CN107769816B - A kind of Chirp spread spectrum communication system receiver time synchronization system and method - Google Patents

Abstract

The invention provides a time synchronization system and method for a receiver of a Chirp spread spectrum communication system. The signal-to-noise ratio enhancement module of the system includes a modulo, a gain acquisition, a delay, a combination, and the modulo eliminates the phase difference between symbols, and the gain Acquire the enhanced useful signal; the combined spread spectrum modulation module uses combined Chirp signal for spread spectrum modulation; after acquiring the signal that has undergone combined spread spectrum modulation, matched filtering reception, and signal-to-noise ratio enhancement, it passes the noise threshold set in the judgment module, The peak search of the narrowband pulse after matched filtering is reduced to one symbol period, and the peak search module realizes time synchronization at the moment when the peak value is captured in this symbol period. The invention solves the misjudgment of the synchronization moment caused by the difference between the actual synchronization and the ideal synchronization by an integer multiple of the symbol period by adding a judgment module in the synchronization module, and improves the timing precision. At the same time, the narrowing of the search range at the synchronization moment also reduces the complexity of the algorithm.

Description

A kind of Chirp spread spectrum communication system receiver time synchronization system and method

technical field

The invention relates to the field of low power consumption wide area network, and more particularly, to a receiver time synchronization system and method of a Chirp spread spectrum communication system.

Background technique

Among the many wireless technologies related to the Internet of Things, LoRa technology in low power wide area network (LPWAN) is a hot spot in recent years. LoRa is a long-distance wireless transmission scheme based on spread spectrum technology promoted by Semtech in the United States. The characteristics of power consumption, low cost, wide coverage, and large connection, and the spread spectrum technology is first used in military communication due to its strong anti-interference ability, multi-access communication, good confidentiality, and anti-multipath interference. It has received great attention, and has gradually been promoted in civilian communications.

There are four main methods of current spread spectrum technology: direct sequence spread spectrum, frequency hopping spread spectrum, time hopping spread spectrum and linear frequency modulation. Linear pulse frequency modulation (CSS) belongs to linear frequency modulation and is the key technology of the physical layer of the LoRa protocol. Chirp spread spectrum means that in a symbol period, the carrier frequency of the system linearly "sweeps" a frequency range, and then forms a large-bandwidth swept frequency signal. Chirp spread spectrum communication adopts CCS as the physical layer transmission technology, which has the advantages of low power consumption, long distance, low complexity and strong anti-interference ability.

In a digital communication system, the quality of the synchronization algorithm will directly affect the reliability of the communication system. When the performance of the synchronization algorithm is poor, the entire communication system may not work properly. After the Chirp system is matched and filtered, the main lobe width of the Chirp signal output pulse is inversely proportional to the signal bandwidth. Because the CSS communication system has a very wide signal bandwidth and short pulse duration, confirming the optimal sampling time of the narrowband pulse requires the time synchronization of the Chirp receiver. The algorithm has better performance.

In the Chirp receiver, the commonly used time synchronization algorithms are the threshold method and the peak search method. The threshold method is to set a larger threshold first. When the output value of the matched filter is greater than the threshold, this moment is used as the synchronization moment. The feature of this method is that the structure is relatively simple, and it is easy to implement in hardware, but it is easily disturbed by noise. , when the noise interference is strong, it may occur that the value at the moment of the noise interference is greater than the threshold, resulting in a misjudgment of the peak of the compressed pulse. In addition, when the signal transmission environment is relatively bad, the peak value of the compressed pulse of the matched output will be smaller than the threshold value, and the phenomenon of packet loss will occur at this time. Therefore, the threshold method is not applicable when the signal-to-noise ratio is low and the environment is too harsh. In the peak search method, the initial value takes a value greater than the noise power threshold. Two registers R ₁ and R ₂ are set, R ₁ stores the initial value, and R ₂ is initialized to 0, which is used to store the maximum value after comparing the input value I greater than R ₁ with R ₂ . When a symbol is sampled at N points, the peak finding method requires N comparisons, but it is more difficult to implement in hardware. Since then, some studies have introduced the interference suppression technology into the CSS system, which has enhanced the anti-interference ability to a certain extent, but this method has a defect. This will lead to misjudgment of the synchronization moment. Therefore, in the receiver of the Chirp spread spectrum communication system, it is of great significance to find a method with strong anti-interference ability and can solve the synchronization misjudgment caused by the sampling error of an integer multiple of the symbol period.

SUMMARY OF THE INVENTION

The invention is a receiver time synchronization system of Chirp spread spectrum communication system with strong anti-interference ability.

Another object of the present invention is to provide a time synchronization method for a receiver of a Chirp spread spectrum communication system, which has low algorithm complexity and high timing precision.

In order to achieve above-mentioned technical effect, technical scheme of the present invention is as follows:

A receiver time synchronization system of a Chirp spread spectrum communication system, comprising a signal-to-noise ratio enhancement module, a combined Chirp modulation module, a peak search module, and a decision module; The modulo eliminates the phase difference between the symbols, and the gain acquisition strengthens the useful signal; the combined spread spectrum modulation module uses the combined Chirp signal for spread spectrum modulation; after acquiring the combined spread spectrum modulation, matched filter reception, signal-to-noise ratio enhanced signal Then, through the noise threshold set in the decision module, the search for the peak value of the narrowband pulse after matched filtering is reduced to one symbol period, and the peak search module realizes time synchronization at the moment when the peak value is captured in this symbol period.

Further, the combined Chirp modulation module multiplies the M mapping symbols with M different Chirp spread spectrum signals respectively to achieve spectrum spread; at the receiving end, pulse compression is performed on the spread spectrum received signal with a corresponding matched filter. , and then send the M despread signals to the demapper to obtain the original transmission data.

Preferably, the value of M is 4.

Further, the signal-to-noise ratio enhancement module eliminates the phase difference by performing a modulo operation on the narrowband compressed pulse after passing through the matched filter, so that the useful signal in one symbol period has a similar waveform, and then the signal is processed by the feedback module. Reinforcement to achieve suppression of interference noise.

A method for time synchronization of a Chirp spread spectrum communication system receiver, comprising the following steps:

S1: Perform a modulo operation on the narrowband compressed pulse output after matched filtering, and the modulo signal is strengthened through a feedback link;

S2: Let each cycle symbol have N sampling points, and perform 0, N, 2N, and 3N sampling point delay processing on the received digital baseband signal respectively;

S3: Divide the digital baseband signal processed by the delay through the corresponding matched filter, and then send it to the judgment module;

S4: judge the signal entering the judgment module, and obtain the output signal after judgment, that is, the symbol used for peak search;

S5: perform modulo operation on the judged signal to eliminate the influence of the symbol phase difference, and then combine the signals;

S6: Use the received digital baseband signal of S2 again, perform a modulo operation on the received baseband signal, add a delay of 1 sampling point, and subtract the signal with a delay of 4N sampling points to combine;

S7: delay the signals in S6 by 0, N, 2N, and 3N sampling points respectively, and then perform the merging operation;

S8: Use the modulo of the signal obtained in S5 to divide by the signal combined in S7 to achieve normalization processing to obtain a decision variable;

S9: introduce a timing metric through the decision variable of S8 to measure the synchronization accuracy of the time synchronization algorithm;

S10: The peak search module performs a peak search in the symbol output after passing through the judgment module, and the time corresponding to the peak value is the synchronization time.

Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

The signal-to-noise ratio enhancement module of the present invention includes modulo acquisition, gain acquisition, delay device, combination, and modulo acquisition eliminates the phase difference between symbols, and the gain acquisition strengthens the useful signal; the combined spread spectrum modulation module uses combined Chirp signals to spread spectrum Modulation; after obtaining the signal that has undergone combined spread spectrum modulation, matched filtering reception, and signal-to-noise ratio enhancement, the search for narrow-band pulses after matched filtering is reduced to one symbol period through the noise threshold set in the judgment module, and the peak search module is in the The timing at which the peak value is captured within this symbol period is time-synchronized. The invention solves the misjudgment of the synchronization moment caused by the difference between the actual synchronization and the ideal synchronization by an integer multiple of the symbol period by adding a judgment module in the synchronization module, and improves the timing precision. At the same time, the reduction of the search range at the synchronization moment also reduces the complexity of the algorithm.

Description of drawings

Fig. 1 is the block diagram of combined Chirp spread spectrum modulation system;

Fig. 2 is the synchronization module in the Chirp receiver;

Fig. 3 is the time-frequency relation diagram of combined Chirp signal;

Fig. 4 is the parameter diagram of CSS communication system;

Fig. 5 is the decision module structure diagram in the peak search method;

6 is a block diagram of an improved interference suppression time synchronization algorithm;

Figure 7 is a comparison diagram of hardware resources of different algorithms;

Figure 8 shows a comparison of timing metrics for synchronization algorithms.

Detailed ways

The accompanying drawings are for illustrative purposes only, and should not be construed as limitations on this patent;

In order to better illustrate this embodiment, some parts of the drawings are omitted, enlarged or reduced, which do not represent the size of the actual product;

It will be understood by those skilled in the art that some well-known structures and their descriptions may be omitted from the drawings.

The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

Example

The improvement of the present invention to the time synchronization method of the Chirp spread spectrum system receiver is mainly to add a decision module to the peak search method, thereby reducing the complexity of the algorithm. In addition, a combined spread spectrum modulation and signal-to-noise ratio enhancement module is also involved.

Different from the commonly used binary orthogonal keying (BOK) and direct modulation (DM) spread spectrum modulation methods in spread spectrum modulation, the present invention uses a combination of spread spectrum signals for modulation. The idea of combined spread spectrum is to multiply M mapped symbols with M different Chirp spread spectrum signals respectively to realize spread spectrum. At the receiving end, the corresponding matched filter is used to compress the received signal after the spread spectrum, and then the M despread signals are sent to the demapper to obtain the original transmitted data, which will increase the system link capacity and improve the Power efficiency, the block diagram of the combined Chirp spread spectrum modulation system is shown in FIG. 1 , and the value of M in the present invention is 4.

The signal-to-noise ratio enhancement module eliminates the phase difference by taking the modulo operation on the narrow-band compressed pulse after the matched filter, so that the useful signal in one symbol period has a similar waveform, and then the signal is strengthened through the feedback module, so as to achieve for the purpose of suppressing interference noise.

The idea of the decision module is to use the uncorrelation between the signal and the noise. By setting a noise threshold, when there is a sample value in the synchronization symbol that is integrated with the noise and is smaller than the noise threshold, it will be set to zero, so that the peak search method will be searched. The range is reduced to one symbol period. For L symbols, the traditional interference suppression synchronization algorithm needs to perform L×N comparisons. The synchronization algorithm that adds the decision module only needs to perform L+N comparisons on the decision module. In the case of a large number of symbols, the consumption is less. The hardware resources are also reduced, and the complexity is also reduced.

Specific steps are as follows:

S1: Signal-to-noise ratio enhancement module

S1.1 performs the modulo operation on the narrowband compressed pulse output after matched filtering;

The signal after modulo S1.2 is enhanced through a feedback link.

S2: Decision module step

S2.1 Assuming that each periodic symbol has N sampling points, perform 0, N, 2N, and 3N sampling point delay processing on the received digital baseband signal respectively;

S2.2 respectively passes through the corresponding matched filter to the digital baseband signal processed by the delay, and then sends it to the judgment module;

S2.3 judges the signal entering the judgment module, and obtains the output signal after judgment, that is, the signal used for peak search;

S2.4 performs modulo operation on the judged signal to eliminate the influence of the symbol phase difference, and then combines the signals;

S2.5 uses the digital baseband signal received by S2.1 again, performs modulo operation on the received baseband signal, adds a delay of 1 sampling point, and subtracts the signal with a delay of 4N sampling points to combine;

S2.6 is similar to step 2.1, delaying the signals in S2.5 by 0, N, 2N, and 3N sampling points respectively, and then performing the merging operation;

S2.7 utilizes the modulo of the signal obtained in S2.4 to divide the combined signal in S2.6 to achieve normalization processing to obtain a decision variable.

The complete system of Chirp spread spectrum communication consists of transmitter, channel and receiver. The specific steps of the system process are as follows:

S3: The binary data goes through a splitter (1:2), parallel-to-serial conversion, coding, serial-to-parallel conversion, and QPSK modulation in turn, and then becomes complex-valued data.

S4: Perform differential modulation on the complex-valued data modulated by QPSK, that is, multiply the current QPSK constellation point by the QPSK constellation point delayed by 3 symbol periods.

S5: realize the modulation of the combined Chirp signal, that is, multiply the complex symbol obtained in S4 by the corresponding spreading function in the combined Chirp signal.

S6: Pulse shaping operation is performed on the signal after spread spectrum modulation, which will well suppress out-of-band leakage and prevent inter-symbol crosstalk.

S7: Convert the digital signal into an analog signal through D/A conversion, and then up-convert to realize radio frequency modulation, so that the signal can be transmitted through the antenna. After the signal is transmitted through the channel, it is received by the antenna at the receiving end, and then the digital baseband signal is obtained after down-conversion and A/D conversion.

S8: Synchronize the received baseband signal, which will involve the steps of S1 and S2. The position of the synchronization module in the Chirp receiver is shown in Figure 2.

S9: The synchronized signal undergoes signal detection, differential demodulation, parallel-to-serial conversion, decoding, serial-to-parallel conversion in sequence, and the combiner restores binary data.

Combined spread spectrum signal

The combined spread spectrum signal used in the present invention contains 4 sub-Chirp signals, the sub-Chirp signals have the same signal bandwidth and symbol period, there are two frequency modulation slopes used, and the other two frequencies are obtained by taking the inverse of the slope, which is beneficial to the signal When the compressed pulse peak is up-sampled, the interference in the matched filter at the receiving end will be reduced. The time-frequency relationship diagram of the combined spread spectrum signal is shown in Figure 3.

The Chirp signal in one symbol period is described as follows:

_f0 is the center frequency of the Chirp signal, A is the amplitude of the signal, _θ0 is the initial phase, T is the symbol period, and μ is the frequency modulation slope. B=μT is the bandwidth that the signal "sweeps" over a symbol period T.

At the receiving end, the matched filter impulse response h(t) of the Chirp signal is the inverted conjugate of the signal s(t), h(t)=ks*(-t)=ks(-t), and k is the matching The gain of the filter, * represents the conjugate operation, and the output after matched filtering is y(t):

The combined Chirp signal S ₀ (t) is defined as:

T _n,k =(K+1)T _sub +nT _chrip (4)

为映射之后的复数符号，w_k为第k个子Chirp的中心角频率，ξ_k为角频率的斜率符号，T_n,k为子Chirp信号产生的开始时间，T_chrip是组合Chirp信号的平均持续时间，T_sub为子Chirp的持续时间。In equations (3) and (4), n is the sequence number of the combined Chirp signal to be sent, k={0,1,2,3} is the sub-Chirp index,

is the complex symbol after mapping, w _k is the center angular frequency of the kth sub-Chirp, ξ _k is the slope symbol of the angular frequency, T _n,k is the start time of the sub-Chirp signal generation, T _chrip is the average duration of the combined Chirp signal time, T _sub is the duration of the child Chirp.

In this process, μ=2π×7.1358×10 ¹² rad/s ² , T _sub =1.1875us The specific values of each sub-spread spectrum signal parameter are shown in FIG. 4 .

SNR enhancement

Assuming that the channel is stable in L symbol periods, after the received L consecutive symbols are pulse compressed, the useful signal will be strengthened due to the strong correlation of the useful signal. Use r _i (t), i = {0,1,...,L-1} denotes the ith pulse-compressed useful signal, r ₀ (t)=r ₁ (t)=...=r _L-1 (t). The useful signal part is r(t)=(1+α ² +α ³ +...+α ^L-1 )r ₀ (t) after L iterations, and the corresponding noise is _ni (t) after passing through the matched filter , i={0,1,...,L-1} means that the noise has zero mean and the variance is Gaussian white noise of σ ^2. Because the noises are not correlated, the value of N(t) after L iterations is :

N(t)=α ^L-1 n ₀ (t)+α ^L-2 n ₁ (t)+…+n _L-1 (t) (5)

The mean of N(t) is zero, the variance is (1+α ² +α ³ +...+α ^L-1 ) ² , and the useful signal energy gain is:

r(t)=(1+α ² +α ⁴ +...+α ^2(L-1) )σ ² (6)

After L iterations, the signal-to-noise ratio is:

When α < 0 and L is infinite,

That is, when α approaches 1, the signal-to-noise ratio is larger. The T _p (d) signal of the combined signal after matched filtering can be expressed as:

In the above formula, T _p (d) is the d-th output value of the p-th matched filter, p={0, 1, 2, 3}, r _x is the received baseband signal, m _{p, n} is the p-th The nth coefficient of each filter, N is the number of sampling points of a sub-Chirp signal, d is the time coefficient of the starting point of the window composed of N sampling points, L=4 in this scheme, the signal after passing through four matched filters The combined value is P(d):

According to the normalization of the matched filtered signal in step 2.7, the decision variable can be obtained

Decision module structure

The internal structure of the decision module is shown in Figure 5. The L data symbols are sent to the decider and compared with the noise threshold V. The block diagram of the improved interference suppression time synchronization algorithm is shown in Figure 6. The signal-to-noise ratio enhancement unit includes the modulo, feedback, delay, and addition of the received baseband signal, the signal-to-noise ratio enhancement unit and the decision module, and the peak search constitutes the time synchronization module of the CSS system.

In order to verify the effect of the present invention on time synchronization, a corresponding simulation comparison is made with the traditional interference suppression synchronization algorithm. There are two kinds of synchronization algorithms for interference suppression for comparison, algorithm A and algorithm B. Their ideas are respectively: The signal-to-noise ratio is increased by using a sub-spread spectrum to carry out conjugate multiplication of the compressed pulse after matched filtering, and use two adjacent sub-spread spectrum to conjugate the compressed pulse after passing through the corresponding sub-matched filter to increase the signal-to-noise ratio. To achieve the suppression of noise interference. The timing metric is used as the criterion to judge the pros and cons of the effect of the scheme. The timing metric is defined as the square value of the judgment variable. The larger the timing metric value, the better the effect. The hardware resource comparison between the present invention and the other two interference suppression algorithms is shown in FIG. 7 , and the relationship between the timing metric and the signal-to-noise ratio is shown in FIG. 8 .

In Fig. 7, since complex conjugate matching is used, all three algorithms need 8 matched filters. Compared with the other two algorithms, the algorithm of the present invention has more comparators, but does not need to use multipliers. Therefore, according to the corresponding required hardware resources and simulation description, the synchronization algorithm proposed by the present invention is superior to the other two interference suppression algorithms in terms of using hardware resources and timing metrics.

The same or similar reference numbers correspond to the same or similar parts;

The positional relationship described in the accompanying drawings is only for exemplary illustration, and should not be construed as a limitation on this patent;

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the embodiments of the present invention. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. There is no need and cannot be exhaustive of all implementations here. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (4)

1. a Chirp spread spectrum communication system receiver time synchronization system, it is characterized in that, comprise signal-to-noise ratio enhancement module, combine Chirp modulation module, peak search module, judgment module; Signal-to-noise ratio enhancement module comprises modulo, gain acquisition, Delay, combination, and modulo eliminate the phase difference between symbols, and gain acquisition strengthens the useful signal; the combined spread spectrum modulation module uses combined Chirp signal for spread spectrum modulation; after the acquisition is combined spread spectrum modulation, matched filter reception, signal After the signal with enhanced noise ratio passes the noise threshold set in the judgment module, the peak search of the matched filtered narrowband pulse is reduced to one symbol period, and the peak search module realizes time synchronization at the moment when the peak value is captured in this symbol period. ; The combined Chirp modulation module multiplies the M mapping symbols with M different Chirp spread spectrum signals respectively to achieve spectrum spread; at the receiving end, the corresponding matched filter is used to pulse compress the spread spectrum received signal, and then The M despread signals are sent to the demapper to obtain the original transmission data. 2 . The receiver time synchronization system of the Chirp spread spectrum communication system according to claim 1 , wherein the value of M is 4. 3 . 3. Chirp spread spectrum communication system receiver time synchronization system according to claim 2, is characterized in that, described signal-to-noise ratio enhancement module eliminates phase difference by carrying out modulo operation to narrowband compressed pulse after passing through matched filter , so that the useful signal in a symbol period has a similar waveform, and then the signal is strengthened through the feedback module to suppress the interference noise. 4. a synchronization method of Chirp spread spectrum communication system receiver time synchronization system as claimed in claim 1, is characterized in that, comprises the following steps: S1: Perform a modulo operation on the narrowband compressed pulse output after matched filtering, and the modulo signal is enhanced through a feedback link; S2: Let each cycle symbol have N sampling points, and perform 0, N, 2N, and 3N sampling point delay processing on the received digital baseband signal respectively; S3: The digital baseband signal processed by the delay is respectively passed through the corresponding matched filter, and then sent to the judgment module; S4: judge the signal entering the judgment module, and obtain the output signal after judgment, that is, the symbol used for peak search; S5: perform a modulo operation on the judged signal to eliminate the influence of the symbol phase difference, and then combine the signals; S6: Use the digital baseband signal received in S2 again, perform a modulo operation on the received baseband signal, add a delay of 1 sampling point, and subtract the signal with a delay of 4N sampling points to combine; S7: delay the signals in S6 by 0, N, 2N, and 3N sampling points respectively, and then perform the merging operation; S8: Use the modulo of the signal obtained in S5 to divide by the signal combined in S7 to achieve normalization processing to obtain a decision variable; S9: introduce a timing metric through the decision variable of S8 to represent the metric of the synchronization accuracy of the time synchronization algorithm; S10: The peak search module performs peak search on the symbols output after passing through the judgment module, and the time corresponding to the peak is the synchronization time.

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