A backscatter communication demodulation method and system based on environmental cellular OFDM signals

Abstract

The invention discloses a backscatter communication demodulation method based on an environmental cellular OFDM signal, and relates to the technical field of wireless communication. The method comprises the following steps: S10, a transmitting end sends an environmental cellular OFDM signal to a receiving end; S20, a backscatter tag modulates the collected Internet of Things information on the environmental cellular OFDM signal; S30, a receiving end demodulates the environmental cellular OFDM signal and a second reflected signal; the method comprises the following steps: the receiving end demodulates the frequency domain information of the original environmental cellular OFDM signal on the frequency band; receives the frequency domain information of the first reflected signal and the frequency domain information of the second reflected signal on the reflected frequency band after frequency shifting; uses the demodulated environmental cellular OFDM signal and the frequency domain information of the first reflected signal, and combines with a known pilot pattern to perform cascade channel estimation and equalization; S40, restores the bit information transmitted by the second reflected signal through a demodulation algorithm; the beneficial effect of the invention is that reliable, high-speed and wide-coverage backscatter communication can be provided for users.

Description

Technical Field

[0001] The present invention relates to the field of wireless communication technology, and more particularly to a backscatter communication demodulation method and system based on environmental cellular OFDM signals. Background Art

[0002] The emergence of the Internet of Things has brought a huge impact on human society. It makes people's lives more convenient, but also brings more opportunities and challenges. Through the Internet of Things, people can more conveniently obtain massive amounts of information, and can also manage and control various devices and systems more efficiently.

[0003] Wireless communication technology is an important cornerstone supporting the transmission of the Internet of Things. The biggest problem with classic active wireless communication technology in the application scenarios of the Internet of Things is that the equipment consumes a lot of power and the maintenance cost of replacing batteries is high. In addition, active wireless communication requires more complex equipment components, which increases the cost of the equipment. Some backscatter communication technologies require a dedicated RF source as an excitation source, which also increases the deployment cost of low-power IoT devices. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a backscatter communication demodulation method and system based on environmental cellular OFDM signals.

[0005] The technical solution adopted by the present invention to solve the technical problem is: the present invention provides a backscatter communication demodulation method based on environmental cellular OFDM signals, and the improvement thereof is that the method comprises the following steps:

[0006] S10, the transmitting end sends the ambient cellular OFDM signal to the receiving end;

[0007] S20, the backscatter tag modulates the collected IoT information on the ambient cellular OFDM signal, including the following steps:

[0008] The backscatter tag detects synchronization information from the ambient cellular OFDM signal sent by the transmitter, and achieves coarse synchronization with the ambient cellular OFDM signal;

[0009] The backscatter tag switches the tag radio frequency impedance to generate a first reflected signal according to a pilot pattern known to the receiving end;

[0010] The backscatter tag switches the tag radio frequency impedance to generate a second reflected signal according to the IoT information bit;

[0011] S30, the receiving end demodulates the ambient cellular OFDM signal and the second reflected signal, including the following steps:

[0012] The receiving end demodulates the frequency domain information of the original environmental cellular OFDM signal frequency band;

[0013] Receiving the first reflected signal frequency domain information and the second reflected signal frequency domain information in the reflected frequency band after the frequency shifting;

[0014] Using the demodulated ambient cellular OFDM signal and the frequency domain information of the first reflected signal, combined with a known pilot pattern, cascade channel estimation and equalization are performed;

[0015] S40: Recover the bit information transmitted by the second reflected signal through a demodulation algorithm.

[0016] Further, in step S20, the backscatter tag detects synchronization information from the ambient cellular OFDM signal sent by the transmitting end to achieve coarse synchronization with the ambient cellular OFDM signal, including:

[0017] The primary synchronization signal PSS and the secondary synchronization signal SSS in the ambient cellular OFDM signal are used to achieve coarse synchronization.

[0018] Furthermore, in step S20, the switching frequency of the tag radio frequency impedance is twice the sampling frequency of the ambient cellular OFDM signal.

[0019] Furthermore, in step S10, the frequency band signal of the ambient cellular OFDM signal is s lte [n], define f c is the carrier frequency of the environmental cellular OFDM signal, s[n] is the cellular baseband signal, and the frequency band signal is expressed as: Where n is the baseband sampling point, j is the imaginary unit, and t is the continuous time variable.

[0020] Furthermore, in step S10, the number of FFT points of the ambient cellular OFDM signal is n fft , used to represent the number of sampling points in the time domain of a cellular OFDM symbol excluding the cyclic prefix CP;

[0021] The sampling frequency of the ambient cellular OFDM signal is T s , the subcarrier spacing is Δf, where the sampling frequency is T s is the duration of a cellular sampling point,

[0022] Further, in step S20, the reflection coefficient modulus of the backscatter tag is ||α tag ||, the switching range of the tag RF impedance is -||α tag || and ||α tag ||; the switching frequency of the tag RF impedance is f tag ;

[0023] The modulus of the equivalent label reflection coefficient is α t ' ag ,and

[0024] The number of sampling points of the backscatter tag modulation is n tag , indicating that in one environmental OFDM symbol, the backscatter tag changes n tag sampling points;

[0025] The number of bits transmitted by the backscatter tag is n bit , indicating that in one environment OFDM symbol, the backscatter tag transmits n bit bits;

[0026] The number of times the backscatter tag repeats encoding is M, which means that during the encoding process of the backscatter tag transmitting information, each information bit is repeated M times, and

[0027] In step S30, the number of frequency domain subcarriers used in the demodulation process at the receiving end is n. f , represents the number of subcarriers used in frequency domain analysis of the first reflected signal and demodulation of the second reflected signal.

[0028] Furthermore, in step S20, the initial phase of the backscatter tag at the nth sampling point is θ n , the information transmitted by the backscattered tag is transmitted with an initial phase θ n Different values ​​of correspond to different information bits;

[0029] In step S20, the backscatter tag modulates the IoT information into the ambient cellular OFDM signal by switching the tag RF impedance, thereby adding an initial phase θ to each sampling unit of the ambient cellular OFDM signal. n , and by the initial phase θ n Transmit backscatter tag information;

[0030] The cellular baseband signal s[n] is sampled at a frequency T s The value inside remains constant, and the backscattered tag is at the sampling frequency T s The tag RF impedance is continuously changed to form a square wave, which is expanded according to the Fourier series and expressed by the following formula:

[0031]

[0032] Where k is the constant of the series expansion and t is the continuous time;

[0033] In formula (1), s tag The first harmonic of [n] is expressed as:

[0034] s′ tag [n] = 2α′ tag cos(2πf tag t+θ n ) (2);

[0035] f tag is the frequency domain value where the tag is removed, and

[0036] When only the first reflected signal and the second reflected signal of the first harmonic frequency component are considered, the first reflected signal and the second reflected signal are the product of the reflection coefficient and the ambient cellular OFDM signal, which can be expressed as:

[0037]

[0038] Take the lower sideband of formula (3), due to the initial phase θ n There are only two possible values: 0 and π. and The value of has the same effect, both are -1, then the first reflected signal and the second reflected signal received from the frequency band signal are expressed as:

[0039]

[0040] According to formula (4), the baseband of the first reflected signal and the second reflected signal is the result of the cellular baseband signal s[n] receiving BPSK modulation, and the carrier frequency is offset by f tag .

[0041] Furthermore, in step S30, the receiving end demodulates the frequency domain information of the original environment cellular OFDM signal frequency band, including:

[0042] The signal received by the receiving end is subjected to down-conversion, carrier frequency offset correction, delay correction, channel estimation and equalization in sequence to restore the bit information transmitted by the sending end.

[0043] In step S30, the use of the demodulated ambient cellular OFDM signal and the first reflected signal frequency domain information, combined with a known pilot pattern, to perform cascade channel estimation and equalization includes:

[0044] According to the receiving end, the frequency domain information of the original environment cellular OFDM signal is demodulated on the frequency band of the original environment cellular OFDM signal to obtain the frequency domain value X of the original environment cellular OFDM signal at a certain time and frequency point. k (n), after receiving the frequency domain information of the first reflected signal in the reflected frequency band after the frequency shift, the frequency domain value Y of the first reflected signal is obtained k (n);

[0045] Perform channel estimation to obtain the frequency domain channel at time n and the kth subcarrier. Using the LS channel estimation criterion, the frequency domain channel value

[0046] Perform channel equalization to restore the frequency domain subcarrier information of the reflected ambient cellular OFDM signal, and use the LS channel equalization method to obtain the frequency domain value of the second reflected signal after channel equalization.

[0047] Further, in step S40, the step of restoring the bit information transmitted by the second reflected signal by using a demodulation algorithm includes:

[0048] Using the frequency domain value and DFT matrix of the ambient cellular OFDM signal, the dimension is n. f The left side of the equation group is the theoretical calculation value, and the right side is the actual demodulation value. The equation group is robustly solved for the 72-dimensional useful vector and simplified to the following formula:

[0049]

[0050] Among them, A is determined by the DFT matrix, the cellular OFDM frequency domain value and the time domain offset. f ×n tag A matrix of dimension a is n f dimensional vector, calculated from the actual observations, is the n to be solved tag dimensional vector;

[0051] Perform a substitution transformation to convert the variable into 0, 1 values ​​and recode it; For each row there are n tag The repeated coding matrix of n rows has 1 elements and the rest are 0; Written Substitute into formula (5), let in Yes f dimensional all-1 vector, we get the following formula:

[0052]

[0053] is a vector whose elements are either 0 or 1, i.e., the bits transmitted by the second reflected signal to be solved. The minimum norm solution to the problem in formula (6) is obtained.

[0054] The minimum norm solution of formula (6) is obtained by pseudo-inverse as follows:

[0055]

[0056] After solving, The value of each element is compared with 0 or 1 by distance, and the minimum distance decoding criterion is used to perform bit judgment decoding to obtain the Internet of Things information bit of the backscatter tag transmitted by the second reflection signal.

[0057] The present invention also provides a backscatter communication demodulation system based on environmental cellular OFDM signals, the improvement of which is that it includes a transmitting end, a backscatter tag and a receiving end;

[0058] The transmitting end is used to send the ambient cellular OFDM signal to the receiving end;

[0059] The backscatter tag is used to modulate the collected Internet of Things information on the environmental cellular OFDM signal; including the backscatter tag detecting synchronization information from the environmental cellular OFDM signal sent by the transmitting end to achieve coarse synchronization with the environmental cellular OFDM signal; the backscatter tag switches the tag radio frequency impedance according to the pilot pattern known to the receiving end to generate a first reflection signal; the backscatter tag switches the tag radio frequency impedance according to the Internet of Things information bit to generate a second reflection signal;

[0060] The receiving end is used to demodulate the environmental cellular OFDM signal and the second reflected signal; including the receiving end demodulating the frequency domain information of the original environmental cellular OFDM signal on the frequency band; receiving the first reflected signal frequency domain information and the second reflected signal frequency domain information on the reflected frequency band after frequency shifting; using the demodulated environmental cellular OFDM signal and the first reflected signal frequency domain information, and combining with the known pilot pattern, to perform cascade channel estimation and equalization; and restoring the bit information transmitted by the second reflected signal through a demodulation algorithm.

[0061] The beneficial effects of the present invention are: the original environmental cellular OFDM signal is frequency-shifted; repetition coding is used to reduce the reflected signal bandwidth and demodulation variable dimension during information modulation; matrix equation demodulation is established during demodulation to restore label information; reliable, high-speed, and wide-coverage backscatter communication can be provided for users, facilitating the widespread deployment of low-cost, low-power Internet of Things. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 The present invention is a principle block diagram of a backscatter communication demodulation system based on environmental cellular OFDM signals. Figure 2 The present invention is a flowchart of a backscatter communication demodulation method based on an environmental cellular OFDM signal.

[0063] Figure 3 This is a modulation principle diagram of backscatter communication using environmental cellular OFDM signals in the present invention (taking the tag switching rate as twice the sampling frequency as an example).

[0064] Figure 4 This is a structural diagram of the backscatter tag modulation information in the present invention.

[0065] Figure 5 The present invention is a specific flow chart of the demodulation of a backscatter communication demodulation system based on environmental cellular OFDM signals.

[0066] Figure 6 This is a test result diagram of the bit error rate of the reflected signal of the backscatter tag in the present invention at different code rates. DETAILED DESCRIPTION

[0067] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0068] The following will clearly and completely describe the concept, specific structure and technical effects of the present invention in combination with the embodiments and drawings, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by technicians in this field without creative work are all within the scope of protection of the present invention. In addition, all the connection / connection relationships involved in the patent do not refer to the direct connection of components, but refer to the formation of a better connection structure by adding or reducing connection accessories according to the specific implementation situation. The various technical features in the invention can be combined interchangeably without conflicting with each other.

[0069] Example 1

[0070] The present invention discloses a backscatter communication demodulation system based on an environmental cellular OFDM signal, the system comprising a transmitting end, a backscatter tag and a receiving end; Figure 1 As shown, in this embodiment, the sending end is Figure 1 The base station in the backscatter tag is Figure 1 The receiving end is the reflection device in the signal receiving apparatus, and its name can also be defined as a receiver or a terminal receiver.

[0071] In the system of the present invention, the transmitting end is used to send the ambient cellular OFDM signal to the receiving end; Figure 1As shown, the ambient cellular OFDM signal sent by the transmitter can reach the receiver through a direct link, or it can be reflected by a backscatter tag and reach the receiver along a reflection link. Furthermore, in this embodiment, the backscatter tag is used to modulate the collected IoT information on the ambient cellular OFDM signal; including the backscatter tag detecting synchronization information from the ambient cellular OFDM signal sent by the transmitter to achieve coarse synchronization with the ambient cellular OFDM signal; the backscatter tag switches the tag RF impedance according to the pilot pattern known by the receiver to generate a first reflection signal; the backscatter tag switches the tag RF impedance according to the IoT information bit to generate a second reflection signal.

[0072] In the above-mentioned embodiment, the receiving end is used to demodulate the environmental cellular OFDM signal and the second reflected signal; including the receiving end demodulating the frequency domain information of the original environmental cellular OFDM signal in the frequency band; receiving the first reflected signal frequency domain information and the second reflected signal frequency domain information in the reflected frequency band after frequency shifting; using the demodulated environmental cellular OFDM signal and the first reflected signal frequency domain information, and combining with the known pilot pattern, cascade channel estimation and equalization; finally, through the demodulation algorithm, recovering the bit information transmitted by the second reflected signal.

[0073] Based on this, the present invention provides a backscatter communication demodulation system based on environmental cellular OFDM signals, which frequency-shifts the original cellular OFDM signal, uses repetition coding to reduce the reflected signal bandwidth and demodulation variable dimension during information modulation, and establishes a matrix equation to demodulate and restore label information during demodulation. It can provide users with reliable, high-speed, and wide-coverage backscatter communication, and promote the widespread deployment of low-cost, low-power Internet of Things.

[0074] Example 2

[0075] Combination Figures 2 to 6 As shown, the present invention provides a backscatter communication demodulation method based on an environmental cellular OFDM signal. In this embodiment, the method includes the following steps:

[0076] S10, the transmitting end sends an environmental cellular OFDM signal to the receiving end;

[0077] In this embodiment, in step S10, the frequency band signal of the ambient cellular OFDM signal is s lte [n], define f c is the carrier frequency of the environmental cellular OFDM signal, s[n] is the cellular baseband signal, and the frequency band signal is expressed as: Where n is the baseband sampling point, j is the imaginary unit, and t is the continuous time variable.

[0078] In addition, in step S10, the number of FFT points of the ambient cellular OFDM signal is nfft , used to represent the number of sampling points in the time domain of a cellular OFDM symbol excluding the cyclic prefix CP;

[0079] The sampling frequency of the ambient cellular OFDM signal is T s , the subcarrier spacing is Δf, where the sampling frequency is T s is the duration of a cellular sampling point,

[0080] In a specific example, step S10 uses the following parameter set commonly used in LTE as the environmental OFDM signal: 128 sampling points, 72 useful subcarriers, and 9 cyclic prefix (CP) length.

[0081] S20, the backscatter tag modulates the collected IoT information on the ambient cellular OFDM signal, including steps S201-S203:

[0082] S201, the backscatter tag detects synchronization information from the ambient cellular OFDM signal sent by the transmitter, and realizes coarse synchronization with the ambient cellular OFDM signal; in this step, the primary synchronization signal PSS and the secondary synchronization signal SSS in the ambient cellular OFDM signal are used to realize coarse synchronization; the backscatter tag tracks the energy change of the ambient cellular OFDM signal through envelope detection, filtering and comparison circuits, so as to synchronize between systems and avoid the tag information from destroying the structure of the cyclic prefix;

[0083] S202, the backscatter tag switches the tag radio frequency impedance according to the pilot pattern known to the receiving end to generate a first reflected signal;

[0084] S203, the backscatter tag switches the tag radio frequency impedance to generate a second reflected signal according to the IoT information bit;

[0085] In step S20, the reflection coefficient modulus of the backscatter tag is ||α tag ||, the switching range of the tag RF impedance is -||α tag || and ||α tag ||; the switching frequency of the tag RF impedance is f tag ;

[0086] The modulus of the equivalent label reflection coefficient is α t ' ag ,and

[0087] The number of sampling points of the backscatter tag modulation is n tag , indicating that in one environmental OFDM symbol, the backscatter tag changes n tag sampling points;

[0088] The number of bits transmitted by the backscatter tag is n bit , indicating that in one environment OFDM symbol, the backscatter tag transmits n bit bits;

[0089] The number of times the backscatter tag repeats encoding is M, which means that during the encoding process of the backscatter tag transmitting information, each information bit is repeated M times, and

[0090] Furthermore, in step S20, the initial phase of the backscatter tag at the nth sampling point is θ n In step S202 and step S203, the information transmitted by the backscatter tag is transmitted with an initial phase θ n Different values ​​of correspond to different information bits;

[0091] In step S20, the backscatter tag modulates the IoT information into the ambient cellular OFDM signal by switching the tag RF impedance, thereby adding an initial phase θ to each sampling unit of the ambient cellular OFDM signal. n , and by the initial phase θ n Transmit backscatter tag information;

[0092] The cellular baseband signal s[n] is sampled at a frequency T s The value inside remains constant, and the backscattered tag is at the sampling frequency T s The tag RF impedance is continuously changed to form a square wave, which is expanded according to the Fourier series and expressed by the following formula:

[0093]

[0094] Where k is the constant of the series expansion and t is the continuous time;

[0095] In formula (1), s tag The first harmonic of [n] is expressed as:

[0096] s′ tag [n] = 2α′ tag cos(2πf tag t+θ n ) (2);

[0097] f tag is the frequency domain value where the tag is removed, and

[0098] When only the first reflected signal and the second reflected signal of the first harmonic frequency component are considered, the first reflected signal and the second reflected signal are the product of the reflection coefficient and the ambient cellular OFDM signal, which can be expressed as:

[0099]

[0100] Take the lower sideband of formula (3), due to the initial phase θ n There are only two possible values: 0 and π. and The value of has the same effect, both are -1, then the first reflected signal and the second reflected signal received from the frequency band signal are expressed as:

[0101]

[0102] According to formula (4), the baseband of the first reflected signal and the second reflected signal is the result of the cellular baseband signal s[n] receiving BPSK modulation, and the carrier frequency is offset by f tag .

[0103] In a specific example, in step S202 and step S203, the environmental cellular OFDM signal at each of the 128 time domain sampling points carries the same number of pilots or information as the number of useful subcarriers of the environmental cellular OFDM signal. In addition, in step S202, 2 OFDM symbols in a subframe are used as tag pilots: 1 OFDM symbol estimates the cascade channel, and 1 OFDM symbol estimates the offset error. The remaining 12 OFDM symbols of a subframe are used to transmit useful information. Furthermore, in step S202 and step S203, the switching frequency of the tag RF impedance is twice the sampling frequency of the environmental cellular OFDM signal. And during bit mapping, the phases 0 and π of the tag RF impedance correspond to the transmitted bits 0 and 1, respectively.

[0104] S30, the receiving end demodulates the environmental cellular OFDM signal and the second reflected signal;

[0105] In step S30, the number of frequency domain subcarriers used in the demodulation process at the receiving end is n. f , represents the number of subcarriers used in frequency domain analysis of the first reflected signal and demodulation of the second reflected signal.

[0106] In this embodiment, step S30 includes steps S301-S303:

[0107] S301, the receiving end demodulates the frequency domain information of the original environment cellular OFDM signal frequency band; in this embodiment, the signal received by the receiving end is down-converted, carrier frequency offset corrected, delay corrected, channel estimated and equalized in sequence to restore the bit information transmitted by the transmitting end;

[0108] S302, receiving frequency domain information of a first reflected signal and frequency domain information of a second reflected signal in a reflected frequency band after frequency shifting;

[0109] In the present invention, the environmental signal used is a limited bandwidth OFDM signal. When the value of each sampling unit is changed by +1 or -1 times in the time domain, the first reflection signal and the second reflection signal generated are both OFDM signals with extended bandwidth. However, a general OFDM receiver can only receive part of the subcarriers. The present invention will collect this part of the subcarrier information for subsequent demodulation of the Internet of Things information transmitted by the tag. Specifically, according to a specific implementation method of an embodiment of the present invention, the center frequencies of the first reflection signal and the second reflection signal are moved to f c -f tag In this frequency band, through down-conversion, carrier frequency offset correction, and delay correction, some subcarriers of the reflected signal that have not been channel-equalized can be obtained.

[0110] S303, using the demodulated ambient cellular OFDM signal and the frequency domain information of the first reflected signal, and combining with a known pilot pattern, to perform cascade channel estimation and equalization;

[0111] Since the first reflected signal is a broadband OFDM signal, it is affected by the multipath effect in both channels. In the reflection process, the cyclic prefix CP is not destroyed, so the cyclic prefix CP can still be used to combat the inter-subcarrier interference and inter-code interference caused by the multipath effect. However, the multipath effect also causes frequency selective fading, so it is necessary to estimate the cascaded channel on each subcarrier. Specifically, Figure 4 As shown in FIG. 1 , from the perspective of the time domain structure, the label of the symbol where the PSS and SSS are located and the symbol after it always maintains 0 phase change; therefore, these OFDM symbols only shift the carrier frequency, and the subcarrier frequency domain value does not change.

[0112] In this embodiment, step S303 includes S3031-S3033:

[0113] S3031, according to the receiving end, demodulates the frequency domain information of the original environment cellular OFDM signal in the frequency band, and obtains the frequency domain value X of the original environment cellular OFDM signal at a certain time and frequency point k (n), after receiving the frequency domain information of the first reflected signal in the reflected frequency band after the frequency shift, the frequency domain value Y of the first reflected signal is obtained k (n);

[0114] S3032, perform channel estimation to obtain the frequency domain channel on the kth subcarrier at time n, using the LS channel estimation criterion, and the frequency domain channel value

[0115] S3033, perform channel equalization, restore the frequency domain subcarrier information of the ambient cellular OFDM signal after reflection, and use the LS channel equalization method to obtain the frequency domain value of the second reflected signal after channel equalization

[0116]

[0117] S40: Recover the bit information transmitted by the second reflected signal through a demodulation algorithm.

[0118] The core of demodulation is to first describe the changes of the tag to the cellular OFDM signal in the time domain, and express the reflected baseband OFDM time domain sampling value that is not affected by the channel. Then, the time domain is transformed to the frequency domain through IFFT to obtain the frequency domain value of the baseband OFDM subcarrier modulated by the tag, and compared with the actual frequency domain value restored by the terminal receiver demodulation. In theory, by traversing all possible tag modulation combinations, the modulation method with the smallest error between the theoretical value and the actual value can be found, but the complexity of this method is exponential O(2 n ) is too high, so the present invention demodulates the tag information by establishing a frequency domain equation and finding the least square solution of the equation group.

[0119] In addition, there are two points to note. First, due to the presence of multiple harmonics in the square wave of the tag impedance switching, the spectrum of the reflected signal is expanded and aliased. Therefore, when performing time domain and frequency domain baseband processing, the aliasing effect can be achieved by increasing the number of sampling points and other methods to improve the approximation accuracy of the theoretical value. Second, when demodulating, n tag The minimum norm solution of the linear equations is obtained by introducing repetitive coding during modulation to reduce the dimension of unknown numbers, which is beneficial for reliable demodulation and reliable transmission.

[0120] In this embodiment, in step S40, when demodulating, the demodulation is performed in the same manner as the demodulation of a commercial cellular receiver, and only 72 effective subcarriers centered on the carrier frequency in the second reflected signal are used to establish a frequency domain equation for demodulation. The remaining subcarriers are not used in demodulation because they are on both sides of the bandwidth and are seriously affected by spectrum spread interference. The step S40 includes the following steps:

[0121] Using the frequency domain value and DFT matrix of the ambient cellular OFDM signal, the dimension is n. f The left side of the equation group is the theoretical calculation value, and the right side is the actual demodulation value. The equation group is robustly solved. tag The useful vector of dimension simplifies to the following formula:

[0122]

[0123] Among them, A is determined by the DFT matrix, the cellular OFDM frequency domain value and the time domain offset. f ×n tagA matrix of dimension a is n f dimensional vector, calculated from the actual observations, is the n to be solved tag dimensional vector;

[0124] Perform a substitution transformation to convert the variable into 0, 1 values ​​and recode it; For each row there are n tag The repeated coding matrix of n rows has 1 elements and the rest are 0; Written Substitute into formula (5), let in Yes f dimensional all-1 vector, we get the following formula:

[0125]

[0126] The vector whose value is 0 or 1 is the bit transmitted by the second reflected signal to be solved, and the minimum norm solution of the problem in formula (6) is obtained;

[0127] The minimum norm solution of formula (6) is obtained by pseudo-inverse as follows:

[0128]

[0129] After solving, The value of each element is compared with 0 or 1 by distance, and the minimum distance decoding criterion is used to perform bit judgment decoding to obtain the Internet of Things information bit of the backscatter tag transmitted by the second reflection signal.

[0130] Combination Figure 6 The figure shows the test results of the bit error rate of the second reflected signal of the backscatter tag at different code rates. The test equipment and configuration are as follows: NI's USRP2952R is used as the transceiver, the transceiver is 30 meters away, the backscatter tag is 10 meters away from the transmitter, and the 5MHz bandwidth LTE system FDD downlink signal is used as the environmental cellular OFDM signal. The carrier frequency is 847.5MHz, and the average transmission power is 15dBm; the backscatter tag consists of a synchronization circuit, an impedance switching circuit, and an FPGA.

[0131] Example 3

[0132] The present invention discloses a backscatter communication demodulation system based on an environmental cellular OFDM signal, the system comprising a transmitting end, a backscatter tag and a receiving end; Figure 1 As shown, in this embodiment, the sending end is Figure 1 The base station in the backscatter tag is Figure 1The receiving end is the reflection device in the signal receiving process, and its name can also be defined as a receiver or a terminal receiver.

[0133] On the basis of the embodiments 1 and 2, a specific embodiment is provided for the specific composition of the transmitting end, the backscattering tag and the receiving end, wherein the transmitting end is a signal source for sending the ambient cellular OFDM signal, which is used to send the ambient cellular OFDM signal of 128 FFT sampling points; the backscattering tag is responsible for the backscattering modulation transmission of the data to be transmitted, and the backscattering tag adopts 2ASK modulation to generate the first reflected signal and the second reflected signal with 2PSK modulation information and frequency shifting; the receiver is responsible for receiving the first reflected signal and the second reflected signal and demodulating the information transmitted by the second reflected signal of the backscattering tag.

[0134] In step S10 of embodiment 2, the following parameter set commonly used in LTE is used as the environmental OFDM signal: 128 sampling points, the number of useful subcarriers is 72, and the cyclic prefix (CP) length is 9. In steps S202 and S203 of the embodiment, on each OFDM signal of 128 time domain sampling points, the same number of pilots or information as the number of useful subcarriers of the environmental signal is carried. In step S202, 2 OFDM symbols in a subframe are used as label pilots: 1 OFDM symbol is used to estimate the cascade channel, and 1 OFDM symbol is used to estimate the offset error. The remaining 12 OFDM symbols in a subframe are used to transmit useful information (see Figure 4 ). In steps S202 and S203, the switching frequency of the tag RF impedance is twice the sampling frequency of the ambient cellular OFDM signal. And during bit mapping, the phases 0 and π of the tag RF impedance correspond to the transmitted bits 0 and 1, respectively. When demodulating in step S304, the demodulation of the commercial cellular receiver only uses the 72 effective subcarriers centered on the carrier frequency in the second reflected signal.

[0135] In this embodiment, both the transmitting end and the receiving end use NI's USRP2952R, and the 5MHz bandwidth LTE system FDD downlink signal is used as the environmental cellular OFDM signal. The carrier frequency is 847.5MHz, and the average transmission power is 15dBm. The structure of the backscatter tag is composed of a synchronization circuit, an impedance switching circuit and an FPGA.

[0136] Through the above embodiments, the backscatter communication demodulation method and system based on the environmental cellular OFDM signal of the present invention shifts the frequency of the original cellular OFDM signal; uses repetition coding to reduce the bandwidth of the reflected signal and the dimension of the demodulation variable during information modulation; and establishes a matrix equation demodulation to restore the label information during demodulation. Such a method and system can provide users with reliable, high-speed, and wide-coverage backscatter communication, and facilitate the widespread deployment of low-cost, low-power Internet of Things.

[0137] Furthermore, an embodiment of the present invention can achieve a communication rate of up to 2.2 Mbps within a communication range of 30 meters. This rate is related to the bandwidth of the environmental signal used. For example, a higher rate can be achieved by using an environmental OFDM signal with a larger bandwidth. In addition, the backscatter tag can also achieve backscattering based on cellular OFDM signals by using a lower frequency shift. The core advantage of this system is the faster communication rate and the low power consumption of the backscatter tag at the microwatt level, which is one of the trends in the future design of green Internet of Things and 6G networks.

[0138] The above is a specific description of the preferred implementation of the present invention, but the invention is not limited to the embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A backscatter communication demodulation method based on environmental cellular OFDM signals, characterized in that: The method comprises the following steps: S10, the transmitting end sends the ambient cellular OFDM signal to the receiving end; S20, the backscatter tag modulates the collected IoT information on the ambient cellular OFDM signal, including the following steps: The backscatter tag detects synchronization information from the ambient cellular OFDM signal sent by the transmitter, and achieves coarse synchronization with the ambient cellular OFDM signal; The backscatter tag switches the tag radio frequency impedance to generate a first reflected signal according to a pilot pattern known to the receiving end; The backscatter tag switches the tag radio frequency impedance to generate a second reflected signal according to the IoT information bit; S30, the receiving end demodulates the ambient cellular OFDM signal and the second reflected signal, including the following steps: The receiving end demodulates the frequency domain information of the original environmental cellular OFDM signal frequency band; Receiving the first reflected signal frequency domain information and the second reflected signal frequency domain information in the reflected frequency band after the frequency shifting; Using the demodulated ambient cellular OFDM signal and the frequency domain information of the first reflected signal, combined with a known pilot pattern, cascade channel estimation and equalization are performed; S40: Recover the bit information transmitted by the second reflected signal through a demodulation algorithm.

2. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 1, characterized in that: In step S20, the backscatter tag detects synchronization information from the ambient cellular OFDM signal sent by the transmitting end to achieve coarse synchronization with the ambient cellular OFDM signal, including: The primary synchronization signal PSS and the secondary synchronization signal SSS in the ambient cellular OFDM signal are used to achieve coarse synchronization.

3. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 1, characterized in that: In step S20, the switching frequency of the tag radio frequency impedance is twice the sampling frequency of the ambient cellular OFDM signal.

4. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 1, characterized in that: In step S10, the frequency band signal of the ambient cellular OFDM signal is s lte [n], define f c is the carrier frequency of the environmental cellular OFDM signal, s[n] is the cellular baseband signal, and the frequency band signal is expressed as: Where n is the baseband sampling point, j is the imaginary unit, and t is the continuous time variable.

5. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 4, characterized in that: In step S10, the number of FFT points of the ambient cellular OFDM signal is n fft , used to represent the number of sampling points in the time domain of a cellular OFDM symbol excluding the cyclic prefix CP; The sampling frequency of the ambient cellular OFDM signal is T s , the subcarrier spacing is Δf, where the sampling frequency is T s is the duration of a cellular sampling point, 6. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 5, characterized in that: In step S20, the reflection coefficient modulus of the backscatter tag is ||α tag ||, the switching range of the tag RF impedance is -||α tag || and ||α tag ||; the switching frequency of the tag RF impedance is f tag ; The modulus of the equivalent label reflection coefficient is α′ tag ,and The number of sampling points of the backscatter tag modulation is n tag , indicating that in one environmental OFDM symbol, the backscatter tag changes n tag sampling points; The number of bits transmitted by the backscatter tag is n bit , indicating that in one environment OFDM symbol, the backscatter tag transmits n bit bits; The number of times the backscatter tag repeats encoding is M, which means that during the encoding process of the backscatter tag transmitting information, each information bit is repeated M times, and In step S30, the number of frequency domain subcarriers used in the demodulation process at the receiving end is n. f , represents the number of subcarriers used in frequency domain analysis of the first reflected signal and demodulation of the second reflected signal.

7. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 6, characterized in that: In step S20, the initial phase of the backscatter tag at the nth sampling point is θ n , the information transmitted by the backscattered tag is transmitted with an initial phase θ n Different values ​​of correspond to different information bits; In step S20, the backscatter tag modulates the IoT information into the ambient cellular OFDM signal by switching the tag RF impedance, thereby adding an initial phase θ to each sampling unit of the ambient cellular OFDM signal. n , and by the initial phase θ n Transmit backscatter tag information; The cellular baseband signal s[n] is sampled at a frequency T s The value inside remains constant, and the backscattered tag is at the sampling frequency T s The tag RF impedance is continuously changed to form a square wave, which is expanded according to the Fourier series and expressed by the following formula: Where k is the constant of the series expansion and t is the continuous time; In formula (1), s tag The first harmonic of [n] is expressed as: s′ tag [n]=2α′ tag cos(2πf tag t+θ n ) (2); f tag is the frequency domain value where the tag is removed, and When only the first reflected signal and the second reflected signal of the first harmonic frequency component are considered, the first reflected signal and the second reflected signal are the product of the reflection coefficient and the ambient cellular OFDM signal, which can be expressed as: Take the lower sideband of formula (3), due to the initial phase θ n There are only two possible values: 0 and π. and The value of has the same effect, both are -1, then the first reflected signal and the second reflected signal received from the frequency band signal are expressed as: According to formula (4), the baseband of the first reflected signal and the second reflected signal is the result of the cellular baseband signal s[n] receiving BPSK modulation, and the carrier frequency is offset by f tag .

8. The backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 1, characterized in that: In step S30, the receiving end demodulates the frequency domain information of the original environment cellular OFDM signal frequency band, including: The signal received by the receiving end is sequentially subjected to down-conversion, carrier frequency offset correction, delay correction, channel estimation and equalization to restore the bit information transmitted by the sending end; In step S30, the use of the demodulated ambient cellular OFDM signal and the first reflected signal frequency domain information, combined with a known pilot pattern, to perform cascade channel estimation and equalization includes: According to the receiving end, the frequency domain information of the original environment cellular OFDM signal is demodulated on the frequency band of the original environment cellular OFDM signal to obtain the frequency domain value X of the original environment cellular OFDM signal at a certain time and frequency point. k (n), after receiving the frequency domain information of the first reflected signal in the reflected frequency band after the frequency shift, the frequency domain value Y of the first reflected signal is obtained k (n); Perform channel estimation to obtain the frequency domain channel at time n and the kth subcarrier. Using the LS channel estimation criterion, the frequency domain channel value Perform channel equalization to restore the frequency domain subcarrier information of the reflected ambient cellular OFDM signal, and use the LS channel equalization method to obtain the frequency domain value of the second reflected signal after channel equalization.

9. A backscatter communication demodulation method based on environmental cellular OFDM signals according to claim 8, characterized in that: In step S40, the step of restoring the bit information transmitted by the second reflected signal by using a demodulation algorithm includes: Using the frequency domain value and DFT matrix of the ambient cellular OFDM signal, the dimension is n. f The left side of the equation group is the theoretical calculation value, and the right side is the actual demodulation value. The equation group is robustly solved for the 72-dimensional useful vector and simplified to the following formula: Among them, A is determined by the DFT matrix, the cellular OFDM frequency domain value and the time domain offset. f ×n tag A matrix of dimension a is n f dimensional vector, calculated from the actual observations, is the n to be solved tag dimensional vector; Perform a substitution transformation to convert the variable into 0, 1 values ​​and recode it; For each row there are n tag The repeated coding matrix of n rows has 1 elements and the rest are 0; Written Substitute into formula (5), let in Yes f dimensional all-1 vector, we get the following formula: is a vector whose elements are either 0 or 1, i.e., the bits transmitted by the second reflected signal to be solved. The minimum norm solution to the problem in formula (6) is obtained. The minimum norm solution of formula (6) is obtained by pseudo-inverse as follows: After solving, The value of each element is compared with 0 or 1 by distance, and the minimum distance decoding criterion is used to perform bit judgment decoding to obtain the Internet of Things information bit of the backscatter tag transmitted by the second reflected signal.

10. A backscatter communication demodulation system based on ambient cellular OFDM signals, characterized in that: It includes a transmitter, a backscatter tag and a receiver; The transmitting end is used to send the ambient cellular OFDM signal to the receiving end; The backscatter tag is used to modulate the collected Internet of Things information on the environmental cellular OFDM signal; including the backscatter tag detecting synchronization information from the environmental cellular OFDM signal sent by the transmitting end to achieve coarse synchronization with the environmental cellular OFDM signal; the backscatter tag switches the tag radio frequency impedance according to the pilot pattern known to the receiving end to generate a first reflection signal; the backscatter tag switches the tag radio frequency impedance according to the Internet of Things information bit to generate a second reflection signal; The receiving end is used to demodulate the environmental cellular OFDM signal and the second reflected signal; including the receiving end demodulating the frequency domain information of the original environmental cellular OFDM signal on the frequency band; receiving the first reflected signal frequency domain information and the second reflected signal frequency domain information on the reflected frequency band after frequency shifting; using the demodulated environmental cellular OFDM signal and the first reflected signal frequency domain information, and combining with the known pilot pattern, to perform cascade channel estimation and equalization; and restoring the bit information transmitted by the second reflected signal through a demodulation algorithm.

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