A tracking timing method for wideband spread spectrum communication

Abstract

The present invention discloses a tracking and timing method for broadband spread spectrum communication, and relates to the field of satellite communication systems. The present invention compares the power of normalized lagging and leading data of auxiliary data, outputs a frequency control word after entering a second-order loop filter, and controls a digital interpolator to adjust the sampling rate of demodulated data, thereby realizing pseudo code tracking and timing synchronization. The method does not need to adjust the accumulation length according to different spread spectrum ratios, uses pilot data for pseudo code tracking, is conducive to low signal-to-noise ratio demodulation, is suitable for situations where the chip rate is high and the chip clock cannot be generated by adjusting the high-multiple clock, and can be used for broadband satellite anti-interference communication.

Description

Technical Field

[0001] The invention relates to the field of satellite communication, and in particular to a tracking timing method for broadband spread spectrum communication, which is suitable for improving the anti-interference capability of a spread spectrum system in broadband satellite communication. Background Art

[0002] In the communication process, the transmitting and receiving clocks are inconsistent, there is relative motion between the transmitting station and the receiving station, and there are delay changes in the propagation path. These factors will cause real-time phase jitter deviations between the transmitting sequence and the local sequence. Therefore, after completing the capture of the DSSS signal, it is necessary to continuously adjust the local sequence phase to keep it consistent with the phase change of the receiving sequence, so as to achieve precise synchronization of the DSSS signal.

[0003] Traditional spread spectrum code tracking synchronization uses a local spread spectrum code clock adjustment method based on a sooner-and-late loop. This method has stable performance, but in the field of broadband spread spectrum, the spread spectrum code clock is too high and it is impossible to use a high-multiple code clock to adjust and generate the local code clock. Summary of the invention

[0004] In order to overcome the limitations of existing spread spectrum transmission waveforms and improve anti-interference capabilities, the present invention provides a tracking timing method suitable for broadband spread spectrum communications. The present invention achieves timing synchronization of high-speed spread spectrum signals and a strong anti-interference function of the communication system through a variety of technologies such as specific frame structure design, synchronization sequence capture and tracking, and has the survivability and robustness of the spread spectrum system, and can improve the anti-interference capability of the satellite communication system.

[0005] The object of the present invention is achieved in that:

[0006] A tracking timing method for broadband spread spectrum communication comprises the following steps:

[0007] (1) Design a frame structure. Each frame includes a synchronization sequence, control information, service information, pilot, and reserved data. The service information is spread using a spreading sequence, and other information is scrambled using a spreading sequence. The content of the control information indicates the modulation and spreading ratio of the service data. One frame is one cycle.

[0008] (2) The satellite transponder scrambles and transmits the spread spectrum sequence according to the frame structure;

[0009] (3) The ground station terminal performs data interpolation processing on the received signal with a sampling rate of 4 times the data rate according to the error, selects the first sampling point of the interpolated data to determine as the optimal sampling point output, and implements code tracking timing adjustment; the initial error is 0, and the subsequent error is the error extracted in step (6);

[0010] (4) performing initial frame capture of the timing-adjusted signal according to a predetermined spread spectrum sequence to achieve capture synchronization, and at the same time starting the code generator to generate a spread spectrum sequence;

[0011] (5) After acquiring synchronization, the signal is descrambled using the spread spectrum sequence, and the control information is searched to locate the control information. Then, the pilot position is determined according to the arrangement of each data type in the frame structure.

[0012] (6) After the pilot position is determined, the pilots of the leading, middle, and lagging branches are accumulated according to the pilot length L, and the multiple pilot accumulation results are subjected to non-coherent accumulation of length N, where N ≥ 32, and error extraction is performed on the three non-coherent accumulation results. The error extraction results are then subjected to second-order loop filtering to obtain weighted timing error information adapted to the operating frequency of the interpolation filter in step (3), and the error information is sent to step (3).

[0013] Furthermore, in step (1), the spread spectrum sequence adopts a pseudo-random sequence, the synchronization sequence is used for data frame positioning and pseudo-code synchronization, the control information is used for positioning service information, pilot and reserved data, the control information adopts a group of unique words, different unique words indicate different rates, modulation modes, and spread spectrum ratio information used for demodulation, the pilot is used for code tracking timing and carrier recovery, the reserved data is used for data adjustment, and the reserved data is filled with 0.

[0014] Furthermore, the specific method of step (3) is:

[0015] (301) Coefficient selection: design the interpolator coefficients to be waveform matched filter coefficients with a roll-off of 0.35 and store them in the cache;

[0016] (302) Control word generation: periodically accumulating the input error and the initial frequency control word to obtain a 32-bit frequency control word fcw;

[0017] (303) Data interpolation: The upper 7 bits of fcw are taken as the integer part to control the selection of data, and the lower 25 bits are taken as the decimal part to control the selection of interpolator coefficients. The data is then filtered using the interpolator coefficients to obtain the interpolated coefficients.

[0018] (304) Codeword adjustment indication: When the accumulated error exceeds half a chip period, a codeword adjustment indication is issued to adjust the local code generator at the reserved data position. If the data is ahead of the codeword, the codeword is adjusted forward for output; if it lags behind, the codeword is adjusted for output with a delay.

[0019] Compared with the background technology, the present invention has the following advantages:

[0020] 1. The present invention is very suitable for the spread spectrum sequence capture and pseudo code tracking timing of high-speed spread spectrum transmission signals, and the frame structure design in step 1 is very suitable for FPGA implementation.

[0021] 2. The present invention is capable of multi-rate mixed demodulation and has the characteristics of variable data rate, spread spectrum ratio, and modulation mode.

[0022] 3. The tracking error extraction algorithm adopted by the present invention is independent of the spread spectrum ratio and has universality, and the interpolation timing used for spread spectrum timing is suitable for the situation where the code chip rate is high and the code chip clock cannot be generated by adjusting the high-multiple clock. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the transmission frame structure of the present invention.

[0024] Figure 2 It is a schematic diagram of the code tracking timing synchronization process of the present invention. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0026] A tracking timing method for broadband spread spectrum communication comprises the following steps:

[0027] (1) Design a frame structure. Each frame includes a synchronization sequence, control information, service information, pilot, and reserved data. The service information is spread using a spreading sequence, and other information is scrambled using a spreading sequence. The content of the control information indicates the modulation and spreading ratio of the service data. One frame is one cycle.

[0028] (2) The satellite transponder scrambles and transmits the spread spectrum sequence according to the frame structure;

[0029] (3) The ground station terminal performs data interpolation processing on the received signal with a sampling rate of 4 times the data rate according to the error (initial error is 0) extracted by code tracking in step (6), selects the first sampling point of the interpolated data as the optimal sampling point output, and implements code tracking timing adjustment;

[0030] (4) performing initial frame capture of the timing-adjusted signal according to a predetermined spread spectrum sequence to achieve capture synchronization, and at the same time starting the code generator to generate a spread spectrum sequence;

[0031] (5) After acquiring synchronization, the signal is descrambled using the spread spectrum sequence, and the control information is searched to locate the control information. Then, the pilot position is determined according to the arrangement of each data type in the frame structure.

[0032] (6) After the pilot position is determined, the pilots of the leading, middle, and lagging branches are accumulated according to the pilot length L, and the multiple pilot accumulation results are subjected to non-coherent accumulation of length N, where N ≥ 32, and error extraction is performed on the three non-coherent accumulation results. The error extraction results are then subjected to second-order loop filtering to obtain weighted timing error information adapted to the operating frequency of the interpolation filter in step (3), and the error information is sent to step (3).

[0033] Furthermore, in step (1), the spread spectrum sequence adopts a pseudo-random sequence, the synchronization sequence is used for data frame positioning and pseudo-code synchronization, the control information is used for positioning service information, pilot and reserved data, the control information adopts a group of unique words, different unique words indicate different rates, modulation modes, and spread spectrum ratio information used for demodulation, the pilot is used for code tracking timing and carrier recovery, the reserved data is used for data adjustment, and the reserved data is filled with 0.

[0034] Furthermore, the specific method of step (3) is:

[0035] (301) Coefficient selection: design the interpolator coefficients to be waveform matched filter coefficients with a roll-off of 0.35 and store them in the cache;

[0036] (302) Control word generation: periodically accumulate the timing error extracted by the input code tracking and the initial frequency control word to obtain a 32-bit frequency control word fcw;

[0037] (303) Data interpolation: The upper 7 bits of fcw are taken as the integer part to control the selection of data, and the lower 25 bits are taken as the decimal part to control the selection of interpolator coefficients. The data is then filtered using the interpolator coefficients to obtain the interpolated coefficients.

[0038] (304) Codeword adjustment indication: When the accumulated error exceeds half a chip period, a codeword adjustment indication is issued to adjust the local code generator at the reserved data position. If the data is ahead of the codeword, the codeword is adjusted forward for output; if it lags behind, the codeword is adjusted for output with a delay.

[0039] Here is a more specific example:

[0040] A tracking timing method for wideband spread spectrum communication, referring to Figure 1 to Figure 2 , the transmission frame structure of the satellite communication system is as follows Figure 1 As shown, the satellite communication system transponder sends information according to the transmission frame structure; the capture synchronization and tracking timing synchronization process is shown in Figure 2 As shown, after the satellite communication system ground station is turned on, it must first complete the capture synchronization of the spread spectrum sequence, and then perform code tracking timing synchronization based on the pilot signal.

[0041] The specific steps of this method are as follows:

[0042] (1) To facilitate spread spectrum signal transmission, a special frame structure must be designed. Each frame includes a synchronization sequence, control information, service information, pilot, and reserved data. The control information changes according to the modulation and spread spectrum ratio, and one frame is one cycle.

[0043] The spreading sequence uses a pseudo-random sequence, namely, an m-sequence, or other sequences with good correlation performance. In order to ensure normal operation under a threshold signal-to-noise ratio, the pilot length L will change with the change of the maximum spreading ratio.

[0044] The synchronization sequence is used for data frame positioning and pseudo code synchronization. The control information is used to indicate the positioning of service information, pilot and reserved data. At the same time, the rate, modulation mode, spread spectrum ratio and other information are analyzed for demodulation. The pilot is used for code tracking timing and carrier recovery. The reserved data is used for data adjustment.

[0045] (2) The satellite transponder scrambles and transmits the spread spectrum sequence according to the frame structure;

[0046] (3) The ground station terminal performs data interpolation processing on the received signal with a sampling rate of 4 times the data rate based on the error extracted by code tracking to achieve code tracking timing adjustment; the specific method is as follows:

[0047] 1) Coefficient selection: First, the interpolator coefficients are designed as waveform matched filter coefficients with a roll-off of 0.35 and stored in the cache.

[0048] 2) Control word generation: The timing error extracted by input code tracking and the initial frequency control word are periodically accumulated to obtain a 32-bit frequency control word fcw.

[0049] 3) Data interpolation. The upper 7 bits of fcw are taken as the integer part to control the selection of data, and the lower 25 bits are taken as the decimal part to control the selection of interpolator coefficients. The interpolator coefficients are then used to filter the data to obtain the interpolated coefficients.

[0050] 4) Codeword adjustment indication: When the accumulated error is too large, the local code generator needs to be adjusted. This operation is completed in the reserved data segment, so as not to affect the correctness of the demodulated data.

[0051] (4) performing initial frame capture of the timing-adjusted signal according to a predetermined spread spectrum sequence to achieve capture synchronization, and at the same time starting the code generator to generate a spread spectrum sequence;

[0052] (5) After acquiring synchronization, the signal is descrambled using the spread spectrum sequence, and the control information is searched and the data type information is located to locate the pilot position;

[0053] (6) After the pilot position is determined, the pilots of the leading, middle, and lagging branches are accumulated according to the length L, and the accumulated results of multiple pilots are non-coherently accumulated with a length of N to improve the anti-interference capability. The non-coherent accumulated values ​​of the leading, middle, and lagging branches are expressed by C1, C0, and C -1 Represented by, and error extraction is performed on the three-way incoherent accumulation results, the formula is The extracted error Δτ is subjected to second-order loop filtering to extract error information, thereby completing code tracking error extraction.

[0054] The method completes waveform design in step 1, implements interpolation tracking timing in step 3, and is used to extract error information in step 6, thereby providing step 3 with continuous tracking timing.

[0055] In summary, the present invention aims at the timing synchronization problem of variable rate and variable spread spectrum ratio in broadband satellite spread spectrum communication system, and proposes a pseudo code tracking timing method based on interpolation, which can adopt unified processing for the timing of received signals with different spread spectrum ratios, different data rates and different modulation modes in communication. According to the normalized lagging and leading data power comparison of auxiliary data, the method outputs the frequency control word after entering the second-order loop filter, controls the digital interpolator to adjust the sampling rate of the demodulated data, and thus realizes pseudo code tracking timing synchronization. The method does not need to adjust the accumulation length according to different spread spectrum ratios, uses pilot data for pseudo code tracking, is conducive to low signal-to-noise ratio demodulation, and is suitable for situations where the chip rate is high and the chip clock cannot be generated by adjusting the high-multiple clock. The present invention can be used for broadband satellite anti-interference communication, and is very suitable for FPGA implementation.

Claims

1. A tracking timing method for broadband spread spectrum communication, characterized in that: The following steps are involved: (1) Design a frame structure. Each frame includes a synchronization sequence, control information, service information, pilot, and reserved data. The service information is spread using a spreading sequence, and other information is scrambled using a spreading sequence. The content of the control information indicates the modulation and spreading ratio of the service data. One frame is one cycle. (2) The satellite transponder scrambles and transmits the spread spectrum sequence according to the frame structure; (3) The ground station terminal performs data interpolation processing on the received signal with a sampling rate of 4 times the data rate according to the error, selects the first sampling point of the interpolated data to determine as the optimal sampling point output, and implements code tracking timing adjustment; the initial error is 0, and the subsequent error is the error extracted in step (6); (4) performing initial frame capture of the timing-adjusted signal according to a predetermined spread spectrum sequence to achieve capture synchronization, and at the same time starting the code generator to generate a spread spectrum sequence; (5) After acquiring synchronization, the signal is descrambled using the spread spectrum sequence, and the control information is searched to locate the control information. Then, the pilot position is determined according to the arrangement of each data type in the frame structure. (6) After the pilot position is determined, the pilots of the leading, middle, and lagging branches are accumulated according to the pilot length L, and the multiple pilot accumulation results are subjected to non-coherent accumulation of length N, where N ≥ 32, and error extraction is performed on the three non-coherent accumulation results. The error extraction results are then subjected to second-order loop filtering to obtain weighted timing error information adapted to the operating frequency of the interpolation filter in step (3), and the error information is sent to step (3).

2. A tracking timing method for broadband spread spectrum communication according to claim 1, characterized in that: In step (1), the spread spectrum sequence adopts a pseudo-random sequence, the synchronization sequence is used for data frame positioning and pseudo-code synchronization, the control information is used for positioning service information, pilot and reserved data, the control information adopts a group of unique words, different unique words indicate different rates, modulation modes, and spread spectrum ratio information used for demodulation, the pilot is used for code tracking timing and carrier recovery, the reserved data is used for data adjustment, and the reserved data is filled with 0.

3. A tracking timing method for broadband spread spectrum communication according to claim 1, characterized in that: The specific method of step (3) is: (301) Coefficient selection: design the interpolator coefficients to be waveform matched filter coefficients with a roll-off of 0.35 and store them in the cache; (302) Control word generation: periodically accumulating the input error and the initial frequency control word to obtain a 32-bit frequency control word fcw; (303) Data interpolation: The upper 7 bits of fcw are taken as the integer part to control the selection of data, and the lower 25 bits are taken as the decimal part to control the selection of interpolator coefficients. The data is then filtered using the interpolator coefficients to obtain the interpolated coefficients. (304) Codeword adjustment indication: When the accumulated error exceeds half a chip period, a codeword adjustment indication is issued to adjust the local code generator at the reserved data position. If the data is ahead of the codeword, the codeword is adjusted forward for output; if it lags behind, the codeword is adjusted for output with a delay.

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