DSSS enhanced synchronization method combining PMF-FFT and improved Costas ring

Abstract

The invention discloses a DSSS enhanced synchronization method combining PMF-FFT and an improved Costas ring, and is applied to the technical field of wireless communication. In a traditional receiver design, an independent frequency offset estimation and carrier synchronization method is usually adopted and is not suitable for processing a high dynamic environment and a low signal-to-noise ratio condition; according to the method, a maximum frequency offset estimation algorithm based on partial matched filter-fast Fourier transform is adopted; fFT conversion is carried out on a received signal, and the frequency offset of the signal is accurately estimated, so that the influence of the frequency offset on the synchronization performance is effectively reduced; on the basis of frequency offset estimation, a receiver system designed by the invention combines a Costas loop to carry out carrier synchronization tracking; and high-precision carrier recovery is realized by locking the phase of the input signal.

Description

Technical Field

[0001] The invention belongs to the technical field of wireless communications, and in particular relates to a radio signal carrier synchronization technology. Background Art

[0002] In modern wireless communication systems, receiver synchronization is crucial. The quality of synchronization directly affects the system's bit error rate, transmission rate, and overall communication quality. In traditional receiver design, independent frequency offset estimation and carrier synchronization methods are usually used. However, these methods still have certain limitations in handling high dynamic environments and low signal-to-noise ratio conditions. Processing frequency offset estimation and carrier synchronization separately not only increases the complexity of the system, but may also lead to a negative impact of frequency offset estimation errors on carrier synchronization. Summary of the invention

[0003] In order to overcome these challenges, the present invention designs a fast synchronization receiver system based on maximum frequency offset estimation, which achieves more efficient carrier synchronization by combining advanced algorithms and feedback mechanisms.

[0004] The technical solution adopted by the present invention is: a DSSS (Direct Sequence Spread Spectrum) enhanced synchronization method combining PMF-FFT and improved Costas loop, the frequency deviation estimation process is implemented by using a PMF-FFT algorithm module, the PMF-FFT algorithm module includes: a bandpass filter, a squarer, a pulse shaping filter, a PMF operation unit, and an FFT operation unit connected in sequence; after the received IQ signal passes through the PMF-FFT algorithm module, a maximum frequency deviation rough estimation value is obtained; and a low-pass filter is also included, the input of the low-pass filter is the output of the bandpass filter;

[0005] The carrier synchronization process is implemented by an improved Costas loop based on a PMF-FFT loop. The improved Costas loop includes: a despreading module, a phase detector, a loop filter, and a digitally controlled oscillator. The input of the despreading module is the output of the low-pass filter in the PMF-FFT algorithm module and the output of the digitally controlled oscillator. The output of the despreading module is used as the input of the phase detector. The output of the phase detector is used as the input of the loop filter. The output of the loop filter and the coarse estimation value of the maximum frequency deviation are used as the input of the digitally controlled oscillator.

[0006] When the phase detector error is 0, locking is performed and a synchronized demodulated output is obtained.

[0007] Beneficial effects of the present invention: The receiver system proposed in the present invention adopts a maximum frequency deviation estimation algorithm based on partial matching filter-fast Fourier transform (PMF-FFT); the algorithm accurately estimates the frequency deviation of the signal by performing FFT transformation on the received signal, thereby effectively reducing the impact of the frequency deviation on the synchronization performance. Compared with the traditional frequency deviation estimation algorithm, the PMF-FFT algorithm has higher accuracy and faster convergence speed, and can provide more reliable frequency deviation estimation in a dynamically changing channel environment;

[0008] On the basis of frequency offset estimation, the receiver system designed in the present invention combines the Costas loop for carrier synchronization tracking. The Costas loop is a carrier synchronization technology widely used in digital communication systems, which achieves high-precision carrier recovery by locking the phase of the input signal. However, in a low signal-to-noise ratio environment, the performance of the traditional Costas loop will be significantly reduced. In order to solve this problem, the present invention introduces the despread output in the direct sequence spread spectrum system as a feedback mechanism in the Costas loop. Through this feedback mechanism, the system can effectively improve the synchronization performance of the Costas loop in a low signal-to-noise ratio environment and significantly reduce the bit error rate;

[0009] Compared with the traditional receiver synchronization scheme, the system designed by the present invention shows significant advantages in both frequency offset estimation and carrier synchronization. The present invention combines frequency offset estimation and carrier synchronization, and utilizes the high precision of the frequency offset estimation algorithm and the enhanced effect of the feedback mechanism to achieve a tighter synchronization process. At the same time, the design of the present invention can still maintain excellent synchronization performance in low signal-to-noise ratio and high dynamic environment, reflecting strong robustness and adaptability;

[0010] The method of the present invention has the following advantages:

[0011] 1. The present invention adopts PMF-FFT combined with the Costas loop carrier synchronization method, which greatly improves the speed of carrier synchronization. When the received information signal-to-noise ratio is 0 dB, compared with the traditional Costas loop at a carrier frequency deviation of 10 kHz, the traditional Costas carrier synchronization method takes 27.7 milliseconds to complete carrier synchronization, while the present invention takes 3.4 milliseconds to complete carrier synchronization;

[0012] 2. The present invention adopts a method of fine synchronization of the carrier of a spread spectrum Costas loop, which greatly improves the effect of carrier synchronization of the receiver under extremely low signal-to-noise ratio conditions. For example, when the signal-to-noise ratio is -10 dB, the receiving bit error rate of the traditional Costas loop is 0.4, while the receiving bit error rate of the present invention is 0.15. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a system structure block diagram of the present invention.

[0014] Figure 2 It is the PMF-FFT structure diagram of the present invention.

[0015] Figure 3 It is a spread spectrum Costas structure diagram of the present invention.

[0016] Figure 4 It is a specific flow chart of step 1 of a specific embodiment of the present invention.

[0017] Figure 5 It is a specific flow chart of step 2 of a specific embodiment of the present invention.

[0018] Figure 6 It is a specific flow chart of step 3 of a specific embodiment of the present invention.

[0019] Figure 7 This is a graph showing the bit error rate of the present invention compared with the traditional Costas under different signal-to-noise ratios.

[0020] Figure 8 It is a graph showing the synchronous speed of the present invention compared with the traditional Costas under different frequency deviations. DETAILED DESCRIPTION

[0021] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is further explained below with reference to the accompanying drawings.

[0022] The present application provides a fast synchronous receiver system based on the maximum frequency deviation estimation algorithm of PMF-FFT and improved Costas loop carrier synchronization. By adopting the PMF-FFT algorithm for frequency deviation estimation, the algorithm quickly and accurately determines the frequency deviation of the signal through FFT transformation and partial matching filter estimation. Next, the frequency deviation information is input into the Costas loop for carrier synchronization. The Costas loop achieves high-precision carrier recovery through phase detection, filtering and voltage-controlled oscillator adjustment. In order to improve performance in a low signal-to-noise ratio environment, the system introduces the despread output as a feedback signal, which enhances the synchronization capability of the Costas loop, reduces the error of carrier synchronization and further reduces the bit error rate. When the carrier frequency deviation is 10kHz, the bit error rate performance of the present invention compared to the traditional Costas under different signal-to-noise ratios is as shown in the figure. Figure 7 For example, when the signal-to-noise ratio is -10 dB, the receiving bit error rate of the traditional Costas loop is 0.4, and the receiving bit error rate of the present invention is 0.15. The system structure block diagram is as follows: Figure 1 .

[0023] In the PMF-FFT algorithm module, the received IQ (In Phase & Quadrature Phase) signal first passes through a bandpass filter to filter out out-of-band noise. The baseband spectrum interference caused by the code element can be offset by the squarer, and the signal passing through the pulse shaping filter can filter out the noise generated by the squarer.

[0024] By extracting the change peak value of the sampling point within the FFT calculation period through PMF, the maximum frequency deviation peak value can be calculated more accurately.

[0025] The peak value after FFT (fast Fourier transform) calculation is the maximum frequency deviation range, and the frequency deviation is fed back to the NCO (numerical controlling oscillator, NCO) loop.

[0026] In the despreading loop of the Costas and DSSS systems, the mixing signal generated by the NCO is mixed with the output signal of the low-pass filter in the PMF-FFT preprocessing loop to offset part of the frequency deviation. The despreading module tracks the mixed signal by the sliding correlation method. Since the spread spectrum sequence has a good cross-correlation characteristic, the receiver can recover the information at a very low signal-to-noise ratio and suppress noise. The Costas loop synchronization through the despread data can greatly improve the tracking speed and stability.

[0027] The phase detector performs phase error identification on the despread signal, and after passing through the loop filter, stable NCO control parameters can be obtained.

[0028] The control parameters of NCO are composed of the PMF-FFT maximum frequency deviation estimation system and the Costas loop synchronization tracking system based on spread spectrum improvement. PMF-FFT can calculate larger frequency deviations to generate fast control parameters, ensuring that the system can quickly complete coarse carrier synchronization. The Costas loop synchronization tracking system based on spread spectrum improvement can calculate accurate frequency deviations to generate accurate and stable control parameters, ensuring that the system can complete accurate carrier synchronization.

[0029] The specific synchronization process is:

[0030] The information is transmitted using spread spectrum modulation, and the code element is mapped to the [-1,1] channel as an additive white Gaussian noise channel. The received signal is converted into a digital signal after AD conversion, and then the digital signal is converted to the baseband through digital down-conversion, becoming a baseband digital signal with frequency deviation. This baseband signal IQ can be expressed as:

[0031] S[n]=AM[nT s ]G[nT s ]cos(Δω(n)nTs )+N[nT s ]

[0032] Where A is the amplitude of the received signal, T s is the sampling time, Δω(n) is the angular frequency difference between the received signal and the local NCO at the current moment, G[nT s ] is the spreading code at the sampling time, N[nT s ] is the Gaussian noise sampling signal, M[nT s ]Data modulation signal.

[0033] Follow these steps:

[0034] Step 1. Signal preprocessing, square S[n]:

[0035] S 2 [n] = A 2 M 2 [nT s ]G 2 [nT s ]cos 2 (ΔωnT s )+N 2 [nT s ]

[0036] After the pulse shaping filter, M 2 [nT s ]=1,G 2 [nT s ]=1, we have:

[0037] S 2 [n] = A 2 cos 2 (ΔωnT s )+N 2 [nT s ]

[0038] It can be seen that after the pulse shaping filter, only the 2 times frequency deviation component 2Δω and the frequency component in the noise are left, which will be sent to the PMF. After PMF filtering, FFT transformation is performed to analyze the transformed signal spectrum. The peak value output of the matched filter is squared and then FFT operation is performed. The output of the PMF operation is:

[0039]

[0040] Among them, n and k represent the serial numbers of the sampled signals, c[n] is the pseudo-random sequence of the spread spectrum code, and the peak output of PMF can be regarded as the output of a despreading method. During despreading, the spectrum of the interference signal is broadened by the spread spectrum sequence, so that the interference signal power is evenly distributed over the entire frequency band. The processing process of step 1 is as follows: Figure 4 shown.

[0041] Step 2. PMF and FFT operation, the PMF-FFT structure diagram is as follows Figure 2 . The FFT algorithm has the characteristic of fast response speed when estimating the signal frequency deviation. The number of FFT calculation points has a direct impact on the calculation time and calculation resolution. The more FFT calculation points, the higher the resolution of the calculation result, but the longer the calculation time. Conversely, when the FFT calculation points are fewer, the resolution of the calculation result is lower, but the calculation time is shorter. For fast synchronization systems, there are very high requirements for synchronization time, and the number of calculation points needs to be reduced. For high-precision synchronization systems, there are very high requirements for the accuracy of frequency deviation estimation, and the number of FFT calculation points needs to be increased. The calculation accuracy and calculation time are in an inverse relationship. To meet system requirements, a trade-off must be made on the number of FFT calculation points.

[0042] Performing an N-point FFT operation on the PMF output signal f[n] yields:

[0043]

[0044] The frequency calculation formula of FFT is:

[0045] F[n]=n·(f s / N)

[0046] Where F[n] is the frequency corresponding to the nth point of the FFT result, n is the number of points calculated by the FFT point, and f s is the sampling rate, and N is the number of FFT calculation points. The collected signal is downsampled, and the FFT operation is also performed after downsampling. The frequency calculation formula obtained is:

[0047] F[n]=n·(f s / (MN))

[0048] Here, M is the downsampling point interval, and the resolution of the calculation result is increased by M times.

[0049] At a sampling rate of f s =30.72MHz, sampling interval is ΔT=128T s =128 / f s , when the number of FFT calculation points is fftn=512, the calculation frequency range is [-120KHz, 120KHz], the frequency calculation resolution is Δf=1 / (fftnΔT)=468.75Hz, and the total sampling time in a single FFT operation is about 2.1ms. If the signal is processed by FFT, assuming that the position where the maximum value of |F[n]| is located is n=k m , then the maximum frequency offset rough estimate is It can be expressed as:

[0050]

[0051] The frequency offset estimation algorithm based on PMF-FFT can calculate the approximate frequency offset range, but the accuracy of the estimation result is not high. For example, the accuracy of the frequency offset range calculated in the above example is 468.75Hz. This frequency offset range will still affect the performance of the receiver, so it is necessary to make an accurate estimation and compensation for this offset frequency. The processing process of step 2 is as follows: Figure 5 shown.

[0052] Step 3. Despread the received signal and synchronize the Costas carrier. The NCO of the loop is controlled by PMF-FFT and Costas phase detection results. The data after carrier synchronization is sent to the despreading module, and the despread sliding correlation result is output. It is output to the NCO through the BPSK (Binary Phase Shift Keying) phase detector. The principle block diagram is shown in the figure. Figure 3 .

[0053] When the signal-to-noise ratio of the received signal is very low, directly using it as the calculation parameter of Costas cannot obtain a good calculation result. The present invention designs a design scheme combining direct sequence spread spectrum communication with Costas.

[0054] The input signal of the direct sequence despreading system is the output after carrier synchronization. The input signal x c [n] is:

[0055] x c [n]=I[n]cosφ+Q[n]sinφ

[0056] Where φ is the signal phase, calculate the input signal x c The correlation between [n] and the locally generated pseudo-random sequence c[n], the sliding correlation function is defined as:

[0057] R[τ]=R[τ-1]+x c [n]·c[n-τ]

[0058] R[τ] is the correlation result. The sliding window moves on different τ values ​​to find the value of n that maximizes R[τ]. Finally, the correlation value after despreading is output to the Costas loop for carrier synchronization. The despreading output r[n] is:

[0059] r[n]=R[n]

[0060] The sliding correlation method can well detect whether the input signal of the despreading module has completed synchronization, and can be well implemented in logic devices. The sliding correlation capture algorithm uses the autocorrelation characteristics of the pseudo-random code to perform correlation detection. When the time difference Δt generated by the pseudo-random code at the receiving end and the pseudo-random code at the transmitting end is zero, the correlation result is the peak value of the autocorrelation function, that is, the sequence length n. When Δt≠0, the correlation result drops sharply to near 0. According to this characteristic, it can be judged whether the receiving end has completed synchronization with the transmitting end. If the correlation value is lower than the set threshold, that is, the transmitting and receiving ends are not synchronized, then change the local controlled clock phase, and do a step slide of T / 2, and then do a correlation operation until the correlation peak is found, and it is judged that the synchronization is completed. The set threshold here is the pseudo-code synchronization judgment threshold in spread spectrum communication. The threshold value is the prior art, and the present invention is not described in detail here.

[0061] Let the real part of r[n] be I[k] and the imaginary part be Q[n], we have,

[0062] I[k]=i[k]cos(2πf d kT s +θ n )

[0063] -q[k]sin(2πf d kT s +θ n )

[0064] Q[k]=q[k]cos(2πf d kT s +θ n )

[0065] +i[k]sin(2πf d kT s +θ n )

[0066] Where f d is the Doppler frequency shift, θ n is the random phase caused by the channel, i[k] is the in-phase component of the demodulated output symbol integration, q[k] is the orthogonal component of the demodulated output symbol integration, T s =1 / f s is the sampling period, and through the Costas feedback loop, we can get:

[0067] U I [k]=I[k]cosφ+Q[k]sinφ

[0068] =i[k]cos(2πf d kT s +θ n -φ)

[0069] -q[k]sin(2πf d kT s +θ n -φ)

[0070] U q [k]=Q[k]cosφ-I[k]sinφ

[0071] =q[k]cos(2πf d kT s +θ n -φ)

[0072] +i[k]sin(2πf d kT s +θ n -φ)

[0073] In the formula θ e is the phase error. Frequency offset residual and phase error Δφ=2πf d kT s +θ n -φ, we know that when Δφ≈0, cos(Δφ)=1, sin(Δφ)=0, we can get the phase detector error U pd [k]:

[0074] U pd [k]=U q [k]·sgn{U i [k]}

[0075] ={q[k]cos(Δφ)+i[k]sin(Δφ)}·sgn(i[k])

[0076] When the loop is in tracking state, the frequency offset residual and phase error Δφ approach 0, then:

[0077] U pd [k] = q[k]·sgn(i[k]) = 0

[0078] Loop pair U pd [k] is used for detection and discrimination. At the beginning of synchronization, the residual frequency difference is large and the loop performs fast tracking. pd When [k] gradually approaches 0, synchronization is considered successful and locking is performed. The processing process of step 3 is as follows: Figure 6 shown.

[0079] For the present invention, PMF-FFT quickly tracks and locks the coarse frequency deviation of the received signal, and the NCO controls the signal frequency deviation to quickly drop to a small range based on the feedback result of PMF-FFT. The spread spectrum synchronization can restore the baseband information and filter out the noise under the condition of extremely low signal-to-noise ratio. The Costas loop uses the spread spectrum output result as the detection parameter to quickly complete the fine carrier synchronization.

[0080] When the received information signal-to-noise ratio is 0 dB, the synchronization speed performance of the present invention compared with the traditional Costas loop at a carrier frequency deviation of 10 kHz is shown in the figure below: Figure 8 The traditional Costas carrier synchronization method takes 27.7 milliseconds to complete carrier synchronization, while the present invention takes 3.4 milliseconds to complete carrier synchronization.

[0081] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop, characterized in that: The frequency deviation estimation process is implemented by a PMF-FFT algorithm module, which includes: a bandpass filter, a squarer, a pulse shaping filter, a PMF operation unit, and an FFT operation unit connected in sequence; after the received IQ signal passes through the PMF-FFT algorithm module, a maximum frequency deviation rough estimation value is obtained; and a low-pass filter is also included, and the input of the low-pass filter is the output of the bandpass filter; The carrier synchronization process is implemented by an improved Costas loop based on a PMF-FFT loop. The improved Costas loop includes: a despreading module, a phase detector, a loop filter, and a digitally controlled oscillator. The input of the despreading module is the output of the low-pass filter in the PMF-FFT algorithm module and the output of the digital controlled oscillator. The output of the despreading module is used as the input of the phase detector, and the output of the phase detector is used as the input of the loop filter. The output of the loop filter and the maximum frequency deviation rough estimate are used as the input of the digital controlled oscillator: When the phase detector error is 0, locking is performed and a synchronized demodulated output is obtained.

2. The DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop according to claim 1, characterized in that: The method also includes down-sampling the output signal of the PMF operation unit.

3. The DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop according to claim 2, characterized in that: After the downsampled signal is processed by the FFT operation unit, the frequency calculation formula is: F[n]=n·(f s / (MN)) Where M is the sampling point interval, N is the number of FFT calculation points, and f s is the sampling rate.

4. The DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop according to claim 3, characterized in that: After being processed by the FFT operation unit, the position k where the maximum value of |F[n]| is located m = k, and the maximum frequency offset rough estimate is obtained. The maximum frequency offset rough estimate is but The corresponding calculation formula is:

5. The DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop according to claim 4, characterized in that: The despreading module uses a sliding correlation method to detect whether the input signal of the despreading module has completed synchronization.

6. The DSSS enhanced synchronization method combining PMF-FFT and improved Costas loop according to claim 5, characterized in that: The synchronized demodulated output is expressed as: r[n]=R[n] Wherein, r[n] represents the despread output, and R[n] represents the maximum value of the sliding correlation function.

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