High-speed maneuvering low-orbit satellite Doppler signal tunable frequency offset compensation method and device based on photon frequency shift

Abstract

The invention discloses a high-speed maneuvering low-orbit satellite Doppler signal tunable frequency offset compensation method and device based on photon frequency shift. When the high-speed maneuvering low-orbit satellite operates at a certain speed and communicates with the ground at a certain carrier frequency, the ground receives a satellite communication signal to be compensated. A to-be-compensated signal enters the system to realize carrier frequency measurement to determine the current carrier frequency and the Doppler frequency to be compensated, and a sawtooth driving signal is generated and input into the modulator to realize phase modulation; a signal to be compensated is modulated to an optical domain and generates a signal compensated by the optical domain together with frequency-shift optical carrier waves generated by phase modulation in a beat frequency manner, so that frequency compensation is realized. When the low-orbit satellite communicates with other carrier frequencies, the current compensation frequency can be calculated through the device for compensation, so that the tunable frequency offset compensation of the Doppler signal of the high-speed maneuvering low-orbit satellite is realized.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic technology, and particularly relates to a method and device for compensating tunable frequency offset of Doppler signals of high-speed mobile low-orbit satellites based on photon frequency shift. Background Art

[0002] In a satellite mobile communication system, due to the relative motion between the satellite and the mobile terminal, the Doppler effect will occur. For a low-orbit satellite mobile communication system, the satellite runs at a high speed, and a large Doppler frequency offset will be generated during communication, making the demodulation and other processes at the receiving end very difficult, directly resulting in a reduction in system performance. To ensure the reliability of communication, it is necessary to analyze the Doppler frequency offset and its variation law of the mobile terminal within the visible range of the satellite, so as to compensate for the Doppler effect and improve the communication quality.

[0003] In a mobile communication system, due to the relative motion between the satellite and the ground mobile terminal, the frequency of the electromagnetic wave received by the receiving end will shift. The magnitude of the Doppler frequency shift where F is the carrier frequency, v is the satellite motion speed, θ is the angle between the incident direction of the electromagnetic wave and the satellite motion direction, and c = 3×10 8 is the speed of light. It can be seen that the Doppler offset value is proportional to the relative speed between the transmitter and the receiver, and proportional to the frequency of the electromagnetic wave. In addition, it is also related to the angle between the line connecting the transmitter and the receiver and the motion direction. Through analysis, it can be obtained that when the receiving end and the transmitting end move towards each other, the Doppler frequency shift value is positive; when the receiving end and the transmitting end move away from each other, the Doppler frequency shift value is negative. If the satellite motion direction and the relative running speed are small, the Doppler frequency shift is small, and this influence can be ignored. If there is a large relative radial motion, a large Doppler frequency shift will be generated, and at this time, its influence must be considered. Moreover, the frequency bands used in the current mobile communication network are gradually increasing, and the Doppler frequency shift will also increase with the increase of the frequency (i.e., the decrease of the wavelength), which is also one of the important factors causing large Doppler frequency shifts. In a satellite mobile communication system, when there is a large Doppler frequency shift between the frequency received by the receiving end and the actual frequency of the wave source, the receiving end cannot correctly demodulate, resulting in a reduction in communication performance. To avoid the influence of the Doppler effect on communication, it is necessary to deeply study its characteristics and variation laws, and then propose corresponding treatment measures.

[0004] Traditional Doppler signal compensation methods involve extracting the carrier frequency from the signal. For example, a pilot signal is inserted at the transmitting end, and the receiving end extracts it through a narrowband filter to obtain the carrier frequency. However, this method is rarely adopted because the transmission of the pilot signal increases the transmission power. Another method is to use a loop (such as a Costas loop) to track the carrier, but it requires a very complex and high-precision carrier recovery loop. Microwave photonics is a new technology that combines microwave technology and photon technology. Based on photon frequency shift, it can utilize the inherent advantages of photon technology, such as large bandwidth, low frequency-dependent loss, and electromagnetic interference resistance, to achieve high-quality transmission and processing of microwave signals. Summary of the Invention

[0005] In view of the above, the present invention provides a tunable frequency offset compensation method and device for Doppler signals of high-speed maneuvering low-earth orbit satellites based on photon frequency shift.

[0006] The tunable frequency offset compensation device for Doppler signals of high-speed maneuvering low-earth orbit satellites based on photon frequency shift includes: a signal with Doppler frequency offset at the receiving end, a 180° hybrid coupler, a mixer, a high-speed ADC, an FPGA, a local oscillator, a laser, a dual-parallel dual-drive Mach-Zehnder modulator, an optical filter, an optical amplifier, an optical filter, and a photodetector.

[0007] The high-speed maneuvering low-earth orbit satellite operates at a certain speed and communicates with the ground at a certain carrier frequency to obtain a satellite communication signal to be compensated. The signal to be compensated enters the mixer and is mixed with the swept-frequency signal generated by the FPGA-controlled local oscillator to an intermediate frequency. After being acquired by the high-speed ADC, it enters the FPGA to calculate the current carrier frequency and determine the magnitude of the Doppler compensation frequency. The Doppler signal passes through a 180° hybrid coupler to generate two signals with opposite phases, and is modulated onto the optical carrier generated by the laser through the sub-modulator 2 of the dual-parallel dual-drive Mach-Zehnder modulator to generate ±1-order optical sidebands. The two same-frequency and in-phase sawtooth signals generated by the FPGA according to the magnitude of the current compensation frequency perform phase modulation on the input optical signal through the sub-modulator 1. Then, it sequentially passes through an optical filter to filter out the useless sidebands, an optical amplifier to amplify the signal, and an optical filter to filter out the amplified out-of-band noise. Finally, it is input to the photodetector, so that the microwave sideband and the frequency-shifted optical carrier are detected in the optical domain and the beat frequency is converted into an electrical signal to achieve frequency offset compensation. When the low-earth orbit satellite communicates at other carrier frequencies, this device can calculate the magnitude of the current compensation frequency for compensation, thereby realizing the tunable frequency offset compensation of the Doppler signals of high-speed maneuvering low-earth orbit satellites.

[0008] Further, the satellite communication signal to be compensated is a communication signal with Doppler frequency offset received by the ground receiving system when the low-earth orbit satellite operates at a certain speed and communicates with the ground at a certain carrier frequency.

[0009] Further, the frequency range of the frequency-swept signal generated by the FPGA-controlled local oscillator includes

[0010] the carrier frequency change range of the satellite; the signal to be compensated is mixed with the frequency-swept signal to the intermediate frequency, and after being collected by the high-speed ADC, it enters the FPGA for FFT operation to calculate the carrier frequency F of the received signal 1 , determine the running speed v according to the current running state of the satellite, the included angle θ between the electromagnetic wave incident direction and the satellite movement direction, the speed of the vacuum electromagnetic wave c = 3 * 10 8 m / s, according to F d = F - F 1 (away from the receiving end) estimate the magnitude F of the original carrier frequency and the magnitude F of the compensation frequency d .

[0011] Further, the Doppler signal generates two signals with opposite phases through a 180° hybrid coupler, and is modulated onto the optical signal generated by the laser through the sub-modulators of the dual-parallel dual-drive Mach-Zehnder modulator. The FPGA generates two sawtooth signals with the same frequency and the same phase according to the estimated magnitude of the Doppler compensation frequency, and the frequency is the magnitude F of the Doppler frequency compensation d , and the peak-to-peak value V pp is 2 times the half-wave voltage V of the dual-parallel dual-drive Mach-Zehnder modulator π , and the two signals perform phase modulation on the input optical signal through sub-modulator 1.

[0012] Further, sub-modulator 1 in the dual-parallel dual-drive Mach-Zehnder modulator works at the maximum point, and the two input sawtooth signals with the same frequency and the same phase perform phase modulation on the optical carrier, causing the optical carrier to generate a frequency offset, and the magnitude of the generated frequency offset is the magnitude F of the current compensation frequency d ; sub-modulator 2 works at the minimum point, and the two signals generated by the signal to be compensated through the 180° hybrid coupler are modulated onto the ±1st order sidebands.

[0013] Further, the optical filter filters out the +1st order sideband, retains the frequency-shifted optical carrier and the -1st order sideband, the optical amplifier (amplifies the optical signal, and the optical filter (realizes the filtering of the out-of-band noise of the amplification. The frequency-shifted optical carrier and the -1st order sideband in the photodetector are detected in the optical domain to generate a beat frequency and converted into an electrical signal to achieve frequency compensation. When a low-earth orbit satellite communicates at other carrier frequencies, the magnitude of the current compensation frequency can be calculated through this device for compensation, so as to realize the tunable frequency offset compensation of the Doppler signal of the high-speed maneuvering low-earth orbit satellite. Description of the Drawings

[0014] Figure 1 is a schematic structural diagram of the Doppler compensation system device of the present invention.

[0015] Figure 2For a low-earth orbit satellite with a carrier frequency of 8 GHz, a symbol rate of 10 M symbols / s, the time-domain waveform diagram of the QPSK signal with a frequency offset of 25 kHz received at the receiving end after being compensated by the above structure.

[0016] Figure 3 It is the spectrum diagram of the QPSK signal.

[0017] Figure 4 It is the constellation comparison diagram before and after compensating the frequency offset signal.

[0018] In the figure: 1 - The signal with Doppler frequency offset at the receiving end, 2 - 180° hybrid coupler, 3 - Mixer, 4 - High-speed ADC, 5 - FPGA, 6 - Local oscillator, 7 - Laser, 8 - Dual-parallel dual-drive Mach-Zehnder modulator, 9 - Optical filter, 10 - Optical amplifier, 11 - Optical filter, 12 - Photoelectric detector. Detailed implementation manner

[0019] In order to describe the present invention more specifically, the technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation manners. Figure 1 It is the Doppler compensation module. Figure 2 It is the Doppler compensation system. As Figure 1 Figure 2 As shown, the devices adopted in the method include the signal 1 with Doppler frequency offset at the receiving end, 180° hybrid coupler 2, mixer 3, high-speed ADC 4, FPGA 5, local oscillator 6, laser 7, dual-parallel dual-drive Mach-Zehnder modulator 8, optical filter 9, optical amplifier 10, optical filter 11, and photoelectric detector 12.

[0020] In this embodiment, as Figure 1 shown, the satellite communication signal 1 to be compensated is the communication signal received by the ground receiving system with a Doppler frequency offset F when a low-earth orbit satellite operates at a certain

[0021] speed and communicates with the ground at a certain carrier frequency. d

[0022] In this embodiment, as Figure 1 shown, the frequency range of the swept-frequency signal generated by the FPGA 5 controlling the local oscillator 6 includes the carrier frequency change range of the satellite; the signal 1 to be compensated is mixed with the swept-frequency signal to the intermediate frequency, and after being collected by the high-speed ADC 4, it enters the FPGA 5 for FFT operation to calculate the current carrier frequency F 1 , if the satellite carrier frequency is F, the satellite operating speed is v (away from the receiving end), the speed of vacuum electromagnetic wave is c, and the included angle between the electromagnetic wave incident direction and the satellite movement direction is θ, according to

[0023]

[0024] It can be known that When receiving at the far end, F d = F - F 1 , then the magnitude of the Doppler frequency offset can be estimated by the following formula.

[0025]

[0026] In this embodiment, as Figure 1 shown, the Doppler signal 1 passes through the 180° hybrid coupler 2 to generate two signals with opposite phases, which are divided into two paths and enter the two RF ports of the sub-modulator 2 to achieve carrier-suppressed double-sideband modulation (operating at the minimum point) and generate ±1-order sidebands. Assume that the electric field expression of the optical signal generated by the laser 7 is:[[]]

[0027]

[0028] where E is the amplitude of the optical signal, and f c is the frequency of the optical signal. The sub-modulator 2 operates at the minimum point, and the electric field at the output end can be expressed as:[[]]

[0029]

[0030] where J n (x) is the Bessel function of the first kind of order n, is the modulation index, V RF and F 1 are respectively the amplitude and frequency of the microwave signals input to each RF port of the modulator.

[0031] The two sawtooth signals with the same frequency and phase generated by the FPGA 5 perform phase modulation on the input optical signal through the sub-modulator 1. Assume that the ideal sawtooth time-domain expression is:[[]]

[0032]

[0033] where A is the signal amplitude, Ts is the signal frequency. After phase modulation, the electric field output by the sub-modulator 1 can be expressed as:[[]]

[0034]

[0035] where V π is the half-wave voltage of the modulator, t ff is the insertion loss, ε is the extinction ratio of the modulator. When the peak-to-peak value of the sawtooth wave is 2V π , and the frequency is the calculated Doppler frequency offset f d , causing the optical carrier to have a frequency shift, then the above formula can be rewritten as:[[]]

[0036]

[0037] If the bias voltage of the main modulator in the dual-parallel dual-drive modulator 8 is set to 0, the output electric field can be expressed as:

[0038]

[0039] The output optical signal enters the optical filter 9, where the -1st order sideband and the frequency-shifted optical carrier are retained, and the +1st order sideband is filtered out. The optical amplifier 10 amplifies the optical signal, and the electrical signal detected by the photodetector 11 can be expressed as:

[0040]

[0041] where α is the suppression amount introduced by the optical filter 9 at the frequency of the frequency-shifted optical carrier, and G OA is the gain of the optical amplifier 10. The -1st order sideband is beat with the frequency-shifted optical carrier, and the photocurrent generated on the photodetector 11 is given by the following formula:

[0042]

[0043] In this embodiment, as Figure 1 shown. The frequency compensation is realized by detecting the beat of the frequency-shifted optical carrier and the -1st order sideband in the optical domain by the photodetector 11 and converting it into an electrical signal.

[0044] Taking the low-earth orbit satellite carrier frequency of 8 GHz, symbol rate of 10 M symbols / s, and frequency offset of 25 kHz received at the receiving end as an example, Figure 2 is the time-domain waveform of the QPSK signal after being compensated by the above structure; Figure 3 is the QPSK signal spectrum; Figure 4 is the constellation comparison diagram before and after the frequency offset signal compensation.

[0045] When the low-earth orbit satellite communicates with other carrier frequencies, this device can calculate the magnitude of the current compensation frequency for compensation, so as to realize the tunable frequency offset compensation of the Doppler signal of the high-speed mobile low-earth orbit satellite.

Claims

1. A tunable frequency deviation compensation device for Doppler signals of high-speed maneuverable low-orbit satellites based on photon frequency shift, comprising: The receiving end has a Doppler frequency deviation signal (1), a 180° hybrid coupler (2), a mixer (3), a high-speed ADC (4), an FPGA (5), a local oscillator (6), a laser (7), a dual-parallel dual-drive Mach-Zehnder modulator (8), an optical filter (9), an optical amplifier (10), an optical filter (11), and a photodetector (12). The invention is characterized in that: when a high-speed maneuverable low-orbit satellite runs at a certain speed and communicates with the ground at a certain carrier frequency, a satellite communication signal (1) to be compensated is obtained; the signal (1) to be compensated enters the mixer (3), is mixed with a swept frequency signal generated by a local oscillator (6) controlled by the FPGA (5) to an intermediate frequency, and after being collected by the high-speed ADC (4), enters the FPGA (5) to calculate the current carrier frequency and determine the magnitude of the Doppler compensation frequency. The Doppler signal (1) is passed through a 180° hybrid coupler (2) to generate two signals of positive and negative phases, and is modulated onto an optical carrier generated by a laser (7) through a sub-modulator 2 of a dual-parallel dual-drive Mach-Zehnder modulator (8) to generate ±1-order optical sidebands; the FPGA (5) generates two sawtooth wave signals of the same frequency and phase according to the current compensation frequency, and phase modulates the input optical signal through the sub-modulator 1; then the signals are sequentially passed through an optical filter (9) to filter out useless sidebands, an optical amplifier (10) to amplify signals, and an optical filter (11) to filter out amplified out-of-band noise; finally, the signals are input into a photodetector (12), so that the microwave sideband and the frequency-shifted optical carrier are detected in the optical domain and converted into electrical signals to achieve frequency deviation compensation; when a low-orbit satellite communicates with other carrier frequencies, the current compensation frequency can be calculated by this device to compensate, thereby achieving tunable frequency deviation compensation of the Doppler signal of a high-speed maneuvering low-orbit satellite.

2. The device according to claim 1, characterized in that: The satellite communication signal (1) to be compensated is a communication signal with Doppler frequency deviation received by a ground receiving system when a low-orbit satellite runs at a certain speed and communicates with the ground at a certain carrier frequency.

3. The device according to claim 1, characterized in that: The frequency range of the swept frequency signal generated by the local oscillator (6) controlled by the FPGA (5) includes the carrier frequency variation range of the satellite; the signal to be compensated (1) is mixed with the swept frequency signal to an intermediate frequency, and after being collected by the high-speed ADC (4), it enters the FPGA (5) for FFT calculation to obtain the carrier frequency F1 of the received signal, and the operating speed v (away from the receiving end), the angle θ between the incident direction of the electromagnetic wave and the direction of satellite movement, and the vacuum electromagnetic wave speed c are determined according to the current operating state of the satellite, and the size F of the original carrier frequency and the size F of the compensation frequency are estimated. d .

4. The device according to claim 1, characterized in that: The Doppler signal (1) passes through 180° The hybrid coupler (2) generates two signals of positive and negative phases, and the signals are modulated onto the optical signal generated by the laser (7) through the sub-modulator 2 of the dual-parallel dual-drive Mach-Zehnder modulator (8). The FPGA (5) generates two sawtooth wave signals of the same frequency and phase according to the estimated Doppler compensation frequency. The frequency is the Doppler frequency compensation frequency F. d , peak-to-peak value V pp is twice the half-wave voltage V of the dual parallel dual drive Mach-Zehnder modulator (8) π , the two signals phase-modulate the input optical signal through sub-modulator 1.

5. The device according to claim 1, characterized in that: The sub-modulator 1 in the dual parallel dual drive Mach-Zehnder modulator (8) operates at the maximum point, and the two input sawtooth wave signals with the same frequency and phase modulate the phase of the optical carrier, so that the optical carrier generates a frequency deviation, and the magnitude of the generated frequency deviation is the magnitude of the current compensation frequency F d ; Sub-modulator 2 works at the minimum point, and the two signals generated by the signal to be compensated (1) through the 180° hybrid coupler (2) are modulated onto the ±1-order sidebands.

6. The device according to claim 1, characterized in that: The optical filter (9) filters out the +1-order sideband, retains the frequency-shifted optical carrier and the -1-order sideband, the optical amplifier (10) amplifies the optical signal, the optical filter (10) filters out the amplified out-of-band noise, and the photoelectric detector (11) detects the beat frequency of the frequency-shifted optical carrier and the -1-order sideband in the optical domain and converts it into an electrical signal to achieve frequency compensation; when the low-orbit satellite communicates with other carrier frequencies, the current compensation frequency can be calculated by this device to perform compensation, thereby achieving tunable frequency deviation compensation of the Doppler signal of the high-speed maneuvering low-orbit satellite.

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