Frequency offset estimation and correction method for short-wave communication and short-wave communication system

Abstract

The invention belongs to the technical field of wireless communication, and relates to a frequency offset estimation and correction method for short-wave communication and a short-wave communication system. On the basis of an FPGA (Field Programmable Gate Array) platform of a short-wave receiver, received signals are processed in two paths on the platform, one path completes digital frequency mixing, filtering, down-conversion and frequency offset estimation, and the other path completes digital frequency mixing, filtering and down-conversion processing after frequency offset correction and sends baseband data after down-conversion to a DSP (Digital Signal Processor) module, so that the correction of carrier frequency offset is completed, and the frequency offset of the received signals is corrected. The demodulation of the received signal is completed, so that the establishment of communication under the selective calling function is ensured; moreover, under the condition of not additionally adding hardware and on the premise of not influencing the normal use of other functions, the selective calling function can be realized only by redesigning the short-wave receiver FPGA, and the method has the characteristics of low cost and short development period.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and relates to shortwave communication, and more particularly to a method for frequency offset estimation and correction in shortwave communication, and a shortwave communication system. Background Technology

[0002] Selective Call is a technology that enables directional calling through coded signals, widely used in aviation, walkie-talkies, and radio communications. When a ground station communicates with an aircraft via shortwave using Selective Call, it first transmits the target aircraft's serial number. This serial number consists of four low-frequency signals (300Hz~3000Hz), which are AM modulated with an intermediate frequency carrier (2MHz~30MHz) of a predetermined frequency before being transmitted. The shortwave receiver on the target aircraft receives this signal and sends the signal, after multiple stages of processing including mixing, filtering, and down-conversion, to the system decoder. The system decoder then identifies whether the signal matches its own serial number, determining whether it is the ground station's communication target. During this communication process, issues such as the accuracy error of the local oscillators of both devices, the time-varying nature and non-uniformity of the shortwave channel can cause frequency offsets. However, the required frequency error for the system decoder to match the low-frequency signal received by the shortwave receiver with its own serial number is within 1Hz. Therefore, after receiving the intermediate frequency signal from the ground station, the shortwave receiver needs to correct the frequency offset before proceeding with further processing to obtain the correct signal to send to the system decoder, so that the target aircraft can correctly determine whether the ground station's communication target is itself.

[0003] Shortwave communication primarily relies on ionospheric reflection for long-distance communication, offering advantages such as low cost, high resilience, and no need for repeaters. However, the ionosphere's time-varying and inhomogeneous nature causes Doppler shift in the transmitted signal. Furthermore, the local oscillators of both devices exhibit accuracy errors. These combined factors result in carrier frequency offset in the received signal. Under selective calling functionality, existing shortwave receivers do not correct for this carrier frequency offset. Consequently, under current technological conditions, directly adding selective calling functionality to existing shortwave receivers will fail to demodulate the correct frequency signal, preventing communication between the two parties.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for frequency offset estimation and correction in shortwave communication, as well as a shortwave communication system.

[0006] To achieve the above objectives, the present invention provides the following technical solution: On one hand, this invention provides a frequency offset estimation and correction method for shortwave communication, applied to a shortwave communication system. It corrects carrier frequency offset while simultaneously demodulating the received signal to establish communication under selective calling functionality. In the shortwave communication system, the shortwave receiver FPGA divides the received signal into two paths for processing: the first path is used for frequency offset estimation, and the second path is used for frequency offset correction and, after multi-stage processing, outputs the final baseband signal to the DSP. The first signal undergoes multiple processing stages before being transformed from the time domain to the frequency domain using an FFT to determine the observation frequency f where the peak amplitude of the spectrum is located. ob Define the observation frequency f ob The difference between the frequency and the nominal carrier frequency is the frequency offset estimate F. d .

[0007] Specifically, the second signal is used to correct frequency offset and, after multi-stage processing, outputs the final baseband signal to the DSP. The specific process is as follows: First, based on the local carrier frequency F c With the frequency offset estimate F d The corrected carrier frequency F is calculated, and based on the carrier frequency F, a single-tone signal sin and a single-tone signal cos are generated by DDS. These signals are then mixed with the second signal, filtered, and down-converted to obtain two baseband signals DI and DQ. Finally, they are synthesized into a single signal Dout by Hilbert transform and sent to the DSP.

[0008] Specifically, the formula for calculating the corrected carrier frequency F is as follows: F=F c ±F d Formula (1) In formula (1), F c For the local carrier frequency, F d This is the frequency offset estimate.

[0009] Specifically, the frequency offset estimate F d The acquisition process is as follows: The shortwave receiver FPGA generates single-tone signals sin and cos through a DDS, with a frequency of F. c -f am , of which F c The local carrier frequency, with values ​​ranging from 2MHz to 30MHz, f am The nominal carrier frequency; During selective calling, the received first signal is mixed with the single-tone signals sin and cos generated by the DDS to obtain two signals DI and DQ. The two signals [DI, DQ] are concatenated into a single signal S(n), which is then subjected to an N-point FFT to obtain the frequency domain signal S(k). The location of the peak amplitude of the spectrum is determined, and the observation frequency f is calculated. ob ; Where DI = Din × sin, DQ = Din × cos, and Din is the received signal (including the first signal and the second signal in the embodiment); the formula for calculating the frequency domain signal S(k) is as follows: Formula (4) In formula (4), k = (0, 1, 2, ..., N-1), and N is the number of FFT points.

[0010] Calculate the frequency offset estimate F d The frequency offset estimate F d For the observation frequency f ob With the nominal carrier frequency f am The difference.

[0011] Specifically, the observation frequency f ob The calculation formula is as follows: f ob =peak×f s / N formula (2) In formula (2), peak is the location of the peak value of the spectral amplitude, and f s The sampling rate is [value].

[0012] Specifically, the calculated frequency offset estimate is converted into a frequency word, and the specific calculation formula is as follows: F req =F d / f sys ×2 32 Formula (3) In formula (3), f sys For the FPGA system master clock, F d This is the frequency offset estimate.

[0013] Specifically, under the selective call function, the DSP directly performs D / A conversion on the received baseband signal to obtain an analog signal, one of which is directly output to the headset and the other is directly output to the system decoder.

[0014] On the other hand, the present invention also provides a shortwave communication system that applies some or all of the frequency offset estimation and correction methods described above. The shortwave communication system is applied in a ground-to-air communication scenario with selective calling function to establish a reliable link between the ground station and the aircraft.

[0015] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects: Based on a shortwave receiver FPGA platform, this invention processes the received signal in two paths. One path performs digital mixing, filtering, down-conversion, and frequency offset estimation, while the other path, after correcting the frequency offset, performs digital mixing, filtering, and down-conversion, and sends the down-converted baseband data to the DSP module. This achieves both carrier frequency offset correction and received signal demodulation, thus ensuring communication establishment under the selective calling function. Furthermore, without adding additional hardware or affecting the normal operation of other functions, the selective calling function can be implemented simply by redesigning the shortwave receiver FPGA, featuring low cost and short development cycle. Attached Figure Description

[0016] The accompanying drawings are incorporated in and form part of this specification, and together with the description serve to explain the principles of the invention.

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a method for frequency offset estimation and correction in shortwave communication provided by this invention; Figure 2 The frequency offset estimation flowchart provided by this invention; Figure 3 A flowchart illustrating the specific implementation of frequency offset estimation provided in this example; Figure 4 The frequency offset correction flowchart provided by this invention; Figure 5 The flowchart illustrates the specific implementation of frequency offset correction in this embodiment. Detailed Implementation

[0019] Exemplary embodiments will now be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples consistent with some aspects of the invention as detailed in the appended claims.

[0020] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0021] Terminology Explanation: DSP - Digital Signal Processor; D / A - Digital-to-Analog Conversion; FPGA - Field Programmable Gate Array; FFT - Fast Fourier Transform; DDS - Direct Digital Frequency Synthesizer. Example

[0022] See Figure 1 As shown, this embodiment provides a frequency offset estimation and correction method for shortwave communication, which is implemented in the FPGA of a shortwave receiver. The shortwave receiver divides the received signal into two paths for processing on the FPGA: the first path performs digital mixing, filtering, down-conversion and frequency offset estimation, and the second path performs digital mixing, filtering and down-conversion after frequency offset correction, and sends the down-converted baseband data to the DSP.

[0023] As per design requirements, the maximum frequency offset f that the shortwave receiver can correct is... am The standard frequency is 100Hz. Carrier frequency deviations exceeding 100Hz are treated as 100Hz, with an error within 1Hz. The parameter settings for this example model are as follows: Local carrier frequency F is issued. c1 The frequency is 10MHz. An AM modulated signal is emitted by a radio frequency signal source to simulate a ground station. The AM modulated signal is: ; In the above formula, the frequency offset estimate F is set. d =50Hz, then the signal source frequency (carrier frequency) F c2 =F c1 +F d =10000050Hz, baseband frequency F b Set the frequency to 1kHz, the modulation depth m to 30%, and the carrier amplitude A to 1mV.

[0024] Specifically, this embodiment is designed in the following two aspects: 1) Frequency offset estimation See the flowchart for frequency offset estimation. Figure 2 As shown, the corresponding frequency offset estimation algorithm is as follows: The shortwave receiver FPGA generates single-tone signals sin and cos through a DDS, with a frequency of F. c -f am , of which F c For local carrier frequencies (2MHz~30MHz), f am The nominal carrier frequency (i.e., the maximum frequency offset estimate, which is 100Hz in this embodiment); In selective calling mode, the radio frequency signal is sampled by an A / D converter and converted into a digital signal DIN at a sampling rate of 100MHz. The received signal DIN is mixed with the local carrier signal generated by the DDS to obtain two signals, I and Q, at a sampling rate of 100MHz: DI = Din × sin, DQ = Din×cos, DDS frequency is 10MHz-100Hz; after low-pass filtering and down-conversion (consisting of one CIC filter and 6 FIR filters), the 100MHz sampling rate I and Q signals DI and DQ are decimated by a factor of 25×5×5×5×4×4×4=200000, finally obtaining the 500Hz sampling rate I and Q signals DI_FIR6 and DQ_FIR6; DI_FIR6 and DQ_FIR6 are combined into a single channel {DI_FIR6, DQ_FIR6} and a 1024-point FFT is performed; the FFT output is {DI_FFT, DQ_FFT}, and the peak position is found by comparing the membrane values ​​(since the FFT spectrum is symmetrical, only the peak of the first 512 points is needed). The membrane value is calculated as follows: ; Observation frequency f ob The calculation formula is: Where peak is the peak position, f s The signal sampling rate is 500Hz, and N is the number of FFT points, 1024 (the resolution is required to be within 1Hz; in this embodiment, the sampling rate is 500Hz, and the resolution is 500 / 1024 = 0.48828125Hz, which meets the requirement); finally, the frequency offset estimate F is calculated. d =f ob -f am And convert it into frequency word F req =F d / f sys ×2 32 f sys For the FPGA system master clock, see [link / reference] Figure 3 .

[0025] In the absence of frequency offset, the frequency of the signal {DI_FIR6, DQ_FIR6} is the nominal carrier frequency of 100Hz. Due to the addition of a 50Hz frequency offset, its frequency is 100 + 50 = 150Hz, which is the calculated observed frequency f. ob Therefore, it can be deduced that the correct theoretical value of peak should be 150×1024 / 500=307.

[0026] 2) Frequency offset correction See the flowchart for frequency offset correction. Figure 4 First, based on the local carrier frequency F c The calculated frequency offset estimate F d The corrected carrier frequency is calculated, wherein the carrier frequency F = Fc1 +F d =10MHz + 50Hz = 10000050Hz. Based on the carrier frequency F, single-tone signals sin and cos are generated by DDS, and then mixed with the second signal to obtain two signals with a sampling rate of 100MHz: DI = Din × sin, DQ = Din × cos. After filtering and down-conversion (consisting of one CIC filter and four FIR filters), the two signals DI and DQ with a sampling rate of 100MHz are decimated by a factor of 25 × 5 × 5 × 5 × 4 = 12500, finally obtaining two baseband signals DI_FIR4 and DQ_FIR4 with a sampling rate of 8kHz. Finally, after passing through a Hilbert filter and synthesizing a single signal Dout, it is sent to the DSP. See [link to DSP documentation]. Figure 5 .

[0027] In summary, the technical solution provided by this invention processes the received signal in two paths. One path performs multi-stage processing, including mixing, filtering, and down-conversion, to transform the time-domain signal into the frequency domain using FFT, finding the frequency point corresponding to the maximum spectral amplitude. The difference between this point and the nominal carrier frequency is the frequency offset estimate. The other path adds or subtracts the calculated carrier frequency offset from the local frequency to correct the frequency offset, and then performs subsequent mixing, filtering, and down-conversion processing before sending the signal to the DSP, thereby enabling the target aircraft to successfully establish communication with the ground station.

[0028] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention.

[0029] It should be understood that the present invention is not limited to the content already described above, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for frequency offset estimation and correction in shortwave communication, characterized in that, Applied to shortwave communication systems, this technology corrects carrier frequency offset while demodulating the received signal to establish communication under selective calling functionality. In this system, the shortwave receiver FPGA splits the received signal into two paths for processing: the first path is used for frequency offset estimation, and the second path is used for frequency offset correction. After multi-stage processing, the final baseband signal is output to the DSP. The first signal undergoes multiple processing stages before being transformed from the time domain to the frequency domain using an FFT to determine the observation frequency f where the peak amplitude of the spectrum is located. ob Define the observation frequency f ob The difference between the frequency and the nominal carrier frequency is the frequency offset estimate F. d .

2. The frequency offset estimation and correction method for shortwave communication according to claim 1, characterized in that, The second signal is used to correct frequency offset and, after multi-stage processing, outputs the final baseband signal to the DSP. The specific process is as follows: First, based on the local carrier frequency F c With the frequency offset estimate F d The corrected carrier frequency F is calculated, and based on the carrier frequency F, a single-tone signal sin and a single-tone signal cos are generated by DDS. These signals are then mixed with the second signal, filtered, and down-converted to obtain two baseband signals DI and DQ. Finally, they are synthesized into a single signal Dout by Hilbert transform and sent to the DSP.

3. The frequency offset estimation and correction method for shortwave communication according to claim 2, characterized in that, The formula for calculating the corrected carrier frequency F is as follows: F=F c ±F d Formula (1) In formula (1), F c For the local carrier frequency, F d This is the frequency offset estimate.

4. The frequency offset estimation and correction method for shortwave communication according to claim 1, characterized in that, The frequency offset estimate F d The acquisition process is as follows: The shortwave receiver FPGA generates single-tone signals sin and cos through a DDS, with a frequency of F. c -f am , of which F c The local carrier frequency, with values ​​ranging from 2MHz to 30MHz, f am The nominal carrier frequency; During selective calling, the received first signal is mixed with the single-tone signals sin and cos generated by the DDS to obtain two signals DI and DQ. The two signals [DI, DQ] are concatenated into a single signal S(n), which is then subjected to an N-point FFT to obtain the frequency domain signal S(k). The location of the peak amplitude of the spectrum is determined, and the observation frequency f is calculated. ob ; Calculate the frequency offset estimate F d The frequency offset estimate F d For the observation frequency f ob With the nominal carrier frequency f am The difference.

5. The frequency offset estimation and correction method for shortwave communication according to claim 4, characterized in that, The observation frequency f ob The calculation formula is as follows: f ob =peak×f s / N formula (2) In formula (2), peak is the location of the peak value of the spectral amplitude, and f s The sampling rate is [value].

6. The frequency offset estimation and correction method for shortwave communication according to claim 4, characterized in that, The calculated frequency offset estimate is converted into a frequency word, and the specific calculation formula is as follows: F req =F d / f sys ×2 32 Formula (3) In formula (3), f sys For the FPGA system master clock, F d This is the frequency offset estimate.

7. The method for frequency offset estimation and correction of shortwave communication according to any one of claims 1 to 6, characterized in that, In the selective calling function, the DSP directly performs D / A conversion on the received baseband signal to obtain an analog signal, one of which is directly output to the headset and the other is directly output to the system decoder.

8. A shortwave communication system applying the frequency offset estimation and correction method according to any one of claims 1 to 6, characterized in that, The shortwave communication system is used in air-to-ground communication scenarios with selective calling capabilities to establish a reliable link between the ground station and the aircraft.

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