Backscattering method based on ambient OFDM WiFi

Abstract

The application discloses a backscattering processing method based on environment OFDM WiFi and belongs to the field of Internet of Things, which comprises the following steps: taking a standard wireless signal of an environment OFDM WiFi device as an excitation signal; forming a backscattering signal by reflecting the excitation signal through a backscattering tag; modulating tag data in a binary Golay code and phase shift keying combined mode by using a reference signal found from the backscattering signal; receiving the backscattering signal through a backscattering receiver, separating tag phases from common phase errors in an iterative mode after pretreatment, recombining OFDM symbols according to estimated sampling time offset values in the iteration, and finally mapping the estimated phases to a constellation diagram of phase shift keying modulation to demodulate the tag data. The method uses a virtual 0th subcarrier as a reference signal, can directly demodulate the tag data from the backscattering symbol, and has lower error in modulation by using binary Golay code.

Description

[0001] This application claims priority to the Chinese Patent Application No. 202211350265.5, filed on October 31, 2022, and entitled “Backscatter processing method based on environmental OFDM WiFi”, the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the field of Internet of Things communication, and in particular to a backscatter processing method based on environmental OFDM WiFi. BACKGROUND

[0003] Environmental backscatter systems have gained increasing attention in recent years as they have the potential to provide ultra-low power communication for billions of tiny computing devices. Unlike traditional radio frequency identification (RFID) technology, environmental backscatter systems have three advantages. First, they use uncontrolled environmental signals as wireless carriers, extending the excitation source from a special reader to rich environmental wireless signals; second, they support higher backscatter transmission throughput; and finally, unlike the duplex receiver required to decode RFID signals, they only need a standard radio device that supports general wireless protocols as a receiver, such as WiFi, Bluetooth, etc.

[0004] One of the key features of environmental backscatter systems is that the excitation signal is separate from the receiver, so the environmental signal used for excitation is uncontrolled. In wireless communication, the demodulation operation essentially finds the difference between the received signal and the reference signal. As in active communication, a binary phase-shift keying (BPSK) receiver uses a continuous wave (CW) as a reference signal. When using uncontrolled environmental signals for excitation, finding a reference signal is a challenge.

[0005] Existing environmental backscatter systems use the original environmental signal as the reference signal. For example, in HitchHike, if the received backscatter symbol is “1” and the original environmental symbol is “0”, the tag symbol will be demodulated as “1”, which indicates that the phase rotation modulated by the tag has converted the environmental symbol “0” to “1”. Other existing environmental backscatter systems such as FreeRider, PLoRa, and LScatter extend this idea to 802.11n, ZigBee, Bluetooth, LoRa, and LTE signals. However, these methods all have the feature described above: using the original environmental signal as the reference signal. This means that their data demodulation not only depends on the quality of the environmental data, but also requires additional receivers, increasing hardware costs and synchronization costs. This feature greatly limits the practicality of these environmental backscatter systems.

[0006] In addition, the existing ambient backscatter system often applies phase rotation to the backscatter symbol to modulate the tag data when modulating the tag data. In order to ensure that the backscatter signal can be correctly received and the backscatter data can be demodulated, only binary phase modulation is often used, and only 1 bit of tag data can be modulated in each modulation symbol, which limits the throughput rate of the backscatter system.

[0007] In view of this, the present application is proposed. SUMMARY

[0008] The purpose of the present application is to provide an ambient OFDM WiFi-based backscatter processing method, which not only can find reference signals in the backscatter signal to reduce hardware cost and synchronization overhead, but also can realize high-order PSK modulation to modulate more tag data in each symbol, thereby significantly improving the throughput rate of the system, and further solving the above technical problems existing in the prior art.

[0009] The purpose of the present application is achieved by the following technical solutions:

[0010] An ambient OFDM WiFi-based backscatter processing method, comprising:

[0011] An ambient OFDM WiFi device is used as a backscatter excitation source, and a standard wireless signal of the ambient OFDM WiFi device is used as an excitation signal for backscatter;

[0012] A backscatter signal is formed by reflecting the excitation signal of the backscatter excitation source by a backscatter tag, and a reference signal found from the backscatter signal is used to modulate tag data in a binary Golay code and phase shift keying combined manner for data transmission;

[0013] The backscatter signal of the backscatter tag is received by a backscatter receiver, frequency and time synchronization is performed, partial carrier frequency offset and sampling time offset are estimated and eliminated, then WiFi channel estimation is performed, then the tag phase of the backscatter tag is separated from the common phase error in an iterative manner, and the OFDM symbol is reorganized according to the estimated sampling time offset value in the iteration, finally the estimated phase is mapped to the constellation diagram of the phase shift keying modulation to demodulate the tag data.

[0014] Compared with the prior art, the ambient OFDM WiFi-based backscatter processing method provided by the present application has the following beneficial effects:

[0015] Compared with the prior art system requiring an ambient signal as a reference signal, the tag data can be directly demodulated from the backscatter signal by using the virtual 0th subcarrier obtained from the backscatter signal as a reference signal, and binary Golay code is used in the modulation, which is lower in error than the conventional binary code. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort.

[0017] Figure 1 The flowchart of the backscatter processing method based on the ambient OFDM WiFi provided by the embodiments of the present application.

[0018] Figure 2 The comparison chart of the influence of the 0th subcarrier phase estimation under different SFOs on the tag data error rate in the method provided by the embodiments of the present application.

[0019] Figure 3 The comparison chart of the influence of separating CPE and not separating CPE on the tag data error rate in the method provided by the embodiments of the present application.

[0020] Figure 4 The comparison chart of the influence of symbol recombination on the tag data error rate in the method provided by the embodiments of the present application.

[0021] Figure 5 The comparison chart of the difference between the ambient backscatter system used in the method provided by the embodiments of the present application and the prior art ambient backscatter system; wherein, (1) is the principle diagram of the backscatter system used in the prior art method; (2) is the principle diagram of the backscatter system used in the method of the present application.

[0022] Figure 6 The 8-PSK constellation diagram of the binary Golay code obtained by using the conventional binary code and the method provided by the embodiments of the present application; wherein, (a) is the constellation diagram of the conventional binary code; (b) is the constellation diagram of the binary Golay code.

[0023] Figure 7 The block diagram of the non-ideal receiving end provided by the embodiments of the present application. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the specific contents of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application, which do not constitute a limitation of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the present application.

[0025] Firstly, the terms possibly used in the present application are described as follows:

[0026] The term "and / or" means either of the two or both, for example, X and / or Y means three cases including "X" or "Y" or "X and Y".

[0027] The terms "include", "contain", "have", "possess" or other similar semantic descriptions should be interpreted as non-exclusive inclusion. For example: including a technical feature element (such as raw materials, components, ingredients, carriers, dosage forms, materials, sizes, parts, components, mechanisms, devices, steps, processes, methods, reaction conditions, processing conditions, parameters, algorithms, signals, data, products or articles, etc.) should be interpreted as not only including the explicitly listed technical feature element, but also including other technical feature elements not explicitly listed in the art.

[0028] The term "consisting of" means excluding any technical feature element not explicitly listed. If this term is used in the claims, the term will make the claim closed, so that it does not contain technical feature elements other than the explicitly listed technical feature elements, except for conventional impurities related thereto. If the term only appears in a certain clause of the claim, it is only limited to the elements explicitly listed in that clause, and the elements described in other clauses are not excluded from the overall claim.

[0029] Unless otherwise specifically specified or limited, the terms "mount", "connect", "connect", "fix", and the like should be broadly understood, for example: it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be connected inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0030] The terms "central", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like indicate the orientation or positional relationship shown in the drawings, which are only for the convenience of description and simplification of description, and do not mean that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting herein.

[0031] The environment OFDM WiFi-based backscatter PSK modulation method provided by the present application will be described in detail below. The contents not described in detail in the embodiments of the present application belong to the prior art known to those skilled in the art. If no specific conditions are specified in the embodiments of the present application, the conventional conditions or the conditions recommended by the manufacturer are used. If no manufacturer of the reagent or instrument used in the embodiments of the present application is specified, it is a conventional product that can be purchased on the market.

[0032] As shown in Figure 1 The embodiments of the present application provide an environment OFDM WiFi-based backscatter processing method, which comprises the following steps:

[0033] Step S1: using an environment OFDM WiFi device as a backscatter excitation source, and using a standard wireless signal of the environment OFDM WiFi device as an excitation signal for backscatter;

[0034] Step S2: forming a backscatter signal by reflecting the excitation signal of the backscatter excitation source through a backscatter tag, and modulating tag data in a binary Golay code and phase shift keying combined manner using a reference signal found from the backscatter signal for data transmission;

[0035] Step S3: receiving the backscatter signal of the backscatter tag through a backscatter receiver, performing frequency and time synchronization, estimating and eliminating part of the carrier frequency offset and the sampling time offset, then performing WiFi channel estimation, then separating the tag phase of the backscatter tag from the common phase error in an iterative manner, and recombining the OFDM symbol according to the estimated sampling time offset value in the iteration, and finally mapping the estimated phase to a constellation diagram of the phase shift keying modulation to demodulate the tag data.

[0036] In step S2 of the above method, the backscatter tag modulates the tag data in a binary Golay code and phase shift keying combined manner using the reference signal found from the backscatter signal, which comprises:

[0037] The average value of pilot subcarriers for channel estimation in a standard wireless signal of an OFDM WiFi device takes the 0th subcarrier of an uncontrolled excitation data packet as a reference signal.

[0038] Different tag data is modulated into binary Golay codes by applying different phase rotation modulations to the reference signal.

[0039] In the above method, 2 m - applying different phase rotations to the reference signal in a phase shift keying modulation mode to modulate different tag data into binary Golay codes, comprising:

[0040] Each OFDM symbol modulates a group of m-bit tag data, m being a positive integer, and the phase rotation applied to the i-th OFDM symbol by the tag data is denoted as , then is determined by the following formula:

[0041]

[0042] wherein n i is the result of converting the m-bit tag data of the i-th OFDM symbol from a binary Golay code into a decimal number, is the phase interval of the phase shift keying modulation, being 2 m - in the phase shift keying modulation mode

[0043] In the above method, the backscatter receiver uses the WiFi preamble field for frequency and time synchronization common to conventional WiFi receivers, estimates and eliminates part of the carrier frequency offset and sampling time offset, and then performs channel estimation of the conventional WiFi receiver. After the channel estimation is completed, the backscatter receiver starts to demodulate the tag data in an iterative manner.

[0044] In step S3 of the above method, the tag phase of the backscatter tag is separated from the common phase error in the following iterative manner, comprising:

[0045] In step S31, the common phase error estimation value of the i-th symbol is calculated If i = 1, let If 1 < i ≤ l (l is a parameter), the average value of the common phase error of the previous i-1 symbols is taken as the estimation result, If i > l, the average value of the common phase error of the previous l consecutive symbols is taken as the estimation result,

[0046] In step S32, the phase of the 0th subcarrier of the i-th symbol is estimated using the least squares method The slope β1 of the linear regression is used to determine whether OFDM symbol reconstruction is performed when the absolute value of the slope β1 exceeds a preset threshold. The threshold is set to... Q represents the number of subcarriers in the symbol;

[0047] Step S33, based on the phase of subcarrier 0 of the i-th symbol Tag phase and common phase error relational formula use The tag phase is calculated to obtain the separation common phase error of the i-th symbol. according to Definite Finding the mapping relationship with the tag modulation phase The corresponding optimal n i This is converted into binary Gray code, which is the tag data for the symbol. Then, using... A definite n i With phase The correspondence relationship is used to derive the actual tag phase of the i-th symbol.

[0048] Step S34, using the formula The tag phase of the i-th symbol is calculated. And update i to i+1;

[0049] Step S35: Determine whether all symbols have been processed. If not, repeat steps 31 to 34. If yes, end the processing.

[0050] In step S32 above, the phase β0 of subcarrier 0 is obtained by performing weighted least squares estimation on the i-th symbol using the following formula:

[0051]

[0052] Where, n p It is the number of pilot subcarriers in the symbol; A i ,Φ i ,x i β1 represents the amplitude, phase, and index of the i-th pilot subcarrier, respectively; β0 represents the estimated phase of the 0th subcarrier; β1 represents the slope of the linear regression, the sign of which indicates the sign of the sampling time offset, and its absolute value reflects the degree of sampling time offset.

[0053] In step S32 above, OFDM symbol recombination is performed in the following manner, including:

[0054] If the slope β1 of the linear regression is a positive number, one sample point is discarded, if the slope β1 of the linear regression is a negative number, one sample point is copied;

[0055] For each discarded or copied sample point, the slope β1 of the linear regression is corrected to Q is the number of subcarriers in a symbol.

[0056] As can be seen from the above, the method of the embodiment of the application can demodulate the tag data directly from the backscatter symbol because the virtual 0th subcarrier is used as the reference signal, compared with the existing system which needs an ambient signal as the reference signal; and the binary Golay code is used in the modulation, and the error is lower than that of the traditional binary code.

[0057] In order to more clearly show the technical solutions provided by the application and the technical effects produced, the backscatter processing method based on ambient OFDM WiFi provided by the embodiment of the application is described in detail below with specific embodiments.

[0058] Embodiment 1

[0059] As shown in the formula (1), the embodiment of the application provides a backscatter processing method based on ambient OFDM WiFi, which is a backscatter modulation and demodulation method. Figure 1 The environmental backscatter system involved in the method includes three parts. The backscatter excitation source provides a carrier wave for backscatter, which is usually a standard wireless signal, such as WiFi, Bluetooth. The backscatter tag transmits data by backscattering the excitation signal. The backscatter receiver demodulates the tag data from the backscatter signal. In the traditional RFID system, the backscatter excitation source and the backscatter receiver are the same device, which is usually an RFID reader. In the environmental backscatter system, the backscatter excitation source can be an existing wireless infrastructure. Orthogonal frequency division multiplexing (OFDM) is a data transmission technology that uses multiple orthogonal subcarrier frequencies with overlapping spectra to transmit data. The more advanced WiFi protocols (802.11a / g / n / ac / ax) in the 802.11 protocol family all use OFDM technology because of its high transmission rate and ability to resist errors such as frequency-selective fading. With the widespread deployment of WiFi infrastructure, there are sufficient OFDM WiFi signals in the environment that can provide an excitation source for the backscatter system.

[0060] Specifically, the processing method of the application includes the following parts:

[0061] (1) Obtain the reference signal from the excitation signal:

[0062] The average value of pilot sub-carriers in OFDM WiFi signal is used to estimate the 0th sub-carrier of uncontrolled excitation data packet as a reference signal for modulation of tag data.

[0063] The reference signal is selected because the inventors found that there is a constant amount in OFDM WiFi signal, which can be regarded as a kind of continuous wave. For example, the OFDM symbol of 802.11n signal with bandwidth of 20MHz contains 64 sub-carriers, 52 of which are used for data transmission, 4 are used for channel estimation, 1 is used as direct current, and 7 are used as guard band. Since the 4 pilot sub-carriers are known and fixed, a virtual sub-carrier can be created in the middle of the frequency band, which is exactly empty, and can be called 0th sub-carrier. Its phase and amplitude are estimated according to the pilot sub-carrier. The average value of the 4 pilot sub-carriers is used to estimate the 0th sub-carrier of uncontrolled excitation data packet, and the result is a 250kHz continuous wave signal. This virtual 0th sub-carrier is an ideal reference signal for modulation of tag data.

[0064] (2) Tag data modulation:

[0065] The present application modulates different tag data by applying different phase rotations to the carrier signal, i.e. using phase shift keying (PSK) modulation. Let denote the phase rotation applied by the backscattering tag to the i-th OFDM symbol, The value of is related to the i-th set of backscattering tag data. In the present application, binary Golay code and PSK modulation are combined. For a 2 m -PSK modulation encoding scheme (where m is a positive integer), There are 2 m values, and each value corresponds to a set of m-bit tag data. The encoding rule is as follows:

[0066]

[0067]

[0068] where n i is the result of converting the m-bit tag data of the i-th OFDM symbol from binary Golay code to decimal number, is the phase interval of phase shift keying modulation.

[0069] The conversion of Golay code and traditional binary, decimal encoding is shown in the following table:

[0070]

[0071] Gray code has a distinct advantage: the difference between two consecutive coded Gray codes has and only has 1 bit. For a high-order PSK modulation system, the bit error rate tends to rise with the increase of the order, but Gray code can minimize the bit error rate of PSK. It is assumed that most of the decoding mapping errors occur between two adjacent phases. Under the traditional binary coding, the decoding mapping error of adjacent phases is 2 m -PSK coding can introduce at most m bits of error. But if Gray code coding is used, it will only cause 1 bit error under the same condition.

[0072] The tag data adds a phase rotation to the backscattering signal by inputting a square wave signal with a phase offset to the radio frequency switch. The principle and specific implementation method have been proved in the existing work HitchHike, which will not be repeated here.

[0073] (3) Tag data demodulation:

[0074] As mentioned earlier, the core of demodulating tag data lies in estimating the phase of the 0th subcarrier. The wireless communication channel in the real scene always has various non-ideal factors, including channel imbalance, I / Q imbalance, carrier frequency offset (CFO), sampling time offset (STO), sampling frequency offset (SFO), etc. These non-ideal factors can not only introduce amplitude imbalance, interfere with the estimation of the phase of the 0th subcarrier, but also introduce phase errors, and mix with the tag modulation phase, so that the tag data decoding fails. The present application solves the above problems by using the following methods, including:

[0075] a) 0th subcarrier phase estimation

[0076] The present application uses a weighted least squares-based 0th subcarrier phase estimation. For the kth symbol, the following formula is used to perform weighted least squares estimation:

[0077]

[0078] where n p is the number of pilot subcarriers in the symbol, A i ,Φ i , x i is the amplitude, phase and subscript of the i th pilot subcarrier.

[0079] After solving the weighted least squares, β0 is the estimated phase of the 0th subcarrier, β1 is the slope of linear regression, which indicates how STO affects the pilot subcarrier.

[0080] b) Common Phase Error (CPE) separation:

[0081] CFO will introduce CPE into the symbol of the received signal, CPE not only entangles with the phase of the tag modulation, but also changes constantly between different symbols. The method of directly mapping the phase estimated from the 0th subcarrier to the tag data is only effective when the channel phase is small enough to be ignored.

[0082] To solve this problem, the present application processes in the way of iteratively eliminating CPE, specifically:

[0083] Let denote the phase of the 0th subcarrier of the i-th symbol, the tag phase and the CPE. Then Usually, is small and will slowly accumulate with the increase of i. In view of this, it is assumed that is very small, and when the positive integer l is very small, is also very small. The core idea of this way is to average the CPE of the i-1, i-2, …, i-l consecutive symbols as the CPE estimation value of the i-th symbol Use to estimate the tag phase of the i-th symbol, and then use to map the tag data, and then use the tag data to deduce Let d = M(θ) denote the tag data mapped by the phase θ, and θ = M -1 (d) denotes the phase corresponding to the data d as θ, then Finally, calculate Then iterate the next symbol.

[0084] c) Sampling recombination symbol

[0085] The inventor found that SFO will not only introduce phase error, but also cause symbol combination error. The occurrence of SFO is because the sampling frequency of the sending end and the sampling frequency of the receiving end are different. This difference will introduce additional STO, and will accumulate over time, resulting in insufficient or over-sampling of symbols. For example, for a WiFi receiver of 20MHz transmission, a 4us symbol should have 80 sampling points. After sampling multiple points at the receiving end, if the STO is greater than 50ns, the combined symbol will be lost or contain additional sampling points from adjacent symbols.

[0086] To solve this problem, the present application adopts symbol sampling and recombination symbol technology. Beta 1 is used as the recombination index. When the absolute value of beta 1 exceeds a preset threshold, a sampling point is discarded or copied according to the sign of beta 1. For each discarded or copied sampling point, beta 1 can be corrected Here, Q is the number of subcarriers in a symbol. The preset threshold is set to Thus, the case of dramatic changes in beta 1 is dealt with.

[0087] In summary, the tag data demodulation process of the present application is as follows. After the receiver receives the backscattered signal, the receiver performs frequency and time synchronization, estimates and eliminates part of the CFO and STO, and then performs WiFi channel estimation. Then, the receiver estimates the phase of the 0th subcarrier for each symbol using the least squares method and iteratively separates the tag phase from the CPE. In the iteration process, the OFDM symbol is recombined according to the estimated STO value. Finally, the estimated phase is mapped to the 0th subcarrier to demodulate the tag data.

[0088] To better reflect the method of the present application, first study the influence of different 0th subcarrier phase estimation methods on the tag data bit error rate. Different SFOs are added at the receiving end, and then the tag data bit error rate is measured. The experimental results are shown in Figure 2 In addition to the weighted linear regression proposed in the present application, five other methods are used, including simple average, weighted average, least squares, and cubic spline interpolation. Figure 2 The results show that the decoding effect of weighted linear regression is the best and is robust under different SFOs. Specifically, for different SFOs, the bit error rate of weighted regression is always less than 0.16%. In addition, when there is no SFO, the tag data bit error rate of weighted regression is 0.022%, which is 135.7, 5.4, 135.7, and 319.7 times lower than that of simple average, weighted average, linear regression, and cubic spline interpolation.

[0089] Next, the influence of CPE separation on tag data demodulation is studied. BPSK, QPSK, and 16-PSK modulated data are used, and the results are shown in Figure 3

[0090] Two conclusions are drawn from this. First, the CPE separation method of the present application significantly improves the demodulation accuracy of the tag data. When BPSK is used, the tag data bit error rate with CPE separation is 0.011%, while without CPE separation it increases to 4.2%, thus achieving a performance gain of 383 times. Second, the CPE separation effect of the present application decreases as the PSK order increases. This is similar to the high-order modulation of active radio. However, the tag data bit error rate is still less than 0.97% when 16-PSK is used, proving the effectiveness of the iterative CPE separation of the present application.​

[0091] In addition, to verify the impact of the sampling recombination symbol on the bit error rate of the tag data. Set different SFO, and then calculate the bit error rate with and without using the sampling recombination symbol, and the results are shown in Figure 4 It can be seen from the above that the sampling recombination symbol of the present application greatly reduces the bit error rate of the tag data. For example, when the SFO is -40 kHz, without using the sampling recombination, the bit error rate is as high as 36%, and after using it, it is reduced to 0.011%. In addition, when using the symbol recombination, the bit error rate is always lower than 0.024%.

[0092] Figure 5 The difference between the system used in the method of the present application and the existing system is embodied. Compared with the existing system which needs an environmental signal as a reference signal, the present system uses a virtual 0th subcarrier as a reference signal, and the system can directly demodulate the tag data from the backscatter symbol.

[0093] Figure 6 The advantage of using Gray code in PSK modulation is shown. The black dot is an example of a mapping error. When the adjacent constellation mapping is wrong, the conventional code can have at most 3-bit error, but the Gray code encoding only has 1-bit error.

[0094] Figure 7 The non-ideal signal receiving process and the error that can be introduced in each step are shown.

[0095] Embodiment 2

[0096] The WiFi receiver uses ZedBoard ZYNQ-7000 and AD-FMCOMMS3 for prototyping, and this development board and radio frequency front end can support 2x2 multiple input multiple output in the 40MHz frequency band. In this embodiment, GNU Radio is used to realize signal receiving and tag data demodulation, and all baseband signal processing algorithms, such as tag and environment data demodulation, are realized in real time on the development board.

[0097] The backscatter tag uses Xilinx ZYNQ-7010 FPGA, 25MHz crystal oscillator, modulator and energy detector. For high-order PSK modulation, the modulator is connected to a 16-to-1 multiplexer. The circuit impedance is modulated between 16 impedance states according to the mapped tag data. The impedance state is selected by the reflection coefficient , where Z a is the antenna impedance, and Z b is the complex circuit impedance. In this way, ultra-low power modulation can be achieved.

[0098] The above description is merely preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any changes or substitutions easily conceived by those skilled in the art within the technical scope disclosed by the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims. The information disclosed in the background section of the present application is merely intended to deepen the understanding of the general background of the present application, and should not be regarded as acknowledging or implying in any form that the information constitutes the prior art known to those skilled in the art.

Claims

1. A method for backscattering processing based on environmental OFDM WiFi, characterized in that, The application relates to a method for transmitting data by a backscatter tag, comprising the following steps: S1, using a standard wireless signal of an ambient OFDM WiFi device as an excitation signal of backscatter, and taking the ambient OFDM WiFi device as an excitation source of backscatter; S2, forming a backscatter signal by reflecting the excitation signal of the backscatter excitation source through the backscatter tag, and using a reference signal found from the backscatter signal to modulate tag data in a binary Golay code and phase shift keying combined mode for data transmission; In the step S2, the backscatter tag uses the reference signal found from the backscatter signal to modulate tag data in a binary Golay code and phase shift keying combined mode for data transmission in the following mode: Taking the average value of pilot subcarriers for channel estimation in the standard wireless signal of the OFDM WiFi device as a reference signal of the 0th subcarrier of uncontrolled excitation data packets; Different tag data modulated in a binary Golay code is obtained by applying different phase rotations to the reference signal; S3, receiving the backscatter signal of the backscatter tag through a backscatter receiver, performing frequency and time synchronization, estimating and eliminating partial carrier frequency offset and sampling time offset, then performing WiFi channel estimation, then separating the tag phase of the backscatter tag from common phase error in an iterative mode, and recombining OFDM symbols according to the estimated sampling time offset value in the iteration, finally mapping the estimated phase to a constellation diagram of phase shift keying modulation to demodulate the tag data; In the step S3, the tag phase of the backscatter tag is separated from common phase error in the following iterative mode, comprising: Step S31, find the common phase error estimation value of the first symbol : if , let ; if (α is a parameter), take the average of the common phase errors of the previous symbols as the estimation result, i.e. ; if , take the average of the common phase errors of the previous consecutive symbols as the estimation result, i.e. ; Step S32, using least square method to estimate the phase of 0th subcarrier of the th symbol and the slope of linear regression , judging to perform OFDM symbol recombination when the absolute value of the slope of linear regression exceeds a preset threshold value , wherein the threshold value is set as , wherein N is the number of subcarriers in the symbol Step S33, according to the first The phase of the 0th subcarrier of the symbol Tag phase and common phase error relation ,use Calculation yields the first Tag-modulated phase with separation of common phase error of each symbol ,according to Definite Phase modulation with tag Finding the mapping relationship The corresponding optimal This is converted into binary Gray code, which is the tag data for the symbol. Then, using... Definite With phase The correspondence relationship is derived to obtain the first The actual label phase of each symbol ; Step S34, the label phase of the first symbol is calculated by formula and updated to ;​​​ S35, judging whether all symbols are processed, if not, repeating the steps 31-34, and if yes, ending the processing.

2. The method of claim 1, wherein the method is based on an environmental OFDM WiFi backscatter process. Adopting - applying different phase rotations to the reference signal in a phase-shift keying modulation scheme to modulate different tag data into binary Golay codes, comprising: Each OFDM symbol modulates a set of tag data bits, is a positive integer, the phase rotation imposed by the tag data on the th OFDM symbol is denoted by , then is determined by ； wherein, is the number of OFDM symbols of the bit label data is converted from a binary Gray code to a decimal number, is the phase interval of the phase-shift keying modulation, - in the phase-shift keying modulation scheme .​ 3. The method of claim 1, wherein the method is based on an ambient OFDM WiFi backscatter process. In the step S32, the phase of the 0th subcarrier of the 1st symbol is estimated using the least square method with the following equation .​ ； wherein is the number of pilot subcarriers in the symbol; are the amplitude, phase and index of the th pilot subcarrier, respectively; is the estimated phase of the 0th subcarrier; is the slope of the linear regression, whose sign indicates the sign of the sampling time offset and whose absolute value reflects the extent of the sampling time offset.

4. The method of claim 1, wherein the method is based on an environmental OFDM WiFi backscatter process. In the step S32, the OFDM symbol recombination is performed in the following mode, comprising: if the slope of the linear regression is positive select discarding one sample point if the slope of the linear regression is positive select copying one sample point if the slope of the linear regression is negative For each discarded or copied sample point, the slope of the linear regression is corrected to .

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