Satellite-borne ADS-B signal low signal-to-noise ratio receiving method

Abstract

The application provides a kind of spaceborne ADS-B signal low SNR receiving method, comprising: input data buffer module;Frequency offset estimation and compensation module;Matching filter module;Matching data buffer module;Phase synchronization module;Amplitude estimation module;Bipolar transformation module;Correlation operation module;Peak detection module;Bit decision module;Confidence sorting module;CRC brute-force decoding module.The application uses the spectral characteristics of ADS-B signal, and proposes a kind of low complexity ADS-B coherent demodulation method.The application uses specially designed block processing structure, and places burst frame detection after coherent demodulation, improves frame header detection performance.The application combines CRC brute-force decoding, and finally reaches E b / N o When equal to 6.5dB, frame accuracy is more than 90%.

Description

Technical Field

[0001] This invention belongs to the field of satellite communication technology, and relates to spaceborne ADS-B signals, specifically to a method for receiving spaceborne ADS-B signals with low signal-to-noise ratio. Background Technology

[0002] Automatic Dependent Broadcast Surveillance System (ADS-B) is the primary future air traffic control surveillance technology identified by the International Civil Aviation Organization (ICAO). It uses an open broadcast method to periodically broadcast information such as the target's position and speed. This allows air traffic control centers to move beyond solely relying on radar for control and surveillance, while also enabling other aircraft to receive the signals, providing effective data support for situational awareness and aviation safety. However, due to limitations in ground-based deployment, its surveillance range is generally confined to land and coastal areas.

[0003] Spaceborne ADS-B refers to the use of ADS-B receivers on satellites to extend monitoring coverage to areas inaccessible by traditional ground-based equipment, such as oceans, polar regions, and remote areas, achieving global coverage. The Iridium NEXT satellite constellation carries ADS-B payloads to expand global applications. In September 2015, the National University of Defense Technology's Micro-Nano Satellite Engineering Center launched the "TianTuo-3" satellite, whose onboard ADS-B receiver, through low-noise design and narrow bandwidth compression, achieved a sensitivity of -93dBm.

[0004] Due to the large distance between the satellite and the spacecraft, the signal from the satellite-based ADS-B receiver is weak and the signal-to-noise ratio is low. Calculations show that a minimum power of -102 dBm signal needs to be processed, converted to E... b / N o At a power level of 7dB, the frame correct reception rate (PCR) needs to reach over 90%. Ground-based ADS-B receivers can only handle signals with a minimum power of -90dBm, and generally employ incoherent demodulation, the demodulation performance of which cannot meet the requirements for spaceborne reception. Therefore, it is necessary to research high-sensitivity spaceborne ADS-B reception methods.

[0005] Coherent demodulation of ADS-B signals requires addressing two main challenges: First, due to the Doppler effect, low-Earth orbit satellite links exhibit significant frequency offsets (approximately ±40kHz). Improving sensitivity through coherent demodulation necessitates carrier synchronization, including frequency and phase offset estimation and compensation. Second, ADS-B signals have a burst frame structure, requiring accurate detection of burst frame initiation for carrier synchronization, brute-force decoding, and other processing. Currently proposed coherent demodulation methods for ADS-B generally have high implementation complexity and employ incoherent burst frame detection followed by carrier synchronization, which reduces frame header detection performance. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals, thereby solving the technical problem that existing spaceborne ADS-B signal reception methods struggle to balance complexity and frame header detection performance.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A method for receiving low signal-to-noise ratio (SNR) satellite-borne ADS-B signals, the method comprising the following steps:

[0009] Step 1, Input data caching module:

[0010] For a complex signal r[n] that has been down-converted to zero intermediate frequency, the sampled data is first buffered, and then read out using a sliding window, with each output having a length of (α+β)*N. s The sampled data block.

[0011] In the formula:

[0012] n represents the sampling point number;

[0013] N s This indicates the number of sampling points for each ADS-B symbol;

[0014] α*N s Indicates the size of the sliding window;

[0015] β*N s Indicates the amount of overlap between blocks;

[0016] Step 2, Frequency Offset Estimation and Compensation Module:

[0017] For each sampled data block obtained in step one, perform a frequency offset estimation once to obtain the frequency offset estimate f. cw Frequency offset compensation is performed on the entire data block to obtain the compensated sampling signal.

[0018] Step 3, Matched Filtering Module:

[0019] Using N s A square wave with a 2-point radius is used as a matched filter to compensate the sampled signal after step two. The signal is filtered to obtain the matched filter output signal u[n].

[0020] Step 4, Match the data caching module:

[0021] Store the matched filter output signal u[n] from step three into the matched data buffer, and let address T. num =0.

[0022] Step 5, Phase Synchronization Module:

[0023] From address T in step four num Start reading 120*N consecutively from the matching data cache s Each sampling point is used to perform phase estimation and compensation in segments, and then the phase-synchronized unipolar signal v[n] is output.

[0024] Step 6, Amplitude Estimation Module:

[0025] Using phase synchronization to output the δ*N at the end s The average of the sampled data is used to obtain the signal amplitude estimate. Where δ represents N s Multiples of; N s This indicates the number of sampling points for each ADS-B symbol.

[0026] Step 7, Bipolar Conversion Module:

[0027] Using the amplitude estimate obtained in step six Then, the unipolar signal v[n] output in step five is transformed into a bipolar signal w[n].

[0028] Step 8, related calculation modules:

[0029] With N s / 2 represents the interval at which 26 sampling points are extracted, and then compared with the local frame header v in the bipolar signal w[n] obtained in step seven. uw Perform relevant calculations to obtain the relevant value x[T] num ].

[0030] Step 9, Peak Detection Module:

[0031] If the relevant value x[T] obtained in step eight num In the context, if address T num =α*N s If -1, then search for the peak value from the relevant value x[n] obtained in step eight, where 0≤n≤(α*N) s -1)) to obtain the position ρ corresponding to the peak; then, with (ρ+8*N) s ) is the starting address, N s With an interval of / 2, 224 sampling points are read from the matching data buffer as the detected ADS-B burst frame y[n], where 0≤n≤223. Otherwise, address T num =T num +1, from address T num Start reading 120*N consecutively from the matching data cache s Sampling points, skip to step four.

[0032] Step 10, Bit Decision Module:

[0033] For each ADS-B symbol, bit decision and confidence are calculated based on the values ​​of the first half and the second half of the symbol to obtain the decision bit z[n] and confidence p[n], where 0≤n≤111.

[0034] Step 11, Confidence Ranking Module:

[0035] Sort the confidence scores p[n] obtained in step 10 in a monotonically increasing order to obtain the confidence score sequence q[n] and its position sequence π[n] in the original sequence p[n].

[0036] Step 12, CRC brute-force decoding module:

[0037] Select the N bits with the lowest confidence level, and then select all combinations of 0 to 5 bits from them. After bit flipping, calculate the CRC value. Use the bit sequence with CRC of zero as the decoded ADS-B message d[n], where 0≤n≤111.

[0038] Compared with the prior art, the present invention has the following technical effects:

[0039] (I) This invention utilizes the spectral characteristics of ADS-B signals to propose a low-complexity ADS-B coherent demodulation method.

[0040] (II) The present invention adopts a specially designed block processing structure and places burst frame detection after coherent demodulation, thereby improving the frame header detection performance.

[0041] (III) This invention combines CRC brute-force decoding to ultimately achieve E b / N o At a value of 6.5dB, the frame accuracy exceeds 90%. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the burst frame structure of an ADS-B signal.

[0043] Figure 2 This is a schematic diagram of the architecture for low signal-to-noise ratio (SNR) reception processing of ADS-B signals.

[0044] Figure 3 A diagram illustrating block-based output with input data buffered.

[0045] Figure 4 This is a schematic diagram of carrier phase segmentation tracking and compensation processing.

[0046] Figure 5 This is a schematic diagram of the output signal of the phase synchronization module.

[0047] Figure 6This is a schematic diagram of the output signal of the bipolar converter module.

[0048] The specific content of the present invention will be further explained in detail below with reference to the embodiments. Detailed Implementation

[0049] It should be noted that, unless otherwise specified, all devices and techniques in this invention are based on those known in the prior art.

[0050] The 1090ES is an ADS-B communication protocol based on a secondary surveillance radar's Mode S transponder, an extension of the traditional 56-bit Mode S short message. The 1090ES data link operates at a frequency of 1090MHz, with a symbol width of 1µs and a data transmission rate of 1Mb / s. A complete 1090ES ADS-B signal lasts 120µs and consists of two parts: an 8µs header and a 112µs data segment. The header consists of four 0.5µs rectangular pulses located at 0µs, 1µs, 3.5µs, and 4.5µs respectively. The data segment module contains a total of 112 bits of information, encoded using Pulse Position Modulation (PPM). Specifically, each 1µs bit time is divided into two 0.5µs chips; a high level in the preceding chip represents a bit "1", and a high level in the following chip represents a bit "0".

[0051] The 112 bits of information contained in the data segment can be divided into five parts: DF field, CA field, AA field, ME field, and PI field. For the ADS-B signal transmitted by a Mode S transponder, DF is set to 17 (binary 10001), the CA field represents the capability and status of the transmitting device, the AA field is the globally unique address assigned to each aircraft by the International Civil Aviation Organization, the 56-bit ME field contains flight information such as the aircraft's position, speed, heading, and intention, and the last 24 bits are the Cyclic Redundancy Check (CRC) code, used to detect whether data transmission is erroneous. The ADS-B signal burst frame structure is as follows: Figure 1 As shown.

[0052] Theoretically, the sensitivity equation for an ADS-B receiver is:

[0053] S min = -174(dBm / Hz) + NF(dB) + E b / N o (dB)+10lgR b (bps)

[0054] In the formula:

[0055] S min Indicates the sensitivity of the ADS-B receiver;

[0056] NF represents the receiver's noise figure, which is generally equal to 2dB in engineering.

[0057] E b Represents the energy of a binary bit;

[0058] N o Indicates the noise power spectral density;

[0059] E b / N o This represents the ratio of binary bit energy to noise power spectral density.

[0060] R b This indicates the equivalent bit rate.

[0061] E b / N o This corresponds to the signal-to-noise ratio when PCR is 90%. Considering that the ADS-B signal is PPM modulated, the pulse width is equal to 0.5 μs, and the sensitivity is characterized by peak power, the equivalent bit rate R is... b The speed is 2Mb / s. Therefore, to achieve a sensitivity of -102dBm for the spaceborne ADS-B receiver, the demodulation threshold E... b / N o The requirement is less than or equal to 7dB, and the key points are carrier synchronization and frame header detection.

[0062] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0063] Example:

[0064] This embodiment provides a method for receiving low signal-to-noise ratio (SNR) satellite-borne ADS-B signals, such as... Figure 2 As shown, the method includes the following steps:

[0065] Step 1, Input data caching module:

[0066] For a complex signal r[n] that has been down-converted to zero intermediate frequency, the sampled data is first buffered, and then read out using a sliding window, such as... Figure 3 As shown, the output length for each iteration is (α+β)*N. s The sampled data block.

[0067] In the formula:

[0068] n represents the sampling point number;

[0069] N s This indicates the number of sampling points for each ADS-B symbol;

[0070] α*N s Indicates the size of the sliding window;

[0071] β*N s Indicates the amount of overlap between blocks;

[0072] Step 2, Frequency Offset Estimation and Compensation Module:

[0073] For each sampled data block obtained in step one, perform a frequency offset estimation once to obtain the frequency offset estimate f. cw Frequency offset compensation is performed on the entire data block to obtain the compensated sampling signal.

[0074] In this step, the ADS-B signal is treated as an approximate single-carrier signal. Frequency offset is estimated by performing delay correlation on the signal, and then frequency offset compensation is performed on the data.

[0075] In step two, the frequency offset estimate f cw for:

[0076]

[0077] In the formula:

[0078] d represents the delay value;

[0079] k represents the sampling point number used for parameter estimation;

[0080] L represents the number of sampling points used for parameter estimation;

[0081] r[] represents a complex signal;

[0082] arg[] represents the argument operation function for complex numbers;

[0083] conj() represents the conjugate function for complex numbers;

[0084] start indicates the starting position.

[0085] In this specific embodiment, α = 80, β = 40, and N are selected. s =8. The sliding window outputs a sampling data block of length 960 each time, and the frequency offset is estimated using the last 320 sampling points, i.e.: start = 640, d = 8, L = 320.

[0086] Step 3, Matched Filtering Module:

[0087] Using N s A square wave with a 2-point radius is used as a matched filter to compensate the sampled signal after step two. The signal is filtered to obtain the matched filter output signal u[n].

[0088] Step 4, Match the data caching module:

[0089] Store the matched filter output signal u[n] from step three into the matched data buffer, and let address T. num =0.

[0090] Step 5, Phase Synchronization Module:

[0091] From address T in step four num Start reading 120*N consecutively from the matching data cache s Each sampling point is used to perform phase estimation and compensation in segments, and then the phase-synchronized (real part) unipolar signal v[n] is output.

[0092] In this step, for the matched filter output signal u[n], due to errors in frequency offset estimation, the residual frequency offset will cause a linear increase in the carrier phase, requiring carrier phase tracking during demodulation. Here, a segmented phase estimation and compensation technique is used, with each segment containing a block of sampling points, estimating the accumulated phase error Δθ within the segment. k This involves performing phase compensation on this data segment to achieve carrier phase synchronization. The carrier phase segmentation tracking and compensation process is as follows: Figure 4 As shown.

[0093] In this step, since the ADS-B signal is regarded as an approximate single-carrier signal, no demodulation processing is required during carrier tracking; instead, maximum likelihood estimation is performed directly.

[0094] In this embodiment, the output signal of the phase synchronization module is shown in the diagram below. Figure 5 As shown.

[0095] In step five, when performing phase estimation segment by segment, the phase estimate for the k-th segment is:

[0096]

[0097] In the formula:

[0098] Δθ k This indicates the phase offset of the k-th segment;

[0099] n represents the sequence number of the sampling point;

[0100] block indicates the number of sampling points;

[0101] k represents the segment number;

[0102] u[] represents the output signal of the matched filter;

[0103] In this embodiment, block = 160 is set. The estimated phase offset Δθ is used. kPhase compensation is performed on the 160 sampling points of the k-th segment to complete the phase error correction.

[0104] Step 6, Amplitude Estimation Module:

[0105] Using phase synchronization to output the δ*N at the end s The average of the sampled data is used to obtain the signal amplitude estimate. Where δ represents N s Multiples of; N s This indicates the number of sampling points for each ADS-B symbol.

[0106] In this embodiment, coherent detection includes steps six to nine, namely: an amplitude estimation module, a bipolar transformation module, a correlation operation module, and a peak detection module. Coherent detection is performed once for each received phase synchronization output v[n] sampled data block.

[0107] In step six, for the Tth... num The amplitude estimate is obtained by averaging the last 320 samples of a unipolar signal v[n].

[0108] Step 7, Bipolar Conversion Module:

[0109] Using the amplitude estimate obtained in step six Then, the unipolar signal v[n] output in step five is transformed into a bipolar signal w[n].

[0110] In this embodiment, the output signal diagram of the bipolar conversion module is shown below. Figure 6 As shown.

[0111] Step 8, related calculation modules:

[0112] With N s / 2 represents the interval at which 26 sampling points are extracted, and then compared with the local frame header v in the bipolar signal w[n] obtained in step seven. uw Perform relevant calculations to obtain the relevant value x[T] num ].

[0113] In step eight, the relevant values

[0114] In the formula:

[0115] n represents the sampling point number;

[0116] w[] represents a bipolar signal;

[0117] v uw=[1,0,1,0,0,0,0,1,0,1,0,0,0,0,0,0,1,0,0,1,0,1,0,1,1,0].

[0118] Step 9, Peak Detection Module:

[0119] If the relevant value x[T] obtained in step eight num In the context, if address T num =α*N s If -1, then search for the peak value from the relevant value x[n] obtained in step eight, where 0≤n≤(α*N) s -1)) to obtain the position ρ corresponding to the peak. Then, (ρ+8*N) s ) is the starting address, N s With an interval of / 2, 224 sampling points are read from the matching data buffer as the detected ADS-B burst frame y[n], where 0≤n≤223; otherwise, the address T is used. num =T num +1, from address T num Start reading 120*N consecutively from the matching data cache s Sampling points, skip to step four.

[0120] Step 10, Bit Decision Module:

[0121] For each ADS-B symbol, bit decision and confidence are calculated based on the values ​​of the first half and the second half of the symbol to obtain the decision bit z[n] and confidence p[n], where 0≤n≤111.

[0122] Step 11, Confidence Ranking Module:

[0123] Sort the confidence scores p[n] obtained in step 10 in a monotonically increasing order to obtain the confidence score sequence q[n] and its position sequence π[n] in the original sequence p[n].

[0124] Step 12, CRC brute-force decoding module:

[0125] Select the N bits with the lowest confidence, and then select all combinations of 0 to 5 bits from them. After bit flipping, calculate the CRC (Cyclic Redundancy Check) value. Use the bit sequence with zero CRC as the decoded ADS-B message d[n], where 0≤n≤111.

Claims

1. A method for receiving low signal-to-noise ratio (SNR) satellite-borne ADS-B signals, characterized in that, The method includes the following steps: Step 1: Input data caching; Step 2, Frequency Offset Estimation and Compensation: For each sampled data block obtained in step one, perform a frequency offset estimation once to obtain the frequency offset estimate. Frequency offset compensation is performed on the entire data block to obtain the compensated sampling signal. ;in, Indicates the sampling point number; Step 3, matched filtering; Step 4, Match Data Cache: The matched filter output signal from step three Store in the matching data cache, let address ; Step 5, Phase Synchronization: From the address in step four Start reading continuously from the matching data cache Each sampling point is used to perform phase estimation and compensation in segments, and then a unipolar signal with phase synchronization is output. ; Step 6, Amplitude Estimation: Using phase synchronization to output the end The average of the sampled data is used to obtain the signal amplitude estimate. ;in, express Multiples of; This indicates the number of sampling points for each ADS-B symbol; Step 7, Bipolar Conversion: Using the amplitude estimate obtained in step six Then the unipolar signal output in step five Transformed into a bipolar signal , ; Step 8, related calculations: by 26 sampling points were extracted at intervals, and then compared with the bipolar signal obtained in step seven. Local frame header Perform relevant calculations to obtain relevant values. ; Step 9, Peak Detection: If the relevant value obtained in step eight In the middle, if the address The relevant values ​​obtained from step eight The peak search volume in the middle, among which, Indicates the size of the sliding window. ), to obtain the position corresponding to the peak. Then, with For the starting address, At intervals, 224 sampling points are read from the matching data buffer as the detected ADS-B burst frames. Otherwise, address ， From address Start reading continuously from the matching data cache After sampling one point, proceed to step four; Step 10, Bit Decision: For each ADS-B symbol, bit decision and confidence are calculated based on the values ​​of the first and second halves of the symbol to obtain the decision bits. and confidence level ,in, The sequence number representing the decision bit. ; Step 11, Confidence Ranking: The confidence level obtained in step ten Sort the results according to monotonically increasing order to obtain the confidence sequence. and in the original sequence Position sequence in ; Step 12, CRC brute-force decoding: Select the one with the lowest confidence level Each bit is selected, and all combinations of 0 to 5 bits are chosen. After bit flipping, the CRC value is calculated. The bit sequence with a CRC of zero is used as the decoded ADS-B message. ,in, .

2. The low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals as described in claim 1, characterized in that, The method includes the following steps: Step 1, Input data caching: Complex signals down-converted to zero intermediate frequency by digital conversion First, the sampled data is buffered, and then read out using a sliding window, with each output being a length of [length missing]. The sampled data block; In the formula: This indicates the number of sampling points for each ADS-B symbol; Indicates the amount of overlap between blocks; Step 3, matched filtering: use A square wave is used as a matched filter to compensate the sampled signal after step two. The filtered signal is obtained by performing filtering. .

3. The low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals as described in claim 2, characterized in that, The method includes the following steps: In step two, the frequency offset estimate is: ； In the formula: Indicates the delay value; Indicates the sampling point number used for parameter estimation; This indicates the number of sampling points used for parameter estimation; Represents a complex signal; Functions for argument operations of complex numbers; The function representing the conjugate operation of complex numbers; Indicates the starting position.

4. The low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals as described in claim 2, characterized in that, In step five, when performing phase estimation piecewise, the first... The phase estimate of the segment is: In the formula: Indicates the first The phase deviation of the segment; Indicates the sequence number of the sampling point; Indicates the number of sampling points; Indicates the sequence number of the segment; This represents the output signal of the matched filter.

5. The low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals as described in claim 2, characterized in that, In step six, for the first A sampled unipolar signal The signal amplitude estimate is obtained by averaging the last 320 sampled data points. .

6. The low signal-to-noise ratio (SNR) reception method for spaceborne ADS-B signals as described in claim 2, characterized in that, In step eight, the relevant values ; In the formula: Indicates the sampling point number; Indicates a bipolar signal; 。

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