Amplitude coded modulation linear frequency modulation signal demodulation method

Abstract

The invention provides an amplitude code modulation linear frequency modulation signal demodulation method, which comprises the following steps of: 1, designing an amplitude code modulation linear frequency modulation signal; for a random communication sequence {an}, an encoding signal a (t) is made to be a unipolar non-return-to-zero signal of the encoding sequence {an}, a linear frequency modulation signal is set to be s (t), and when the encoding signal is used for conducting amplitude modulation on the linear frequency modulation signal, an obtained modulated signal is x (t) = a (t) * s (t); 2, designing a related reference signal; setting the width of a code element as TB, and cutting off the chirp signal by taking the TB as the width to obtain a plurality of sub chirp signals with coherent phases; the nth sub-linear frequency modulation signal obtained by truncation is set as sn (t), and the related reference signal y (t) is the sum of all sub-linear frequency modulation signals; 3, performing correlation operation and sampling judgment; and performing correlation operation on the modulated signal x (t) and a preset correlation reference signal y (t), sampling a correlation result at a specific moment, and for a given threshold value, when a sampling value is greater than the threshold value, determining that the symbol is 1, otherwise, determining that the symbol is 0.

Description

[Technical field]

[0001] The invention relates to a method for demodulating an amplitude coded modulated linear frequency modulation signal, belongs to the field of radar communication integration, and specifically relates to a method for amplitude coded modulated linear frequency modulation signal and demodulating the modulated signal to recover communication information. [Background technology]

[0002] With the rapid development of electronic information technology, radar sensing and wireless communication technology have made great progress in their specific tasks and application scenarios. Radar technology is mainly used for target detection, identification and tracking, while wireless communication technology focuses on the efficient transmission of information. However, with the increasingly tight spectrum resources, the signal interference problem between radar and communication systems has become increasingly prominent, which not only limits the performance of both, but also triggers an urgent need for the effective use of spectrum resources.

[0003] In order to meet the above challenges, radar communication integration has become a research hotspot in recent years. The core concept of radar communication integration is to meet the needs of radar sensing and wireless communication by sharing the same signal waveform, so as to improve spectrum utilization and reduce signal interference. At present, the focus of integration research is mainly on orthogonal frequency division multiplexing integration and linear frequency modulation signal integration, but both routes inevitably face the situation where radar functions and communication functions are mutually restricted or even contradictory, such as high bit error rate in the communication link or reduced radar target detection capability and other undesirable effects.

[0004] The present invention utilizes the coded signal in the communication system to perform amplitude modulation on the linear frequency modulation signal. The receiving end can effectively restore the original coded information by performing correlation operation on the received modulated signal and the preset correlation reference signal and making a judgment based on the result of the correlation operation, and can reduce the bit error rate when the input signal-to-noise ratio is low. [Summary of the invention]

[0005] The technical problem to be solved by the present invention is that the existing integrated signal is affected by the mutual constraints between the radar function and the communication function, and has defects such as high bit error rate. The present invention modulates the coded signal on the linear frequency modulation signal in the form of amplitude, performs correlation operation on the received modulated signal and the preset related reference signal at the receiving end, makes a judgment based on the correlation operation result, and finally restores the coded sequence.

[0006] The present invention relates to a method for demodulating an amplitude coded modulation linear frequency modulation signal, and the technical scheme adopted is as follows:

[0007] The first step is to design the linear frequency modulation signal of amplitude coded modulation.

[0008] For a random communication sequence {a n}, let the coded signal a(t) be the coded sequence {a n} unipolar non-return-to-zero signal, "0", "1" symbols correspond to rectangular signals with amplitudes of 0 and +1 respectively. Assuming that the linear frequency modulation signal is represented by s(t), when the linear frequency modulation signal is amplitude modulated by the coded signal, the modulated signal obtained is x(t) = a(t) · s(t), and x(t) can be regarded as a plurality of phase-coherent linear frequency modulation signals.

[0009] The second step is to design relevant reference signals.

[0010] Assume that the width of a symbol (rectangular signal width) is T B , for linear frequency modulation signal with T B The width is truncated to obtain several phase-coherent sub-linear frequency modulation signals. Assume that the nth sub-linear frequency modulation signal obtained by truncation is s n (t), the related reference signal y(t) is the sum of all sub-linear frequency modulation signals.

[0011] The third step is related operations and sampling decisions.

[0012] According to the first and second steps, the modulated signal x(t) is correlated with the preset correlation reference signal y(t), and the correlation result is sampled at a specific time. For a given threshold value, when the sampled value is greater than the threshold value, it is judged as a "1" symbol, otherwise it is determined as a "0" symbol.

[0013] The beneficial effects of the present invention mainly include:

[0014] First, a demodulation method for amplitude coded modulated linear frequency modulation signals is proposed. The coded signal is modulated on the linear frequency modulation signal in the form of amplitude, and the communication information carried is restored using correlation operation, which can reduce the bit error rate under low signal-to-noise ratio conditions.

[0015] Second, a correlation reference signal model is designed. By segmenting the complete linear frequency modulation signal, several phase-coherent sub-linear frequency modulation signals are obtained, and the correlation reference signal is synthesized by adding all the sub-linear frequency modulation signals.

[0016] Third, the bit error rate simulation experiment was completed, which verified that this demodulation method can reduce the bit error rate by adjusting the sampling rate and the bit rate.

Brief Description of the Drawings

[0017] Figure 1 It is the processing flow of the demodulation process.

[0018] FIG. 2( a ) and FIG. 2( b ) are waveform diagrams of a coded signal and an amplitude coded modulated linear frequency modulation signal, respectively.

[0019] FIG3(a) and FIG3(b) are waveform diagrams of a sub-linear frequency modulation signal and a related reference signal, respectively.

[0020] FIG4(a) and FIG4(b) are the correlation operation result and the coded signal after sampling and decision, respectively.

[0021] Figure 5 It is the bit error rate curve under different sampling frequencies.

Specific implementation method

[0023] The present invention is further described below in conjunction with the accompanying drawings. The present invention relates to a method for demodulating an amplitude coded modulation linear frequency modulation signal, the steps of which are as follows:

[0024] The first step is to design the linear frequency modulation signal of amplitude coded modulation.

[0025] communication sequence {a n} is composed of random "0" and "1" symbols, and the corresponding coding signal a(t) is the coding sequence {a n} is a unipolar non-return-to-zero signal, recorded as:

[0026]

[0027] Among them, T B is the symbol width, rect(·) is a rectangular pulse with a height of 1 and a width of 1, recorded as:

[0028]

[0029] The linear frequency modulation signal s(t) is:

[0030]

[0031] Wherein, T is the signal pulse width, μ=B / T is the frequency modulation slope, B is the signal bandwidth, and j represents the imaginary unit.

[0032] The amplitude coded modulated linear frequency modulation signal x(t) can be expressed as:

[0033] x(t)=a(t)·s(t) (4)

[0034] The second step is to design relevant reference signals.

[0035] For the complete linear frequency modulation signal s(t), T B The width is segmented and truncated to obtain several phase-coherent sub-linear frequency modulation signals, and the nth sub-linear frequency modulation signal is recorded as s n (t) is:

[0036]

[0037] The related reference signal y(t) is the sum of all sub-linear frequency modulation signals and is expressed as:

[0038]

[0039] The third step is related operations and sampling decisions.

[0040] According to the first and second steps, the modulated signal x(t) is correlated with the related reference signal y(t), and the obtained correlation result R(τ) is:

[0041]

[0042] Where τ represents the time delay, m and n represent the subscript numbers, y*(t) represents the conjugate signal of y(t), and R nm (τ) is s n (t) and s m The correlation function of x(t) and y(t) can also be regarded as the matched filtering result of x(t) and y(t). k =kT B When sampling, formula (7) is simplified to:

[0043]

[0044] Where R kk (0) represents the value of the autocorrelation function of the k-th sub-linear frequency modulation signal at time 0, and also represents the power of the k-th sub-linear frequency modulation signal, R km (0) indicates that the cross-correlation function of the kth sub-linear frequency modulation signal and the mth sub-linear frequency modulation signal takes a value at time 0. When the kth code element is 1 (a k =1), the sampling value is composed of the autocorrelation function and the cross-correlation function; when the kth code element is 0 (a k =0), the sampled values ​​consist only of the cross-correlation function.

[0045] When any two different sub-linear frequency modulation signals are uncorrelated, then

[0046]

[0047] Satisfaction When , formula (8) is further simplified to:

[0048]

[0049] In digital computing, assuming that N points are sampled within a pulse width T, there are M code elements, and when the amplitude of x(t) is A, If the average power of additive white Gaussian noise is 2σ 2 , the correlation function R between the noise n(t) and the related reference signal y(t)n (τ k ) obeys the normal distribution, and the variance has an upper limit:

[0050]

[0051] where s m [k] represents the discrete expression of the mth linear frequency modulation signal. In addition, The phase is affected by multiple parameters (B, T, N, M, etc.). After taking the modulus of 2π, it can be approximately considered that the phase values ​​are independent; on the other hand, the number of sampling points N is generally large, and it can be approximately considered that the phase is uniformly distributed on [0,2π]. Therefore, according to the central limit theorem, Therefore, when Gaussian white noise exists, equation (10) can be rewritten as:

[0052]

[0053] Among them, R n (τ k ) has a mean of 0 and a variance of 2Nσ 2 If the real part is selected for judgment, equation (12) can be regarded as a complex normal distribution with amplitude The noise intensity is Nσ 2 The coherent demodulation is equivalent to the input signal-to-noise ratio The effect of coherent demodulation, where r = A 2 / (2σ 2 ) is the actual input signal-to-noise ratio. Therefore, the decision threshold level is:

[0054]

[0055] Assume that the coding sequence {a n}, the probability of "0" and "1" code elements appearing is equal, the bandwidth of the linear frequency modulation signal s(t) is B = 10MHz, the pulse width is T = 10μs, and the code element width is T B =1μs, sampling frequency f s =10MHz, modulated signal amplitude A=1, actual input signal-to-noise ratio r=5dB.

[0056] according to Figure 1 The simulation is performed by the flowchart shown in FIG2(a). The coded signal is shown in FIG2(a). The coded signal performs amplitude modulation on the linear frequency modulation signal, and the modulated signal is shown in FIG2(b). BThe width is truncated to obtain several sub-linear frequency modulation signals, among which the 1st, 2nd and 3rd sub-linear frequency modulation signals are shown in Figure 3(a). All sub-linear frequency modulation signals are summed to obtain the correlation reference signal shown in Figure 3(b). The modulated signal is correlated with the preset correlation reference signal, and the correlation result is shown in Figure 4(a). The correlation result is sampled regularly and compared with the decision threshold, and the recovered coding signal is shown in Figure 4(b). It is not difficult to see that the recovered coding signal is consistent with the original coding signal. The sampling frequency is adjusted to 10MHz, 20MHz and 50MHz respectively, the actual input signal-to-noise ratio is adjusted, and multiple simulations are performed to obtain the relationship curve between the bit error rate and the sampling frequency as shown in Figure 5 It can be seen that by increasing the sampling frequency, the bit error rate can be reduced, and low bit error rate demodulation under low signal-to-noise ratio can be achieved.

Claims

1. A method for demodulating an amplitude coded modulation linear frequency modulation signal, characterized in that: The steps include: The first step is to design the linear frequency modulation signal of amplitude coded modulation; For a random communication sequence {a n }, let the coded signal a(t) be the coded sequence {a n } unipolar non-return-to-zero signal, "0" and "1" symbols correspond to rectangular signals with amplitudes of 0 and +1 respectively; let the linear frequency modulation signal be s(t), when the linear frequency modulation signal is amplitude modulated by the coded signal, the modulated signal obtained is x(t)=a(t)·s(t); The second step is to design relevant reference signals; Assume a code element width is T B , for linear frequency modulation signal with T B The width is truncated to obtain a number of phase-coherent sub-linear frequency modulation signals; let the nth sub-linear frequency modulation signal obtained by truncation be s n (t), the related reference signal y(t) is the sum of all sub-linear frequency modulation signals; The third step is related operation and sampling decision; According to the first and second steps, the modulated signal x(t) is correlated with the preset correlation reference signal y(t), and the correlation result is sampled at a specific time. For a given threshold value, when the sampled value is greater than the threshold value, it is determined to be a "1" symbol, otherwise it is determined to be a "0" symbol.

2. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1, characterized in that: In the first step, the communication sequence {a n } is composed of random "0" and "1" symbols, and the corresponding coded signal a(t) is the coded sequence {a n } is a unipolar non-return-to-zero signal, recorded as: Among them, T B is the symbol width, rect(·) is a rectangular pulse with a height of 1 and a width of 1, recorded as:

3. A method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1 or 2, characterized in that: In the first step, the linear frequency modulation signal s(t) is: Where, T is the signal pulse width, μ=B / T is the frequency modulation slope, and B is the signal bandwidth; The amplitude coded modulated linear frequency modulation signal x(t) is expressed as: x(t)=a(t)·s(t) (4).

4. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1, characterized in that: In the second step, the complete linear frequency modulation signal s(t) is subjected to T B The width is segmented and truncated to obtain several phase-coherent sub-linear frequency modulation signals, and the nth sub-linear frequency modulation signal is recorded as s n (t) is:

5. A method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1 or 4, characterized in that: In the second step, the relevant reference signal y(t) is the sum of all sub-linear frequency modulation signals, expressed as:

6. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1, characterized in that: In the third step, the modulated signal x(t) is correlated with the related reference signal y(t), and the correlation result R(τ) is: Among them, R nm (τ) is s n (t) and s m The correlation result can also be regarded as the matched filtering result of x(t) and y(t).

7. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 6, characterized in that: In the third step, the symbol start time τ is selected k =kT B When sampling, formula (7) is simplified to:

8. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 7, characterized in that: In the third step, when any two different sub-linear frequency modulation signals are uncorrelated, then, Satisfaction When , formula (8) is further simplified to:

9. A method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 1 or 8, characterized in that: In the third step, in digital operation, assume that N points are sampled within a pulse width T, there are M code elements, and when the amplitude of x(t) is A, If the average power of additive white Gaussian noise is 2σ 2 , the correlation function R between the noise n(t) and the related reference signal y(t) n (τ k ) obeys the normal distribution, and the variance has an upper limit:

10. The method for demodulating an amplitude coded modulation linear frequency modulation signal according to claim 9, characterized in that: In the third step, The phase of is affected by multiple parameters. After taking the modulus of 2π, the phase values ​​are considered to be independent; the phase is considered to be uniformly distributed on [0,2π]. Therefore, according to the central limit theorem, Therefore, when Gaussian white noise exists, it is: Among them, R n (τ k ) has a mean of 0 and a variance of 2Nσ 2 If the real part is selected for judgment, equation (12) is regarded as a complex normal distribution with amplitude The noise intensity is Nσ 2 The coherent demodulation is equivalent to the input signal-to-noise ratio The effect of coherent demodulation, where r = A 2 / (2σ 2 ) is the actual input signal-to-noise ratio; therefore, the decision threshold level is:

Images