CN109842430B - Demodulation method of LoRa modulated signal under multipath channel - Google Patents

Abstract

The invention discloses a demodulation method of LoRa modulation signals under a multipath channel, which mainly comprises the following stepsThe problem that in the prior art, the performance of directly demodulating LoRa modulation signals is poor is solved. The implementation scheme is as follows: directly demodulating the down-sampled preamble signal and the load signal to obtain a directly demodulated preamble signal A and a directly demodulated load signal D; searching the maximum value in A, reserving M points around the maximum value to obtain a preamble signal C with the length of (2M +1) after interception, and making C smaller than the maximum value

Resetting the point data to 0 to obtain a new leading signal B; carrying out matched filtering on the new preamble signal B and the load signal D after direct demodulation to obtain a new load signal E; and E, searching a position P corresponding to the maximum value of each symbol, and sequentially carrying out decimal to binary conversion and gray inverse mapping on the position P to obtain modulation information bits. The invention improves the demodulation performance of the system, and can be used for processing the LoRa modulation signal at the receiving end under the condition of a multipath channel.

Description

Demodulation method of LoRa modulated signal under multipath channel

technical field

The invention belongs to the technical field of communications, and in particular relates to a method for demodulating LoRa modulated signals, which can be used for processing LoRa modulated signals at a receiving end in the case of multipath channels.

Background technique

With the development of Internet users from people to things, the Internet of Things (IoT) has aroused great interest. The Internet of Things enables objects around us to be connected to each other, and to send and receive messages to and from the Internet. To meet the various demands of IoT applications, low-power wide-area LPWANs provide novel communication paradigms. LPWAN technology includes ultra-narrowband technology SigFox, long-range technology LoRa, etc. These technologies are proposed to meet the wide area connection of the Internet of Things from several kilometers to tens of kilometers, and realize the application of low data rate, low power consumption and low throughput. Among the recently proposed and dominant LPWAN technologies, semiconductor manufacturer Semtech has introduced a wide range of advanced spread spectrum technologies for its long-range LoRa ^™ product line. Unlike direct sequence spread spectrum DSSS, which modulates the message signal on a pseudo-random code, LoRa uses a swept signal with a frequency that increases up-chirp or decreases down-chirp over time to encode the message signal. It can be seen that LoRa technology is essentially A derivative of chirp spread spectrum modulation.

Chirp spread spectrum modulation technology CSS neither uses pseudo random code as spread spectrum code nor frequency hopping pattern, but uses matched filtering and pulse compression characteristics of chirp signal to realize spread spectrum communication. In the field of wireless communication, chirp spread spectrum modulation technology is still a new technology. Although it has been listed as one of the physical standards of IEEE 802.15.4, it still has many shortcomings. For example, if the chirp spread spectrum technique is used alone for information transmission without other modulation techniques, the link capacity is generally low, and the improvement of the data transmission rate depends on shortening the duration of the chirp signal, or using overlapping techniques. However, whether it is Either shortening the duration of the chirp signal or overlapping the signals increases the complexity and power consumption of the system.

In the LoRa modulation system, the traditional direct demodulation method is generally used for signal demodulation, as shown in Figure 6, that is, the received signal is multiplied by the original down-chirp, and then the fast Fourier transform FFT is performed, and finally Determine the maximum value. The direct demodulation method can obtain better demodulation performance under the single-path channel, but when the channel is multi-path, the demodulation performance of the system will drop sharply, that is, the bit error performance will decrease, resulting in the system reliability is reduced.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a method for demodulating LoRa modulated signals under multipath channels in view of the deficiencies of the above technologies, so as to improve the bit error performance of the receiving end, thereby improving the reliability of the system.

For achieving the above object, technical scheme of the present invention is as follows:

(1) Do the corresponding dot multiplication between the down-sampled preamble signal and the original down-chirp signal, and then perform FFT and find the absolute value in turn to obtain the preamble signal A with a length of 2 ^SF after direct demodulation, where SF is the spread frequency factor;

(2) The preamble signal after direct demodulation is intercepted and the data is reset to obtain a new preamble signal B:

(2a) Find the maximum value of the leading signal A after direct demodulation, retain M points around the maximum value, and intercept the leading signal C with a length of (2M+1) points. The size of M is valued according to the channel conditions. The value range is 1 to 2 ^SF-1 -1;

进行比较：当某一点的值大于最大值的

时，则该点的值保持不变，当某一点的值小于最大值的

时，则将该点的值重置为0，得到新的前导信号B，其中P的大小根据信道的情况取值，

(2b) Calculate the difference between the value of each point of the leading signal C after the interception and the maximum value

To compare: when the value of a point is greater than the maximum value

When the value of the point remains unchanged, when the value of a point is less than the maximum value

, then reset the value of this point to 0 to obtain a new preamble signal B, where the size of P is valued according to the channel conditions,

(3) each symbol in the down-sampling payload signal is multiplied by the corresponding point with the original down-chirp signal, then FFT is performed in turn and the absolute value of each symbol is obtained to obtain the payload signal D after the direct demodulation;

(4) each symbol in the load signal D after the direct demodulation and the new leading signal B are carried out matched filtering operation to obtain the new load signal E;

(5) Find the position corresponding to the maximum value of each symbol in the new payload signal E, and perform decimal-to-binary conversion and Gray inverse mapping in turn to obtain modulation information bits.

Compared with the prior art, the present invention has the following advantages:

1. Improve the reliability of the system

In the present invention, at the receiving end, the channel information contained in the preamble signal is fully utilized through the matched filtering operation between the preamble signal and the payload signal, thereby reducing the influence of the channel; Compared with the traditional direct demodulation method, which directly demodulates the received payload signal, the present invention is less affected by the channel and noise, and the obtained demodulation Higher performance, which can improve the reliability of the LoRa modulation signal demodulation system.

2. Good bit error performance

Experiments show that under the condition that the channel is set to two fixed paths and SF=7, the performance of the present invention is 1.5dB higher than that of the traditional direct demodulation method at the bit error rate of 10 ^-3 , which is comparable to the additive white Gaussian noise. The performance under the channel is only 0.5dB different.

Description of drawings

Fig. 1 is the LoRa signal frequency modulation diagram used in the present invention;

Fig. 2 is the LoRa signal modulation diagram used in the present invention;

Fig. 3 is the realization block diagram of the present invention;

Fig. 4 is the realization flow chart of leading signal and load signal during demodulation among the present invention;

Fig. 5 is the realization principle diagram of leading signal interception and data reset during demodulation in the present invention;

Fig. 6 is the schematic diagram of demodulating LoRa signal with traditional demodulation method;

Fig. 7 is the four methods of the present invention, the traditional direct demodulation method and the matched filtering condition, the preamble signal is not intercepted and intercepted and not set to zero. At SF=7, the channel is the bit error under two fixed paths. Performance comparison chart;

Fig. 8 is the four methods of the present invention, the traditional direct demodulation method and the matched filtering method, the preamble signal is not intercepted and intercepted and not set to zero. When SF=8, the channel is the bit error under two fixed paths. Performance comparison chart.

Detailed ways

The objects, technical solutions and advantages of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

The present invention relates to a demodulation method for LoRa signals, and LoRa signals use up-chirp or down-chirp frequency sweep signals to encode signal information.

Referring to FIG. 1 , the frequency modulation of the LoRa signal includes the frequency curve 1 of the unmodulated LoRa signal, that is, the up-chirp signal, and the frequency curve 2 of the modulated LoRa signal.

Referring to FIG. 2 , the LoRa signal modulation includes an unmodulated LoRa signal diagram a and a modulated LoRa signal diagram b.

At present, most of the demodulation of LoRa modulated signals is the traditional direct demodulation method. This method can obtain better performance in single-path channels, but in multi-path channels, the bit error performance will be significantly reduced. , the reliability of the system deteriorates. The present invention just solves the problem of demodulation of LoRa modulated signal under multipath channel.

Referring to Figure 3, the implementation steps of the present invention are as follows:

Step 1, obtain a preamble signal and a payload signal.

Downsampling the received signal and the unmodulated LoRa signal, that is, the original up-chirp and the original down-chirp;

According to the number of symbols of the preamble signal and the payload signal known at the receiving end, the down-sampled received signal is divided to obtain the preamble signal F and the payload signal P1.

Step 2, acquire new preamble signal B and payload signal D.

4, the specific implementation of this step is as follows:

(2a) Obtain a new preamble signal B:

(2a1) Do point multiplication with the unmodulated LoRa signal after down-sampling, that is, the original down-chirp, and then perform FFT and find the absolute value in turn to obtain the leading signal A with a length of 2 ^SF ;

(2a2) Intercept the preamble signal A with a length of 2 ^SF :

Referring to Figure 5, the implementation of this step is: save M points about the maximum value, obtain a signal C with a length of (2M+1) points, find the maximum value from the directly demodulated preamble signal A, and determine the The subscript MAX of the position corresponding to the maximum value, compare MAX and M in size:

When MAX≤M, use the subscript of the preamble signal A as 2 ^SF -(MAX-M) to the subscript of 2 ^SF as the first half of the intercepted signal, and subscript the preamble signal A as 1 to the subscript MAX+ This section of signal M is used as the second half of the intercepted signal, and the two sections before and after constitute the leading signal C;

When ^MAX≥2SF -M, the subscript of the preamble signal A is MAX-M to the subscript of ^2SF as the first half of the intercepted signal, and the subscript of the preamble signal A is 1 to the subscript MAX+M- 2 The signal of ^SF is used as the second half of the intercepted signal, and the first and second segments constitute the leading signal C;

When MAX does not meet the above two conditions, the preamble signal A is subscripted as MAX-M to MAX+M, which constitutes the preamble signal C;

(2a3) Reset the intercepted preamble signal C:

作大小比较，当某一点的值小于最大值的

时，该点数据重置为0，当某点的值大于等于最大值的

时，保持该点的值不变，得到新的前导信号B，其中P的大小根据信道的情况取值，

The difference between the value of each point in the preamble signal C after truncating and the maximum value

For size comparison, when the value of a point is less than the maximum value

, the point data is reset to 0, when the value of a point is greater than or equal to the maximum value

When , keep the value of this point unchanged, and get a new preamble signal B, where the size of P takes a value according to the channel condition,

(2b) Obtain a new load signal D:

Perform the corresponding point multiplication of each symbol in the down-sampled payload signal P1 with the unmodulated LoRa signal, that is, the original down-chirp signal, and then perform FFT and take the absolute value of each symbol in turn to obtain the payload after direct demodulation. signal D.

Step 3, acquiring a new load signal E.

Perform matched filtering on each symbol in the new preamble signal B and the payload signal D, while keeping each symbol in the payload signal D fixed, perform a point-by-point sliding point multiplication with the new preamble signal B. When the preamble signal B and the last point of each symbol in the payload signal D are coincident, each symbol in the payload signal D is moved from the starting point and placed behind the last point, until the new preamble signal B and payload signal D When the starting points of 1 and 2 coincide again, the operation is terminated, and the result obtained is the new load signal E.

Step 4, obtaining modulation information bits.

(5a) Find the subscript max of the position corresponding to the maximum value of each symbol in the new load signal E, calculate the value of max+M-MAX, and perform the remainder operation with the calculation result and ^2SF to eliminate the starting position The influence of MAX-M, the actual position P of each symbol maximum value in the new load signal E is obtained;

(5b) Divide P obtained in (5a) by 2, take the remainder as the coefficient d0 of the lowest bit of the binary Gray code, then continue to divide the quotient by 2, and the remainder is used as the coefficient d1 of the second-lowest bit of the binary Gray code, and repeat , until the quotient is 0, the binary Gray code b is obtained;

(5c) Retain the highest digit of the binary Gray code b as the highest digit of the natural binary code, and use the result obtained after the highest digit of the natural binary code and the second highest gray code are XORed as the second highest digit of the natural binary code;

(5d) According to the same method as (5c), the remaining bits of the natural binary code are sequentially obtained to obtain the modulation information bits.

Below in conjunction with the simulation experiment, the technical effect of the present invention is described in further detail:

Experiment 1,

1.1) Experimental conditions: set SF=7, that is, the symbol length is 128 points, the sampling frequency is Fs=10MHz, the upsampling multiple is 8, the signal bandwidth is B=Fs/8, and the byte size of the input modulation information is Byte=64, The channel is two fixed paths, and the delay time between the two paths is 0.6us, and 16 points on the left and right sides of the maximum value of the preamble signal A are reserved.

1.2) Experiment content

Under the conditions of 1.1) above, the LoRa signal is demodulated by the method of the present invention and the traditional demodulation method, and the result is shown in Figure 7:

As can be seen from Fig. 7, compared with the traditional demodulation method, the method of the present invention achieves a 1.5dB improvement in the bit error performance at a bit error rate of ^10-3 , and the preamble signal with 33 points not set to 0 and the preamble signal with 128 points long In comparison, the bit error performance is improved, and the bit error performance of the 33-point long preamble signal with 0 is also improved compared with the 33-point long preamble signal that is not set to 0. From the point of view of reliability, under the same SNR, the lower the bit error rate, the better the reliability of the system.

It can also be seen from the five curves in FIG. 7 that the performance of the method of the present invention is close to the performance under the theoretical case of additive white Gaussian noise AWGN, that is, there is only a gap of 0.5dB.

Experiment 2,

2.1) Experimental conditions: set SF=8, that is, the symbol length is 256 points, the sampling frequency is Fs=10MHz, the upsampling multiple is 8, the signal bandwidth is B=Fs/8, and the byte size of the input modulation information is Byte=64, The channel is two fixed paths, and the delay time between the two paths is 0.6us, and 16 points on the left and right sides of the maximum value of the preamble signal A are reserved.

2.2) Experimental content

Under the above-mentioned 2.1) conditions, the method of the present invention and the traditional demodulation method are used to demodulate the LoRa signal, and the result is shown in Figure 8:

It can be seen from Fig. 8 that, compared with the traditional demodulation method, the method of the present invention achieves a 1.5dB improvement in the bit error performance at a bit error rate of ^10-3 , and the 33-point unset preamble signal and the 256-point long preamble signal In comparison, the bit error performance is improved, and the bit error performance of the 33-point long preamble signal with 0 is also improved compared with the 33-point long preamble signal that is not set to 0.

From the point of view of reliability, under the same SNR, the lower the bit error rate, the better the reliability of the system.

To sum up, compared with the traditional direct demodulation method of LoRa modulated signal under a multipath channel of the present invention, under the condition of the same SF and fixed two paths, the bit error rate is The bit error performance at ^10-3 is improved by 1.5dB, and the preamble after setting 0 and the preamble not set to 0 at the same time, as well as the preamble not truncated, and the unmatched method have the best performance, which is additive with the theoretical situation. The performance difference under the AWGN of Gaussian white noise is only 0.5dB, which shows that the present invention has better demodulation performance under the multipath channel, and the reliability of the system is higher.

Claims (4)

1. A LoRa modulation signal demodulation method under a multipath channel is characterized by comprising the following steps:

(1) performing corresponding point multiplication on the down-sampled preamble signal and the original down-chirp signal, and then sequentially performing FFT and absolute value calculation to obtain the length of 2 after direct demodulation^SFThe preamble signal a of (1), wherein SF is a spreading factor;

(2) intercepting and resetting data of the preamble signal after direct demodulation to obtain a new preamble signal B:

(2a) searching the maximum value of the preamble signal A after direct demodulation, reserving M points around the maximum value, intercepting the preamble signal C with the length of (2M +1) points, wherein the size of M is taken according to the condition of a channel and the range of M is 1 to 2^SF-1-1；

(2b) The value of each point of the preamble signal C is compared with the maximum value

And (3) comparison: when the value at a certain point is greater than or equal to the maximum value

When the value of a certain point is less than the maximum value, the value of the point is kept unchanged

Then, the value of the point is reset to 0 to obtain a new preamble signal B, wherein the value of Z is taken according to the condition of the channel,

(3) carrying out corresponding point multiplication on each symbol in the load signal after down sampling and the original down-chirp signal, and then sequentially carrying out FFT (fast Fourier transform) and taking an absolute value of each symbol to obtain a load signal D after direct demodulation;

(4) carrying out matched filtering operation on each symbol in the load signal D after direct demodulation and the new preamble signal B to obtain a new load signal E;

(5) and searching a position P corresponding to the maximum value of each symbol in the new load signal E, and sequentially carrying out decimal to binary conversion and Gray code inverse mapping on the position P to obtain modulation information bits.

2. The method of claim 1, wherein step (2a) is performed for a length of 2^SFIntercepting the leading signal A, storing M points around the maximum value to obtain a signal C with the length of (2M +1) points, finding out the maximum value from the leading signal A after direct demodulation, determining a subscript MAX of a position corresponding to the maximum value, and comparing the size of the MAX with that of the M:

when MAX is less than or equal to M, subscript of preamble signal A is 2^SF- (MAX-M) to subscript 2^SFThe section of signal is taken as the first half section of the intercepted signal, the section of signal with the subscript of the leading signal A ranging from 1 to the subscript MAX + M is taken as the second half section of the intercepted signal, and the front section and the rear section form a leading signal C;

when MAX is more than or equal to 2^SFwhen-M, index of preamble signal A is MAX-M to index is 2^SFThe signal of the section is taken as the first half section of the intercepted signal, and the subscript of the leading signal A is 1 to the subscript MAX + M-2^SFThis segment of the signalAs the second half section of the intercepted signal, the front section and the rear section form a preamble signal C;

when MAX is not more than MAX and is not less than MAX and not less than 2^SFIn both cases-M, the preamble signal A is indexed MAX-M to MAX + M to form the preamble signal C.

3. The method of claim 1, wherein the step (4) of performing matched filtering on each symbol in the new preamble signal B and the payload signal D is to perform point-by-point sliding dot multiplication on the new preamble signal B while keeping each symbol in the payload signal D fixed, and when the new preamble signal B coincides with the last point of each symbol in the payload signal D, the symbols in the payload signal D are shifted from the starting point to the end of the last point one by one until the starting points of the new preamble signal B and the payload signal D coincide again, and the operation is terminated.

4. The method of claim 1, wherein the step (5) of maximum value detection is performed on the new load signal E after matching, and gray code inverse mapping is performed on the finally generated binary system, and the steps are implemented as follows:

(5a) finding the subscript MAX of the position corresponding to the maximum value of each symbol in the new load signal E, calculating the value of MAX + M-MAX, and comparing the calculation result with 2^SFPerforming remainder operation to eliminate the influence of the initial position MAX-M and obtain the actual position P of the maximum value of each symbol in the new load signal E;

(5b) dividing P obtained in the step (5a) by 2, taking the rest numbers as the coefficient d0 of the lowest bit of the binary gray code, then continuously dividing the quotient by 2, taking the remainder as the coefficient d1 of the lowest bit of the binary gray code, and repeating the steps until the quotient is 0, so as to obtain the binary gray code b;

(5c) reserving the highest bit of the binary gray code b as the highest bit of the natural binary code, and taking the result of the exclusive OR of the highest bit of the natural binary code and the second highest bit of the gray code as the second highest bit of the natural binary code;

(5d) and (5) sequentially solving the rest bits of the natural binary code according to the same method as the method (5c) to obtain modulation information bits.

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