CN114759947B - Method for detecting spreading factor of multi-path linear spread spectrum signal under parallel transmission - Google Patents
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Abstract
The invention discloses a method for detecting a spreading factor of a multi-path linear spread spectrum signal under parallel transmission, which mainly solves the problem that the prior art does not detect the spreading factor SF of a gateway received signal. The implementation scheme is as follows: setting detectors of 7 different SF (sulfur factor) from 6 to 12 at the gateway, and sequentially inputting a gateway receiving signal A to the detectors; intercepting a signal B according to the corresponding SF value of each detector, and demodulating the signal B; taking different numbers of symbols before and after the demodulated signal to perform sliding addition average and maximum value average operation to obtain a decision value set E; if in E there is p in the first half 1 Each is greater than or equal to the front threshold m and has a second half part with p 2 If the number of the signals is larger than or equal to the rear threshold n, the chirp signal of the SF corresponding to the detector exists in the signal A, otherwise, the chirp signal of the SF corresponding to the detector does not exist in the signal A. The invention has high false detection rate performance, expands the application range of linear spread spectrum signal scenes and can be used for signal identification.
Description
Technical Field
The invention belongs to the technical field of communication, and particularly relates to a method for detecting a spreading factor SF, which can be used for signal identification.
Background
With the rise of IOT, IOT has been widely used in various fields of industry, medicine, education, etc. for providing environmental sensing, event monitoring, monitoring and controlling. In these applications, a large number of IOT devices need to be connected, so the macro connection is the most significant feature in the service scenario of the internet of things. To meet the various demands of the internet of things, low power wide area LPWANs provide a novel communication paradigm. The LPWAN technology comprises an ultra-narrow band technology SigFox, a remote technology LoRa and the like, the technologies meet the requirement of wide area connection of the Internet of things from several kilometers to dozens of kilometers, and the application of low data rate, low power consumption and low throughput is realized. LoRa, an emerging technology in the application scenario, is a kind of frequency offset modulation of chirp signals of linear spread spectrum signals, and the chirp spread spectrum modulation technology has been listed as one of the physical standards of IEEE 802.15.4.
The macro connection is taken as a typical service scene of the internet of things, an unauthorized communication mode is adopted, and a plurality of users in an active state simultaneously send pilot frequency and data to the gateway in any given time, so that the purposes of low time delay and low cost are achieved. In the scene, the user equipment sends out chirp spread spectrum signals with different spread spectrum factors SF according to the distance between the user equipment and the gateway, the user equipment close to the gateway selects the minimum spread spectrum factor SF to achieve the highest data rate for information transmission, the user far away from the gateway selects the larger SF, the user corresponding to the lower data rate performs information transmission to ensure the reliability, so as to embody the compromise of the data transmission rate and the reliability, and in addition, the user can randomly change the spread spectrum factor SF of each signal transmission according to the self requirement.
In the system using chirp spread spectrum signal, most of the existing research describes how to separate the multiple parallel transmission signals using the same spreading factor SF under the condition that the Gateway knows the SF, such as two papers of Real-Time LoRa colloid Decoding with Peak Tracking and Online current transmission at LoRa Gateway published in the International Conference on Computer Communications Conference. In addition, under the condition that the gateway knows SF, the gateway utilizes the capturing effect OF chirp signals to separate chirp signals with different spreading factors SF, such as A Novel MAC Protocol applying conversion for Massive LoRa Connectivity published in JOURNAL OF COMMUNICATIONS AND NETWORKS. However, in practical applications, multiple ues randomly select chirp spread spectrum signals with different spreading factors SF according to their own requirements for transmission, and the gateway is unknown about which spreading factors SF are used by the user of the transmitting terminal. Therefore, when multiple parallel transmission signals reach the gateway, spreading factor SF possibly existing in a signal sent by a user terminal needs to be detected first, and then spreading signals of different users can be separated by using the existing technology, but a method for detecting the spreading factor SF does not exist at present, so that the gateway cannot demodulate a received signal, and cannot separate the signals sent by different users by using the single-frequency characteristic of the demodulated chirp signal, so that the chirp spreading signal is limited in practical application.
Disclosure of Invention
The present invention aims to provide a method for detecting a spreading factor SF of a multipath linear spread spectrum signal under parallel transmission, so as to make a chirp spread spectrum signal more widely applied in an actual scene, in view of the above-mentioned deficiencies of the prior art.
In order to achieve the purpose, the technical scheme of the invention comprises the following steps:
(1) Setting the signal of each terminal user to be composed of N chirp signals up-chirp with the same SF spreading factor and with the frequency increasing along with time and M chirp signals down-chirp with the frequency decreasing along with time, wherein N is more than or equal to 1, and M is more than or equal to 1;
(2) Superposing signals transmitted by a plurality of users in parallel into a path of receiving signal A at a gateway;
(3) Setting 6-12 detectors with different spreading factors SF at the gateway;
(4) And carrying out spreading factor identification on the received signal A by using an SF detector:
(4a) Sending the received signal A to each independent SF detector, and intercepting the received signal A with the length of (N + M) multiplied by 2 according to the SF size corresponding to each detector SF When the length of the received signal A is less than (N + M). Times.2 SF When the detection is finished, the detector finishes the detection;
(4b) Sequentially carrying out corresponding point multiplication for N times on the first N symbols in the intercepted signal B and chirp signal down-chirp signals with the frequency of SF corresponding to the detector decreasing along with time, and sequentially carrying out 2 times on each symbol SF Point Fast Fourier Transform (FFT) and absolute value operation are carried out to obtain demodulation signals C1 of the first N symbols, and then the last M symbols are processed according to 2 SF Sequentially performing corresponding point multiplication for M times on the upchirp signal of SF corresponding to the detector, sequentially performing Fast Fourier Transform (FFT) and absolute value operation to obtain demodulated signals C2 of the last M symbols, and finally obtaining a demodulated signal formed by connecting signals C1 and C2 in seriesNumber C = [ C1C 2 =]；
(4c) The first N symbols of the demodulated signal C in each SF detector are divided by 2 SF The point length sliding addition is used for averaging, namely two adjacent symbols are added and averaged to obtain (N-1) symbols P after the sliding average, and then the M symbols are equally divided by 2 SF The point length sliding addition is averaged to obtain (M-1) symbols Q after the sliding average, and finally, a signal D = [ P Q ] after the sliding average formed by connecting symbols P and Q in series is obtained]；
(4d) Calculating the maximum value and the average value of each symbol in the signal D after the moving average, and dividing the maximum value of each symbol by the average value to obtain a decision value s of the symbol, and finally obtaining a set E containing (N + M-2) decision values;
(4e) Setting a front threshold M and a rear threshold N, and comparing a front (N-1) judgment value set E1 and a rear (M-1) judgment value set E2 in the set E with the front threshold M and the rear threshold N respectively:
if there is p in the set E1 1 The value is greater than or equal to m, and p is in the set E2 2 If the value is larger than or equal to n, the chirp signal of the detector corresponding to the spreading factor SF is considered to exist in the signal A,
otherwise, the chirp signal of the detector corresponding to the spreading factor SF is not present in the signal a.
The invention has the following advantages:
1. has wider application in practical application
In the invention, as the 7 detectors with different spreading factors SF are arranged at the receiving end, different SF detection is carried out on the linear spreading signals transmitted in parallel in a multipath way, and the gateway detects the SF existing at the position from chirp superposed signals of a plurality of different spreading factors SF.
2. Good error detection rate performance
Experiments prove that the false detection rate of different SFs of linear spread spectrum signals is 10 under AWGN -3 The signal-to-noise ratio of the signal is 10 in the error rate of the signal under the known SF -3 Has a small value and a signal-to-noise ratio which is different between the two with a larger SFThe larger the chirp signal demodulation method is, the requirements of chirp signal demodulation on the signal-to-noise ratio are completely met.
Drawings
FIG. 1 is a flow chart of an implementation of the present invention;
fig. 2 is a schematic diagram of up-chirp symbol demodulation used in the present invention;
fig. 3 is a schematic diagram of down-chirp symbol demodulation used in the present invention;
FIG. 4 is a schematic diagram of an implementation of the present invention for determining a decision value of a demodulated signal;
fig. 5 is a simulation diagram of SF false detection performance for two superimposed signals of SF =7 and SF =11 when the channel is AWGN;
fig. 6 is a simulation diagram of SF false detection performance of four superimposed signals of SF =11, SF =9, SF =7, and SF =6 in the case of AWGN as a channel according to the present invention.
Detailed Description
Embodiments and effects of the present invention will be described in further detail below with reference to the accompanying drawings. Obviously, all other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without any inventive work belong to the protection scope of the present invention.
Referring to fig. 1, the implementation steps of this example are as follows:
step 1, acquiring a sending signal of a terminal user.
In a macro-connection scenario, each active user generally performs channel estimation by sending a plurality of linear spread spectrum chirp signals to a gateway as preambles, in this example, the preamble signals are used for spread spectrum factor detection, and the preamble signals of each user are configured to include N chirp signals up-chirp with a frequency increasing with time and M chirp signals down-chirp with a frequency decreasing with time, where the spreading factors SF are the same, N is greater than or equal to 1, and M is greater than or equal to 1, where the up-chirp signals and the down-chirp signals are basic signals or modulated signals.
And 2, acquiring a receiving signal A of the gateway.
When the preamble signals of a plurality of users reach the gateway through parallel transmission, the gateway changes the multipath signals into a path of received signal a in a superposition mode, which is expressed as follows:
A＝∑X i H i +Z
wherein, X i Signal representing the ith user, H i Represents channel information of the ith user, and Z represents noise.
And 3, setting a spreading factor detector of the gateway.
Before the gateway separately demodulates the signals of different users, it needs to know which linear spread spectrum chirp signals of spreading factor SF are included in the received signal, so it needs to set 6-12 detectors of 7 different spreading factor SFs at the gateway, that is: spreading factor SF =6 for the first detector, spreading factor SF =7 for the second detector, spreading factor SF =8 for the third detector, spreading factor SF =9 for the fourth detector, spreading factor SF =10 for the fifth detector, spreading factor SF =11 for the sixth detector, and spreading factor SF =12 for the seventh detector.
And 4, acquiring the intercepted signal B and the demodulated signal C in each SF detector.
(4.1) obtaining the intercepted signal B in each SF detector:
before each detector detects the spreading factor, the signal for detecting the spreading factor SF is acquired, so that the length of (N + M) multiplied by 2 is acquired from the received signal A according to the corresponding spreading factor SF of the detector SF Intercepting a signal B;
if the length of the signal A is less than (N + M). Times.2 SF If the linear spread spectrum chirp signal corresponding to the spreading factor SF does not exist in the received signal a, the detector ends the detection;
(4.2) obtaining the demodulated signal C in each SF detector:
the detection of the spreading factor SF of the linear spread spectrum chirp signal utilizes the single frequency characteristic after chirp signal demodulation, so that the intercepted signal B needs to be demodulated, and the detection is realized as follows:
(4.2.1) referring to fig. 2, each symbol of the first N linear spread chirp symbols in the intercepted signal B from the SF detector is dot multiplied by the down-chirp of the spreading factor SF corresponding to the detector, FFT operation is performed, and the absolute value is taken to obtain the demodulated signals C1 of the first N symbols:
wherein, C1 n' Is the N' th symbol, A, of the demodulated signal C1 of the first N symbols n' The nth symbol of the signal A is received by the gateway, and N' is more than or equal to 1 and less than or equal to N;
(4.2.2) referring to fig. 3, performing point multiplication, FFT operation, and taking an absolute value on each symbol of the last M linear spread chirp symbols in the intercepted signal B in the SF detector and the up-chirp of the spreading factor SF corresponding to the detector to obtain demodulated signals C2 of the last M symbols:
wherein, C2 n” Is the n' th symbol, A, of the demodulated signal C2 of the last M symbols n” The nth symbol of the signal A received by the gateway is more than or equal to 1 and less than or equal to M;
(4.2.3) serially combining the demodulated signals C1 of the first N symbols and the demodulated signals C2 of the last M symbols to finally obtain the demodulated signal C = [ C1C 2].
And step 5, acquiring a moving average signal D in each SF detector.
In order to improve the accuracy of the detection of the spreading factor SF, the demodulated signal C is subjected to a sliding addition averaging, which is implemented as follows:
(5.1) the first N symbols of the demodulated signal C in each SF detector are multiplied by 2 SF And (3) carrying out point length sliding addition averaging, namely averaging two adjacent symbol additions to obtain the symbols P after the first (N-1) sliding averages:
wherein, P m' Represents the m 'th symbol of P, wherein m' is more than or equal to 1 and less than or equal to N-1;
(5.2) the last M symbols are also given 2 SF The dot length sliding sums are averaged to obtain the last (M-1) symbols Q after sliding averaging, which is expressed as follows:
wherein Q is m” Represents the mth symbol of Q, wherein M is more than or equal to 1 and less than or equal to M-1;
(5.3) the first (N-1) moving average post-symbols P and the last (M-1) moving average post-symbols Q are connected in series, resulting in a moving average signal D = [ P Q ].
And 6, acquiring a set E in each SF detector.
After obtaining the moving average signal D, it is necessary to further obtain a decision value for detecting the spreading factor SF,
referring to fig. 4, this step is implemented as follows:
(6.1) calculating the average value of each symbol in the signal D after moving average:
wherein d is n' Is the average of the n' th symbol of the signal D, D n',l Is the l-th value of the N '-th symbol of the signal D, N' is more than or equal to 1 and less than or equal to N + M-2;
and (6.2) dividing the maximum value of each symbol by the average value of the symbol to obtain a decision value s of the symbol, and finally obtaining a set E containing (N + M-2) decision values.
And 7, judging the chirp signal of SF corresponding to the SF detector.
Setting a front threshold m and a rear threshold n of each spreading factor SF detector according to the ratio of the maximum value to the average value of the demodulated SF chirp signals;
comparing the front (N-1) decision value sets E1 and the rear (M-1) decision value sets E2 in the set E with a front threshold M and a rear threshold N respectively:
if the set E1In which is p 1 The value is greater than or equal to m, and p is in the set E2 2 If the value is larger than or equal to n, the chirp signal of the detector corresponding to the spreading factor SF is considered to exist in the signal A,
otherwise, the chirp signal of the detector corresponding to the spreading factor SF is not present in the signal a.
The technical effects of the invention are further explained in detail by combining simulation experiments as follows:
experiment 1:
setting the system bandwidth as B =125KHz, the number of users as 2, respectively adopting SF =11 and SF =7, the channel as AWGN, the number of upchirp symbols N =8, the number of downchirp symbols as M =2, the threshold of judgment as M =2, p 1 ＝4，n＝2.5，p 2 ＝1。
Under the above conditions, different SF were tested by the method of the present invention, and the results are shown in FIG. 5.
In fig. 5, the dotted line is the error rate of chirp signal demodulation under the known SF, the solid line is the false detection rate of SF detection under the invention, and it is obvious from fig. 5 that when two signals arrive at the gateway at the same time, the false detection rate of spreading factor of the invention is 10 -3 The corresponding signal-to-noise ratio is obviously lower than the demodulation error rate of the linear spread spectrum chirp signal under the known SF which is 10 -3 The corresponding signal-to-noise ratio shows that under two paths of signals, the method completely meets the requirements of linear chirp signal demodulation on the signal-to-noise ratio.
Experiment 2:
the system bandwidth is set to be B =125KHz, the number of users is set to be 4, SF =11, SF =9, SF =7, SF =6, the channel is AWGN, the number of upchirp symbols N =8, the number of downchirp symbols M =2 of each user, the threshold of judgment is set to be M =2 1 ＝4，n＝2.5，p 2 ＝1。
Under the above conditions, different SFs were tested by the method of the present invention, and the results are shown in fig. 6:
in fig. 6, the dotted line is the error rate of chirp signal demodulation under the known SF, the solid line is the false detection rate of SF detection under the invention, and when four paths of signals reach the gateway at the same time, the false detection rate of spreading factor SF and the linear spreading under the known SF are comparedThe demodulation error rates of the frequency chirp signals are all 10 -3 Signal to noise ratio of time. As is apparent from fig. 6, the signal-to-noise ratios required for detecting the spreading factors of the chirp signals SF =11 and SF =9 are lower, and the signal-to-noise ratios required for detecting the spreading factors of the chirp signals SF =7 and SF =6 are almost the same, which indicates that the signal-to-noise ratio requirements for demodulating the linearly chirped chirp signals are completely satisfied also in the four-way signals.
In summary, the method for detecting spreading factor SF of multi-path linear spread spectrum signals under parallel transmission can realize the detection of spreading factor SF in multi-path superposed signals at a gateway, greatly improve the application range of linear spread spectrum chirp signals in actual scenes, and simultaneously, under multi-path parallel signals, the SF false detection rate is 10 -3 The signal-to-noise ratio of the time is completely satisfied with the same SF at the error rate of 10 -3 The signal-to-noise ratio requirement of time indicates the reliability of the invention.
Claims (8)
1. A method for detecting a spreading factor of a plurality of linear spread spectrum signals under parallel transmission is characterized by comprising the following steps:
(1) Setting the signal of each terminal user to be composed of N chirp signals up-chirp with the same SF spreading factor and with the frequency increasing along with time and M chirp signals down-chirp with the frequency decreasing along with time, wherein N is more than or equal to 1, and M is more than or equal to 1;
(2) Superposing signals transmitted by a plurality of users in parallel into a path of receiving signal A at a gateway;
(3) Setting 6-12 detectors with different spreading factors SF at the gateway;
(4) And carrying out spreading factor identification on the received signal A by using an SF detector:
(4a) Sending the received signal A to each independent SF detector, and intercepting the received signal A with the length of (N + M) multiplied by 2 according to the SF size corresponding to each detector SF When the length of the signal A is less than (N + M). Times.2 SF When the detection is finished, the detector finishes the detection;
(4b) Sequentially carrying out corresponding point multiplication for N times on the first N symbols in the intercepted signal B and chirp signal down-chirp signals with the frequency of SF corresponding to the detector decreasing along with time, and sequentially carrying out 2 times on each symbol SF Point fast fourier transform FFT sumAbsolute value operation is carried out to obtain demodulation signals C1 of the first N symbols, and then the last M symbols are processed according to 2 SF Sequentially performing M times of corresponding dot multiplication on the upchirp signal of SF corresponding to the detector, sequentially performing Fast Fourier Transform (FFT) and absolute value operation to obtain demodulated signals C2 of the last M symbols, and finally obtaining demodulated signals C = [ C1C 2] formed by serially connecting signals C1 and C2]；
(4c) The first N symbols of the demodulated signal C in each SF detector are divided by 2 SF The point length sliding addition is used for averaging, namely two adjacent symbols are added and averaged to obtain (N-1) symbols P after the sliding average, and then the M symbols are equally divided by 2 SF The point length sliding addition is averaged to obtain (M-1) symbols Q after the sliding average, and finally, a signal D = [ P Q ] after the sliding average formed by connecting symbols P and Q in series is obtained]；
(4d) Calculating the maximum value and the average value of each symbol in the signal D after the moving average, and dividing the maximum value of each symbol by the average value to obtain a decision value s of the symbol, and finally obtaining a set E containing (N + M-2) decision values;
(4e) Setting a front threshold M and a rear threshold N, and comparing a front (N-1) judgment value set E1 and a rear (M-1) judgment value set E2 in the set E with the front threshold M and the rear threshold N respectively:
if there is p in the set E1 1 The value is greater than or equal to m, and p is in the set E2 2 If the value is larger than or equal to n, the chirp signal of the detector corresponding to the spreading factor SF is considered to exist in the received signal A,
otherwise, the chirp signal of the detector corresponding to the spreading factor SF is not present in the received signal a.
2. The method according to claim 1, wherein (2) the gateway superposes signals transmitted by a plurality of users in parallel into a received signal a, which is expressed as follows:
A＝∑X i H i +Z
wherein, X i Signals representing the ith user, H i Represents channel information of the ith user, and Z represents noise.
3. The method according to claim 1, wherein the (3) sets 6-12 detectors with different spreading factors SF at the gateway, that is, the spreading factor SF =6 for the first detector, the spreading factor SF =7 for the second detector, the spreading factor SF =8 for the third detector, the spreading factor SF =9 for the fourth detector, the spreading factor SF =9 for the fifth detector, the spreading factor SF =10 for the fifth detector, the spreading factor SF =11 for the sixth detector, and the spreading factor SF =12 for the seventh detector.
4. The method of claim 1, wherein the demodulated signals C1 of the first N symbols in (4 b) are represented as follows:
wherein, C1 n' Is the nth symbol, A, of the demodulated signal C1 of the first N symbols n' The nth symbol of the signal A is received by the gateway, and N' is more than or equal to 1 and less than or equal to N.
5. The method of claim 1, wherein the demodulated signal C2 of the last M symbols in (4 b) is represented as follows:
wherein, C2 n” Is the n' th symbol, A, of the demodulated signal C2 of the last M symbols n” The nth symbol of the signal A received by the gateway is more than or equal to 1 and less than or equal to n and less than or equal to M.
6. The method according to claim 1, wherein the first (N-1) moving average post-symbol P obtained in (4 c) is represented as follows:
wherein, P m' Represents the m 'th symbol of P, wherein 1 is not less than m' and not more than N-1.
7. The method of claim 1, wherein the last (M-1) moving average last symbols Q obtained in (4 c) are represented as follows:
wherein Q m” The M-th symbol representing Q is represented by 1. Ltoreq. M.ltoreq.M-1.
8. The method according to claim 1, wherein the average value of each symbol in the moving-average signal D is calculated in (4D), and the formula is as follows:
wherein d is n' Is the average of the n' th symbol of the signal D, D n',l Is the l-th value of the N '-th symbol of the signal D, 1 < N' < N + M-2.
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