CN116961679A - Dual-band WiFi backscattering system and implementation method - Google Patents
Dual-band WiFi backscattering system and implementation method Download PDFInfo
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Abstract
The application provides a dual-band WiFi backscattering system and an implementation method thereof, wherein the system comprises a dual-band ultra-low power modulator and a dual-band tag data decoder; the dual-band ultra-low power modulator adjusts the reflection coefficient of the radio frequency front end according to the load data of the tag, and the data is carried on an incident WiFi signal; the dual-band tag data decoder restores tag data by analyzing the reflected WiFi signals. According to the application, the reflection tag supports WiFi dual-band at the same time, so that higher throughput can be realized by using more excitation signals, and meanwhile, the reliability of reflection communication is improved by using the fact that almost no other equipment except WiFi works in the 5GHz band; meanwhile, the method has the characteristic of low power consumption of the backscattering system, so that communication of a large number of Internet of things equipment is guided, and support is provided for development of the Internet of things field.
Description
Technical Field
The application relates to the technical field of the Internet of things, in particular to a dual-band WiFi backscattering system and an implementation method.
Background
The internet of things has rapidly penetrated into various industries such as logistics warehouse, intelligent retail, intelligent manufacturing, etc. Backscatter communication technology is receiving widespread attention because of its passive reflection of incoming wireless signals with micro-watt power consumption, whereas conventional reflection communication-based RFID technology requires custom dedicated transceivers with high deployment costs. The WiFi backscattering can utilize widely deployed WiFi equipment to perform data transmission, so that the deployment difficulty can be effectively reduced.
Patent document CN115499106B discloses a tag data decoding method based on a codeword-converted WiFi backscatter system, which calculates the intensity of a reflected signal to infer the modulation type of a tag, so as to select a demodulation mode corresponding to the modulation type at a receiver. That is, the receiver demodulates the BPSK/QPSK signal by calculating the similarity of the reflected signal and the excitation signal, and demodulates the 16QAM signal using a differential summing or majority voting method. Patent document CN115833925a discloses a backscattering processing method based on environmental OFDM WiFi, which separates phase information of a tag from an accepted reflected signal in an iterative manner, so as to correctly demodulate tag data. Patent document CN116232443a discloses an ambient WiFi backscatter system and method based on a single commercial AP receiver, which uses a single AP to infer all possible tag data and excitation signal data from the reflected signal, after which the correct tag data and excitation signal data are selected from all data to be selected by a CRC check.
Although the mainstream WiFi protocols, such as 802.11n/ac/ax, support both 2.4GHz and 5GHz dual bands, current WiFi backscatter communication systems can only operate in the 2.4GHz band. On one hand, due to the fact that the frequency interval between the two frequency bands of the WiFi is too large, a proper commercial device is difficult to find to design the dual-frequency-band tag, and secondly, compared with the 5GHz frequency band, a 2.4GHz radio frequency circuit is easier to design. However, since the 2.4GHz band is very limited in bandwidth, the 2.4GHz WiFi throughput is limited, and many wireless devices such as bluetooth, zigBee, wireless keyboard and the like all work in the 2.4GHz band, so that the 2.4GHz WiFi noise is serious co-channel interference, so the current WiFi device generally works in the 5GHz band with cleaner frequency spectrum. This situation makes it impossible for a WiFi reflective tag that can only operate at 2.4GHz to obtain enough excitation signals to carry its own data, and at the same time, suffers serious co-channel interference, so that the communication quality cannot be guaranteed.
Disclosure of Invention
Aiming at the defects in the prior art, the application aims to provide a dual-band WiFi backscattering system and an implementation method.
The dual-band WiFi backscattering system comprises a dual-band ultralow power consumption modulator and a dual-band tag data decoder;
the dual-band ultra-low power modulator adjusts the reflection coefficient of the radio frequency front end according to the load data of the tag, and the data is carried on an incident WiFi signal;
the dual-band tag data decoder restores tag data by analyzing the reflected WiFi signals.
Preferably, the dual-band ultra-low power modulator changes the reflection coefficient by using a pHEMT transistor with impedance regulated by bias voltage, so that the dual-band ultra-low power modulator has the same constellation diagram for different working frequency bands.
Preferably, the pHEMT transistor has different bias voltage-impedance transformation relations under different working frequencies, and the dual-band ultra-low power modulator is switched to a corresponding bias voltage sequence according to the working frequency band;
the bias voltage sequence is obtained through measurement in advance, corresponding information of the bias voltage sequence is stored in a memory of the tag, and the bias voltage sequence is unchanged for a certain working frequency band;
the pre-measurement refers to analyzing an electric field model of a reflected signal with a receiver visual angle, and corresponding tag data components in the electric field model to a desired constellation diagram, so as to calculate bias voltage corresponding to a required reflection coefficient;
the receiver view refers to that the homodyne receiver filters static components in a received signal, and keeps dynamic components which change, wherein the dynamic components are caused by the reflection coefficient of the radio frequency front end of frequent switching of the tag.
Preferably, the dual-band tag data decoder searches for a suitable amplitude threshold and a suitable phase threshold after receiving the reflected signal, and demodulates the PAM symbol modulated by the tag.
Preferably, the selection of the amplitude threshold depends on the reflective tag not changing the WiFi preamble so that the high energy preamble has a fixed energy difference from the reflected PAM symbol, and the receiver counts the energy of the preamble to obtain the appropriate amplitude threshold.
Preferably, the fixed energy difference means that the energy of the signal at the preamble is the same as the symbols 0, 3 of the 4-PAM modulation, but 9.5dB higher than the symbols 1, 2.
Preferably, the phase threshold value is acquired by setting pi/2 after eliminating the phase error in the received WiFi signal;
the phase error is eliminated by embedding a training sequence into data to be transmitted through a tag, and a receiver eliminates the phase error by observing the phase of the training sequence.
The implementation method of the dual-band WiFi backscattering system provided by the application comprises the following steps:
step S1: mapping bit streams to be transmitted on a tag into 4-PAM symbols, and selecting bias voltages corresponding to PAM according to the working frequency band of the current tag, so as to complete modulation of data;
step S2: and after the WiFi receiver detects the WiFi signal, the average energy of each OFDM symbol is counted, and the phase of the pilot frequency is calculated and used for demodulating the tag data.
Preferably, the step S1 includes:
step S1.1: measuring reflection parameters of the pHEMT under different bias voltages and different working frequency bands by using a vector network analyzer;
step S1.2: establishing a reflected signal model and separating out dynamic components in the reflected signal;
step S1.3: mapping the 4-PAM constellation point track with a dynamic component track, and selecting a reflection coefficient sequence corresponding to the 4-PAM constellation point on the dynamic component track;
step S1.4: finding out a bias voltage sequence corresponding to the reflection coefficient sequence obtained in the step S1.3 in the reflection signal model, and storing the bias voltage sequence to a memory of the tag;
step S1.5: the tag maps the data to be transmitted into 4-PAM symbols, and selects corresponding bias voltage to be applied to the pHEMT transistor according to the working frequency band;
step S1.6: the tag needs to shift the reflected signal to another separate WiFi channel.
Preferably, the step S2 includes:
step S2.1: after the WiFi receiver detects the reflected WiFi signal according to the short-segment training sequence in the WiFi frame, the frequency offset of the received signal is corrected by using the short-segment training sequence;
step S2.2: the WiFi receiver extracts long training in the received signal to perform channel estimation, and the estimated channel information is used for equalizing the channel;
step S2.3: the WiFi receiver counts the energy of the preamble and calculates an amplitude threshold;
step S2.4: the WiFi receiver estimates the phase errors of the first 16 OFDM symbols of the WiFi frame by using a training sequence embedded by a tag, and obtains the change rate of the phase errors along with time by using a linear fitting mode so as to correct the phase errors of all subsequent OFDM symbols;
step S2.5: the WiFi receiver calculates the average phase of the pilot frequency in each OFDM symbol;
step S2.6: the decoder demodulates the PAM symbols with pi/2 as the phase threshold and using the calculated amplitude threshold.
Compared with the prior art, the application has the following beneficial effects:
according to the method, wiFi backscatter communication can be enabled, the excitation signals of WiFi dual frequency bands are simultaneously utilized, communication reliability is improved, throughput is increased, communication of a large number of Internet of things devices is guided, and support is provided for development of the Internet of things field.
Drawings
Other features, objects and advantages of the present application will become more apparent upon reading of the detailed description of non-limiting embodiments, given with reference to the accompanying drawings in which:
FIG. 1 is a flowchart of the operation of a dual band WiFi backscatter system of the present application;
FIG. 2 is a modulation and frequency shift principle;
FIG. 3 is a graph showing the effect of reducing the bit error rate;
fig. 4 is a graph of the effect of increasing throughput.
Detailed Description
The present application will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the present application, but are not intended to limit the application in any way. It should be noted that variations and modifications could be made by those skilled in the art without departing from the inventive concept. These are all within the scope of the present application.
Example 1:
the dual-band WiFi backscattering system comprises a dual-band ultralow power consumption modulator and a dual-band tag data decoder;
the dual-band ultra-low power modulator adjusts the reflection coefficient of the radio frequency front end according to the load data of the tag, and the data is carried on an incident WiFi signal;
the dual-band tag data decoder restores tag data by analyzing the reflected WiFi signals.
The dual-band ultra-low power consumption modulator changes the reflection coefficient by using a pHEMT transistor with the impedance regulated by bias voltage, so that the dual-band ultra-low power consumption modulator has the same constellation diagram for different working frequency bands;
the pHEMT transistor has different bias voltage-impedance transformation relations under different working frequencies, and the dual-band ultra-low power consumption modulator is switched to a corresponding bias voltage sequence according to the working frequency band;
the bias voltage sequence is obtained through the prior measurement, corresponding information of the bias voltage sequence is stored in a memory of the tag, and the bias voltage sequence is unchanged for a certain working frequency band;
the pre-measurement refers to analyzing an electric field model of the reflected signal with a receiver viewing angle, and corresponding tag data components in the electric field model to a desired constellation diagram, so as to calculate a bias voltage corresponding to a required reflection coefficient.
The receiver view angle means that the homodyne receiver can filter static components in a received signal, and keep changing dynamic components, wherein the dynamic components are caused by the reflection coefficient of the radio frequency front end of frequent switching of the tag.
The dual-band tag data decoder can find a proper amplitude threshold and a proper phase threshold of the received reflected signal to demodulate the PAM symbol modulated by the tag.
The selection of the amplitude threshold depends on the reflection tag not changing the preamble of the WiFi, so that the high-energy preamble and the reflected PAM symbol have a fixed energy difference, and the receiver counts the energy of the preamble to obtain a proper amplitude threshold.
The fixed energy difference means that the energy of the signal at the preamble is the same as the symbols 0, 3 of the 4-PAM modulation, but 9.5dB higher than the symbols 1, 2.
The phase threshold is obtained by setting pi/2 after eliminating the phase error in the received WiFi signal.
The phase error is eliminated by embedding a certain amount of training sequence into the data to be transmitted through the tag, and the receiver eliminates the phase error by observing the phase of the training sequence.
As shown in fig. 1, the specific implementation flow includes:
step S1: mapping bit streams to be transmitted on a tag into 4-PAM symbols, and selecting bias voltages corresponding to PAM according to the working frequency band of the current tag, so as to complete modulation of data;
step S2: and after the WiFi receiver detects the WiFi signal, the average energy of each OFDM symbol is counted, and the phase of the pilot frequency is calculated and used for demodulating the tag data.
Specifically, the step S1 includes:
step S1.1: measuring reflection parameters of the pHEMT under different bias voltages and different working frequency bands by using a vector network analyzer;
step S1.2: establishing a reflected signal model and separating out dynamic components in the reflected signal;
step S1.3: mapping the 4-PAM constellation point track with a dynamic component track, and selecting a reflection coefficient sequence corresponding to the 4-PAM constellation point on the dynamic component track;
step S1.4: finding out a bias voltage sequence corresponding to the reflection coefficient sequence obtained in the step S1.3 in the reflection signal model, and storing the bias voltage sequence to a memory of the tag;
step S1.5: the tag maps the data to be transmitted into 4-PAM symbols, and selects corresponding bias voltage to be applied to the pHEMT transistor according to the working frequency band;
step S1.6: to avoid interference with the excitation signal, the tag shifts the reflected signal to another separate WiFi channel.
Specifically, the step S2 includes:
step S2.1: after the WiFi receiver detects the reflected WiFi signal according to the short-segment training sequence in the WiFi frame, the frequency offset of the received signal is corrected by using the short-segment training sequence;
step S2.2: the WiFi receiver extracts long training in the received signal to perform channel estimation, and the estimated channel information is used for equalizing the channel;
step S2.3: the WiFi receiver counts the energy of the preamble and calculates an amplitude threshold;
step S2.4: the WiFi receiver estimates the phase errors of the first 16 OFDM symbols of the WiFi frame by using a training sequence embedded by a tag, and obtains the change rate of the phase errors along with time by using a linear fitting mode so as to correct the phase errors of all subsequent OFDM symbols;
step S2.5: the WiFi receiver calculates the average phase of the pilot frequency in each OFDM symbol;
step S2.6: the decoder demodulates the PAM symbols with pi/2 as the phase threshold and using the calculated amplitude threshold.
Example 2:
example 2 is a preferred example of example 1.
WiFi backscatter requires the tag to shift the reflected signal to another channel independent of the excitation signal. To avoid self-interference of both. The process of the tag encoding and frequency shifting the excitation signal is as follows:
step S1: assume that four symbols of 4-PAM are s respectively 1 ,s 2 ,s 3 ,s 4 The corresponding bias voltages are v 1 ,v 2 ,v 3 ,v 4 . Wherein s is 1 Sum s 4 The energy of (2) is greater than s 2 Sum s 3 ；s 1 Sum s 2 The corresponding symbols have the same phase and are identical to s 3 Sum s 4 And (3) inverting.
Step S2: generating a control waveform having a variable frequency f s Is a square wave of (c).
Specifically, the step S2 includes
Step S2.1: the frequency shift is set to f s When the tag transmits the symbol s 1 When the label generates an initial level v 1 Level v after jump 4 Frequency f s The square wave of (2) controls the pHEMT transistor.
Step S2.2: the frequency shift is set to f s When the tag transmits the symbol s 2 When the label generates an initial level v 2 Level v after jump 3 Frequency f s The square wave of (2) controls the pHEMT transistor.
Step S2.3: the frequency shift is set to f s When the tag transmits the symbol s 3 When the label generates an initial level v 3 Level v after jump 2 Frequency f s The square wave of (2) controls the pHEMT transistor.
Step S2.4: the frequency shift is set to f s When the tag transmits the symbol s 4 When the label generates an initial level v 4 Level v after jump 1 Frequency f s The square wave of (2) controls the pHEMT transistor.
This modulation process is shown in fig. 2, and one skilled in the art can understand this embodiment as a more specific illustration of the modulator behavior in embodiment 1.
Example 3:
example 3 is a preferred example of example 1.
The double-frequency-band WiFi backscattering system can utilize 5GHz frequency band to possess cleaner frequency spectrum and realize reliable reflection communication, utilizes the WiFi equipment to improve throughput in the data volume of the transmission of 5GHz frequency band far exceeds the data volume of 2.4GHz frequency band transmission in certain time simultaneously. The specific implementation flow comprises the following steps:
step S1: the reflective tag is configured to operate in the 2.4GHz band for 1 mm and in the 5GHz band for the remaining 9 milliseconds with a period of 10 milliseconds.
Step S2: the tag modulates its own load data using the modulator and piggybacks the data to the WiFi signal in the environment.
Step S3: the WiFi receiver demodulates the tag data according to the calculated phase threshold value and the calculated amplitude threshold value, and bit error rate and throughput results obtained at different distances are shown in figures 3 and 4.
In fig. 3, the error rate of the dual-band WiFi backscatter system of the present application is 7 to 10 times lower than that of the conventional single-band WiFi backscatter system. The dual-band WiFi backscatter system depicted in fig. 4 has a 8 to 11 times higher throughput than the conventional single-band WiFi backscatter system.
Those skilled in the art will appreciate that the systems, apparatus, and their respective modules provided herein may be implemented entirely by logic programming of method steps such that the systems, apparatus, and their respective modules are implemented as logic gates, switches, application specific integrated circuits, programmable logic controllers, embedded microcontrollers, etc., in addition to the systems, apparatus, and their respective modules being implemented as pure computer readable program code. Therefore, the system, the apparatus, and the respective modules thereof provided by the present application may be regarded as one hardware component, and the modules included therein for implementing various programs may also be regarded as structures within the hardware component; modules for implementing various functions may also be regarded as being either software programs for implementing the methods or structures within hardware components.
The foregoing describes specific embodiments of the present application. It is to be understood that the application is not limited to the particular embodiments described above, and that various changes or modifications may be made by those skilled in the art within the scope of the appended claims without affecting the spirit of the application. The embodiments of the application and the features of the embodiments may be combined with each other arbitrarily without conflict.
Claims (10)
1. The dual-band WiFi backscattering system is characterized by comprising a dual-band ultralow-power modulator and a dual-band tag data decoder;
the dual-band ultra-low power modulator adjusts the reflection coefficient of the radio frequency front end according to the load data of the tag, and the data is carried on an incident WiFi signal;
the dual-band tag data decoder restores tag data by analyzing the reflected WiFi signals.
2. The dual band WiFi backscatter system of claim 1 wherein the dual band ultra low power modulator varies the reflection coefficient by using pHEMT transistors whose impedance is regulated by a bias voltage to achieve the same constellation for different operating bands.
3. The dual-band WiFi backscatter system of claim 2 wherein the pHEMT transistors have different bias voltage-impedance transformation relationships at different operating frequencies, the dual-band ultra-low power modulator switching to a corresponding bias voltage sequence according to the operating frequency band;
the bias voltage sequence is obtained through measurement in advance, corresponding information of the bias voltage sequence is stored in a memory of the tag, and the bias voltage sequence is unchanged for a certain working frequency band;
the pre-measurement refers to analyzing an electric field model of a reflected signal with a receiver visual angle, and corresponding tag data components in the electric field model to a desired constellation diagram, so as to calculate bias voltage corresponding to a required reflection coefficient;
the receiver view refers to that the homodyne receiver filters static components in a received signal, and keeps dynamic components which change, wherein the dynamic components are caused by the reflection coefficient of the radio frequency front end of frequent switching of the tag.
4. The dual-band WiFi backscatter system of claim 1 wherein the dual-band tag data decoder, upon receiving the reflected signal, looks for suitable amplitude and phase thresholds to demodulate the PAM symbol after tag modulation.
5. The dual-band WiFi backscatter system of claim 4 wherein the amplitude threshold is selected in dependence on the reflective tag not changing the WiFi preamble such that the high energy preamble has a fixed energy difference from the reflected PAM symbol and the receiver counts the energy of the preamble to obtain the appropriate amplitude threshold.
6. The dual band WiFi backscatter system of claim 5 wherein the fixed energy difference refers to the energy of the signal at the preamble being the same as the sign 0, 3 of the 4-PAM modulation and 9.5dB higher than the sign 1, 2.
7. The dual-band WiFi backscatter system of claim 6, wherein the acquisition of the phase threshold is set to pi/2 after eliminating phase errors in the received WiFi signal;
the phase error is eliminated by embedding a training sequence into data to be transmitted through a tag, and a receiver eliminates the phase error by observing the phase of the training sequence.
8. A method for implementing a dual-band WiFi backscatter system according to any one of claims 1 to 7, comprising:
step S1: mapping bit streams to be transmitted on a tag into 4-PAM symbols, and selecting bias voltages corresponding to PAM according to the working frequency band of the current tag, so as to complete modulation of data;
step S2: and after the WiFi receiver detects the WiFi signal, the average energy of each OFDM symbol is counted, and the phase of the pilot frequency is calculated and used for demodulating the tag data.
9. The method according to claim 8, wherein the step S1 includes:
step S1.1: measuring reflection parameters of the pHEMT under different bias voltages and different working frequency bands by using a vector network analyzer;
step S1.2: establishing a reflected signal model and separating out dynamic components in the reflected signal;
step S1.3: mapping the 4-PAM constellation point track with a dynamic component track, and selecting a reflection coefficient sequence corresponding to the 4-PAM constellation point on the dynamic component track;
step S1.4: finding out a bias voltage sequence corresponding to the reflection coefficient sequence obtained in the step S1.3 in the reflection signal model, and storing the bias voltage sequence to a memory of the tag;
step S1.5: the tag maps the data to be transmitted into 4-PAM symbols, and selects corresponding bias voltage to be applied to the pHEMT transistor according to the working frequency band;
step S1.6: the tag needs to shift the reflected signal to another separate WiFi channel.
10. The method according to claim 8, wherein the step S2 includes:
step S2.1: after the WiFi receiver detects the reflected WiFi signal according to the short-segment training sequence in the WiFi frame, the frequency offset of the received signal is corrected by using the short-segment training sequence;
step S2.2: the WiFi receiver extracts long training in the received signal to perform channel estimation, and the estimated channel information is used for equalizing the channel;
step S2.3: the WiFi receiver counts the energy of the preamble and calculates an amplitude threshold;
step S2.4: the WiFi receiver estimates the phase errors of the first 16 OFDM symbols of the WiFi frame by using a training sequence embedded by a tag, and obtains the change rate of the phase errors along with time by using a linear fitting mode so as to correct the phase errors of all subsequent OFDM symbols;
step S2.5: the WiFi receiver calculates the average phase of the pilot frequency in each OFDM symbol; step S2.6: the decoder demodulates the PAM symbols with pi/2 as the phase threshold and using the calculated amplitude threshold.
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