CN115834315B - High-speed OFDM subcarrier environment back scattering communication method and system - Google Patents
High-speed OFDM subcarrier environment back scattering communication method and system Download PDFInfo
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Abstract
The invention discloses a high-speed OFDM subcarrier environment backscatter communication method and a system, belonging to the field of wireless communication. The transmitting end tag encodes based on information acquired from the environment or the host computer. When an OFDM signal capable of being used for transmission exists in the environment, the transmitting end tag changes the subcarrier arrangement mode of the signal through subcarrier shift keying in the process of reflecting the environment OFDM signal, and different subcarrier arrangement modes are utilized to represent different tag information, so that the transmission of the tag information is realized. The receiving end adopts a receiver with double antennas to respectively receive and demodulate the information carried in the original OFDM signal and the reflected OFDM signal. The receiving end carries out correlation operation on the demodulated information and then searches the peak position of the correlation result. The peak position represents the tag information because the transmitting end tag maps the information into different subcarrier arrangement modes. The receiving end demodulates the tag information to realize the high-speed OFDM subcarrier environment backscatter communication. The minimum modulation unit of the invention is an OFDM subcarrier.
Description
Technical Field
The invention relates to a high-speed OFDM subcarrier environment backscatter communication method and a system, belonging to the field of wireless communication.
Background
An ambient backscatter communication system includes three parts, a radio frequency signal source, a tag, and a receiver. The radio frequency signal source is usually a commercial signal source such as a Wi-Fi node, an LTE base station and the like, and can generate signals to communicate with other commercial equipment; the tag is a hardware system with two characteristics of low power consumption and low cost, and transmits own information by modulating and reflecting signals generated by a radio frequency signal source; the receiver then obtains tag information by receiving and demodulating the radio frequency signal reflected from the tag. Since such a communication process utilizes existing signals in the environment without the need for additional generation of signals, the environmental backscatter communication system has three advantages of low power consumption, low cost, and high spectrum utilization.
The communication process uses an environmental radio frequency signal from a commercial signal source, so that a modulation mode of tag information needs to be specifically designed based on a physical layer modulation mode of the commercial signal. Since Orthogonal Frequency Division Multiplexing (OFDM) transmission systems are widely adopted by various commercial protocols such as Wi-Fi, LTE, NB-IoT, designing a backscatter communication method and system for OFDM transmission systems is focused on by many students, and the present invention is also designed for this aspect. Conventional OFDM backscatter communication methods perform transmission of information by modulating the phase of OFDM symbols. In this way, the tag cannot independently modulate different subcarriers of the OFDM symbol. Therefore, the conventional communication system can only realize symbol-level modulation, has the defect of low transmission rate, and is not suitable for high-speed communication occasions.
Disclosure of Invention
The invention mainly aims to provide a high-speed OFDM subcarrier environment backscattering communication method, a system method and a system, which utilize the spectrum characteristics of an environment OFDM signal to carry out high-speed backscattering communication; when the transmitting end tag reflects an environment OFDM signal, the subcarrier arrangement mode of the signal is changed through subcarrier shift keying, different label information is represented by different subcarrier arrangement modes, and the transmission of the label information is realized; the receiving end is a receiver with dual antennas. The receiver adopts correlation decoding, and utilizes the mapping relation between the subcarrier arrangement mode established by the transmitting terminal tag and the peak position of the correlation result of the received data to realize demodulation of the transmitting terminal tag transmission information, namely realize high-speed OFDM subcarrier environment backscatter communication. The minimum modulation unit of the invention for the environmental OFDM signal is the subcarrier of OFDM, which is far smaller than the minimum modulation unit of the traditional OFDM backscattering communication system. Therefore, compared with the traditional communication system, the invention has the advantages of high transmission rate, simple structure, wide application range, low power consumption, low cost and the like.
The aim of the invention is achieved by the following technical scheme.
The invention discloses a high-speed OFDM subcarrier environment backscatter communication method and a system thereof, wherein a transmitting end tag encodes based on information acquired from an environment or an upper computer. After coding is finished, when an OFDM signal capable of being used for transmission exists in the environment, a transmitting end tag changes the subcarrier arrangement mode of the signal through subcarrier shift keying in the process of reflecting the environment OFDM signal, and different label information is represented by different subcarrier arrangement modes, so that the transmission of the label information is realized. The receiving end adopts a receiver with double antennas to respectively receive and demodulate the information carried in the original OFDM signal and the reflected OFDM signal. After demodulation is completed, the receiving end carries out correlation operation on the demodulated information and searches the peak position of the correlation result. The peak position represents the tag information because the transmitting end tag maps the information into different subcarrier arrangement modes. The receiving end can demodulate the label information, namely realize the high-speed OFDM subcarrier environment back scattering communication.
The invention discloses a high-speed OFDM subcarrier environment backscattering communication method, which comprises the following steps:
step one, a transmitting terminal tag encodes information to be transmitted based on the number of subcarriers in an environment OFDM signal.
Step 1.1, the transmitting end tag acquires information to be transmitted from the environment or an upper computer and encodes the information.
And 1.2, the transmitting end tag maps the encoded information into a numerical value based on the number of subcarriers in the environment OFDM signal.
Step 1.2-1, the label predetermines the subcarrier bit number N of the environment OFDM signal, and groups the subcarriers of the OFDM signal according to no more than log 2 N bits based on N.
Step 1.2-2, the label calculates the numerical value of each group of subcarrier information after grouping.
And step two, the transmitting end tag changes the subcarrier arrangement mode of the signal through subcarrier shift keying in the process of reflecting the environment OFDM signal, and different label information is represented by using different subcarrier arrangement modes, so that the transmission of the label information is realized. Different subcarrier arrangement modes are adopted to represent different label information modes for transmission, the modulating unit is reduced to OFDM subcarriers from OFDM symbols, the transmission rate of the label information is greatly improved, and the application requirements of high-rate communication are met.
And 2.1, after capturing an OFDM signal for transmission in the environment by the transmitting end tag, reflecting the signal and starting to transmit tag information.
And 2.2, the transmitting end tag carries out subcarrier shift keying modulation while reflecting the environment OFDM signal so as to change the subcarrier pattern of the reflected signal, and the tag information is embedded into the reflected signal for transmission.
And 2.2-1, the first reflection antenna of the transmitting end tag reflects the environment OFDM signal and carries out frequency shift. Let the bandwidth of the ambient OFDM signal be Λ, the baseband signal be y ob (t), and the center frequency be f o. For this signal, the first reflective antenna needs to shift its frequency Λ while reflecting. In order to perform such frequency shift on the environmental signal, the switching frequency of the first reflective switch to which the first reflective antenna is connected is set to f 1 =Λ.
Let the antenna impedance be Z A, the corresponding load impedance of the switch in the open state be Z L1, and the corresponding load impedance in the closed state be Z L2, then the reflection coefficient Γ 1 of the antenna in the open state is: The reflection coefficient Γ 2 of the antenna in the closed state is: wherein, Is the conjugate of Z A. Thus, by switching the first reflective switch at the switching frequency Λ, the change in the reflection coefficient of the first reflective antenna will exhibit a square wave y rf1 (t) at frequency Λ. Taking Fourier series expansion into consideration, and taking primary harmonic of the square wave into account to obtain:
Wherein f 1 =Λ is the switching frequency of the first reflective switch to which the first reflective antenna is connected.
Therefore, after the first reflective antenna reflects the environmental OFDM signal, the generated reflected signal y out1 (t) is:
Wherein a 1 is the energy intensity of the reflected signal of the first reflective antenna. As can be seen from y out1 (t), the reflected signal is frequency shifted by Λ compared to the original signal.
Step 2.2-2, the second reflective antenna of the transmitting end tag reflects the environmental OFDM signal and the reflected signal generated by the first reflective antenna, and frequency shifts the signals at the same time.
If the bandwidth of the sub-carrier of the environmental OFDM signal is ζ and the decimal value to be transmitted by the tag is Θ (t), the second reflection antenna needs to perform a frequency shift of Θ (t) ×ζ. For this reason, similarly to the setting in step 2.2-1, the switching frequency of the second reflective switch to which the second reflective antenna is connected is set to ζxΘ (t). Similar to the first reflective antenna, the variation of the reflection coefficient of the second reflective antenna will correspond to a square wave y rf2 (t) of frequency ζxΘ (t) for which fourier analysis is performed and whose primary harmonic is taken into account:
Wherein f 2 =Θ (t) ×ζ is the switching frequency of the second reflective switch.
Thus, the reflected signal y out2 (t) generated by the second reflective antenna is:
Wherein a 2 is the energy intensity of the reflected signal of the second reflective antenna.
As can be seen from y out2 (t), the second reflected antenna frequency shifts the signal by f 2 and f 1+f2 while reflecting it. Let the environment OFDM signal include N sub-carriers, let the bandwidth of the reflection band be Λ, and the center frequency point be f o +Λ. The tag shifts the N- Θ (t) +1-N sub-carriers of the OFDM symbol of the original frequency band into the 1 st- Θ (t) sub-carriers in the reflection frequency band through the frequency shift f 2; the tag frequency shifts the 1 st to N- Θ (t) th subcarriers of the OFDM symbol of the original frequency band to the Θ (t) +1 st to N th subcarriers in the reflection band by the frequency shift f 1+f2.
The minimum modulation unit of the subcarrier shift keying modulation technique described in step 2.2 is a subcarrier of an OFDM symbol instead of an OFDM symbol, so that different subcarrier arrangements are used to represent different tag information for transmission, the modulation unit is reduced from the OFDM symbol to the OFDM subcarrier, the information transmission rate is not affected by the OFDM symbol rate, and the method has the advantage of high transmission rate, and meets the application requirements of high-rate communication.
Step three, the receiving end receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas respectively; the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained; because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication is realized. Because the noise of the environment signal and the noise of the reflected signal have no correlation, the correlation decoding method has better anti-noise performance, and improves the anti-noise performance and the robustness of the communication system.
And 3.1, the receiving end respectively receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas.
Step 3.1-1, setting a center frequency point f o corresponding to a first receiving antenna of the receiving end, wherein the bandwidth of a front-end filter corresponding to the antenna is Λ, so as to receive and demodulate the environmental OFDM signal.
Considering a noise-free and interference-free channel, and simultaneously, ignoring the influence of the reflected signal on the environmental signal because the intensity of the reflected signal is far smaller than that of the environmental signal, the baseband signal received by the first receiving antenna is y ob1(t)＝Aroyob (t), where a ro is the attenuation coefficient of the direct transmission link.
Based on fourier transform, the spectrum Y ob1 (ω) of Y ob1 (t) is:
wherein, τ＝1/ζ，D i is information carried by the ith subcarrier in the N subcarriers of the original OFDM symbol. Thus, the information D ob1 demodulated by the environmental OFDM signal is
Step 3.1-2, setting a center frequency point f o +Λ corresponding to a second receiving antenna of the receiving end, and setting the bandwidth as Λ so as to receive and demodulate the reflected OFDM signal.
The baseband OFDM signal received from the second receiving antenna is:
Wherein, a rb is the attenuation coefficient of the reflection link, and F (t) is the rf front-end filter connected to the second receiving antenna with bandwidth Λ.
Y ob2 (t) is transformed into the frequency domain by fourier transformation, resulting in a spectrum Y ob2 (ω) of:
wherein, Thus, the information carried by the reflected OFDM signal is:
wherein, D 1＝{d1,…,dN and D 3＝{dΘ(t)+1,…,dN are interfering signals;
And (3) carrying the tag information coded in the step (I) as a result of the cyclic shift of the subcarriers of the original OFDM symbol.
And 3.2, the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained. Because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication is realized.
By correlating D ob1 with D ob2, a correlation result Z (m) of
Since Z (m) will appear maximum when D ob1 is aligned with D ob2, the correlation results will peak when m= {0, - Θ (t), Θ (t). Therefore, the demodulation of the label information is completed by locating the peak value which is positioned at the non-origin point in the correlation result, namely, acquiring the value corresponding to the label.
The invention also discloses a high-speed OFDM subcarrier environment backscatter communication system for realizing the high-speed OFDM subcarrier environment backscatter communication method. The high-speed OFDM subcarrier environment backscatter communication system comprises a transmitting terminal tag and a receiving terminal. The transmitting end tag comprises an environment signal detection module, a microprocessor and a transmission state control module. The receiving end is a radio platform with dual antennas.
The environment signal detection module comprises a radio frequency detector and a comparator and is used for detecting whether an OFDM signal which can be used for tag information transmission exists in the environment. The radio frequency detector captures radio frequency signals in the environment and outputs envelope values of the signals, and the comparator judges whether OFDM signals available for transmission exist in the environment or not by comparing the envelope values with amplitude differences of detection thresholds set based on priori knowledge and correspondingly outputs indication signals to the microprocessor. If there is an OFDM signal available for transmission in the environment, the indication signal is high; otherwise, the indication signal is low.
The microprocessor is a low-power consumption microprocessor and is used for realizing the coding of information to be transmitted based on the number of subcarriers in the environment OFDM signal in the step one, namely, the information to be transmitted obtained from the upper computer is used as tag information, the tag information is coded, and the numerical value corresponding to the coding result is calculated. Meanwhile, the microprocessor outputs a control signal based on the indication signal output by the environment signal detection module to control the transmission state control module. When the indication signal is at a high level, the control signal output by the micro-processing is also at a high level, and is used for controlling the transmission state control module to enter a transmission starting state; when the indication signal is at low level, the control signal output by the microprocessor is also at low level, so as to control the transmission state module to enter a 'stop transmission' state.
The transmission state control module is used for correspondingly connecting two radio frequency switches for two antennas working in an ISM wave band, and is used for realizing that the subcarrier arrangement mode of the signal is changed through subcarrier shift keying in the process of reflecting the environment OFDM signal by the transmitting end tag in the step two under the state of 'starting transmission', and different tag information is represented by using different subcarrier arrangement modes. The module is controlled by control signals from a microprocessor. When the module enters a "start transmission" state, the first and second radio frequency switches to which the first and second reflective antennas of the module are connected are switched at different frequencies to cause the reflection coefficient of the antennas to change at the designated frequency. When the module enters a transmission stopping state, the first radio frequency switch and the second radio frequency switch of the transmission state module are both in a closed state, and the transmission of the tag information is not performed.
The receiving end is a software defined radio level station with double antennas for receiving and demodulating the tag information, and is used for realizing step-dependent decoding, namely, demodulating the tag information in the step one, namely, realizing a high-speed OFDM subcarrier environment backscatter communication method and system. The center receiving frequency point of the first receiving antenna of the platform is set as the center frequency point of the environment excitation signal and is used for receiving and demodulating information carried by the OFDM symbol of the environment excitation signal, and the center frequency point of the second receiving antenna of the platform is set as the center frequency point of the environment excitation signal and is offset from the frequency band bandwidth of the OFDM signal and is used for receiving and demodulating information carried by the OFDM symbol of the reflected signal.
Advantageous effects
1. The invention discloses a high-speed OFDM subcarrier environment backscattering communication method and a system, which utilize the frequency spectrum characteristics of an environment OFDM signal to carry out high-speed backscattering communication. The transmitting end tag embeds the tag information into the environment OFDM signal by adopting subcarrier shift keying to realize information transmission, and the receiving end demodulates the tag information by adopting related decoding by utilizing the environment OFDM signal and the reflection OFDM signal to realize the tag information transmission, namely, realize high-speed OFDM subcarrier environment backscattering communication.
2. The invention discloses a high-speed OFDM subcarrier environment backscatter communication method and a system, wherein in the process of transmitting label information by a transmitting end, different subcarrier patterns for representing different label information are constructed by adopting subcarrier shift keying technology. Compared with the traditional OFDM backscattering communication method, the modulation unit is reduced from OFDM symbols to OFDM subcarriers by adopting the method, so that the transmission rate of tag information is greatly improved, and the method is more suitable for application occasions requiring high-rate communication such as remote office work, audio and video live broadcast and the like.
3. The invention discloses a high-speed OFDM subcarrier environment backscatter communication method and a system, wherein a receiving end adopts a correlation decoding method, and the demodulation of label information is completed by acquiring the position of a peak value of a correlation result. The method has better anti-noise performance because the correlation operation is performed and the noise of the environment signal and the reflected signal has no correlation. Therefore, compared with the traditional OFDM environment backscatter communication system, the invention has lower error rate under noise condition, and better anti-noise performance and robustness.
4. The invention discloses a high-speed OFDM subcarrier environment backscattering communication method and a system, which are used for realizing the high-speed OFDM subcarrier environment backscattering communication method. The system can receive the label information only by controlling the radio frequency switch through the microprocessor, and the labels are all low-power-consumption devices, so that the system has the characteristics of simple hardware structure and high realizability.
5. According to the high-speed OFDM subcarrier environment backscatter communication method and system disclosed by the invention, on the basis of realizing the beneficial effects 1-4, the transmitting end tag can complete information transmission by using environment signals conforming to a commercial protocol. Because commercial OFDM excitation sources such as WiFi and LTE exist widely, the method does not need to deploy a special radio frequency signal source, and has the advantages of simple system structure, low deployment difficulty and low complexity.
Drawings
Fig. 1 is a flow chart of a high-speed OFDM subcarrier environment backscatter communication method of the present disclosure.
Fig. 2 is a schematic diagram of a conventional phase modulation based OFDM environment backscatter communication system.
Fig. 3 is a schematic diagram of a high-speed OFDM subcarrier environment backscatter communication method and information transmission of a system transmitting end tag.
Fig. 4 is a schematic diagram of information demodulation at a receiving end of a high-speed OFDM subcarrier environment backscatter communication method and system, where Λ is an environment OFDM signal bandwidth, f o is an environment OFDM signal center frequency point, and N is an environment OFDM signal subcarrier number.
Fig. 5 is a diagram of a high-speed OFDM subcarrier environment backscatter communication method and system architecture according to the present invention.
Fig. 6 is a schematic diagram of a transmission throughput of a test system according to an embodiment of the present invention, where fig. a is a schematic diagram of a system throughput of the test system transmitted in a line-of-sight environment, and fig. b is a schematic diagram of a system throughput of the test system transmitted in a non-line-of-sight environment.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings and examples. The technical problems and the beneficial effects solved by the technical proposal of the invention are also described, and the described embodiment is only used for facilitating the understanding of the invention and does not have any limiting effect.
The embodiment adopts an open-source commercial Wi-Fi platform to generate an environment OFDM signal which accords with an 802.11g protocol as an environment excitation source. The embodiment sets the center frequency point of the signal to 2.412GHz, the transmission rate to 6Mbps and the transmitting power to 20dBm. The method and the system for carrying out high-speed OFDM subcarrier environment backscatter communication by utilizing the environment signal comprise the following specific implementation steps:
As shown in fig. 1, the high-speed OFDM subcarrier environment backscatter communication method disclosed in this embodiment specifically includes the following implementation steps:
step one, a transmitting terminal tag encodes information to be transmitted based on the number of subcarriers in an environment OFDM signal.
Step 1.1, the transmitting end tag acquires information to be transmitted from the environment or an upper computer and encodes the information.
And 1.2, the transmitting end tag maps the encoded information into a numerical value based on the number of subcarriers in the environment OFDM signal.
In this embodiment, since the number of sub-carrier bits n=64 of the environmental OFDM signal conforming to the 802.11g protocol, the transmitting end tag can transmit log 2 (64) =6 bits of information at most by using each OFDM symbol. In order to improve transmission reliability, 3 bits of information are transmitted using one OFDM symbol in this embodiment. Since the OFDM symbol rate is 250kbps, in this way, the transmission rate of tag information is 750kbps. Is 3 times that of the conventional OFDM backscatter communication system.
And step two, the transmitting end tag changes the subcarrier arrangement mode of the signal through subcarrier shift keying in the process of reflecting the environment OFDM signal, and different label information is represented by using different subcarrier arrangement modes, so that the transmission of the label information is realized. Different subcarrier arrangement modes are adopted to represent different label information modes for transmission, the modulating unit is reduced to OFDM subcarriers from OFDM symbols, the transmission rate of the label information is greatly improved, and the application requirements of high-rate communication are met.
And 2.1, after capturing an OFDM signal for transmission in the environment by the transmitting end tag, reflecting the signal and starting to transmit tag information.
And 2.2, the transmitting end tag carries out subcarrier shift keying modulation while reflecting the environment OFDM signal so as to change the subcarrier pattern of the reflected signal, and the tag information is embedded into the reflected signal for transmission.
In this embodiment, since the bandwidth of the environmental OFDM signal is Λ=20 MHz, the subcarrier spacing is ζ=312.5 kHz, and the switching frequency of the first reflective switch connected to the first reflective antenna of the transmitting end tag is set to be 20MHz. Setting the corresponding load impedance of the switch in the opening state to be +infinity, setting the corresponding load impedance in the closing state to be 0, the reflection coefficient in the open state of the switch is 1 and in the closed state is-1. Thus, by switching the first impedance switch, a square wave with a frequency of 20MHz and an amplitude of 1 can be generated. With this square wave, the first reflective antenna shifts the ambient OFDM signal by 20MHz.
Meanwhile, if the square wave uses the tag data decimal value Θ (t) =4 transmitted by one OFDM symbol, the switching frequency of the second reflective switch connected to the second reflective antenna of the transmitting end tag is set to Θ (t) ×ζ=4×0.3125=1.25 MHz. In the present embodiment of the present invention, the corresponding load impedance in the off state of the switch is also set to approach + -infinity, the corresponding load impedance in the closed state is 0. The reflection coefficient of the switch in the open state is 1 and in the closed state is-1. Thus, by switching the second impedance switch, a square wave with a frequency of 1.25MHz and an amplitude of 1 can be generated. With this square wave, the second reflective antenna shifts both the ambient OFDM signal and the OFDM signal reflected by the first reflective antenna by 1.25MHz.
To sum up, the frequency shift of the environmental OFDM signals by 1.25MHz and 21.25MHz respectively is realized.
Step three, the receiving end receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas respectively; the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained; because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication is realized. Because the noise of the environment signal and the noise of the reflected signal have no correlation, the correlation decoding method has better anti-noise performance, and improves the anti-noise performance and the robustness of the communication system.
And 3.1, the receiving end respectively receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas.
And 3.2, the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained. Because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication method and the system are realized.
In this embodiment, a center frequency point corresponding to a first receiving antenna of a receiving end is set to be 2.412GHz, and a bandwidth is set to be 20MHz, so as to receive and demodulate an environmental OFDM signal; and setting the center frequency point corresponding to the second receiving antenna of the receiving end as 2.432GHz and the bandwidth as 20MHz so as to receive and demodulate the information carried by the reflected OFDM signal. In this way, the correlation result |z (m) | will peak at m=0, m=4, and m= -4. Thus, by acquiring the position where the peak other than the origin appears, the decimal value of the tag information, that is, Θ (t) =4, can be demodulated. Further, the tag information can be demodulated by the decimal value.
In the experiment, the designed high-speed OFDM subcarrier environment backscatter communication method and system comprise a transmitting terminal tag and a receiving terminal. The transmitting end tag comprises an environment signal detection module, a microprocessor and a transmission state control module. The receiving end is a radio platform with dual antennas.
The environment signal detection module comprises a radio frequency detector and a comparator and is used for detecting whether an OFDM signal which can be used for tag information transmission exists in the environment. In this embodiment, the radio frequency detector and the comparator are respectively an ANALOG device LT5534 and an ONSEMI NCS2200.
The microprocessor is a low-power consumption microprocessor and is used for realizing the coding of information to be transmitted based on the number of subcarriers in the environment OFDM signal in the step one, namely, the information to be transmitted obtained from the upper computer is used as tag information, the tag information is coded, and the numerical value corresponding to the coding result is calculated. In this embodiment, the microprocessor adopts AGLN FPGA of Igllo Nano company;
The transmission state control module is used for correspondingly connecting two radio frequency switches for two antennas working in an ISM wave band, and is used for realizing that the subcarrier arrangement mode of the signal is changed through subcarrier shift keying in the process of reflecting the environment OFDM signal by the transmitting end tag in the step two under the state of 'starting transmission', and different tag information is represented by using different subcarrier arrangement modes. In this embodiment, the transmission state control module is two VERT2450 RF antennas of Ettus company connected to two ADG902 RF switches of ANALOG DEVICES company.
The receiving end is a software defined radio level station with double antennas for receiving and demodulating the tag information, and is used for realizing the step-dependent decoding, namely, the demodulation of the tag information in the step one, namely, the high-speed OFDM subcarrier environment backscatter communication method and the system are realized. In this embodiment, the receiving end is USRP B210 of Ettus company.
Under the experimental conditions, the effective maximum throughput of the tag information of the system is 743kbps, which is far higher than that of a transmission OFDM back-scattering communication system. It should be emphasized that in the implementation, the transmission distance is further increased by increasing the gain of the reflection antenna of the transmitting end tag, increasing the power of the signal transmitted by the environmental excitation source, and the like. Meanwhile, the throughput can be further improved by selecting an OFDM environment excitation source with more subcarrier numbers or adding more subcarrier patterns selected by a tag to represent different back scattering information and the like, and the method is suitable for occasions needing high-speed communication.
The foregoing is merely a preferred embodiment of the present invention and is not intended to limit the embodiment of the present invention. Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art in light of the foregoing description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.
Claims (4)
1. The high-speed OFDM subcarrier environment backscattering communication method is characterized in that: comprises the following steps of the method,
Step one, a transmitting terminal tag encodes information to be transmitted based on the number of subcarriers in an environment OFDM signal;
Step two, the transmitting end tag changes the subcarrier arrangement mode of the signal through subcarrier shift keying in the process of reflecting the environment OFDM signal, and different label information is represented by different subcarrier arrangement modes, so that the transmission of the label information is realized; transmitting by adopting different subcarrier arrangement modes to represent different label information, and reducing the modulating unit from OFDM symbols to OFDM subcarriers;
the implementation method of the second step is that,
Step 2.1, after capturing an OFDM signal for transmission in the environment by a transmitting terminal tag, reflecting the signal and starting to transmit tag information;
step 2.2, the transmitting end tag carries out subcarrier shift keying modulation while reflecting the environment OFDM signal so as to change the subcarrier pattern of the reflected signal, and the tag information is embedded into the reflected signal for transmission;
2.2-1, a first reflection antenna of a transmitting end tag reflects an environment OFDM signal and carries out frequency shift; let the bandwidth of the environmental OFDM signal be Λ, the baseband signal be y ob (t), the center frequency be f o; for this signal, the first reflective antenna needs to shift its frequency Λ while reflecting; in order to perform such frequency shift on the environmental signal, the switching frequency of the first reflective switch connected to the first reflective antenna is set to f 1 =Λ;
Let the antenna impedance be Z A, the corresponding load impedance of the switch in the open state be Z L1, and the corresponding load impedance in the closed state be Z L2, then the reflection coefficient Γ 1 of the antenna in the open state is: The reflection coefficient Γ 2 of the antenna in the closed state is: wherein, Is the conjugate of Z A; thus, by switching the first reflective switch at the switching frequency Λ, the change in the reflection coefficient of the first reflective antenna will exhibit a square wave y rf1 (t) at frequency Λ; taking Fourier series expansion into consideration, and taking primary harmonic of the square wave into account to obtain:
Wherein f 1 =Λ is the switching frequency of the first reflective switch connected to the first reflective antenna;
therefore, after the first reflective antenna reflects the environmental OFDM signal, the generated reflected signal y out1 (t) is:
Wherein a 1 is the energy intensity of the reflected signal of the first reflective antenna; as can be seen from y out1 (t), the reflected signal is frequency shifted by Λ compared to the original signal;
2.2-2, reflecting the environment OFDM signals and the reflected signals generated by the first reflecting antenna by a second reflecting antenna of the transmitting end tag, and simultaneously carrying out frequency shift on the signals;
If the bandwidth of the sub-carrier wave of the environment OFDM signal is ζ, and the decimal value to be transmitted by the tag is Θ (t), the frequency shift of Θ (t) x ζ is needed by the second reflection antenna; for this purpose, the switching frequency of the second reflective switch connected to the second reflective antenna is ζxΘ (t); similar to the first reflective antenna, the variation of the reflection coefficient of the second reflective antenna will correspond to a square wave y rf2 (t) of frequency ζxΘ (t) for which fourier analysis is performed and whose primary harmonic is taken into account:
wherein f 2 =Θ (t) ×ζ is the switching frequency of the second reflective switch;
thus, the reflected signal y out2 (t) generated by the second reflective antenna is:
Wherein A 2 is the energy intensity of the reflected signal of the second reflective antenna;
As seen by y out2 (t), the second reflective antenna frequency shifts the signal by f 2 and f 1+f2 while reflecting it; let the environment OFDM signal contain N sub-carriers, and let the bandwidth of the reflection band be Λ, the center frequency point be f o +Λ; the tag shifts the N- Θ (t) +1-N sub-carriers of the OFDM symbol of the original frequency band into the 1 st- Θ (t) sub-carriers in the reflection frequency band through the frequency shift f 2; the tag shifts the 1 to N- Θ (t) th subcarriers of the OFDM symbol of the original frequency band into the Θ (t) +1 to N th subcarriers in the reflection frequency band by the frequency shift f 1+f2;
The minimum modulation unit of the subcarrier shift keying modulation technique described in step 2.2 is the subcarrier of the OFDM symbol instead of the OFDM symbol, so that different subcarrier arrangements are used to represent different tag information for transmission, the modulation unit is reduced from the OFDM symbol to the OFDM subcarrier, and the information transmission rate is not affected by the OFDM symbol rate;
Step three, the receiving end receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas respectively; the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained; because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication is realized.
2. The high-speed OFDM subcarrier environment backscatter communication method of claim 1, wherein: the first implementation method of the step is that,
Step 1.1, a transmitting end tag acquires information to be transmitted from an environment or an upper computer and encodes the information;
Step 1.2, the transmitting end tag maps the coded information into numerical values based on the number of subcarriers in the environment OFDM signal;
step 1.2-1, the tag predetermines the subcarrier bit number N of the environmental OFDM signal, grouping subcarriers of the OFDM signal according to no more than log 2 N bits based on N pairs;
Step 1.2-2, the label calculates the numerical value of each group of subcarrier information after grouping.
3. The high-speed OFDM subcarrier environment backscatter communication method of claim 1, wherein: the implementation method of the third step is that,
Step 3.1, the receiving end receives and demodulates the information carried in the original OFDM signal and the reflected OFDM signal by using double antennas respectively;
Step 3.1-1, setting a center frequency point f o corresponding to a first receiving antenna of a receiving end, wherein the bandwidth of a front-end filter corresponding to the antenna is Λ, so as to receive and demodulate an environment OFDM signal;
Considering a noise-free and interference-free channel, and ignoring the influence of a reflected signal on an environmental signal because the intensity of the reflected signal is far smaller than that of the environmental signal, wherein a ro is an attenuation coefficient of a direct transmission link, and the baseband signal received by the first receiving antenna is y ob1(t)＝Aroyob (t);
Based on fourier transform, the spectrum Y ob1 (ω) of Y ob1 (t) is:
wherein, τ＝1/ζ，D i is information carried by the ith subcarrier in the N subcarriers of the original OFDM symbol; thus, the information D ob1 demodulated by the environmental OFDM signal is
Step 3.1-2, setting a center frequency point f o +Λ corresponding to a second receiving antenna of the receiving end, wherein the bandwidth is Λ, so as to receive and demodulate the reflected OFDM signal;
The baseband OFDM signal received from the second receiving antenna is:
Wherein, a rb is an attenuation coefficient of the reflection link, F (t) is a radio frequency front-end filter connected to the second receiving antenna with bandwidth Λ;
y ob2 (t) is transformed into the frequency domain by fourier transformation, resulting in a spectrum Y ob2 (ω) of:
wherein, Thus, the information carried by the reflected OFDM signal is:
Wherein, D 1＝{d1,...,dN and D 3＝{dΘ(t)+1,...,dN are interfering signals;
The result of the sub-carrier cyclic shift of the original OFDM symbol is carried with the tag information coded in the step one;
Step 3.2, the receiving end carries out correlation operation on the information carried by the original OFDM signal and the information carried by the reflected OFDM signal, and a peak value position of a correlation operation result is obtained; because the transmitting end tag maps the information into different subcarrier arrangement modes, the peak position represents the tag information, and the demodulation of the tag information in the step one is realized, namely the high-speed OFDM subcarrier environment backscatter communication is realized;
By correlating D ob1 with D ob2, a correlation result Z (m) of
Since Z (m) will appear maximum when D ob1 is aligned with D ob2, the correlation result will appear peak when m= {0, - Θ (t), Θ (t); therefore, the demodulation of the label information is completed by locating the peak value which is positioned at the non-origin point in the correlation result, namely, acquiring the value corresponding to the label.
4. A high-speed OFDM subcarrier environment backscatter communication system for implementing a high-speed OFDM subcarrier environment backscatter communication method as claimed in claim 1, 2 or 3, characterized by: the method comprises a transmitting terminal label and a receiving terminal; the transmitting end tag comprises an environmental signal detection module, a microprocessor and a transmission state control module; the receiving end is a radio platform with double antennas;
The environment signal detection module comprises a radio frequency detector and a comparator and is used for detecting whether an OFDM signal which can be used for tag information transmission exists in the environment; the radio frequency detector captures radio frequency signals in the environment and outputs envelope values of the signals, and the comparator judges whether OFDM signals available for transmission exist in the environment or not by comparing the envelope values with amplitude differences of detection thresholds set based on priori knowledge and correspondingly outputs indication signals to the microprocessor; if there is an OFDM signal available for transmission in the environment, the indication signal is high; otherwise, the indication signal is low;
The microprocessor is a low-power consumption microprocessor and is used for realizing the first step of coding information to be transmitted based on the number of subcarriers in an environment OFDM signal, namely, the information to be transmitted obtained from an upper computer is used as tag information, the tag information is coded, and a numerical value corresponding to a coding result is calculated; meanwhile, the microprocessor outputs a control signal based on the indication signal output by the environment signal detection module to control the transmission state control module; when the indication signal is at a high level, the control signal output by the micro-processing is also at a high level, and is used for controlling the transmission state control module to enter a transmission starting state; when the indication signal is at a low level, the control signal output by the microprocessor is also at a low level and is used for controlling the transmission state module to enter a transmission stop state;
The transmission state control module is used for correspondingly connecting two radio frequency switches for two antennas working in an ISM wave band, and is used for realizing that the subcarrier arrangement mode of the signal is changed through subcarrier shift keying in the process of reflecting the environment OFDM signal by the transmitting end tag in the step two under the state of 'starting transmission', and different tag information is represented by using different subcarrier arrangement modes; the module is controlled by a control signal from the microprocessor; when the module enters a transmission starting state, the first radio frequency switch and the second radio frequency switch connected with the first reflection antenna and the second reflection antenna of the module are switched according to different frequencies, so that the reflection coefficient of the antenna is changed according to the appointed frequency; when the module enters a transmission stopping state, the first radio frequency switch and the second radio frequency switch of the transmission state module are both in a closed state, and the tag information is not transmitted;
the receiving end is a software defined radio level station with double antennas for receiving and demodulating the tag information, and is used for realizing step-dependent decoding, namely, demodulating the tag information in the step one, namely, realizing a high-speed OFDM subcarrier environment backscatter communication method and system; the center receiving frequency point of the first receiving antenna of the radio platform is set as the center frequency point of the environment excitation signal and is used for receiving and demodulating information carried by the OFDM symbol of the environment excitation signal, and the center frequency point of the second receiving antenna of the radio platform is set as the center frequency point of the environment excitation signal and is offset from the frequency band bandwidth of the OFDM signal and is used for receiving and demodulating information carried by the OFDM symbol of the reflected signal.
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