阅读说明：本技术 一种低轨星座系统终端载波同步方法及载波同步解调装置 (Carrier synchronization method and carrier synchronization demodulation device for low-orbit constellation system terminal ) 是由 周微 张庆业 王力男 王涛 袁旭彬 仲伟强 于 2021-10-19 设计创作，主要内容包括：本发明公开了一种低轨星座系统终端载波同步方法及载波同步解调装置,属于卫星通信领域。载波同步方法包括以下步骤：终端将下行时域信号经过低通滤波和数据降采样后进行时域主同步信号PSS快速捕获,完成时域信号定时和频率的粗估计；终端将粗定时和频率估计信息送入定时调整模块和数字下变频完模块分别完成定时调整和频率补偿；将粗同步后的时域数据进行FFT处理,完成时域信号到频域信号的变换；将频域数据送入细定时同步模块完成定时粗同步估计以及定时细同步估计,利用细定时估计信息对频域数据进行补偿处理,同时对频域数据进行解扩处理,并将解扩后参考信号送入频率估计模块完成载波细频率估计。(The invention discloses a low-orbit constellation system terminal carrier synchronization method and a carrier synchronization demodulation device, and belongs to the field of satellite communication. The carrier synchronization method comprises the following steps: the terminal rapidly captures a time domain primary synchronization signal PSS after low-pass filtering and data down-sampling a downlink time domain signal to finish the rough estimation of time domain signal timing and frequency; the terminal sends the rough timing and frequency estimation information to a timing adjustment module and a digital down-conversion completion module to respectively complete timing adjustment and frequency compensation; performing FFT processing on the time domain data after coarse synchronization to complete the conversion from the time domain signal to the frequency domain signal; and sending the frequency domain data into a fine timing synchronization module to finish timing coarse synchronization estimation and timing fine synchronization estimation, compensating the frequency domain data by using fine timing estimation information, simultaneously performing de-spreading processing on the frequency domain data, and sending a reference signal after de-spreading into a frequency estimation module to finish carrier fine frequency estimation.)

1. A low-orbit constellation system terminal carrier synchronization method is characterized by comprising the following steps:

(1) setting a reference signal in OFDM as a fixed real value, carrying out CDMA spread spectrum processing, mapping the reference signal after spread spectrum to occupy all the positions of subcarriers of a whole OFDM time domain symbol and a frequency domain, wherein the data signal and the reference signal are subjected to scrambling and IFFT conversion processing after being superposed in the frequency domain;

(2) FFT conversion is carried out on the time domain downlink signals after coarse synchronization to obtain frequency domain signals, and then the reverse rotation factor e is completed on each subcarrier signal in the frequency domain^j2πkτ/NCompensation processing, wherein k is 0,1,2, …, N-1, tau is a search timing pre-compensation value with the value range of [ -L.Ts, L.Ts]The value of each step of the tau is Ts/M, wherein Ts is the time interval of sampling points, L and M are positive integers, and N is the number of subcarriers;

(3) pre-compensating each subcarrier by utilizing the tau value of each step in the step (2), then de-spreading the reference signal, de-spreading a reference symbol Q value by N subcarriers, and performing energy modulus operation on the Q value to obtain Q modulus values corresponding to 2LM +1 tau value steps in total;

(4) accumulating a plurality of symbols by using the 2LM +1 step diameter Q modulus values in the step (3), and finding out a maximum peak value from the symbols, wherein the abscissa corresponding to the peak value is the timing offset estimation value tau';

(5) performing each subcarrier e on the frequency domain signal by using the value tau' in the step (4)^j2πkτ’/NTiming precompensation is carried out, the synchronous influence of timing errors on spread spectrum signals is reduced, then fine frequency estimation is carried out on reference signal symbol data after despreading through FFT conversion, and fine frequency offset estimation values f' are obtained by utilizing amplitude peak value information after FFT;

(6) timing adjustment is carried out on externally input baseband time domain signals by utilizing the tau ' value in the step (4), the timing adjustment of a local counter is completed when the tau ' value exceeds an integral multiple timing sampling point threshold value, and meanwhile, the tau ' is updated to a residual decimal sampling point value; and (5) performing digital down-conversion on the baseband time domain signal after timing adjustment by using the fine frequency estimation value f' in the step (5) to finish tracking compensation of the frequency.

2. A low orbit constellation system terminal carrier synchronization demodulation device is characterized by comprising a timing adjustment module, a digital down-conversion module, a digital filter module, a PSS synchronization module, an FFT module, a fine timing synchronization module, a fine frequency estimation module, a demodulation module and a decoding module; wherein:

the timing adjustment module receives the baseband time domain signal, counts sampling points, time slots, subframes and frames of the baseband time domain signal according to coarse timing information output by the PSS synchronization module and a fine timing estimation value tau' output by the fine timing synchronization module, and sends the baseband time domain signal after timing adjustment to the digital down-conversion module;

the digital down-conversion module carries out frequency adjustment according to a coarse frequency estimation value output by the PSS synchronization module and a fine frequency estimation value f' output by the fine frequency estimation module, carries out digital frequency conversion compensation processing on a baseband time domain signal after timing adjustment, and sends the time domain baseband signal after frequency conversion to the digital filter module and the FFT module;

the digital filter module filters the signal sent by the digital down-conversion module, filters out the signal out of the bandwidth of the signal, and sends the filtered time domain signal to the PSS synchronization module after speed reduction and extraction;

the PSS synchronization module carries out time domain correlation capture processing on the signals sent by the digital filter module to complete an initial synchronization process, outputs PSS synchronization information to the FFT module, sends out coarse timing information to the timing adjustment module, and sends out a coarse frequency estimation value to the digital down-conversion module;

the FFT module carries out FFT transformation on the signal output by the digital down-conversion module by using PSS synchronization information to complete the transformation from a time domain signal to a frequency domain signal, and sends the transformed frequency domain signal to the fine timing synchronization module;

the fine timing synchronization module carries out fine timing related timing estimation calculation on the frequency domain signal, and sends the calculated fine timing estimation value tau' to the timing adjustment module to finish the fine timing synchronization adjustment process;

the fine frequency estimation module performs de-spread processing on the frequency domain data of the reference signal after fine timing adjustment, then performs fine frequency estimation by using FFT (fast Fourier transform), and sends a fine frequency estimation value f' to the digital down-conversion module to complete accurate frequency compensation;

the demodulation module performs de-spread processing and channel estimation and equalization on the frequency domain data after fine frequency estimation, and sends the demodulated data to the decoding module to complete channel decoding processing of the data.

Technical Field

The invention belongs to the technical field of satellite mobile communication, and particularly relates to a method for timing and frequency synchronization of a terminal carrier wave in a low signal-to-noise ratio environment of a low earth orbit satellite constellation system.

Background

In satellite mobile communication systems, the TDMA/FDMA technical scheme is generally adopted. The new generation of low-orbit satellite constellation has the satellite-on-load requirement of a base station, and simultaneously, in order to meet the requirement of international telecommunication union on the ground power spectrum of the satellite, the whole downlink power spectral density of the low-orbit satellite constellation is lower, so the requirement of the carrier synchronization technology of the terminal under the low signal-to-noise ratio and interference environment is higher.

At present, in the prior art, a timing and frequency tracking method in a low-orbit constellation is not introduced in detail in a low-orbit satellite scene aiming at an OFDM (orthogonal frequency division multiplexing) combined CDMA (code division multiple access) technical system with high spectral efficiency, the existing low-orbit system is mainly focused on CDMA time domain spread spectrum and TDMA/FDMA (time division multiple access/frequency division multiple access) related transmission technical research, a low-orbit constellation related ground mobile communication terminal is not designed in a commercial miniaturization mode, most of the low-orbit constellation related ground mobile communication terminals are experimental test equipment, and related timing estimation and frequency drift are designed by a hardware clock scheme with high stability and precision.

Disclosure of Invention

In view of this, the present invention provides a carrier synchronization method and a carrier synchronization demodulation apparatus for a low-orbit constellation system terminal, where the apparatus and the method do not depend on hardware conditions, but use a software algorithm to complete timing and frequency tracking compensation in an OFDM combined frequency domain CDMA spread spectrum communication system scene under a weak signal with a low signal-to-noise ratio, so as to effectively reduce a satellite signal ground power spectral density.

In order to achieve the purpose, the invention adopts the technical scheme that:

a low orbit constellation system terminal carrier synchronization method includes the following steps:

(1) setting a reference signal in OFDM as a fixed real value, carrying out CDMA spread spectrum processing, mapping the reference signal after spread spectrum to occupy all the positions of subcarriers of a whole OFDM time domain symbol and a frequency domain, wherein the data signal and the reference signal are subjected to scrambling and IFFT conversion processing after being superposed in the frequency domain;

(2) FFT conversion is carried out on the time domain downlink signals after coarse synchronization to obtain frequency domain signals, and then the reverse rotation factor e is completed on each subcarrier signal in the frequency domain^j2πkτ/NCompensation processing, wherein k is 0,1,2, …, N-1, tau is a search timing pre-compensation value with the value range of [ -L.Ts, L.Ts]The value of each step of the tau is Ts/M, wherein Ts is the time interval of sampling points, L and M are positive integers, and N is the number of subcarriers;

(3) pre-compensating each subcarrier by utilizing the tau value of each step in the step (2), then de-spreading the reference signal, de-spreading a reference symbol Q value by N subcarriers, and performing energy modulus operation on the Q value to obtain Q modulus values corresponding to 2LM +1 tau value steps in total;

(4) accumulating a plurality of symbols by using the 2LM +1 step diameter Q modulus values in the step (3), and finding out a maximum peak value from the symbols, wherein the abscissa corresponding to the peak value is the timing offset estimation value tau';

(5) performing each subcarrier e on the frequency domain signal by using the value tau' in the step (4)^j2πkτ’/NTiming precompensation is carried out, the synchronous influence of timing errors on spread spectrum signals is reduced, then fine frequency estimation is carried out on reference signal symbol data after despreading through FFT conversion, and fine frequency offset estimation values f' are obtained by utilizing amplitude peak value information after FFT;

(6) timing adjustment is carried out on externally input baseband time domain signals by utilizing the tau ' value in the step (4), the timing adjustment of a local counter is completed when the tau ' value exceeds an integral multiple timing sampling point threshold value, and meanwhile, the tau ' is updated to a residual decimal sampling point value; and (5) performing digital down-conversion on the baseband time domain signal after timing adjustment by using the fine frequency estimation value f' in the step (5) to finish tracking compensation of the frequency.

In addition, the invention also provides a low-orbit constellation system terminal carrier synchronization demodulation device, which comprises a timing adjustment module, a digital down-conversion module, a digital filter module, a PSS synchronization module, an FFT module, a fine timing synchronization module, a fine frequency estimation module, a demodulation module and a decoding module; wherein:

the timing adjustment module receives the baseband time domain signal, counts sampling points, time slots, subframes and frames of the baseband time domain signal according to coarse timing information output by the PSS synchronization module and a fine timing estimation value tau' output by the fine timing synchronization module, and sends the baseband time domain signal after timing adjustment to the digital down-conversion module;

the digital down-conversion module carries out frequency adjustment according to a coarse frequency estimation value output by the PSS synchronization module and a fine frequency estimation value f' output by the fine frequency estimation module, carries out digital frequency conversion compensation processing on a baseband time domain signal after timing adjustment, and sends the time domain baseband signal after frequency conversion to the digital filter module and the FFT module;

the digital filter module filters the signal sent by the digital down-conversion module, filters out the signal out of the bandwidth of the signal, and sends the filtered time domain signal to the PSS synchronization module after speed reduction and extraction;

the PSS synchronization module carries out time domain correlation capture processing on the signals sent by the digital filter module to complete an initial synchronization process, outputs PSS synchronization information to the FFT module, sends out coarse timing information to the timing adjustment module, and sends out a coarse frequency estimation value to the digital down-conversion module;

the FFT module carries out FFT transformation on the signal output by the digital down-conversion module by using PSS synchronization information to complete the transformation from a time domain signal to a frequency domain signal, and sends the transformed frequency domain signal to the fine timing synchronization module;

the fine timing synchronization module carries out fine timing related timing estimation calculation on the frequency domain signal, and sends the calculated fine timing estimation value tau' to the timing adjustment module to finish the fine timing synchronization adjustment process;

the fine frequency estimation module performs de-spread processing on the frequency domain data of the reference signal after fine timing adjustment, then performs fine frequency estimation by using FFT (fast Fourier transform), and sends a fine frequency estimation value f' to the digital down-conversion module to complete accurate frequency compensation;

the demodulation module performs de-spread processing and channel estimation and equalization on the frequency domain data after fine frequency estimation, and sends the demodulated data to the decoding module to complete channel decoding processing of the data.

The beneficial effect who adopts above-mentioned technical scheme to obtain lies in:

1. the invention is based on the fact that an OFDM combined frequency domain CDMA spread spectrum system is adopted in a satellite mobile communication system, can effectively complete the timing frequency estimation of satellite signals in weak signal and interference environments by utilizing the characteristics of high OFDM frequency spectrum efficiency and strong CDMA anti-interference capability, can effectively reduce the power spectrum density of low-orbit constellation satellite downlink signals, and effectively improves the carrier synchronization estimation and tracking capability in large Doppler environments.

2. The invention is realized by adopting a mature software algorithm and system flow control without depending on hardware conditions, has higher technical maturity and is simple and reliable to realize.

Drawings

FIG. 1 is a schematic block diagram of an embodiment of the present invention;

FIG. 2 is a sub-carrier mapping chart of a satellite mobile communication system per sub-frame according to an embodiment of the present invention;

FIG. 3 is a diagram of the PSS synchronization module effect in the embodiment of the present invention, with the coordinate precision of the horizontal axis being Ts;

FIG. 4 is a block diagram of an implementation of a fine timing synchronization module in an embodiment of the present invention;

FIG. 5 is a timing estimation effect diagram of the fine timing synchronization module according to the embodiment of the present invention, where the horizontal axis coordinate precision is Ts/8;

fig. 6 is a block diagram of an implementation of a fine frequency estimation module in an embodiment of the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As shown in fig. 1, a low-orbit constellation system terminal carrier synchronization demodulation apparatus includes a timing adjustment module, a digital down-conversion module, a digital filter, a PSS synchronization module, an FFT module, a fine timing synchronization module, and a fine frequency estimation module. The terminal in the PSS synchronization module performs low-pass filtering and data down-sampling on a downlink time domain signal and then performs time domain PSS rapid capture to complete a time domain signal timing and frequency rough estimation process; the fine timing synchronization module is responsible for large-range timing coarse synchronization estimation and small-range timing fine synchronization estimation under low signal-to-noise ratio, compensation processing is carried out on frequency domain data by using fine timing estimation information, de-spreading processing is carried out on the frequency domain data, and reference signals after de-spreading are sent to the frequency estimation module to complete carrier fine frequency estimation.

The carrier synchronization process in the device mainly comprises the following steps:

(1) the terminal carries out low-pass filtering and data down-sampling on the downlink time domain signal, then carries out rapid capture on a time domain Primary Synchronization Signal (PSS), finishes the rough estimation process of time domain signal timing and frequency, PSS time frequency positions and pilot frequency distribution are shown in figure 2, the PSS occupies the 2 nd symbol position of each time slot, pilot frequency symbols and data symbols occupy the rest symbol positions, and all sub-carrier bandwidths are occupied on the frequency;

(2) the terminal sends the rough timing and frequency estimation information to a timing adjustment module and a digital down-conversion completion module to respectively complete timing adjustment and frequency compensation;

(3) performing FFT processing on the time domain data after coarse synchronization to complete the conversion from the time domain signal to the frequency domain signal;

(4) and sending the frequency domain data into a fine timing synchronization module to finish the coarse timing synchronization estimation in a large range and the fine timing synchronization estimation in a small range under the condition of low signal to noise ratio, compensating the frequency domain data by using fine timing estimation information, simultaneously performing de-spreading processing on the frequency domain data, and sending a reference signal after de-spreading into a frequency estimation module to finish the fine carrier frequency estimation.

The following is a more specific carrier synchronization process, which includes the following specific implementation steps:

(1) the terminal sends the downlink time domain signal to the PSS synchronization module after low-pass filtering and data down-sampling, the PSS synchronization module finishes the time domain signal timing capture and frequency rough estimation process, wherein the capture algorithm and the realization process of the PSS are realized by referring to the ground universal standard, the PSS time domain sliding correlation peak value search effect graph is shown in figure 3, and the abscissa corresponding to more peak values is the rough synchronization position.

The PSS detection principle is that by utilizing time-frequency characteristics (constant amplitude and zero autocorrelation) of a PSS sequence, a PSS time domain sequence is generated locally, all PSS sequences are subjected to sliding cross correlation with received signals respectively, the initial position of the PSS sequence is estimated, and the position corresponding to the maximum correlation peak value is judged to be the PSS coarse synchronization timing position. Multiplying the received time domain PSS symbol by using a local PSS sequence conjugate point of a time domain under the condition that PSS timing offset estimation is finished, and removing sequence information; then dividing the frequency offset into two parts, respectively summing, and finally estimating the size of the frequency offset:

wherein r is_pss,iFor the i-th received PSS sequence, s_pssFor the locally generated PSS sequence, N is the OFDM symbol FFT sample length. Meanwhile, the C can be jointly estimated by a plurality of PSS signals_acc＝∑_lC^(l)And calculating the normalized decimal frequency offset as follows:

in the formula, angle () is an angle calculation;

(2) the timing adjustment module carries out timing adjustment on the coarse timing information estimated by the PSS synchronization module through a counter, and the digital down-conversion module compensates the coarse frequency estimated by the PSS synchronization module;

(3) the FFT module carries out FFT processing on the time domain data after coarse synchronization to finish the conversion process from the time domain signal to the frequency domain signal;

(4) the fine timing synchronization module will complete the de-rotation factor e for each sub-carrier signal r (k) in the frequency domain^j2πkτ/NCompensation processes, i.e. r (k, τ) ═ r (k) e^j2πkτ/NWhere k is 0,1,2, …, N-1, τ is search timing precompensation value, and its value range [ -L · Ts, L · Ts]The value of tau per step can be Ts/M, wherein Ts is the time interval of each sampling point, L and M are positive integers, N is the number of subcarriers, and the total number of tau per step is 2LM + 1;

(5) the fine timing synchronization module pre-compensates each subcarrier according to the tau value of each step diameter, and then de-spreads the reference signal RS, namely, a reference symbol Q value is de-spread from every N subcarriers, namelyWherein (c) (k) corresponds to each subcarrier spreading code, and performs energy modular value operation on the Q value, which can total to obtain 2LM +1 step diameter Q modular values | | | Q (τ) |, the calculation process of the specific fine timing synchronization module is shown in fig. 4, which includes FFT, counter-rotation, despreading, modular value calculation and other processes;

(6) the fine timing synchronization module performs accumulation processing among a plurality of symbols on the 2LM +1 step diameters Q (tau) | value calculated on each OFDM symbol, namelyFinding out the maximum peak value from Q '(tau), wherein the position of the horizontal axis corresponding to the maximum value is the value of the timing offset estimation value tau', and the final peak value calculation result distribution is shown in figure 5, and the unit of the horizontal axis is Ts/M;

(7) the fine frequency estimation module carries out per subcarrier e on the frequency domain signal by the timing estimation tau' value^j2πkτ’/NTiming precompensation, i.e. r (k, τ') ═ r (k) e^j2πkτ'/NThen, the reference signal RS is despread Q (τ ') and despread Q (τ ') is obtained ') The data is sent to the FFT kernel to perform fine frequency estimation, and the post-FFT amplitude peak information is used to complete the estimation f' of the fine frequency offset, where the specific processing flow is shown in fig. 6, where the FFT estimation adopts the frequency:

in the above equation fs is the frequency domain signal sampling rate,for the number of FFT sub-carrier points, CP, of the frequency domain signal_LENIs the length of the OFDM symbol CP; when operating with a 2048-point FFT kernel, the frequency estimation accuracy is FFT_fs/2048。

(8) The timing adjustment module judges the fine timing estimation tau ' value, and when the tau ' value exceeds an integral multiple timing sampling point threshold value, the timing adjustment module can be started to complete the timing adjustment of the local counter, and meanwhile, the tau ' is updated to a residual decimal sampling point value; and simultaneously, the digital down-conversion module carries out tracking compensation on the frequency of the fine frequency estimation f'.

In a word, the method is suitable for a scene that an OFDM combined frequency domain CDMA spread spectrum communication system is adopted in low-orbit constellation design, can effectively complete timing estimation and frequency tracking compensation of satellite signals in weak signals and interference environments, can effectively reduce the power spectral density of downlink signals of the satellites of the low-orbit constellation in the OFDM combined frequency domain CDMA spread spectrum system, and effectively improves carrier synchronization estimation and tracking capacity in a large Doppler environment.