CN116405359A - LoRa concurrent communication demodulation method and system based on frequency domain interference iterative cancellation - Google Patents

LoRa concurrent communication demodulation method and system based on frequency domain interference iterative cancellation Download PDF

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CN116405359A
CN116405359A CN202310187039.8A CN202310187039A CN116405359A CN 116405359 A CN116405359 A CN 116405359A CN 202310187039 A CN202310187039 A CN 202310187039A CN 116405359 A CN116405359 A CN 116405359A
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frequency domain
frequency
chirp
signal
lora
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房鼎益
胡王倩
徐丹
陈少杰
潘超逸
陈晓江
牛进平
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NORTHWEST UNIVERSITY
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2649—Demodulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
    • H04L27/2691—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation involving interference determination or cancellation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
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    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

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Abstract

The invention discloses a LoRa concurrent communication demodulation method and system based on frequency domain interference iterative elimination, comprising signal transmission; taking the chirp signal as a processing unit, and transmitting the signal by a node end in a standard LoRa data packet frame format; receiving signals; the gateway receives the LoRa signal in a conflict state by taking the frequency domain characteristics of the pilot frequency part as a basis, wherein the conflict state is that a plurality of data packets are transmitted in the same time period; preprocessing signals; performing low-pass filtering and signal synchronization on the LoRa signal in the collision state, dividing the LoRa signal by taking the chirp length as a step length to obtain a plurality of chirp symbols, and performing carrier frequency offset correction on the chirp symbols; demodulating conflict signals; and D, carrying out frequency domain interference estimation and elimination on the chirp symbol preprocessed in the step three, and completing symbol demodulation. The demodulation method provided by the invention can complete demodulation of the conflict signal under the condition that the signal strength is far lower than the noise strength under the condition that the stronger robustness exists under the condition of low signal-to-noise ratio.

Description

LoRa concurrent communication demodulation method and system based on frequency domain interference iterative cancellation
Technical Field
The invention belongs to the field of communication, and particularly relates to a LoRa concurrent communication demodulation method and system based on frequency domain interference iterative cancellation.
Background
The LoRa is based on a linear spread spectrum modulation technique, where each signal modulated, i.e., chirp, is a sin wave with a linearly increasing (upchirp) or decreasing (downchirp) frequency. The frequency of (-BW/2, BW/2) chirp in the band range is from the initial frequency f 0 The linear rise starts until bw/2 and then returns to the lower band boundary-bw/2, thus sweeping the entire bandwidth. Different chirp corresponds to different initial frequencies, and has 2 SF species. As shown in fig. 1 (a) and 1 (b), time-frequency diagrams of two up-chirp types having different initial frequencies are shown, respectively. From the figure, it can be seen that the frequency of the signal is gradually increased in a certain interval. As shown in fig. 1 (c), is a time-frequency diagram of a standard down-chirp. As can be seen from the figure, the frequency of the signal is changed with time to be gradually reduced. When demodulating, pulse compression is carried out on the original chirp to obtain an initial frequency, and because the upchirp and the downchirp have a conjugation relationship, the upchirp and the standard downchirp can be directly used for multiplying and carrying out Fourier transformation to obtain a frequency energy peak value in the interval. The frequency corresponding to the energy peak is the initial frequency to demodulate.
However, with the massive deployment of LoRa nodes, thousands of LoRa nodes are connected to one LoRa gateway. The severe packet collisions brought by the aggregate network structure result in a large number of packet losses and reduced throughput. In the wireless transmission technology, the processing manner of the collision problem is classified into collision avoidance and collision resolution.
In terms of collision avoidance, where multiple LoRa nodes are transmitted in parallel using multiple PHY techniques, the LoRa nodes may be configured with different radio parameters (e.g., channels, spreading factors, etc.) to mitigate collisions, but this requires cooperation between different operators and service providers, and the LoRa gateway can only support up to 8 LoRa nodes to transmit simultaneously. However, since the LoRa signal is transmitted in an environment below the background noise, it is difficult to perform signal-based channel listening, which results in collision avoidance failure. Meanwhile, channel monitoring requires a great deal of computation power and energy consumption of the nodes, so that the working life of the LoRa node is shortened. The LoRa nodes typically employ simple Aloha-based MACs to avoid collisions, subject to hardware capabilities and power supply limitations. According to the specifications of the protocol, the LoRa node can transmit data packets without channel detection. When the LoRa gateway cannot decode a packet sent from a node due to collision, the packet will be retransmitted after a random back-off time, and back-off retransmission further exacerbates the collision problem in the LoRa network. Meanwhile, unnecessary energy waste caused by multiple transmissions of the node is also brought. Thus, it is more desirable that the node can transmit data packets whenever it wants to transmit, and the gateway decodes the conflicting data packets.
There are also some related efforts in collision decoding, such as Choir, to classify collision frames according to different decimal place hardware frequency offset for each LoRa node. However, in practical application, due to the influence of interference and noise, it is difficult to accurately extract carrier frequency offset caused by hardware, and meanwhile, as the number of nodes increases, the frequency offset of the decimal place hardware is also repeated inevitably, so that classification failure is caused. And the other work mLoRa derives the time offset between two conflict packets according to the designed preamble detection strategy, then obtains a chirp-level conflict-free sample and corresponding frequency, predicts and reconstructs the time domain sample of the conflict signal by utilizing the chirp amplitude of the preamble part and the linear spread spectrum modulation mode of the chirp according to the conflict-free frequency and the samples, subtracts the estimated and reconstructed time domain sample of the conflict signal in the time domain of the conflict signal, and decodes the conflict data packet in one sample by repeating the estimation and subtraction operation. However, as the LoRa signal is transmitted under a low signal-to-noise ratio, the decoding accuracy of the method is greatly reduced under the condition of reduced SNR, and the communication performance of the LoRa is limited. The CoLaRa uses the packet time offset to resolve collisions. The received signal is cut into a series of receive windows, each window having a length equal to the chirp. The symbols are segmented with non-aligned windows. Then, for the signal in each window, collision decoding is performed in proportion to the length of the segment according to the height of the frequency domain peak. However, in the process that the SNR is reduced from 0dB to-15 dB, the system throughput is reduced to half of the original system throughput, and the performance is greatly reduced.
Disclosure of Invention
The invention provides a method and a system for demodulating LoRa concurrent communication based on frequency domain interference iterative cancellation, which can demodulate conflicting LoRa signals under the condition that SNR is less than 0; the method combines sliding window detection, and can ensure accurate data packet detection by utilizing the frequency domain correlation analysis of the received signal after pulse compression and the prior signal even if the intensity of the LoRa signal is far lower than the noise intensity.
In order to achieve the above task, the present invention adopts the following technical scheme, including:
a LoRa concurrent communication demodulation method based on frequency domain interference iterative elimination is carried out according to the following steps:
step one: transmitting signals;
taking the chirp signal as a processing unit, and transmitting the signal by a node end in a standard LoRa data packet frame format;
step two: receiving signals;
the gateway receives the LoRa signal in a conflict state by taking the frequency domain characteristics of the pilot frequency part as a basis, wherein the conflict state is that a plurality of data packets are transmitted in the same time period;
step three: preprocessing signals;
performing low-pass filtering and signal synchronization on the LoRa signal in the collision state, dividing the LoRa signal by taking the chirp length as a step length to obtain a plurality of chirp symbols, and performing carrier frequency offset correction on the chirp symbols;
step four: demodulating conflict signals;
and D, carrying out frequency domain interference estimation and elimination on the chirp symbol preprocessed in the step three, and completing symbol demodulation.
Optionally, in the fourth step, the frequency domain interference estimation comprises the steps of obtaining a frequency domain peak value frequency point and a frequency domain peak value height;
the obtaining of the frequency domain peak frequency point comprises the following steps:
Figure SMS_1
wherein f' A1-1 ，f′ A1-2 The frequency domain peak frequency point, hz, of the interference generated by the chirp symbol A1 to the chirp symbol B1; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the chirp symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the complete time length of the chirp symbol, s;
the obtaining of the frequency domain peak height comprises the following steps:
when (when)
Figure SMS_2
When (1):
Figure SMS_3
when (when)
Figure SMS_4
When (1):
Figure SMS_5
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the chirp symbol A1 to the chirp symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for the chirp symbol A1; h is the peak energy height, hz, of the chirp symbol frequency domain integrity.
Optionally, in the fourth step, after the frequency domain interference is estimated, the symbol demodulation process includes:
the estimated frequency domain interference (f 'in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) Eliminating;
in the frequency domain after the interference location is eliminated, find the frequency point f where the frequency domain peak is highest,
when (when)
Figure SMS_6
When (1):
Figure SMS_7
when (when)
Figure SMS_8
When (1):
Figure SMS_9
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, namely completing the demodulation of the chirp symbol.
Optionally, in the third step, a specific process of correcting the carrier frequency offset is shown in the following formula;
Figure SMS_10
wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Offset frequency, hz, for the carrier frequency; k is the rate of change of frequency, hz/s; t is time, s; j is the imaginary signal.
Optionally, in the third step, the low-pass filtering is performed by using an IIR low-pass filter.
Optionally, in the third step, pulse compression is performed on the frequency domain energy of the low-pass filtered LoRa signal, and a sliding window is utilized to find the position of the maximum energy value after pulse compression, namely the initial position of the effective load of the chirp signal, so that signal synchronization is realized; sliding window with 10 downlink sizes;
and synchronizing signals of the low-pass filtered LoRa signals by using a sliding window, and dividing the Payload signals in the low-pass filtered LoRa signals according to a time unit.
A LoRa concurrent communication demodulation system based on frequency domain interference iterative cancellation, comprising:
the signal transmitting module takes the chirp signal as a processing unit, and the node end transmits the signal in a standard LoRa data packet frame format;
the gateway terminal receives the LoRa signal in a collision state by taking the frequency domain characteristics of the pilot frequency part as a basis, and the collision state is that a plurality of data packets are transmitted in the same time period;
the signal preprocessing module performs low-pass filtering and signal synchronization on the LoRa signal in a collision state, performs segmentation by taking the chirp length as a step length to obtain a plurality of chirp symbols, and performs carrier frequency offset correction on the chirp symbols;
and the conflict signal demodulation module is used for carrying out frequency domain interference estimation and elimination on the chirp symbol preprocessed in the step three, and completing symbol demodulation.
Optionally, in the collision signal demodulation module, the estimating of the frequency domain interference includes obtaining a frequency domain peak frequency point and a frequency domain peak height;
the obtaining of the frequency domain peak frequency point comprises the following steps:
Figure SMS_11
wherein f' A1-1 ，f′ A1-2 The frequency domain peak frequency point, hz, of the interference generated by the chirp symbol A1 to the chirp symbol B1; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the chirp symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the complete time length of the chirp symbol, s;
the obtaining of the frequency domain peak height comprises the following steps:
when (when)
Figure SMS_12
When (1):
Figure SMS_13
when (when)
Figure SMS_14
When (1):
Figure SMS_15
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the chirp symbol A1 to the chirp symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for the chirp symbol A1; h is the peak energy height, hz, of the chirp symbol frequency domain integrity.
Optionally, after the frequency domain interference is estimated, the symbol demodulation process includes:
the estimated frequency domain interference (f 'in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) Eliminating;
finding out the frequency point f where the peak value of the frequency domain is located at the frequency domain after the interference position is eliminated,
when (when)
Figure SMS_16
When (1):
Figure SMS_17
when (when)
Figure SMS_18
When (1):
Figure SMS_19
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, i.e. complete the solution of chirp symbolAnd (5) adjusting.
Optionally, in the signal preprocessing module, a specific process of correcting the carrier frequency offset is shown in the following formula;
Figure SMS_20
wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Offset frequency, hz, for the carrier frequency; k is the rate of change of frequency, hz/s; t is time, s; j is the imaginary signal.
Compared with the prior art, the invention has the following characteristics:
the invention provides a method and a system for demodulating the interference of LoRa signals in concurrent communication on the original communication mechanism of the LoRa. And carrying out iterative estimation and elimination on the frequency domain interference when the LoRa signals collide, so as to complete the demodulation of the LoRa signals under the collision state. The conflict demodulation method does not need an extra hardware module. Meanwhile, the method for demodulating the LoRa signal conflict can be used together with channel parameter optimization and other schemes, so that the network performance is further improved. The idea of implementing the demodulation of the collision signals by carrying out iterative estimation and elimination on the frequency domain interference has never been considered in the previous work. Compared with other methods for demodulating conflict signals based on signal time domain features or signal energy features, the method has stronger robustness under low signal-to-noise ratio, and can complete demodulation of the conflict signals under the condition that the signal strength is far lower than the noise strength.
Drawings
The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification, illustrate the disclosure and together with the description serve to explain, but do not limit the disclosure. In the drawings:
FIG. 1 is a time-frequency plot of three different chirp, (a) up-chirp with an initial frequency of-250 kHz; (b) an up-chirp with an initial frequency of-125 kHz; (c) standard down-chirp;
FIG. 2 is a time-frequency diagram of a LoRa packet in a normal state;
FIG. 3 is a time-frequency diagram of a LoRa packet in a collision state;
fig. 4 shows the accuracy of the collision signal demodulation at different signal-to-noise ratios when sf=8.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. It should be apparent that the embodiments described below are only some, but not all embodiments of the present invention, and the present invention is not limited in any way, and all embodiments using the technical solutions of the present embodiment, including simple changes, fall within the scope of the present invention.
The standard LoRa signal data packet comprises the following parts: a pilot, preamble, for signal detection, a start frame delimiter Start Frame Delimiter (SFD) for signal synchronization, and a Payload data portion Payload that records the original transmission information.
The Payload data part Payload consists of each chirp signal, and the Payload signals are divided according to time units to obtain a plurality of chirp signals; in the signal transmission stage, the transmission signal is not changed, and is carried out according to the standard frame format regulated by the LoRaWAN, so that the original transmission setting and communication protocol are kept, no conflict with the original communication protocol occurs, and the system is ensured to be compatible with the original LoRa communication system;
with the massive deployment of the LoRa nodes, thousands of LoRa nodes are connected to one LoRa gateway. The problem of serious data packet collision caused by the aggregation network structure, which causes a large number of data packets to be lost and the throughput to be reduced, is based on the problem, a demodulation system of the conflicted data packets during concurrent communication is provided, in order to further complete demodulation of the LoRa concurrent transmission conflict signal under the condition of low signal-to-noise ratio, the invention provides a method for carrying out iterative estimation and elimination on the frequency domain interference of the conflict signal, and the invention can demodulate the conflict LoRa signal under the condition that the SNR is less than 0; the method combines sliding window detection, and can ensure accurate data packet detection by utilizing the frequency domain correlation analysis of the received signal after pulse compression and the prior signal even if the intensity of the LoRa signal is far lower than the noise intensity. Comprising the following steps:
step one: the node (signal transmitting end) transmits the data packet in the standard LoRaWAN frame format to the gateway (signal receiving end). For example, the transmitting end selects proper spread spectrum factors SF, frequency bandwidth BW and pilot chirp number according to the communication environment and LoRaWAN protocol, and does not change and limit the signal transmission of the node end;
step two: and (5) receiving signals.
The gateway receives the loRa signal in the collision state shown in fig. 3 according to the frequency domain characteristics of the pilot frequency part, wherein the collision state is that a plurality of data packets are transmitted in the same time period, the time-frequency diagram of the loRa data packet in the normal transmission state is shown in fig. 2, and the time-frequency diagram of the loRa data packet in the collision state is shown in fig. 3; and performing correlation analysis by utilizing the frequency domain characteristics of the pilot signal pulse compressed and the frequency domain characteristics of the prior signal, and if the correlation degree is higher than that of the correlation analysis by utilizing the frequency domain characteristics of the noise and the prior signal, judging the data packet as a LoRa signal, and receiving the data packet.
Step three: and carrying out data preprocessing on the received signal.
Filtering out high-frequency noise of the LoRa signal, dividing the signal according to the time length to obtain each chirp signal and correcting carrier frequency offset;
in the signal preprocessing stage, an IIR low-pass filter is used for filtering the signal so as to remove high-frequency noise outside the bandwidth of the signal. Signal synchronization and segmentation are performed using sliding windows. Carrier frequency offset is due to mismatch between local crystal oscillators between transceivers and doppler shift during signal transmission. Such frequency offset may cause a frequency point value of the LoRa signal after pulse compression to be shifted, thereby affecting the decoding result. And correcting carrier frequency offset in the signal preprocessing stage to obtain the accurate original frequency domain characteristics of the signal.
In step three, a preprocessing operation is required for the data. The first thing to do in this step is signal synchronization andthe problem of accurate interception is solved, a sliding window is used, a signal is multiplied with a Down chirp in the window, the energy in a frequency domain is subjected to pulse compression, and the position of the maximum energy value after pulse compression is found by using the sliding window, namely the initial position of the chirp signal effective load, namely the accurate synchronization of the signal is realized. Then dividing each chirp signal according to the time length of the chirp signal, and finally calculating carrier frequency offset frequency f according to the frequency offset value of the pilot frequency cfo And completing frequency offset correction.
And thirdly, carrying out carrier frequency offset estimation and frequency correction by utilizing the pilot frequency part of the LoRa data packet, thereby obtaining an accurate frequency point value. .
Figure SMS_21
Wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Offset frequency, hz, for the carrier frequency; k is the rate of change of frequency, hz/s; t is time, s; j is the imaginary signal.
Step four: and carrying out frequency domain interference estimation and elimination to complete demodulation.
For the chirp signal after the preprocessing in the step three, calculating the time window offset t generated by the signal arrival time difference at the chirp symbol level according to the initial position between the signal synchronizations 1_2 。
t 1_2 ＝p 2 -p 1 -N×T；
Wherein the time window is offset t 1_2 The value range of (2) is between 0 and T, s; p is p 2 S is the arrival time of the second packet; p is p 1 S is the arrival time of the first packet; n is a positive integer, T is the duration of the chirp symbol, s.
Next, the clean segment of the first chirp symbol A1 where the previous packet a collides with the next packet B is demodulated, and the procedure is as follows: the clean section signal is multiplied by a down chirp with a corresponding length, and the specific process is shown in the following formula;
Figure SMS_22
wherein: cu is upchirp, cd is downchirp, f o The initial frequency of Cu, hz; k is the rate of change of frequency, hz/s; BW is the bandwidth, hz; t is time, and the value range is 0 to t 1_2 S; fBw/2 is Cd initial frequency, hz; j is the imaginary signal.
Then, the multiplied signals are subjected to Fourier transformation to obtain an initial frequency f 0 。
Then according to the initial frequency f of the first chirp symbol A1 of the collision between the previous data packet A and the next data packet B 0 The frequency domain peak frequency point and the frequency domain peak height of the symbol A1 are estimated.
Figure SMS_23
Figure SMS_24
Wherein f A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the symbol A1, and is Hz; h is a A1-1 ，h A1-2 For its corresponding peak height; f (f) 0 The initial frequency of the symbol A1, hz; BW is the bandwidth, hz; h is the peak energy height of the chirp frequency domain integrity.
And calculating the frequency domain peak frequency point and the frequency domain peak height of interference generated by the chirp symbol B1 which can collide with the A1 of the next data packet B according to the frequency domain peak frequency point and the frequency domain peak height estimated by the symbol A1.
Specifically, the frequency domain peak frequency point calculation process of the interference is as follows:
Figure SMS_25
wherein f' A1-1 ，f′ A1-2 Frequency domain peak for interference generated by symbol A1 to B1Frequency point, hz; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the length of time the chirp symbol is complete, s.
Specifically, the frequency domain peak height calculation process of the interference is as follows:
when (when)
Figure SMS_26
When (1):
Figure SMS_27
when (when)
Figure SMS_28
When (1):
Figure SMS_29
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the symbol A1 to the symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for symbol A1; h is the complete peak energy height of the chirp frequency domain, hz; t is t 1_2 S is the time window offset; t is the length of time the chirp symbol is complete, s.
Next, firstly demodulating the chirp symbol B1 of the subsequent packet B colliding with A1, then fourier transforming the multiplied signal to obtain its frequency domain characteristics, and estimating the obtained frequency domain interference (f 'in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) And eliminating.
Finding out the frequency point f where the peak value of the frequency domain is located at the frequency domain after the interference position is eliminated,
when (when)
Figure SMS_30
When (1):
Figure SMS_31
when (when)
Figure SMS_32
When (1):
Figure SMS_33
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, namely completing the demodulation of the chirp symbol.
Finding the initial frequency of the B1 symbol, namely completing the demodulation of the conflict symbol. Similarly, the demodulation of the A2 symbol is completed by repeating the steps 4.4 and 4.5 using the starting frequency of the B1 symbol.
And similarly, estimating and eliminating iterative frequency domain interference on the conflict signals.
Embodiment one:
the embodiment provides a LoRa splicing communication method based on segmented neural network decoding, which comprises the following steps:
step one: the standard LoRa signal data packet comprises the following parts: a pilot, preamble, for signal detection, a start frame delimiter Start Frame Delimiter (SFD) for signal synchronization, and a Payload data portion Payload that records the original transmission information. In the signal transmitting stage, the method does not change the transmitting signal, and is carried out according to the standard frame format regulated by the LoRaWAN, so that the original transmitting setting and communication protocol are kept, no conflict with the original communication protocol occurs, and the system is ensured to be compatible with the original LoRa communication system.
Step two: the gateway receives the conflict signal, and the gateway receives the conflict signal by taking the frequency domain characteristics of the pilot frequency part as the basis; and performing correlation analysis by utilizing the frequency domain characteristics of the pilot signal pulse compressed and the frequency domain characteristics of the prior signal, and if the correlation degree is higher than that of the correlation analysis by utilizing the frequency domain characteristics of the noise and the prior signal, judging the data packet as a LoRa signal, and receiving the data packet.
Step three: and carrying out data preprocessing on the received signal. Filtering out high-frequency noise of LoRa signals to alleviate the influence of out-of-band noise, dividing the signals according to the time length to obtain each chirp signal, and correcting carrier frequency offset of the received signals; .
Step 3.1: first, the signal is filtered using an IIR low-pass filter to remove high-frequency noise.
Step 3.2: signal synchronization and payload segmentation. Since the splice mechanism retains the preamble of the original LoRa signal, the LoRa standard packet detection can be used to ensure signal synchronization. The sliding window is realized by utilizing the window size of 10 downchirp, so that the monitoring of the channel is realized. The sliding window of the channel is then multiplied by 2.25 upchirps and fourier transformed to synchronize the signal. When the peak intensity of the fast Fourier transform of the signal exceeds a preset threshold, the signal synchronization is completed, and the moment is the accurate starting position p of the effective load part of the signal i The peak frequency point is the carrier frequency offset frequency f cfo . From the exact starting position p of the payload part, depending on the length of the signal i The signal in the payload is split to realize the payload acquisition, and the frequency domain peak height h of the chirp symbol is obtained according to the pilot frequency part and calculated by the subsequent steps.
Step 3.3: and estimating carrier frequency offset by using the pilot frequency part of the LoRa data packet and carrying out frequency correction, thereby removing frequency offset generated by mismatch between local crystal oscillators between transceivers and Doppler frequency shift in the signal transmission process.
Figure SMS_34
Wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Is carrier frequency offsetShifting the frequency, hz; k is the rate of change of frequency, hz/s; t is time, s; j is the imaginary signal.
Step four: and carrying out iterative estimation on the frequency domain interference and eliminating the interference to realize accurate demodulation of the conflict signal.
Step 4.1: for the chirp signal after the preprocessing in the step three, calculating the time window offset t generated by the signal arrival time difference at the chirp symbol level according to the initial position between the signal synchronizations 1_2 。
t 1_2 ＝p 2 -p 1 -N×T
Wherein the time window is offset t 1_2 The value range of (2) is between 0 and T, s; p is p 2 S is the arrival time of the second packet; p is p 1 S is the arrival time of the first packet; n is a positive integer, T is the duration of the chirp symbol, s.
Step 4.3: the process of demodulating the clean segment of the first chirp symbol A1 where the previous packet a collides with the next packet B is as follows: the clean section signal is multiplied by a down chirp with a corresponding length, and the specific process is shown in the following formula;
Figure SMS_35
wherein: cu is upchirp, cd is downchirp, f o The initial frequency of Cu, hz; k is the rate of change of frequency, hz/s; BW is the bandwidth, hz; t is time, and the value range is 0 to t i_2 S; fBw/2 is Cd initial frequency, hz; j is the imaginary signal.
Then, the multiplied signals are subjected to Fourier transformation to obtain an initial frequency f 0 。
Step 4.4: the starting frequency f of the first chirp symbol A1 of the collision between the previous data packet A and the next data packet B obtained in the step 4.3 0 The frequency domain peak frequency point and the frequency domain peak height of the symbol A1 are estimated.
Figure SMS_36
Figure SMS_37
Wherein f A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the chirp symbol A1, and is Hz; h is a A1-1 ，h A1-2 For its corresponding peak height; f (f) 0 The initial frequency, hz, for the chirp symbol A1; BW is the bandwidth, hz; h is the peak energy height of the chirp frequency domain integrity.
And 4.5, calculating the frequency domain peak frequency point and the frequency domain peak height of interference generated by the chirp symbol B1 which can collide with the A1 of the next data packet B according to the frequency domain peak frequency point and the frequency domain peak height estimated by the symbol A1.
Specifically, the frequency domain peak frequency point calculation process of the interference is as follows:
Figure SMS_38
wherein f' A1-1 ，f′ A1-2 The frequency domain peak frequency point, hz, of the interference generated by the symbol A1 to the symbol B1; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the length of time the chirp symbol is complete, s.
Specifically, the frequency domain peak height calculation process of the interference is as follows:
when (when)
Figure SMS_39
When (1):
Figure SMS_40
when (when)
Figure SMS_41
When (1):
Figure SMS_42
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the symbol A1 to the symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for symbol A1; h is the complete peak energy height of the chirp frequency domain, hz; t is t 1_2 S is the time window offset; t is the length of time the chirp symbol is complete, s.
Step 4.5: iterative cancellation of frequency domain interference
First, the chirp symbol B1 of the collision between the following packet B and A1 is demodulated, and the procedure is as follows: the B1 signal is multiplied by a down chirp with a corresponding length, and the specific process is shown in the following formula;
Figure SMS_43
wherein: cu is upchirp, cd is downchirp, f o The initial frequency of Cu, hz; k is the rate of change of frequency, hz/s; BW is the bandwidth, hz; t is time, and the value range is 0 to T, s; f (f) BW 2 is Cd initial frequency, hz; j is the imaginary signal.
Then fourier transforming the multiplied signals to obtain frequency domain characteristics, and adding the frequency domain interference (f 'estimated in step 4.4 in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) And eliminating.
Finding out the frequency point f where the peak value of the frequency domain is located at the frequency domain after the interference position is eliminated,
when (when)
Figure SMS_44
When (1):
Figure SMS_45
when (when)
Figure SMS_46
When (1):
Figure SMS_47
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, namely completing the demodulation of the chirp symbol.
Finding the initial frequency of chirp symbol B1, i.e. the demodulation of the collision symbol is completed.
Similarly, the demodulation of the A2 symbol is completed by repeating steps 4.4 and 4.5 using the start frequency of the chirp symbol B1.
And similarly, estimating and eliminating iterative frequency domain interference on the conflict signals.
The method is used for demodulating the LoRa conflict signal under different parameter environments. The same transceiving equipment is adopted for the verification experiments carried out in this section. The transmitting end uses an Arduino module to carry an sx1276 chip to transmit the LoRa signal, and the receiving end adopts USRP-2954 manufactured by America NI company to receive the LoRa signal. The frequency band selected for signal transmission is 915.9MHz, and the signal bandwidth is 500KHz. The demodulation portion of the conflicting data packets is implemented on MATLAB. The experiment adjusts the signal-to-noise ratio of the signal by adjusting the transmitting power of the transmitting end node, changing the transmission distance between the transmitting end and the receiving end, and adding an attenuator at the receiving end and changing the attenuation power.
When SF=8 and SNR is between-5 dB and-10 dB, the conflict signal demodulation accuracy of the method can reach 89.06%. When SF=10 and SNR is between-15 dB and-20 dB, the conflict signal demodulation accuracy of the method can reach 92.94%. As shown in fig. 4, when sf=8, the accuracy of the collision signal demodulation at different signal-to-noise ratios is shown.
The preferred embodiments of the present disclosure have been described in detail above, but the present disclosure is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solutions of the present disclosure within the scope of the technical concept of the present disclosure, and all the simple modifications belong to the protection scope of the present disclosure.
In addition, the specific features described in the foregoing embodiments may be combined in any suitable manner, and in order to avoid unnecessary repetition, the present disclosure does not further describe various possible combinations.
Moreover, any combination between the various embodiments of the present disclosure is possible as long as it does not depart from the spirit of the present disclosure, which should also be construed as the disclosure of the present disclosure.

Claims (10)

1. The LoRa concurrent communication demodulation method based on the frequency domain interference iterative cancellation is characterized by comprising the following steps:
step one: transmitting signals;
taking the chirp signal as a processing unit, and transmitting the signal by a node end in a standard LoRa data packet frame format;
step two: receiving signals;
the gateway receives the LoRa signal in a conflict state by taking the frequency domain characteristics of the pilot frequency part as a basis, wherein the conflict state is that a plurality of data packets are transmitted in the same time period;
step three: preprocessing signals;
performing low-pass filtering and signal synchronization on the LoRa signal in the collision state, dividing the LoRa signal by taking the chirp length as a step length to obtain a plurality of chirp symbols, and performing carrier frequency offset correction on the chirp symbols;
step four: demodulating conflict signals;
and D, carrying out frequency domain interference estimation and elimination on the chirp symbol preprocessed in the step three, and completing symbol demodulation.
2. The method for demodulating the LoRa concurrent communication based on the iterative cancellation of the frequency domain interference according to claim 1, wherein in the fourth step, the estimation of the frequency domain interference includes obtaining a frequency domain peak frequency point and a frequency domain peak height;
the obtaining of the frequency domain peak frequency point comprises the following steps:
Figure FDA0004104214990000011
wherein f' A1-1 ，f′ A1-2 The frequency domain peak frequency point, hz, of the interference generated by the chirp symbol A1 to the chirp symbol B1; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the chirp symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the complete time length of the chirp symbol, s;
the obtaining of the frequency domain peak height comprises the following steps:
when (when)
Figure FDA0004104214990000012
When (1):
Figure FDA0004104214990000013
when (when)
Figure FDA0004104214990000014
When (1):
Figure FDA0004104214990000015
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the chirp symbol A1 to the chirp symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for the chirp symbol A1; h is the peak energy height, hz, of the chirp symbol frequency domain integrity.
3. The method for demodulating the LoRa concurrent communication based on the iterative cancellation of the frequency domain interference according to claim 1 or 2, wherein in the fourth step, after the frequency domain interference is estimated, the symbol demodulation process includes:
the estimated frequency domain interference (f 'in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) Eliminating;
finding out the frequency point f where the peak value of the frequency domain is located at the frequency domain after the interference position is eliminated,
when (when)
Figure FDA0004104214990000021
When (1):
Figure FDA0004104214990000022
when (when)
Figure FDA0004104214990000023
When (1):
Figure FDA0004104214990000024
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, namely completing the demodulation of the chirp symbol.
4. The method for demodulating the LoRa concurrent communication based on the iterative cancellation of the frequency domain interference according to claim 1 or 2, wherein in the third step, the specific procedure of correcting the carrier frequency offset is as follows;
Figure FDA0004104214990000025
wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Offset frequency, hz, for the carrier frequency; k is the rate of change of frequency, hz/s; t is time, s; j is virtualAnd (3) a part signal.
5. The method for demodulating the LoRa concurrent communication based on the frequency domain interference iterative cancellation according to claim 1 or 2, wherein in the third step, the low-pass filtering is performed by using an IIR low-pass filter.
6. The method for demodulating the LoRa concurrent communication based on the frequency domain interference iterative cancellation according to claim 1 or 2, wherein in the third step, pulse compression is performed on the frequency domain energy of the low-pass filtered LoRa signal, and the position of the maximum energy value after pulse compression is found by using a sliding window, namely the initial position of the payload of the chirp signal, so as to realize signal synchronization; sliding window with 10 downlink sizes;
and synchronizing signals of the low-pass filtered LoRa signals by using a sliding window, and dividing the Payload signals in the low-pass filtered LoRa signals according to a time unit.
7. A LoRa concurrent communication demodulation system based on frequency domain interference iterative cancellation, comprising:
the signal transmitting module takes the chirp signal as a processing unit, and the node end transmits the signal in a standard LoRa data packet frame format;
the gateway terminal receives the LoRa signal in a collision state by taking the frequency domain characteristics of the pilot frequency part as a basis, and the collision state is that a plurality of data packets are transmitted in the same time period;
the signal preprocessing module performs low-pass filtering and signal synchronization on the LoRa signal in a collision state, performs segmentation by taking the chirp length as a step length to obtain a plurality of chirp symbols, and performs carrier frequency offset correction on the chirp symbols;
and the conflict signal demodulation module is used for carrying out frequency domain interference estimation and elimination on the chirp symbol preprocessed in the step three, and completing symbol demodulation.
8. The system for the parallel communication demodulation of the LoRa based on the iterative cancellation of the frequency domain interference of claim 7, wherein in the collision signal demodulation module, the estimation of the frequency domain interference comprises obtaining a frequency domain peak frequency point and a frequency domain peak height;
the obtaining of the frequency domain peak frequency point comprises the following steps:
Figure FDA0004104214990000031
wherein f' A1-1 ，f′ A1-2 The frequency domain peak frequency point, hz, of the interference generated by the chirp symbol A1 to the chirp symbol B1; f (f) A1-1 ，f A1-2 The frequency domain peak frequency point is the frequency domain peak frequency point of the chirp symbol A1, and is Hz; BW is the bandwidth, hz; t is t 1_2 S is the time window offset; t is the complete time length of the chirp symbol, s;
the obtaining of the frequency domain peak height comprises the following steps:
when (when)
Figure FDA0004104214990000032
When (1):
Figure FDA0004104214990000033
when (when)
Figure FDA0004104214990000034
When (1):
Figure FDA0004104214990000035
wherein h' A1-1 ，h′ A1-2 The frequency domain peak height of the interference generated by the chirp symbol A1 to the chirp symbol B1; h is a A1-1 ，h A1-2 The frequency domain peak height, hz, for the chirp symbol A1; h is the peak energy height, hz, of the chirp symbol frequency domain integrity.
9. The system for the lorea concurrent communication demodulation based on frequency domain interference iterative cancellation of claim 8 wherein after frequency domain interference estimation, the symbol demodulation process comprises:
the estimated frequency domain interference (f 'in the frequency domain' A1-1 ，h′ A1-1 )、(f′ A1-2 ，h′ A1-2 ) Eliminating;
finding out the frequency point f where the peak value of the frequency domain is located at the frequency domain after the interference position is eliminated,
when (when)
Figure FDA0004104214990000041
When (1):
Figure FDA0004104214990000042
when (when)
Figure FDA0004104214990000043
When (1):
Figure FDA0004104214990000044
wherein: f is the frequency point where the peak value in the frequency domain of the chirp symbol B1 is located after interference is eliminated, and Hz; BW is the bandwidth, hz; f (f) 0-B The initial frequency, hz, of the chirp symbol B1; finding the initial frequency of chirp symbol B1, namely completing the demodulation of the chirp symbol.
10. The system for demodulating the LoRa concurrent communication based on the iterative cancellation of the frequency domain interference according to claim 7, 8 or 9, wherein the specific process of correcting the carrier frequency offset in the signal preprocessing module is as follows;
Figure FDA0004104214990000045
wherein: s' is a chirp symbol after carrier frequency offset correction, S chirp For the original chirp symbol, f 0 For the starting frequency of the original chirp symbol, hz, f cfo Offset frequency, hz, for the carrier frequency; k is the rate of change of frequency, hz/s; t is time, s; j is the imaginary signal.
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