CN114006799B - Passive RFID-oriented spread spectrum and broadband perception enhancement method and system - Google Patents

Passive RFID-oriented spread spectrum and broadband perception enhancement method and system Download PDF

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CN114006799B
CN114006799B CN202111277431.9A CN202111277431A CN114006799B CN 114006799 B CN114006799 B CN 114006799B CN 202111277431 A CN202111277431 A CN 202111277431A CN 114006799 B CN114006799 B CN 114006799B
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rfid
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CN114006799A (en
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赵衰
李镇江
丁菡
王鸽
惠维
赵季中
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Xian Jiaotong University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract

The invention discloses a passive RFID-oriented spread spectrum and broadband perception enhancement method and system, which customize RFID continuous waves by utilizing an orthogonal frequency division multiplexing technology and increase information of perception signal frequency dimension; the time resolution of the sensing signal is improved by optimizing and expanding the bandwidth of the hardware characteristic, the RFID protocol and the communication parameter; two feature extraction algorithms of frequency priority and time resolution priority are adopted, so that the perception features are suitable for different scenes and target requirements. The invention only needs a single set of RFID equipment, is compatible with the RFID protocol, accords with the spectrum allocation regulation, can provide high-granularity (multidimensional, high-speed) sensing characteristics, has the characteristics of universality, light weight, low cost, low expenditure and the like, and has better application value, research prospect and development potential.

Description

Passive RFID-oriented spread spectrum and broadband perception enhancement method and system
Technical Field
The invention belongs to the technical field of Radio Frequency Identification (RFID), and particularly relates to a passive RFID-oriented spread spectrum and broadband perception enhancement method and system.
Background
In recent decades, sensing systems based on passive RFID have received much attention from research teams and manufacturers at home and abroad in terms of motion recognition, material quality detection, target location, and the like. However, the sensing features extracted by most current RFID sensing operations are one-dimensional (spatial or frequency), low-speed (40-200 times per second) metrics, such as phase and strength information of the physical layer of the reply signal of the RFID tag. Such features do not support more fine-grained perceptual requirements, such as diluted/counterfeit/outdated food with subtle differences, fast actions by the user during non-contact human-computer interaction, etc.
In order to improve the RFID perception granularity, a representative solution is to use a plurality of labels (label arrays) to simultaneously perceive the target, so as to improve the spatial dimension information of the perception feature; another solution is to use frequency hopping communication to improve the frequency dimension information of the sensing features.
However, communication collisions between multiple tags and a delay in frequency switching of the RFID hopping pattern further reduce their sensing speed, and the use of multiple tags increases cost investment. Recent research suggests that in the RFID communication process, sensing granularity can be greatly improved by using additional devices to receive and transmit ultra-wideband signals (500-1000 MHz), but the cost of such ultra-wideband devices and the synchronization overhead between two signals are still high, and popularization is difficult.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a method and a system for enhancing passive RFID-oriented spread spectrum and broadband sensing, which are based on a single set of RFID devices, are compatible with RFID protocols, meet the radio frequency spectrum allocation regulations, and can provide a novel RFID sensing technology with high granularity (multidimensional, high-speed) sensing characteristics.
The invention adopts the following technical scheme:
a spread spectrum and broadband perception enhancement method facing to passive RFID comprises the following steps:
s1, designing a generating mechanism of an OFDM symbol, carrying out amplitude control on the generated OFDM symbol and carrying out linear superposition on the OFDM symbol and an RFID continuous wave to form a remodulated continuous wave, transmitting the corresponding remodulated continuous wave during RFID communication, wherein a receiving signal comprises a plurality of orthogonal frequency components, and realizing a spread spectrum mechanism compatible with an RFID protocol;
s2, expanding the communication bandwidth, and reconfiguring the RFID communication parameters through an optimization algorithm, wherein the reconfiguration comprises a maximum value of the bandwidth, the range of the OFDM symbol length in the RFID protocol spread spectrum mechanism realized in the step S1 and other RFID baseband communication parameters;
and S3, according to the RFID spread spectrum and broadband transmission mechanism obtained in the step S2, the RFID equipment is utilized to receive and transmit the broadband signal spread spectrum in the step S2, a receiving end replies a high level and a low level of the signal from the tag, the high level corresponds to an on state, the low level corresponds to an off state, an OFDM symbol is extracted and channel frequency response is calculated, and a characteristic calculation mode with frequency priority and resolution priority is adopted for the channel frequency response aiming at static and dynamic targets, so that the perception enhancement of frequency dimension and time resolution dimension is realized.
Specifically, in step S1, the OFDM symbol generation mechanism is as follows:
grouping pseudo-random binary sequences, each group containing log 2 n binary bits, where n is the order of phase shift keying; mapping each binary bit group to a constellation diagram by adopting nPSK (binary phase Shift keying) to obtain a complex value containing a real part and an imaginary part; performing serial-to-parallel conversion on the N serial complex numbers to obtain N parallel complex numbers; performing a quasi-Fourier transform on the N parallel complex numbers to obtain N parallel complex numbers after the transform; and performing parallel-to-serial conversion on the converted N parallel complex numbers to obtain N serial complex numbers as one OFDM symbol.
Further, at least one complete OFDM symbol exists for each on and off state duration of the RFID tag reply signal:
Figure GDA0003745009410000021
wherein B is bandwidth, mu is label reply bit identification, M is base band signal sampling point number in label circuit state duration, f BLF Is the backscatter link frequency.
Specifically, in step S1, emitting the modulated continuous wave S in a manner of combining an amplitude-controlled linear amplification OFDM symbol, a linear superposition OFDM symbol, and an original continuous wave is:
s＝α×s OFDM +s CW
wherein s is OFDM For the generated OFDM symbol, s CW Alpha is a linear amplification factor for the original continuous wave of the RFID.
Further, the modulated continuous wave s satisfies orthogonality among subcarriers, which is specifically:
Figure GDA0003745009410000031
wherein, d i Is the data carried by the ith subcarrier cos (i ω t), and ω is the subcarrier center angular frequency.
Specifically, in step S2, a target bandwidth B and other communication parameters are obtained by searching and matching through an optimization equation, and then the number of subcarriers is determined by observing the influence of subcarrier spacing on the sensing effect through experiments.
Further, the bandwidth B is determined by searching and matching through the optimization equation as follows:
Figure GDA0003745009410000032
wherein, B u R is the antenna digital-to-analog conversion rate, theta, for the maximum allowable bandwidth of the RFID RFID As communication parameters, N + Is a positive integer.
Specifically, in step S3, a least square algorithm is adopted to calculate a channel frequency response of the received signal to obtain a sensing characteristic, and for a static sensing target, channel frequency responses corresponding to all subcarriers of the OFDM symbols included in the high and low levels of the RFID tag reply signal are respectively calculated, and a difference between the two is calculated as the static sensing characteristic; aiming at a dynamic sensing target, calculating channel frequency response corresponding to each subcarrier of OFDM symbols contained in the high level or the low level of a reply signal of the RFID tag, arranging the channel frequency response according to a time domain sequence to obtain a dynamic sensing characteristic, and improving the time resolution of the sensing characteristic by depending on high bandwidth.
Further, static sensing features
Figure GDA0003745009410000033
Comprises the following steps:
Figure GDA0003745009410000034
wherein the content of the first and second substances,
Figure GDA0003745009410000035
is an N-dimensional vector, N is the number of subcarriers, q ON And q is OFF Respectively the number of high and low levels in the EPC signal replied by the RFID label at one time,
Figure GDA0003745009410000041
and
Figure GDA0003745009410000042
respectively calculating the channel frequency responses in high and low levels;
dynamic perceptual features
Figure GDA0003745009410000043
Comprises the following steps:
Figure GDA0003745009410000044
wherein, the first and the second end of the pipe are connected with each other,
Figure GDA0003745009410000045
the matrix is N multiplied by T, N is the number of subcarriers, and T is the number of high or low levels in the EPC signal replied by the RFID tag at one time in the time domain sequence.
Another technical solution of the present invention is a passive RFID-oriented spread spectrum and broadband perception enhancement system, including:
the spread spectrum module is used for designing a generation mechanism of the OFDM symbol, carrying out amplitude control on the generated OFDM symbol and carrying out linear superposition on the OFDM symbol and the RFID continuous wave to form a remodulated continuous wave, transmitting the remodulated continuous wave corresponding to RFID communication, and realizing a spread spectrum mechanism compatible with an RFID protocol, wherein a received signal contains a plurality of orthogonal frequency components;
the transmission module expands the communication bandwidth, reconfigures RFID communication parameters through an optimization algorithm, and comprises a maximum available value of the bandwidth, the range of OFDM symbol length in an RFID protocol spread spectrum mechanism of the spread spectrum module and other RFID baseband communication parameters, wherein if the RFID communication meets the parameter configuration, a radio frequency signal contains a plurality of orthogonal frequency components in a larger bandwidth range, so that the RFID spread spectrum and broadband transmission mechanism is realized;
and the enhancement module is used for receiving and transmitting the broadband signal spread by the enhancement module by using the RFID equipment according to an RFID spread spectrum and broadband transmission mechanism realized by the transmission module, replying the high level and the low level of the signal from the tag at a receiving end, wherein the high level corresponds to an on state and the low level corresponds to an off state, extracting an OFDM symbol and calculating channel frequency response, and aiming at static and dynamic targets, adopting a characteristic calculation mode with frequency priority and resolution priority for the channel frequency response to realize the perception enhancement of frequency dimension and time resolution dimension.
Compared with the prior art, the invention has at least the following beneficial effects:
the spread spectrum and broadband perception enhancement method facing the passive RFID can break through the dimensionality and rate limitation of the extracted features of the existing RFID perception system, is realized by depending on a single set of RFID equipment, does not need to introduce additional transceiving equipment, is compatible with an RFID protocol, does not influence the normal communication and original functions of the RFID, and does not exceed the allowed communication bandwidth of the RFID in industrial, scientific and medical frequency bands (ISM bands). The invention can provide the sensing characteristics of multiple frequencies (mutual independence) and high bandwidth (high time resolution), and is suitable for the practical application fields of non-invasive material detection, non-contact human-computer interaction, intelligent manufacturing and the like.
Furthermore, the OFDM symbol generated by the orthogonal frequency division multiplexing technology can enable the radio frequency signal to simultaneously contain a plurality of orthogonal frequency components on a specific bandwidth, compared with the traditional RFID sensing method that the signal strength or phase information is obtained from a single frequency for sensing, the method provided by the invention has the advantage that the frequency dimension information of the sensing signal is richer by calculating the channel frequency response of each frequency component of the signal. The invention chooses to enhance the perception based on the RFID, because the RFID is a low-energy-consumption communication technology with high popularization, wide deployment and low cost, and has strong practicability by providing more enhanced (compared with the prior art) perception capability on the basis of not influencing the normal work of the RFID. The RFID label reply signal adopts an On-Off Keying (OOK) modulation mode, the OFDM technology adopts a Phase Shift Keying (PSK) modulation mode, the two modulation modes are mutually independent, and no mutual influence exists during demodulation, so the invention also provides an additional data transmission capability for the RFID system.
Furthermore, the RFID tag reply signal adopts an on-off keying modulation mode, namely, the tag circuit has two states of 'on' and 'off', if the state switching is in a complete OFDM symbol and linear superposition period, larger signal fading can be generated to influence the sensing characteristic, so that when the length of the OFDM symbol is less than one half of the length of the 'on' and 'off' state, a complete OFDM symbol can be ensured to exist in each 'on' and 'off' state period, and the sensing characteristic extracted from the OFDM symbol is more robust.
Furthermore, the original continuous wave of the RFID is directly replaced by the generated OFDM symbol, so that the RFID tag cannot be activated to reply due to insufficient signal energy; if the original continuous wave of the RFID is replaced by the OFDM symbol subjected to linear amplification, when the energy meets the condition of activating the tag to reply, the waveform is noisy due to the overlarge linear amplification coefficient, and the reader-writer cannot correctly decode the data replied by the tag. Therefore, the modulated continuous wave s is transmitted in a mode of combining amplitude control (linear amplification of OFDM symbols) and linear superposition (OFDM symbols and original continuous waves), so that not only is the spread spectrum sensing of multi-frequency components realized, but also the normal communication of the RFID is not influenced.
Furthermore, the modulated integral is set through calculating the content supplementary explanation of the product of the target operator carrier and the received OFDM signal, and the fact that each subcarrier can be correctly separated is found, so that the purpose or the benefit of orthogonality among the subcarriers contained in each wave of the continuous wave s subjected to remodulation is still met, principle analysis is given, mutual interference and influence do not exist, the sensing characteristics extracted from each subcarrier (frequency component) contain independent sensing information, and the effectiveness of the spread spectrum sensing provided by the invention is proved.
Further, step S1 realizes the spread spectrum of the existing RFID system, but actually the RFID bandwidth is narrow (1 to 2 MHz), and if the communication bandwidth (2 to 26 MHz) allowed by the RFID in the industrial, scientific and medical band (ISM band) can be fully utilized to realize the RFID spread spectrum broadband transmission, the frequency dimension of the sensing feature extracted by the invention is wider and the time resolution is higher, but the communication parameter of the RFID is directly affected by the bandwidth conversion, so the reconfiguration needs to be performed according to the hardware characteristic and the protocol. The parameter configuration result determines a target bandwidth and other baseband communication parameters, but the number of subcarriers of an OFDM symbol determines a range, and experimental observation shows that after the number of subcarriers in a specific bandwidth is increased to a certain value, the gain caused by sensing is no longer obvious, and the calculation overhead is continuously increased, so that the specific number of subcarriers needs to be determined according to a sensing scene and experimental experience.
Furthermore, the target bandwidth and the baseband communication parameters are determined by searching and matching an optimization equation, and compared with the trial efficiency through repeated experiments, the method has higher trial efficiency, wherein the upper limit of the bandwidth has a definite reference basis in each country and region, namely the industrial, scientific and medical frequency band (ISM band); the selectable bandwidth in the range is determined by the hardware characteristics of equipment, namely when an extraction and interpolation operation factor D (the ratio between the digital-to-analog conversion rate of the antenna and the target bandwidth) is an integral power of 2 during signal receiving and transmitting, the signal cannot influence the perception effect due to the roll-off problem; in the selectable bandwidth, the RFID baseband communication parameters at least must ensure that the number of baseband sampling points of each command in the protocol is a positive integer, otherwise, the RFID cannot perform stable communication.
Furthermore, as the sending signal, namely the re-modulated continuous wave is a known complex sequence, the receiving signal is directly obtained by the RFID equipment, symbol synchronization can be realized through autocorrelation to obtain each complete OFDM symbol, the change between the receiving complex sequence and the receiving complex sequence is calculated one by one through a least square method, namely the channel frequency response, and the amplitude and phase information of each frequency component is included as the initial sensing characteristic. The RFID system adopts a self-sending and self-receiving communication mode, and the received signals do not need to be subjected to frequency offset estimation and correction, so that the calculation of the channel frequency response is more robust.
Further, since the invention does not affect the normal communication of the RFID, the bit sequence of the tag reply signal can be correctly obtained, so that the complete OFDM contained during the "high" and "low" levels, i.e. the "on" and "off" states, can be obtained and the channel frequency response calculated, respectively. When the high level of the signal is replied by the tag, the tag and the sensing target generate a coupling effect, and meanwhile, the signal is transmitted through the sensing environment, so that the high level signal comprises the sensing target and the environment information; when the tag replies the low level of the signal, the tag is in a non-response state, and the signal propagation also passes through the sensing environment, so that the low level signal only contains the environment information and aims at the static sensing target. Therefore, for a static sensing target, calculating the difference value of the high-level and low-level channel frequency responses as the static sensing characteristic F _ sta can remove the environmental influence from the sensing characteristic and retain the information about the sensing target; aiming at the dynamic sensing target, only the channel frequency response of OFDM symbols contained in the high level or the low level of the reply signal of the RFID tag is calculated, and the dynamic sensing characteristic F _ dyn is obtained by arranging according to the time domain sequence, so the time resolution of the sensing characteristic is improved by the high bandwidth. The invention provides a platform type technology which can sense whether a signal bears data, the number of frequency components (subcarriers), the setting of actual bandwidth, the calculation and optimization of characteristics and the like, and can further adjust or process according to actual application scenes and requirements. The invention can provide multi-frequency high-speed perception characteristics aiming at a single RFID label, and is also suitable for a multi-label (label array) scene, namely, the perception characteristics are enhanced from the space and frequency dimensions at the same time.
In conclusion, the passive RFID-oriented spread spectrum and broadband perception enhancement technology is realized by modulating the RFID continuous wave, expanding the communication bandwidth and extracting the perception characteristics, the perception granularity (frequency dimension and time resolution) of the conventional RFID perception system is greatly improved (2 orders of magnitude), and the passive RFID-oriented spread spectrum and broadband perception enhancement technology can be applied to two perception targets, namely a dynamic perception target and a static perception target and has good application value, research prospect and development potential.
The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a diagram of a basic RFID communication process;
FIG. 3 is a diagram of a reply signal of a tag when an RFID reader transmits different continuous carriers;
FIG. 4 is a diagram of the signal roll-off problem caused by the integrating comb filter;
FIG. 5 is a graph of the frequency spectrum of a signal when original and modulated continuous waves are transmitted;
FIG. 6 is a graph illustrating the effect of determining the number of different sub-carriers on perceptual features over a bandwidth;
FIG. 7 is a graph comparing the accuracy of the present invention and prior art RFID sensing in a fluid identification task;
FIG. 8 is a graph of the accuracy of the present invention versus prior art RFID sensing in a gesture recognition task;
FIG. 9 is a graph of the temporal overhead of two perceptual feature calculations of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it should be understood that the terms "comprises" and/or "comprising" indicate the presence of the described features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
It is also to be understood that the terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the specification of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
It should be further understood that the term "and/or" as used in this specification and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.
Various structural schematics according to the disclosed embodiments of the invention are shown in the drawings. The figures are not drawn to scale, wherein certain details are exaggerated and some details may be omitted for clarity of presentation. The shapes of various regions, layers and their relative sizes and positional relationships shown in the drawings are merely exemplary, and deviations may occur in practice due to manufacturing tolerances or technical limitations, and a person skilled in the art may additionally design regions/layers having different shapes, sizes, relative positions, according to actual needs.
The invention provides a passive RFID-oriented spread spectrum and broadband perception enhancement method, which is characterized in that an orthogonal frequency division multiplexing technology is utilized to customize RFID continuous waves and increase information of perception signal frequency dimension; the time resolution of the sensing signal is improved by optimizing and expanding the bandwidth of the hardware characteristic, the RFID protocol and the communication parameter; two feature extraction algorithms of frequency priority and time resolution priority are adopted, so that the perception features are suitable for different scenes and target requirements. The invention only needs a single set of RFID equipment, is compatible with the RFID protocol, accords with the spectrum allocation regulation, has the characteristics of universality, light weight, low cost, low overhead and the like, and has better application value, research prospect and development potential.
Referring to fig. 1, the present invention provides a passive RFID-oriented spread spectrum and broadband sensing enhancement method, including the following steps:
s1, frequency multiplexing: aiming at a certain specific communication bandwidth, combining an RFID protocol and an orthogonal frequency division multiplexing technology, designing a generation mechanism of an OFDM symbol, carrying out amplitude control on the generated OFDM symbol and carrying out linear superposition on the OFDM symbol and an RFID continuous wave, and realizing a spread spectrum mechanism compatible with the RFID protocol;
orthogonal Frequency Division Multiplexing (OFDM) divides a channel into a plurality of Orthogonal sub-channels, converts a high-speed data signal into parallel low-speed sub-data streams, and modulates the parallel low-speed sub-data streams onto each sub-channel for transmission. The orthogonal signals can be separated by using correlation techniques at the receiving end, which can reduce the mutual interference ICI between the sub-channels. The signal bandwidth on each subchannel is less than the associated bandwidth of the channel, and therefore can be viewed as flat fading on each subchannel, so that intersymbol interference can be eliminated. And since the bandwidth of each sub-channel is only a small fraction of the original channel bandwidth, channel equalization becomes relatively easy.
Each communication process of the RFID mainly includes a reader QUERY command, a tag RN16 reply, a reader ACK command, and a tag EPC reply, as shown in fig. 2. Each communication process runs on a single frequency, and the signal phase and strength information extracted by the existing RFID sensing system is also a sensing characteristic under the single frequency.
Continuous Wave (CW) defined by the RFID protocol is an unmodulated baseband signal with a constant amplitude, and is used to activate the RFID tag after being transmitted, and experiments show that a reply signal of the RFID tag only depends on the frequency of the Continuous Wave transmitted by the reader/writer, and is not sensitive to the waveform of the Continuous Wave, that is, the tag replies the signal as long as the frequency meets the requirement no matter the Continuous Wave transmitted by the reader/writer is a constant value or a random sequence, as shown in fig. 3. Therefore, the invention utilizes the orthogonal frequency division multiplexing technology to modulate the RFID continuous wave, so that the label reply signal comprises a plurality of orthogonal frequency components (subcarriers) which can be used for extracting the perception characteristics under different frequencies in the process of one-time RFID communication.
Specifically, the process of generating the OFDM symbol includes random sequence generation, constellation mapping, serial-to-parallel conversion, inverse fourier transform, and parallel-to-serial conversion: grouping pseudo-random binary sequences, each group containing log 2 n binary bits, where n is the order of Phase Shift Keying (PSK); mapping each binary bit group to a constellation diagram by adopting nPSK (binary phase Shift keying) to obtain a complex value (containing a real part and an imaginary part); performing serial-to-parallel conversion on the N serial complex numbers to obtain N parallel complex numbers; performing a quasi-Fourier transform on the N parallel complex numbers to obtain N parallel complex numbers after the transform; performing parallel-to-serial conversion on the converted N parallel complex numbers to obtain N serial numbersIs called an OFDM symbol.
The length N (upper limit of the number of subcarriers) of the OFDM symbol is limited by the RFID protocol. The RFID label reply signal adopts an on-off keying modulation mode, namely, a label circuit has two states of 'on' and 'off', and if the state switching is in a complete OFDM symbol period, larger signal fading is generated to influence the perception characteristic. The RFID protocol specifies the number M of baseband signal sampling points, the bandwidth B and the backscatter link frequency f in the state duration of a tag circuit BLF And data bit determination:
M＝B/(μ×f BLF ) (1)
where μ =1 when the tag reply data is bit "1", and μ =2 when the tag reply data is bit "0".
If the length of one OFDM symbol is to be used
Figure GDA0003745009410000101
Then it is guaranteed that at least one complete OFDM symbol exists for each "on" and "off" state duration of the RFID tag reply signal:
Figure GDA0003745009410000102
furthermore, the original continuous wave of the RFID is directly replaced by a plurality of OFDM symbols, so that the RFID tag cannot be activated to reply due to insufficient signal energy; if the original continuous wave of the RFID is replaced by a plurality of linearly amplified OFDM symbols, the reader-writer cannot correctly decode the data returned by the tag because the waveform is too noisy. Therefore, the invention transmits modulated continuous waves in a way that combines amplitude control (linear amplification of OFDM symbols) and linear superposition (OFDM symbols and original continuous waves):
s＝α×s OFDM +s CW (3)
where s is the actual transmitted modulated continuous wave, s OFDM For the generated OFDM symbol, s CW Alpha is a linear amplification factor (alpha is more than 1) for the original continuous wave of the RFID. Also, the present invention providesThe RFID continuous wave modulation mode still meets the orthogonality among all subcarriers of the transmission signals:
Figure GDA0003745009410000111
wherein d is i For the data (complex number) carried by the ith subcarrier cos (i ω t), ω is the subcarrier center angular frequency, and the integral of the product between different subcarriers in the transmitted signal remains zero (orthogonality).
Further, because the designed OFDM symbols are the same, each OFDM symbol can be used as a cyclic prefix/suffix of a next/previous symbol, so that an additional cyclic prefix/suffix does not need to be inserted into one OFDM symbol. Meanwhile, since the center subcarrier is affected by direct current offset (crystal oscillator leakage), the center subcarrier is not used for sensing in the invention.
S2, bandwidth expansion: when the communication bandwidth needs to be further expanded, according to the RFID hardware characteristics, protocol requirements and industrial, scientific and medical frequency bands (ISM bands), the RFID communication parameters are reconfigured again through an optimization algorithm, wherein the RFID communication parameters comprise a maximum value of the bandwidth, the range of the OFDM symbol length in the spread spectrum mechanism in the step S1 and other baseband communication parameters of the RFID, so that the RFID spread spectrum and broadband transmission mechanism is realized;
the RFID system is allocated with bandwidths (frequency spectrum resources) in different ranges of 2-26 MHz in different countries and regions of the world, but when the RFID system in real life operates, each complete communication process only occupies the bandwidth of 1-2 MHz, so that the bandwidth resources are wasted, and the perceived time resolution is lower when the RFID system is perceived by using a low-bandwidth signal (the perceived time resolution is equal to the reciprocal of the bandwidth).
Specifically, the invention improves the bandwidth of the signal frequency band by improving the sampling rate of the baseband. The sensing characteristics under different frequencies in a larger range can be obtained by improving the bandwidth of the sensing signal, the frequency dimension information sensed by using a single RFID tag is improved, and when more tags are introduced, the information of the sensing space and the frequency dimension can be improved simultaneously. On the other hand, the sensing is carried out by utilizing a large-bandwidth signal, the sensing time resolution is higher, and a fast moving target can be captured more accurately.
Maximum allowable bandwidth B of RFID according to industrial, scientific and medical frequency band (ISM band) u =26MHz, but the setting of its bandwidth B cannot simply choose the upper limit due to the hardware characteristics of the RFID device. The limitation of the hardware characteristics on the bandwidth setting mainly comes from the integrator-comb filter, and the decimation (decimation) and interpolation (interpolation) operation factor D (the ratio between the antenna digital-to-analog conversion rate r and the baseband sampling rate, i.e. the ratio between the antenna digital-to-analog conversion rate r and the baseband sampling rate during signal transceiving, i.e. the ratio
Figure GDA0003745009410000121
) Should be designed to be an integer power of 2, otherwise a large roll-off will occur around the center frequency of the signal, thereby affecting the perception. As shown in fig. 4, when the factor D is odd, the roll-off effect around the center frequency of the sensing signal generates significant distortion; when the factor D is an even number, the roll-off has relatively little effect; when the factor D is an integer power of 2 (2) i I is an integer), the roll-off effect disappears.
Meanwhile, the setting of the bandwidth B also influences the RFID protocol and the communication parameter theta RFID Arrangement of (a), theta RFID Mainly comprising f BLF TRcal, tari, etc., which together determine the number L (B, θ) of baseband signal samples for each type of RFID command ("QUERY", "RN16", "ACK", "EPC", etc.) RFID ). In order to ensure the processing of the baseband signal and establish a stable RFID communication link, the number of sampling points must be a positive integer. In summary, the setting of the bandwidth B requires searching and matching by the optimization equation:
Figure GDA0003745009410000122
after the target bandwidth B is obtained, the upper limit of the number of subcarriers included in the OFDM symbol may be calculated, and the specific number of subcarriers needs to be determined by observing the influence of the subcarrier spacing on the sensing effect through experiments. As shown in fig. 6, the bandwidth B =25MHz is set, the average difference of the sensing characteristics of the distilled water and the sodium sulfide solution is found in the case of 20,50,100,150 and 200 subcarriers, and it is found that the difference of the sensing characteristics is not improved significantly when the bandwidth B is increased to a certain extent. Therefore, it is more effective to select a moderate bandwidth in consideration of reducing the calculation overhead in practical application.
Further, if only the baseband sampling rate is increased without modulating the original continuous wave of the RFID, the bandwidth of the transmission signal band is still concentrated on the center frequency (about 1 MHz), and therefore, it is necessary to cooperate with the continuous carrier modulation in step S1, as shown in fig. 5.
S3, feature extraction: according to the RFID spread spectrum and broadband transmission mechanism provided in the step S2, the RFID equipment is utilized to transmit and receive the spread spectrum broadband signal, the receiving end transmits and receives the reply signal from the label, the receiving end extracts OFDM symbols from the high and low levels, namely the on and off states, of the reply signal of the label and calculates the channel frequency response, and the channel frequency response is subjected to a characteristic calculation mode with frequency priority and resolution priority aiming at different perception scenes such as static and dynamic targets, so that the perception enhancement of frequency dimension and time resolution dimension is realized.
The frequency domain received signal Y (f) is represented by the frequency domain transmitted signal X (f), the channel frequency response H (f), and the channel noise E:
Y(f)＝H(f)·X(f)+E (6)
the method comprises the steps that f is a subcarrier frequency, received signals are directly obtained by hardware, transmitted signals are used as known quantities for perception, symbol synchronization of the received signals is achieved through an autocorrelation function, after each complete OFDM symbol is obtained, a least square algorithm is adopted to calculate channel frequency response, and the signal frequency response is further processed according to different perception scenes and targets to obtain broadband multi-frequency component perception characteristics.
Specifically, because the technology provided by the invention does not influence the normal communication of the RFID, the high and low levels (the circuit is in an on state and an off state) of the reply signal of the tag can be directly acquired by the RFID equipment. When the high level of the signal is replied by the tag, the signal and the sensing target generate a coupling effect, and the signal is transmitted through the sensing environment, so that the high level signal comprises the sensing target and the environment information; when the label replies the low level of the signal, the label is in a non-response state, the signal propagation also passes through the perception environment, and therefore, the low level signal only contains the environment information.
Aiming at a static sensing target, respectively calculating channel frequency responses corresponding to all subcarriers of OFDM symbols contained in high and low levels of RFID tag reply signals, and calculating the difference value of the two as a static sensing characteristic
Figure GDA0003745009410000131
Figure GDA0003745009410000132
Wherein the content of the first and second substances,
Figure GDA0003745009410000133
is an N-dimensional vector, N is the number of subcarriers, q ON And q is OFF Respectively the number of high and low levels in the EPC signal replied by the RFID label at one time,
Figure GDA0003745009410000141
and
Figure GDA0003745009410000142
the calculated channel frequency responses in the high and low levels, respectively. The purpose of calculating the difference is to remove environmental influences from the perceptual features, preserving information about the perceptual target.
Aiming at the dynamic sensing target, only calculating the channel frequency response corresponding to each subcarrier of OFDM symbols contained in the high level or the low level of the reply signal of the RFID tag, and arranging the channel frequency responses according to the time domain sequence to obtain the dynamic sensing characteristic
Figure GDA0003745009410000143
Figure GDA0003745009410000144
Wherein the content of the first and second substances,
Figure GDA0003745009410000145
the matrix is N multiplied by T, N is the number of subcarriers, and T is the number of high or low levels in EPC signals replied by the RFID tags at one time in time domain sequence.
Dynamic perceptual features
Figure GDA0003745009410000146
From the time dimension, the high bandwidth improves the temporal resolution of the perceptual features.
Furthermore, the RFID system adopts a self-sending and self-receiving communication mode, and the received signals do not need to be subjected to frequency offset estimation and correction, so the sensing characteristics are more robust.
Furthermore, the reply signal of the RFID tag adopts an On-Off Keying (OOK) modulation mode, the OFDM technology adopts a Phase Shift Keying (PSK) modulation mode, the two modulation modes are mutually independent, and no mutual influence exists during demodulation, so the invention also provides the additional data transmission capability for the RFID system.
The two obtained sensing characteristics are high-dimensional (frequency dimension) and high-speed (time resolution) sensing characteristics, have finer-grained sensing capability, can obtain sensing target characteristics in a wider frequency range, and can more accurately capture the sensing target characteristics moving at high speed. Meanwhile, the method is also applicable to a multi-label (label array) scene, namely, the perception features are simultaneously enhanced in the aspects of space, frequency dimension and time resolution.
The invention provides a platform type technology which can sense whether a signal bears data, the number of frequency components (subcarriers), the setting of actual bandwidth, the calculation and optimization of characteristics and the like, and can further adjust or process according to actual application scenes and requirements.
In another embodiment of the present invention, a passive RFID-oriented spread spectrum and broadband perception enhancement system is provided, where the system can be used to implement the passive RFID-oriented spread spectrum and broadband perception enhancement method described above, and specifically, the passive RFID-oriented spread spectrum and broadband perception enhancement method system includes a spread spectrum module, a transmission module, and an enhancement module.
The spread spectrum module is combined with an RFID protocol and an orthogonal frequency division multiplexing technology to design a generation mechanism of an OFDM symbol, amplitude control is carried out on the generated OFDM symbol and the OFDM symbol and an RFID continuous wave are linearly superposed to form a remodulated continuous wave, if the remodulated continuous wave is transmitted correspondingly during RFID communication, a received signal contains a plurality of orthogonal frequency components, and the spread spectrum mechanism compatible with the RFID protocol is realized;
the transmission module expands the communication bandwidth, reconfigures RFID communication parameters through an optimization algorithm according to the RFID hardware characteristics, protocol requirements and industrial, scientific and medical frequency bands (ISM bands), wherein the RFID communication parameters comprise a maximum value of the bandwidth, the range of the OFDM symbol length in an RFID protocol spread spectrum mechanism of the spread spectrum module and other RFID baseband communication parameters;
and the enhancement module is used for receiving and transmitting the broadband signal spread by the enhancement module by using the RFID equipment according to an RFID spread spectrum and broadband transmission mechanism realized by the transmission module, replying the high level and the low level of the signal from the tag at a receiving end, wherein the high level corresponds to an on state and the low level corresponds to an off state, extracting an OFDM symbol and calculating channel frequency response, and aiming at static and dynamic targets, adopting a characteristic calculation mode with frequency priority and resolution priority for the channel frequency response to realize the perception enhancement of frequency dimension and time resolution dimension.
In yet another embodiment of the present invention, a terminal device is provided that includes a processor and a memory for storing a computer program comprising program instructions, the processor being configured to execute the program instructions stored by the computer storage medium. The Processor may be a Central Processing Unit (CPU), or may be other general purpose Processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), an off-the-shelf Programmable gate array (FPGA) or other Programmable logic device, a discrete gate or transistor logic device, a discrete hardware component, etc., which is a computing core and a control core of the terminal, and is adapted to implement one or more instructions, and is specifically adapted to load and execute one or more instructions to implement a corresponding method flow or a corresponding function; the processor of the embodiment of the invention can be used for the operation of the spread spectrum and broadband perception enhancement method facing the passive RFID, and comprises the following steps:
designing a generating mechanism of an OFDM symbol by combining an RFID protocol and an orthogonal frequency division multiplexing technology, carrying out amplitude control on the generated OFDM symbol and carrying out linear superposition on the OFDM symbol and an RFID continuous wave to form a remodulated continuous wave, wherein if the corresponding remodulated continuous wave is transmitted during RFID communication, a receiving signal contains a plurality of orthogonal frequency components, and a spectrum spreading mechanism compatible with the RFID protocol is realized; expanding the communication bandwidth, and reconfiguring RFID communication parameters through an optimization algorithm according to the RFID hardware characteristics, protocol requirements and industrial, scientific and medical frequency bands (ISM bands), wherein the RFID communication parameters comprise a maximum value of the bandwidth, the range of OFDM symbol length in an RFID protocol spread spectrum mechanism and other RFID baseband communication parameters, and if the RFID communication parameters meet the parameter configuration, a radio frequency signal contains a plurality of orthogonal frequency components in a widened bandwidth range, so that the RFID spread spectrum and broadband transmission mechanism is realized; according to an RFID spread spectrum and broadband transmission mechanism, RFID equipment is used for receiving and transmitting spread spectrum broadband signals, high level and low level of signals are replied from a label at a receiving end, the high level corresponds to an on state, the low level corresponds to an off state, OFDM symbols are extracted, channel frequency response is calculated, and aiming at static and dynamic targets, a characteristic calculation mode with frequency priority and resolution priority is adopted for the channel frequency response, so that the perception enhancement of frequency dimension and time resolution dimension is realized.
In still another embodiment of the present invention, the present invention further provides a storage medium, specifically a computer-readable storage medium (Memory), which is a Memory device in a terminal device and is used for storing programs and data. It is understood that the computer readable storage medium herein may include a built-in storage medium in the terminal device, and may also include an extended storage medium supported by the terminal device. The computer-readable storage medium provides a storage space storing an operating system of the terminal. Also, the memory space stores one or more instructions, which may be one or more computer programs (including program code), adapted to be loaded and executed by the processor. It should be noted that the computer-readable storage medium may be a high-speed RAM memory, or may be a non-volatile memory (non-volatile memory), such as at least one disk memory.
One or more instructions stored in a computer-readable storage medium may be loaded and executed by a processor to implement the corresponding steps of the passive RFID-oriented spread spectrum and broadband perception enhancement method in the above embodiments; one or more instructions in the computer readable storage medium are loaded by the processor and perform the steps of:
designing a generating mechanism of an OFDM symbol by combining an RFID protocol and an orthogonal frequency division multiplexing technology, carrying out amplitude control on the generated OFDM symbol and carrying out linear superposition on the OFDM symbol and an RFID continuous wave to form a remodulated continuous wave, wherein if the corresponding remodulated continuous wave is transmitted during RFID communication, a receiving signal contains a plurality of orthogonal frequency components, and a spectrum spreading mechanism compatible with the RFID protocol is realized; expanding the communication bandwidth, and reconfiguring RFID communication parameters through an optimization algorithm according to the RFID hardware characteristics, protocol requirements and industrial, scientific and medical frequency bands (ISM bands), wherein the RFID communication parameters comprise a maximum value of the bandwidth, the range of OFDM symbol length in an RFID protocol spread spectrum mechanism and other RFID baseband communication parameters, and if the RFID communication parameters meet the parameter configuration, a radio frequency signal contains a plurality of orthogonal frequency components in a widened bandwidth range, so that an RFID spread spectrum and broadband transmission mechanism is realized; according to an RFID spread spectrum and broadband transmission mechanism, an RFID device is used for receiving and transmitting a spread spectrum broadband signal, a receiving end replies a high level and a low level of the signal from a tag, the high level corresponds to an on state, the low level corresponds to an off state, an OFDM symbol is extracted, channel frequency response is calculated, and for static and dynamic targets, a frequency-first and resolution-first characteristic calculation mode is adopted for the channel frequency response, so that the perception enhancement of frequency dimension and time resolution dimension is realized.
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Taking liquid identification as an example, experiments were conducted on 18 liquids commonly found in daily life, including several groups of liquids of similar types, such as coca-cola-pepa-sprite, beer-white spirit, skim-whole milk, and the like. For each liquid, it was placed in an identical 200 ml plastic container with the RFID tag attached to the outer surface of the container and a layer of 1 mm thick plastic foam sandwiched between the surface of the container and the tag to ensure proper communication of the tag, with a distance of 50 cm between the reader antenna and the tag (container and liquid). The RFID communication follows EPC Gen2 protocol, a reader-writer is operated by using USRP X310, the bandwidth is set to be 25MHz, the center frequency is set to be 915MHz, the tag model is Alien 9640, two Laird S9028 PCL antennas are used as transceiving devices, and the gain is 8dBi. The classifier adopts random forests instead of more complex neural networks, and mainly aims to embody the high efficiency of the perception features provided by the invention.
Referring to fig. 7, in a liquid identification application scenario, the classification accuracy is compared with that of three existing RFID sensing systems. Wherein PAR-PHA obtains phase information of the label reply signal under single frequency by using a pair of labels, and can only realize classification precision of 57.5 percentDegree; PAR-HOP utilizes a pair of labels, adopts a frequency hopping (8 frequencies) mode to obtain phase information of a label reply signal, and can achieve 87.9% of classification precision; the ARY-HOP utilizes an array consisting of 8 tags to acquire phase information and intensity information of tag reply signals in a frequency hopping (8 frequencies), and the classification precision can reach 94.6%. The invention utilizes a single tag, and adopts frequency multiplexing (150 subcarriers), bandwidth extension (25 MHz) and feature extraction (channel frequency response)
Figure GDA0003745009410000182
) The method achieves the classification precision of 98.2%.
Taking gesture recognition as an example, for 6 common human-computer interaction gestures (clockwise/counterclockwise rotating hand, left/right swinging hand, making a fist or opening a palm), 5 college students are invited to perform experiments for volunteers, and 1200 pieces of experimental data are collected in total. The hardware set-up is the same as for the liquid identification embodiment described above.
Referring to fig. 8, in a gesture recognition application scenario, the classification accuracy of the three existing RFID sensing systems is compared. The PAR-PHA acquires the phase information of the label reply signals under single frequency by using a pair of labels, and can realize the classification precision of 77.3 percent; PAR-HOP utilizes a pair of labels, adopts a frequency hopping (8 frequencies) mode to obtain the phase information of the label reply signal, and can only reach the classification precision of 74.8 percent; the ARY-PHA acquires the phase and intensity information of the label reply signals under single frequency by using an array consisting of 8 labels, and can achieve 92.9% of classification accuracy. The invention utilizes a single tag, and adopts frequency multiplexing (150 subcarriers), bandwidth expansion (25 MHz) and feature extraction (channel frequency response)
Figure GDA0003745009410000181
) The method achieves the classification precision of 97.5%.
Referring to FIG. 9, two sensing characteristics are proposed in the present invention
Figure GDA0003745009410000191
The calculation time expenses required by the bandwidth change are respectively 3.8-29.8 ms and 2.2-18.3 ms, and the calculation time expenses are countedComputation time includes all signal processing and feature computation overhead, i.e., the overall process from the original received signal to the perceptual features is time consuming.
In summary, the method and system for enhancing spread spectrum and broadband sensing for passive RFID of the present invention can break through the limitations of the existing RFID sensing system in terms of feature dimension and time resolution. The invention only needs a single set of RFID equipment, is compatible with an RFID protocol, accords with spectrum allocation regulations, can provide high-granularity (multidimensional, high-speed) sensing characteristics, is applicable to the practical application fields of non-invasive material detection, non-contact human-computer interaction, intelligent manufacturing and the like, has the characteristics of universality, light weight, low cost, low expenditure and the like, and has better application value, research prospect and development potential.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims (4)

1. A spread spectrum and broadband perception enhancement method facing to passive RFID is characterized by comprising the following steps:
s1, designing a generation mechanism of OFDM symbols, and grouping pseudo-random binary sequences, wherein each group contains log 2 n binary bits, where n is the order of phase shift keying; mapping each binary bit group to a constellation diagram by adopting nPSK (binary phase Shift keying) to obtain a complex value containing a real part and an imaginary part; performing serial-to-parallel conversion on the N serial complex numbers to obtain N parallel complex numbers; performing a quasi-Fourier transform on the N parallel complex numbers to obtain N parallel complex numbers after the transform; performing parallel-to-serial conversion on the converted N parallel complex numbers to obtain N serial complex numbers, using the N serial complex numbers as an OFDM symbol, performing amplitude control on the generated OFDM symbol and performing linear superposition on the OFDM symbol and the RFID continuous wave to form a remodulated continuous wave, transmitting the correspondingly remodulated continuous wave during RFID communication, wherein a received signal contains a plurality OF orthogonal frequency components to realize a spread spectrum mechanism compatible with an RFID protocol, and adopting amplitude control linear amplification OFThe method for transmitting modulated continuous waves s by combining the DM symbols, the linear superposition OFDM symbols and the original continuous waves comprises the following steps:
s＝α×s OFDM +s CW
wherein s is OFDM For the generated OFDM symbol, s CW Alpha is a linear amplification coefficient for the original continuous wave of the RFID;
s2, expanding the communication bandwidth, reconfiguring the RFID communication parameters through an optimization algorithm, wherein the configuration comprises a maximum value of the bandwidth, the range of the OFDM symbol length in the RFID protocol spread spectrum mechanism realized in the step S1 and other RFID baseband communication parameters, when the RFID communication meets the parameter configuration, a radio frequency signal contains a plurality of orthogonal frequency components in the enlarged bandwidth range, the RFID spread spectrum and broadband transmission mechanism is realized, a target bandwidth B and other communication parameters are obtained by searching and matching through an optimization equation, then the number of subcarriers is determined by observing the influence of subcarrier intervals on the sensing effect through experiments, and the bandwidth B is determined by searching and matching through the optimization equation as follows:
Figure FDA0003745009400000011
wherein, B u R is the antenna digital-to-analog conversion rate, theta, for the maximum allowable bandwidth of the RFID RFID As communication parameters, N + Is a positive integer, L (B, θ) RFID ) The number of sampling points of the baseband signal;
s3, according to the RFID spread spectrum and the broadband transmission mechanism obtained in the step S2, the RFID equipment is utilized to receive and transmit the broadband signal spread spectrum in the step S2, the receiving end extracts OFDM symbols and calculates channel frequency response from the high level and the low level of the label reply signal, the high level corresponds to the on state, the low level corresponds to the off state, the channel frequency response is calculated by adopting a characteristic calculation mode with frequency priority and resolution priority aiming at static and dynamic targets, the perception enhancement of frequency dimension and time resolution dimension is realized, the least square algorithm is adopted to calculate the channel frequency response of the received signal to obtain perception characteristics, and the static perception targets are respectively calculatedThe channel frequency response corresponding to all subcarriers of the OFDM symbols contained in the low level is calculated, and the difference value of the two is used as the static sensing characteristic; aiming at a dynamic sensing target, calculating channel frequency response corresponding to each subcarrier of OFDM symbols contained in high level or low level of RFID tag reply signals, arranging according to a time domain sequence to obtain dynamic sensing characteristics, and improving time resolution and static sensing characteristics of the sensing characteristics by means of high bandwidth
Figure FDA0003745009400000021
Comprises the following steps:
Figure FDA0003745009400000022
wherein the content of the first and second substances,
Figure FDA0003745009400000023
is an N-dimensional vector, N is the number of subcarriers, q ON And q is OFF The number of high and low levels in the EPC signal returned by the RFID tag at one time respectively,
Figure FDA0003745009400000024
and
Figure FDA0003745009400000025
respectively calculating the channel frequency response in high and low levels;
dynamic perceptual features
Figure FDA0003745009400000026
Comprises the following steps:
Figure FDA0003745009400000027
wherein, the first and the second end of the pipe are connected with each other,
Figure FDA0003745009400000028
is an N multiplied by T matrix and is,n is the number of subcarriers, and T is the number of high or low levels in the EPC signal replied by the RFID label at one time in the time domain sequence.
2. The method according to claim 1, wherein in step S1, there is at least one complete OFDM symbol for each on and off state duration of the RFID tag reply signal:
Figure FDA0003745009400000029
wherein B is bandwidth, mu is tag reply bit identification, M is the number of baseband signal sampling points in tag circuit state duration, f BLF Is the backscatter link frequency.
3. The method according to claim 1, wherein in step S1, orthogonality is satisfied between subcarriers of the modulated continuous wave S, specifically:
Figure FDA0003745009400000031
wherein d is i Is the data carried by the ith subcarrier cos (i ω t), and ω is the subcarrier center angular frequency.
4. A passive RFID-oriented spread spectrum and broadband perception enhancement system is characterized by comprising:
a spread spectrum module for designing OFDM symbol generation mechanism and grouping the pseudo-random binary sequences, wherein each group contains log 2 n binary bits, where n is the order of phase shift keying; mapping each binary bit group to a constellation diagram by adopting nPSK (binary phase Shift keying) to obtain a complex value containing a real part and an imaginary part; performing serial-to-parallel conversion on the N serial complex numbers to obtain N parallel complex numbers; performing quasi-Fourier transform on the N parallel complex numbers to obtain N parallel complex numbers after transformation; performing parallel-to-serial conversion on the converted N parallel complex numbers to obtain N serial numbersThe complex number of (1) is used as an OFDM symbol, the amplitude of the generated OFDM symbol is controlled and linearly superposed with RFID continuous waves to form remodulated continuous waves, the corresponding remodulated continuous waves are transmitted during RFID communication, a received signal contains a plurality of orthogonal frequency components, a spread spectrum mechanism compatible with an RFID protocol is realized, and the modulated continuous waves s are transmitted in a mode of combining amplitude control linear amplification OFDM symbols, linear superposition OFDM symbols and original continuous waves:
s＝α×s OFDM +s CW
wherein s is OFDM For the generated OFDM symbol, s CW Alpha is the linear amplification coefficient of the original continuous wave of the RFID;
the transmission module expands communication bandwidth, reconfigures RFID communication parameters through an optimization algorithm, and comprises a maximum dereferencing value of the bandwidth, a range of OFDM symbol length in an RFID protocol spread spectrum mechanism of the spread spectrum module and other baseband communication parameters of RFID, wherein if the parameter configuration is met during RFID communication, a radio frequency signal contains a plurality of orthogonal frequency components in an enlarged bandwidth range, the RFID spread spectrum and broadband transmission mechanism is realized, a target bandwidth B and other communication parameters are obtained by searching and matching through an optimization equation, then the number of subcarriers is determined by observing the influence of subcarrier intervals on a sensing effect through experiments, and the bandwidth B is determined by searching and matching through the optimization equation as follows:
Figure FDA0003745009400000032
wherein, B u R is the antenna digital-to-analog conversion rate, theta, for the maximum allowable bandwidth of the RFID RFID As communication parameters, N + Is a positive integer, L (B, θ) RFID ) The number of sampling points for baseband signals;
the enhancement module receives and transmits the broadband signal spread by the enhancement module by using the RFID equipment according to the RFID spread spectrum and broadband transmission mechanism realized by the transmission module, replies the high level and the low level of the signal from the label at the receiving end, the high level corresponds to the on state, the low level corresponds to the off state, and extracts the OFDM symbolCalculating channel frequency response, aiming at static and dynamic targets, adopting a characteristic calculation mode with frequency priority and resolution priority to the channel frequency response to realize the perception enhancement of frequency dimension and time resolution dimension, adopting a least square algorithm to calculate the channel frequency response of a received signal to obtain perception characteristics, aiming at the static perception target, respectively calculating the channel frequency response corresponding to all subcarriers of OFDM symbols contained in high and low levels of a reply signal of the RFID tag, and calculating the difference value of the two as the static perception characteristics; aiming at a dynamic sensing target, calculating channel frequency response corresponding to each subcarrier of OFDM symbols contained in high level or low level of RFID tag reply signals, arranging according to a time domain sequence to obtain dynamic sensing characteristics, and improving time resolution and static sensing characteristics of the sensing characteristics by means of high bandwidth
Figure FDA0003745009400000041
Comprises the following steps:
Figure FDA0003745009400000042
wherein the content of the first and second substances,
Figure FDA0003745009400000043
is an N-dimensional vector, N is the number of subcarriers, q ON And q is OFF The number of high and low levels in the EPC signal returned by the RFID tag at one time respectively,
Figure FDA0003745009400000044
and
Figure FDA0003745009400000045
respectively calculating the channel frequency responses in high and low levels;
dynamic perceptual features
Figure FDA0003745009400000046
Comprises the following steps:
Figure FDA0003745009400000047
wherein the content of the first and second substances,
Figure FDA0003745009400000048
the matrix is N multiplied by T, N is the number of subcarriers, and T is the number of high or low levels in EPC signals replied by the RFID tags at one time in time domain sequence.
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