattering method proposed in the present invention is evaluated. First, the effectiveness of the method for converting time domain and frequency domain without information loss is evaluated. The present invention compares the signal recovery error rate and tag decoding error rate of the method proposed in the present invention with those of the traditional method, as shown in the following figure. Figure 8 and Figure 9 As shown. It can be seen that the error rate of the method of the present invention is below 0.2%. The error rate of the traditional method is much higher than that of the method of the present invention, and this error rate is unacceptable in the communication system. Figure 8 and Figure 9 The error rate also shows a downward trend from WiFi 3 to WiFi 6. This is due to the different proportions of different subcarrier types in the protocol. As shown in Table 2, WiFi 3 has a higher proportion of non-data subcarriers. Directly discarding these subcarriers would result in greater information loss and higher error rates.

[0083] Table 2 Comparison of the number of three subcarriers in different protocols:

[0084]

[0085] Then, the backtracking-based channel estimation and equalization method proposed in this invention is evaluated. The present invention compares the tag decoding error rate and data rate of no channel equalization (WCE), using the traditional method for channel equalization (CCE), and using the method of this invention for channel equalization (BCE), as shown in the following figure. Figure 10 and 11 As shown, under all protocols, the error rate of the method of the present invention is lower than 1.2%. In comparison, the error rates of other methods are much higher than that of the method of the present invention. Thanks to the lower error rate, the transmission rate of the system of the present invention is much higher than that of other methods.

[0086] Example

[0087] When implementing the present invention, you can refer to the following steps:

[0088] 1. System construction

[0089] First, build the OFDM WiFi backscatter system. The overall structure of the backscatter system is as follows: Figure 1 As shown, the system consists of three parts: a transmitter, a tag, and a receiver. The transmitter and receiver use commercial WiFi network cards and software-defined radios (SDRs). The commercial device used in the experiment uses a Qualcomm AR938x network card, with CommView software used for transceiver software. The SDR uses a ZedBoard development board with an AD9361 chip as a daughterboard, and Matlab software is used for data analysis. The tag is a prototype built using a field-programmable gate array (FPGA) and is equipped with components such as an amplifier and an RF front-end switch.

[0090] 2. System implementation process

[0091] The overall process of the backscatter system can be found in Figure 12 The working process of the backscatter system is as follows: the transmitting end sends data packets such as WiFi 3, WiFi 5, and WiFi 6. One signal is directly received by the first receiving end, and the other signal is used for backscattering. After passing through the tag, the signal is modulated and carries the tag data. This system adopts a single-sampling-level modulation method, carrying one bit of tag data at each time domain sampling point; it is then received by the second receiving end, and two signals will be received at the two receiving ends. The new channel equalization method proposed in this invention is used to process the signals at the two receiving ends to remove the additional phase offset caused by the channel, and then the phase difference of the time domain signals after the two processes is calculated. Finally, the tag-modulated data can be obtained through data decoding to realize the transmission of tag data.

[0092] 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 embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. It is intended that all variations within the meaning and range of equivalents of the claims be embraced herein, and any reference signs in the claims should not be construed as limiting the claims to which they relate.

[0093] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. An orthogonal frequency division multiplexing WiFi backscattering method based on single sampling level modulation, characterized in that: The backscatter system used includes a transmitter, a tag, and a channel equalization framework having a first receiving end Rx1 and a second receiving end Rx2; the transmitter sends an ambient WiFi signal And sent to the first receiving end and the tag, the tag uses phase modulation to carry the tag data on the received signal, sends a backscattered signal, and sends it to the second receiving end; the signal received by the first receiving end is recorded as , the signal received by the second receiving end is recorded as ; The orthogonal frequency division multiplexing WiFi backscattering method specifically includes: Through fast Fourier transform, and Convert from time domain to frequency domain to get the first frequency domain signal and the second frequency domain signal ; Through channel estimation and equalization, and Processing is performed to remove the extra phase difference and obtain the first channel equalization signal and the second channel equalized signal ; Through the inverse fast Fourier transform, and Convert from frequency domain to time domain to get the first time domain signal and the second time domain signal ;based on and Demodulate tag data to achieve backscattering; Among them, the ambient WiFi signal divides the data stream into multiple subcarriers for transmission; and When converting from the time domain to the frequency domain, all subcarriers modulated by the tag are retained, including data subcarriers, pilot subcarriers, and null subcarriers, so as to retain all tag data; and When converting from the frequency domain to the time domain, the data subcarriers, as well as the pilot subcarriers and null subcarriers retained during the fast Fourier transform are used instead of the pilot subcarriers and null subcarriers set in the standard WiFi protocol, so as to retain all tag data during the conversion between the time domain and the frequency domain; The channel estimation and equalization are respectively and Processing is performed to remove the extra phase difference and obtain the first channel equalization signal and the second channel equalized signal , including: Channel from the sender to the first receiver The channel condition is recorded as the first channel condition , the channel from the sender to the tag The channel condition is recorded as the second channel condition , the channel from the label to the second receiving end The channel condition is recorded as the third channel condition ; Let the first receiving end Rx1 perform backtracking channel equalization to remove the first channel condition Then add a second channel condition The first channel equalization signal is obtained : ； Remove the third channel condition for the second receiving end Rx2 Impact: ； represents the circular convolution of the signal. represents the product of the signals, To perform inverse circular convolution on the signal, Indicates label modulation.

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