CN104181509A - Incoherent scattering radar signal processing method based on frequency hopping and polyphase alternating codes - Google Patents

Abstract

The invention discloses a non-coherent scattering radar signal processing method based on frequency hopping and multi-phase alternating codes, which mainly solves the problems of low height resolution, long detection period and limited coding mode of the existing non-coherent scattering radar. The technical solution is to modulate each group of codes of the polyphase alternating code with different frequencies to generate radar pulses arranged in sequence without time slots, and transmit them through the radar transmitter; Filter to obtain the detection data corresponding to the same height range for each group of polyphase alternating codes, perform post-detection filtering on each group of encoded echo signals, calculate the autocorrelation function, and perform multi-period accumulation and fuzzy correction, and then calculate the ionosphere The power spectral density of the scattered signal. The invention can improve the radar height resolution, reduce the radar detection cycle, expand the signal coding mode, and can be used for ionosphere detection.

Description

Incoherent Scatter Radar Signal Processing Method Based on Frequency Hopping and Multiphase Alternating Codes

technical field

The invention belongs to the technical field of signal and information processing, relates to a signal coding and signal processing method of an incoherent scattering radar, and can be used for ionospheric detection.

Background technique

With the development of science and technology, the impact of the ionosphere on human activities such as radio communication, navigation, broadcasting, and space exploration has become more and more significant. Ionospheric detection technology has gradually attracted the attention of scholars and scientific research institutions at home and abroad. my country has built the first incoherent scattering radar in Asia in Qujing, Yunnan, for ionospheric detection. Incoherent scatter radar is the most effective tool for detecting the ionosphere on the ground. By estimating the autocorrelation function and power spectral density of the echo signal, the parameters of the ionosphere such as electron density, electron temperature, ion temperature, and ion composition can be retrieved. These parameters play a very important role in the study of the characteristics and changing trends of the ionosphere.

Incoherent scatter radar is aimed at ionospheric targets distributed continuously in a large range, and its signal processing method is very different from traditional radar. If the echo signal of the ionosphere is an incoherent scattering signal caused by the reflection of electrons and ions, it is a typical random signal. In a short period of several minutes, the ionospheric echo signal is stationary, and the statistical characteristics of the signal can be characterized by calculating the autocorrelation function and power spectral density. The ionosphere is a typical soft target. Radar echo signals at different heights are aliased with each other. Effective signal coding design and special signal processing algorithms are required to eliminate range ambiguity. Scholars at home and abroad have carried out a lot of research work on incoherent scatter radar signal coding and signal processing.

The earliest incoherent scatter radar coding methods used for ionospheric detection are multi-pulse coding and long-pulse coding. These two encoding methods are easy to implement, but their fuzzy function has a large span and poor height resolution. Subsequently, Barker codes, two-phase alternating codes, etc. were applied to ionospheric detection. Barker codes have better height resolution. Using the traditional matched filtering method, the resolution unit can reach one symbol width, but it can only be used to solve the power profile of the ionosphere. Two-phase alternating code is a commonly used coding mode in modern incoherent scatter radar systems, and its signal processing mainly includes calculating time autocorrelation and ambiguity function correction. Two-phase alternating code modulation can obtain better height resolution, and can calculate autocorrelation function and power spectral density with multiple time delays, but its code length and code group number are strictly limited, which must be an integer power of 2 , so the detection waveform cannot be flexibly designed. The polyphase alternating code can improve the flexibility of the radar transmission pulse length, and reduce the number of groups of transmission pulses while increasing the code length as much as possible. A complete detection of two-phase alternating codes and multi-phase alternating codes needs to transmit and receive multiple sets of codes respectively, and then accumulate the results of multiple sets of echo processing. When the number of groups is large and multiple accumulations are required, the detection time is too long, and for some areas of the ionosphere with rapid changes in parameters, it cannot meet the needs of rapidly changing ionospheric detection, resulting in large detection errors. In addition, the number of symbols in each group of alternating codes is determined. In order to increase the detection distance, the symbol width needs to be increased when the transmission power is constant to increase the transmission energy, but this will lead to a decrease in height resolution.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the existing non-coherent scattering radar signal processing method, and propose a signal coding and signal processing method based on frequency hopping and polyphase alternating codes, so as to extend the signal coding length and reduce the radar detection period. Improve the radar height resolution and operating range to meet the needs of rapidly changing ionospheric detection.

The technical solution of the present invention is: the multi-group codes of polyphase alternating codes are respectively modulated with different frequencies to generate radar pulses arranged in sequence without time slots, which are transmitted through the transmitter; the radar echo signals are modulated according to the transmission time The frequencies are filtered separately to obtain detection data corresponding to the same height range of multiple sets of codes, and the multiple sets of coded echo signals are decoded and blurred respectively, and the autocorrelation function is calculated, and multi-period accumulation is performed to calculate the ionospheric scattering signal. Power Spectral Density. The specific steps include the following:

(1) Signal encoding steps:

(1a) According to the experimental requirements, determine the phase number p and the code length L of the multi-phase alternating code required by the radar;

(1b) According to the polyphase alternating code generation rules and the phase number p and the code length L, first generate N groups of weak polyphase alternating codes, and then convert them into corresponding 2N groups of strong polyphase alternating codes;

(1c) According to the coding group number 2N of the strong multi-phase alternating code that generates, set a group of frequency hopping codes with a length of 2N;

(1d) Modulating each group of strong polyphase alternating codes to each sub-frequency of the frequency hopping code set, so that each group of codes is arranged in order without time slots, and the transmitted signal is obtained;

(1e) Modulate the transmission signal to a radio frequency, and use the transmitter to transmit;

(2) Signal decoding steps:

(2a) The radar receiver receives the echo signal scattered by the ionosphere, down-converts the echo signal to obtain an intermediate frequency signal, and then performs A/D sampling;

(2b) According to the frequency hopping code used in transmission, design a digital bandpass filter bank to filter the echo signal after A/D sampling, and obtain the scattered echo corresponding to each group of codes of the strong multi-phase alternating code used in transmission Signal;

(2c) According to the strong polyphase alternating code used during transmission, a post-detection filter is designed to filter each group of scattered echo signals separately;

(2d) Truncating each group of scattered echo signals into multiple sub-signals, the truncated length is the pulse width of the transmitted signal, the starting time of each truncated signal corresponds to the height of the signal in the target ionosphere, and the scattered echoes at different heights are calculated For the autocorrelation of the wave signal, use the sign correction table to perform sign correction on the autocorrelation of each group of scattered echo signals, and sum the correction results of all groups according to different heights;

(2e) Repeat steps (2a) to (2d), receive and process the echo signals of the next cycle until all cycles are processed, and obtain the autocorrelation results of different height ranges in each cycle, and combine the corresponding heights of all cycles The autocorrelation results are accumulated separately;

(2f) Correct the accumulated autocorrelation results at each height using the corresponding fuzzy function values to obtain the final autocorrelation results, and perform Fourier transform on the autocorrelation results to obtain the ionospheric power spectral density to complete the radar signal deal with.

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

1. The present invention utilizes the signal coding and signal processing method of the non-coherent scattering radar modulated by frequency-hopping coding and multi-phase alternating code, and simultaneously utilizes the advantages of frequency-hopping coding in signal bandwidth and multi-phase alternating code in removing distance ambiguity. Suitable for ionospheric detection;

2. The present invention modulates the multi-phase alternating code through the frequency hopping code, and can complete the ionospheric detection and estimate the autocorrelation function and power spectral density within one transmitting and receiving cycle of the radar. Compared with the traditional incoherent scattering radar signal processing method, the present invention can effectively improve the time resolution of the processing result when the same accumulation times are used;

3. Since the present invention completes the ionospheric detection within one transmission and reception period of the radar, the time for a complete detection is shortened, and the steady-state detection requirements can be satisfied for the ionospheric region whose parameters change rapidly;

4. Since the present invention modulates multiple groups of polyphase alternating codes into one transmission pulse, the time interval between groups when each group is transmitted separately is compared with the traditional non-coherent scattering radar signal processing method. In the same time, It can increase the detection accumulation times and improve the processing signal-to-noise ratio;

5. Since the present invention modulates multiple groups of multi-phase alternating codes into a long transmission pulse, the working efficiency of the radar is improved. Compared with the traditional non-coherent scattering radar processing method, the radar effect can be improved under the condition of the same symbol width. Distance, under the same pulse width condition, the distance resolution of the processing result can be improved.

Description of drawings

Fig. 1 is the realization flowchart of the present invention;

Fig. 2 is a schematic diagram of signal transmission and signal processing in the present invention;

Fig. 3 is a schematic diagram of signal scattering in the present invention;

Fig. 4 is the emulation diagram of the range ambiguity function of one group of 5-phase alternating codes of the present invention with Matlab software;

Fig. 5 is the simulation diagram of the range ambiguity function of a group of 25-bit 5-phase alternating codes of the present invention with Matlab software.

Detailed ways

The present invention is described in further detail below in conjunction with accompanying drawing:

Explanation of related symbols

P: the number of phases of the polyphase alternating code;

m: the index parameter in the corresponding relationship between the phase number of the polyphase alternating code and the code length;

N: number of weak polyphase alternating code encoding groups;

L: coding length of weak polyphase alternating code;

f _n : The modulation frequency corresponding to the nth code of the strong polyphase alternating code.

With reference to Fig. 1, the present invention comprises signal encoding and signal decoding two major parts, and its steps are as follows:

1. Signal encoding

Step 1, select the phase number P and code length L of the polyphase alternating code.

There are two different types of weak polyphase alternating codes:

One is that the symbol phase number P is p, and the code length L is p ^m , where p is a prime number and m is a positive integer;

The second is that the symbol phase number P is p-1, and the code length L is p-1, where p is a prime number;

For these two kinds of weak polyphase alternating codes, the number N of each coding group is equal to its coding length L. According to the detection range and height resolution required by the experiment, determine the appropriate code length L, and then determine a polyphase alternating code type and the corresponding phase number P according to the above two code types. For example, if the code length is selected as 4, the first type can be selected, and the corresponding phase number is 2, or the second type can be selected, and the corresponding phase number is 4.

Step 2, generate weak multi-phase alternating codes and convert them into strong multi-phase alternating codes.

(2a) According to the polyphase alternating code generation rule, generate the phase number as P, the code length is L, and the group number is the weak polyphase alternating code of N groups;

(2b) Copy each of N groups of weak polyphase alternating codes, and multiply the odd-numbered code elements in each group of copied alternating codes by -1, and then combine the copied N groups of weak polyphase alternating codes with the original N groups Weak polyphase alternating codes are combined to form 2N groups of strong polyphase alternating codes.

Step 3, setting a group of frequency hopping codes.

According to the code group number 2N of the generated strong polyphase alternating code, a group of frequency hopping codes with a length of _{2N is} set. f _2N , these 2N frequency-hopping coding symbols correspond to 2N codes of strong polyphase alternating codes.

Step 4, coded signal modulation.

The generated 2N sets of coded signals of strong polyphase alternating codes are respectively modulated by 2N frequency hopping frequencies of frequency hopping codes, and the modulated sets of codes are arranged in order without time slots to form a complete transmission signal. Referring to the coded signal modulation part in Figure 2, it is assumed here that the number of groups of strong polyphase alternating codes is 2N groups, the number of phases is 2, and the code length is 4. In FIG. 2 , a rectangular block represents a symbol, and the symbol is located above or below the time axis, respectively representing that the phase of the symbol is 0 or π. 2N groups of strong polyphase alternating codes are arranged continuously. The 4 symbols in each group are modulated by the same frequency hopping frequency. The frequency selected by the nth group of codes is the nth frequency f _n of the frequency hopping code. After modulation Each group of codes constitutes a complete transmitted signal.

Step 5, modulate the transmission signal to radio frequency, and transmit it through the radar transmitter.

2. Signal decoding

Step 6, down-converting and sampling the echo signal.

The radar receiver receives the echo signal scattered by the ionosphere, down-converts the echo signal to obtain an intermediate frequency signal, and performs A/D sampling on the intermediate frequency signal.

Step 7, designing a filter bank to perform anti-aliasing filtering on the echo signal.

(7a) According to the modulation frequency of each group code of 2N groups of strong polyphase alternating codes when the signal is transmitted, a digital bandpass filter bank is designed, and the digital bandpass filter bank includes 2N sub-filters, and the center frequencies of each sub-filter are respectively Equal to the frequencies f ₁ , f ₂ , ..., f _2N of the frequency hopping code; the bandwidth of each sub-filter is selected according to the modulation frequency of the transmitted signal and the bandwidth parameter of the ionospheric scattering signal, and the passband range of each sub-filter does not overlap ;

(7b) Use a digital band-pass filter bank to filter the echo signal after A/D sampling. Since the modulation frequencies used by each group of strong polyphase alternating codes are different, after filtering by a band-pass filter bank, 2N The signals correspond to 2N codes of strong polyphase alternating codes.

Referring to Figure 3, it is assumed that the number of strong alternating code encoding groups is 4, since the designed incoherent scattering radar encoding waveform has a range resolution equal to its chip width for the ionosphere, the ionosphere is equivalent to a series of equally spaced slices, The pitch is the product of the chip width and the speed of light. It can be seen that within the time slots of the coding width of each group of strong polyphase alternating codes, the echo signal is the superposition of 4 groups of coded scattering signals. Since each group of codes uses different frequency modulation, the bandpass filter corresponding to each group By filtering separately, the scattered signals corresponding to each of the four groups of codes can be separated from the aliased scattered signals.

Step 8: Perform post-detection filtering on the 2N scattered echo signals obtained after anti-aliasing filtering.

(8a) Design a post-detection filter, usually a low-pass filter, and the length of its impulse response is equal to the chip width;

(8b) Use the post-detection filter to perform filtering processing on the scattered echo signals corresponding to each group of 2N groups of strong polyphase alternating codes.

Step 9, intercepting an echo signal at a height.

The 2N post-detection and filtered echo signals obtained in step 3 are respectively intercepted, the interception length is the pulse width of the transmitted signal, and the starting time of the interception signal corresponds to the height of the signal in the target ionosphere.

Step 10, calculate the autocorrelation function, and perform sign correction and accumulation.

(10a) Generate a symbol correction table from 2N groups of strong polyphase alternating codes, and the generation steps are as follows:

(10a1) each group of strong polyphase alternating codes with 2N groups of length L is shifted by n symbols, n=1,2,3,...,L-1;

(10a2) Multiply each group of 2N sets of strong polyphase alternating codes after translation with the corresponding codes before translation to obtain 2N sets of coded symbols with a length of L-n, which are used as each code element of each group at the nth time delay the correction symbol;

(10b) take a symbol width as the time delay interval, calculate respectively the autocorrelation function of the 2N sub-signals intercepted and obtained in step 9, obtain the value of the autocorrelation point at the time delay of each signal L-1;

(10c) Multiply the corrected symbols of each symbol at the nth time delay corresponding to each group of strong polyphase alternating codes in the sign correction table by the corresponding autocorrelation function and the autocorrelation of each symbol position at the nth time delay Point value, get the autocorrelation result after 2N sub-signal symbol correction;

(10d) Summing the corrected autocorrelation results of 2N sub-signal symbols at the nth time delay.

Step 11, calculating autocorrelation functions of other heights.

Repeat steps 9 to 10 to intercept the echo signals in the next height range of each group of signals among the 2N echo signals after post-detection filtering to obtain 2N sub-signals, calculate the autocorrelation function of each sub-signal, and Carry out sign correction and accumulation until the echo signals of all heights are processed.

Step 12, multi-period accumulation.

Repeat steps 6 to 11 to receive and process the echo signals of the next cycle until all cycles are processed, and the autocorrelation results at different heights of each cycle are obtained, and the autocorrelation results obtained at the corresponding heights of all cycles are respectively accumulated , to get the accumulated autocorrelation results at each height.

Step 13, the fuzzy function is corrected, and the power spectral density is calculated.

(13a) Calculate the ambiguity function correction value according to the 2N group codes of the strong polyphase alternating code:

(13a1) Use the post-detection filter to filter each group of codes of 2N groups of strong polyphase alternating codes respectively, and obtain the sequence of amplitude ambiguity function values corresponding to each group of codes;

(13a2) The 2N magnitude ambiguity function value sequences that step (13a1) obtains are shifted respectively by n strong polyphase alternating code symbol widths, n=1,2,3,...,L-1; The 2N sequences of are multiplied with the corresponding sequence before translation respectively, and the distance ambiguity function value sequence of each group of strong polyphase alternating codes at the nth time delay is obtained;

(13a3) Multiply the correction symbols of each symbol at the nth time delay corresponding to each group of strong polyphase alternating codes in the symbol correction table by the distance ambiguity function value at the nth time delay of the corresponding strong polyphase alternating code sequence, and summing the 2N corrected distance ambiguity function value sequences to obtain the final ambiguity function value sequence of each symbol at the nth time delay;

(13b) to the autocorrelation result at each height place that step (12) obtains, use the value of the autocorrelation point of each symbol position of its nth time delay place to be divided by the value of each symbol at the nth time delay place respectively fuzzy function value to get the final autocorrelation result;

(13c) Perform Fourier transform on the final autocorrelation results at each height to obtain the power spectral density at each height of the ionosphere, and complete the processing of the radar signal.

In the present invention, the radar range resolution effect can be illustrated by the following simulation experiments:

1. Simulation conditions: The software used for the simulation is MATLAB.

2. Simulation environment: The simulation is carried out under Windows XP environment.

3. Simulation content:

Simulation 1, the distance ambiguity function of a strong polyphase alternating code with a group number of 5 phases, a code length of 5, and a group number of 10 at a time delay of one symbol width is simulated. Each symbol of the strong polyphase alternating code The phase is shown in Table 1, and the simulation results are shown in Figure 4. The distance ambiguity function of the strong polyphase alternating code in Fig. 4 has 4 peaks in total, which are the distance ambiguity sub-functions at 4 code elements under a chip width time delay.

Table 1. Strong polyphase alternating code table with 5 phases, 5 code lengths and 10 groups

In simulation 2, the distance ambiguity function of a group of strong polyphase alternating codes with a phase number of 5, a code length of 25, and a group number of 50 at a time delay of one symbol width is simulated. The simulation results are shown in Figure 5. The distance ambiguity function of the strong polyphase alternating code in Fig. 5 has 24 peaks in total, which are the distance ambiguity subfunctions at 24 code elements under a chip width time delay.

4. Result analysis:

It can be seen from Figure 4 and Figure 5 that the distance ambiguity function of the multi-phase alternating code has a sharp peak within the corresponding height range of each symbol, and decays to 0 outside the corresponding height of each symbol. In non-coherent scattering radar detection, the range ambiguity function characterizes the range resolution of the radar, which shows that the radar in the present invention can obtain very high range resolution.

Claims (6)

1. A non-coherent scattering radar signal processing method based on frequency hopping and polyphase alternating codes, characterized in that it comprises: (1) Signal encoding steps: (1a) According to the experimental requirements, determine the phase number P and code length L of the multi-phase alternating code required by the radar; (1b) According to the polyphase alternating code generation rules and the phase number P and the code length L, first generate N groups of weak polyphase alternating codes, and then convert them into corresponding 2N groups of strong polyphase alternating codes; (1c) According to the coding group number 2N of the strong multi-phase alternating code that generates, set a group of frequency hopping codes with a length of 2N; (1d) Modulating each group of strong polyphase alternating codes to each sub-frequency of the frequency hopping code set, so that each group of codes is arranged in order without time slots, and the transmitted signal is obtained; (1e) Modulate the transmission signal to a radio frequency, and use the transmitter to transmit; (2) Signal decoding steps: (2a) The radar receiver receives the echo signal scattered by the ionosphere, down-converts the echo signal to obtain an intermediate frequency signal, and then performs A/D sampling; (2b) According to the frequency hopping code used in transmission, design a digital bandpass filter bank to filter the echo signal after A/D sampling, and obtain the scattered echo corresponding to each group of codes of the strong multi-phase alternating code used in transmission Signal; (2c) According to the strong polyphase alternating code used during transmission, a post-detection filter is designed to filter each group of scattered echo signals separately; (2d) Truncating each group of scattered echo signals into multiple sub-signals, the truncated length is the pulse width of the transmitted signal, the starting time of each truncated signal corresponds to the height of the signal in the target ionosphere, and the scattered echoes at different heights are calculated For the autocorrelation of the wave signal, use the sign correction table to perform sign correction on the autocorrelation of each group of scattered echo signals, and sum the correction results of all groups according to different heights; (2e) Repeat steps (2a) to (2d), receive and process the echo signals of the next cycle until all cycles are processed, and obtain the autocorrelation results of different height ranges in each cycle, and combine the corresponding heights of all cycles The autocorrelation results are accumulated separately; (2f) Correct the accumulated autocorrelation results at each height using the corresponding fuzzy function values to obtain the final autocorrelation results, and perform Fourier transform on the autocorrelation results to obtain the ionospheric power spectral density to complete the radar signal deal with. 2. the non-coherent scattering radar signal processing method based on frequency hopping and polyphase alternating code according to claim 1, wherein the frequency hopping coding described in step (1c), the frequency of its each symbol is not equal to each other, respectively f ₁ , f ₂ , ..., f _2N , and correspond to 2N codes of strong polyphase alternating codes. 3. the non-coherent scattering radar signal processing method based on frequency hopping and polyphase alternating code according to claim 1, wherein the digital bandpass filter bank described in step (2b) comprises 2N sub-filters, each sub-filter The center frequency of the filter is equal to the frequency of the frequency hopping code, and the bandwidth is selected according to the modulation frequency of the transmitted signal and the bandwidth parameter of the ionospheric scattering signal; the passband range of each sub-filter does not overlap. 4. the non-coherent scattering radar signal processing method based on frequency hopping and multi-phase alternating code according to claim 1, wherein the post detection filter described in step (2c) is a low-pass filter, and the length of its impulse response It is equal to the chip width of the strong polyphase alternating code. 5. the non-coherent scattering radar signal processing method based on frequency hopping and multi-phase alternating codes according to claim 1, wherein the symbol correction table described in step (2d) is generated by the code table of strong multi-phase alternating codes, generating step as follows: (2d1) each group of strong polyphase alternating codes whose length is L is shifted by n symbols, n=1,2,3,...,L-1; (2d2) Multiply each group of 2N strong polyphase alternating codes after translation with the corresponding codes before translation to obtain 2N groups of coded symbols with a length of L-n, which are used as code elements of each group at the nth time delay correction symbol. 6. the non-coherent scattering radar signal processing method based on frequency hopping and polyphase alternating code according to claim 1, wherein the ambiguity function value described in step (2f) is calculated by the code table of strong polyphase alternating code out, the calculation steps are as follows: (2f1) Use the post-detection filter to filter each group of codes of 2N groups of strong polyphase alternating codes, and obtain the corresponding amplitude ambiguity function value sequence of each group of codes; (2f2) The 2N magnitude ambiguity function value sequences obtained in step (2f1) are shifted by n strong alternating code chip widths, n=1,2,3,...,L-1, and then the shifted sequences are respectively Multiply with the corresponding sequence before translation to obtain the distance ambiguity function value sequence of each group of codes at the nth time delay; (2f3) Correct the range ambiguity function value sequence at the nth time delay of each group of codes using the corrected symbols of each symbol of the corresponding group of codes at the nth time delay in the symbol correction table, and correct the 2N corrected The distance fuzzy function value sequence is summed to obtain the final fuzzy function value sequence of each symbol at the nth time delay.