A frequency synchronization method for a coherent clock system in high-speed data transmission

Abstract

The application discloses a frequency synchronization method of coherent clock system in high-speed data transmission, and a frequency synchronization method and equipment structure for providing high-precision time-frequency reference transmission and 10Gbps data stream transmission. A high-speed modulator is used to generate a radio frequency signal at a local end and a remote end, and the radio frequency signal is received from the opposite end. A carrier tracking technology is used to recover the radio frequency carrier, and a frequency discrimination and phase discrimination value is obtained through a signal processing algorithm in a digital domain. The local frequency discrimination and phase discrimination value is sent to the remote end, the remote end uses the local value and the opposite value to realize joint phase discrimination, and then the joint phase discrimination is sent to a loop filter. The loop filter outputs a voltage control signal, and the remote end uses the voltage control signal to control a voltage-controlled crystal oscillator, so that the frequency synchronization between the remote end and the local end is realized. The method can realize single-fiber single-wavelength bidirectional time-frequency synchronization and high-speed data transmission, can fully utilize the optical wavelength resources of the optical fiber transmission system, and can save the optical wavelength resources compared with the traditional method, and has higher frequency synchronization precision.

Description

Technical Field

[0001] This invention relates to the field of distributed detection, and is applicable to sparsely deployed cross-regional distributed detection systems, providing high-precision time-frequency reference transmission and 10Gbps data stream transmission. It integrates time-frequency and data transmission functions using only a single fiber optic channel, achieving frequency synchronization accuracy on the order of 1E-13@s and 10Gbps data transmission bandwidth. Background Technology

[0002] When the deployment range of a distributed detection system expands across regions, constructing a completely new fiber optic network incurs extremely high costs. Utilizing existing operator fiber optic networks is a crucial approach to achieving cross-regional deployment and a key research direction. Research indicates that operator networks typically consist of a two-tiered structure: a metropolitan area network (MAN) and a wide area network (WAN). MANs are used within cities, typically covering tens to hundreds of kilometers, and are divided into core, aggregation, and access layers. WANs aggregate multiple MANs, covering cities, regions, provinces, and even countries over a range of hundreds to thousands of kilometers. By fully leveraging the operator's existing fiber optic network, a distributed detection system can achieve coverage of hundreds of kilometers.

[0003] Traditional fiber optic time-frequency transmission and data transmission methods require three independent bidirectional channels for time, frequency, and high-speed data services, necessitating a single detection node to have up to six fiber optic channels. This invention envisions leveraging the fiber optic network deployed by operators to achieve single-fiber, single-wavelength bidirectional time-frequency synchronization and high-speed data transmission. It fully utilizes the optical wavelength resources of the fiber optic transmission system, offering picosecond-level time synchronization accuracy and 10Gbps data transmission bandwidth.

[0004] Regarding the integrated implementation of time, frequency, and high-speed data services, some valuable experiences have been gained. Examples include inter-satellite laser calibration synchronization links, the IEEE 1588v2 protocol, and the White Rabbit protocol. However, there is still a significant gap between these and the target. Inter-satellite laser calibration synchronization links and the IEEE 1588v2 protocol use timestamps in data frames to achieve nanosecond-level time synchronization during bidirectional communication, but lack frequency transmission functionality. The White Rabbit protocol uses second pulses and time-stamped data as the time standard carrier and a 125MHz data clock as the frequency standard carrier, achieving a frequency synchronization accuracy on the order of 1E-11@s, which is significantly lower than the 1E-13@s requirement of distributed detection systems. Summary of the Invention

[0005] This invention proposes that when various clock systems (reference clock, AD clock, DA clock, intermediate frequency transceiver local oscillator, and radio frequency transceiver local oscillator) in a high-speed modulator and demodulator possess phase coherence and sufficiently low phase noise, high-precision frequency and phase discrimination required for frequency transmission can be achieved in the digital domain using coherent clock systems. Digital phase-locked loop (PLL) technology can achieve performance comparable to analog frequency transmission. Furthermore, embedding high-precision ranging frames into the data stream on top of frequency synchronization can further achieve time synchronization.

[0006] The technical solution adopted in this invention is as follows:

[0007] A frequency synchronization method utilizing a coherent clock system in high-speed data transmission includes the following steps:

[0008] (1) Generate broadband intermediate frequency signals at the local end and the remote end respectively, and upconvert the intermediate frequency signals to modulate them onto the radio frequency carrier, and then modulate the radio frequency signals onto the optical signals and couple them into the optical fiber channel for transmission to the other end.

[0009] (2) The local end and the remote end receive the optical signal from the other end, recover the radio frequency signal, and use carrier tracking technology to recover the radio frequency carrier. Then, they are mixed, low-pass filtered and integrated in the digital domain with the local radio frequency carrier to obtain the frequency and phase discrimination value.

[0010] (3) The local end frames and modulates the local frequency and phase detection values ​​onto the radio frequency carrier and sends them to the remote end. The remote end uses the frequency and phase detection values ​​of the remote end and the received frequency and phase detection values ​​of the peer end to perform joint phase detection. Then, the joint phase detection values ​​of the remote end are loop filtered to obtain the voltage control signal of the voltage-controlled crystal oscillator.

[0011] (4) The remote end uses voltage control signal to regulate the voltage-controlled crystal oscillator to achieve frequency synchronization between the remote end and the local end. The local end uses the joint phase detection result to detect the frequency synchronization status.

[0012] Furthermore, step (2) specifically includes the following steps:

[0013] The local and remote ends receive optical signals from each other, recover the radio frequency (RF) signals, and perform quadrature down-conversion on the RF signals. The down-converted I and Q signals are then digitally AGC-enabled. The I and Q baseband signals obtained from the digital AGC are noncoherently accumulated and then processed by FFT. The resulting FFT values ​​are used for frequency estimation to determine the frequency Δf. Simultaneously, carrier error extraction is performed to extract the tracking error, resulting in the error frequency control word cw_err. Δf and cw_err are used for carrier phase compensation, and the phase-compensated carrier signal is used for phase detection to obtain the frequency and phase discrimination values. Complex multiplication is performed on the I and Q baseband signals obtained from the digital AGC and the phase-compensated carrier signal. The resulting real part is used as the I data, and the resulting imaginary part is used as the Q data. The carrier error extraction is adjusted based on the I and Q data after complex multiplication, and soft decision and error correction decoding are performed to recover the original data stream.

[0014] Furthermore, in step (3), the remote end uses the frequency and phase detection values ​​from the remote end and the received frequency and phase detection values ​​from the peer end to perform joint phase detection, and then performs loop filtering on the joint phase detection values ​​from the remote end to obtain the voltage control signal of the voltage-controlled crystal oscillator; specifically, it includes the following steps:

[0015] The remote end uses the frequency and phase detection values ​​from the remote end and the received frequency and phase detection values ​​from the peer end to perform comprehensive processing to obtain the phase difference between the two signals. Based on the system requirements, the filter rate and loop bandwidth are designed to obtain specific loop parameters. The voltage control signal of the voltage-controlled crystal oscillator is obtained through correlation calculation using the phase difference and loop parameters.

[0016] The present invention has the following advantages over the prior art:

[0017] The method described in this invention can realize bidirectional time and frequency synchronization and high-speed data transmission functions with single fiber and single wavelength. It can make full use of the optical wavelength resources of the optical fiber transmission system, and has the characteristics of picosecond-level time synchronization accuracy and 10Gbps data transmission bandwidth. Compared with traditional methods, it saves optical wavelength resources and has higher frequency synchronization accuracy. Attached Figure Description

[0018] Figure 1 This is a diagram of the device composition of the present invention.

[0019] Figure 2 This is a design diagram of a high-speed modulator based on a coherent clock system.

[0020] Figure 3 This is a design diagram for a high-speed demodulator.

[0021] Figure 4 This is a design diagram for a single-fiber bidirectional transmission link. Detailed Implementation

[0022] The technical solution of the present invention will be described more clearly and in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto. Figure 1 As shown, this embodiment consists of a digital phase detector and control unit, an electro-optic modulator, a photodetector, a circulator, a long-distance optical fiber, and other equipment.

[0023] The specific implementation steps are as follows:

[0024] (1) Generate broadband intermediate frequency signals at the local end and the remote end respectively, and use a high-speed modulator to upconvert the intermediate frequency signals onto the radio frequency carrier, and then use an electro-optic modulator to modulate the radio frequency signals onto the optical signals and couple them into the optical fiber channel for transmission to the other end.

[0025] like Figure 2 As shown, in this embodiment, the high-speed modulator receives multiple parallel data streams from the sensor nodes. First, the data is encoded by the encoder, then after encoding and shaping filtering, it is sent to the DA converter module to be converted into an analog signal. After analog low-pass filtering and operational amplifier, the signal is sent to the quadrature modulator for quadrature modulation, and finally, it is up-converted to transmit the radio frequency signal.

[0026] The 20GHz intermediate frequency carrier, obtained by multiplying the frequency standard (10MHz / 100MHz) signal to obtain a high-frequency sidetone, carries the characteristics of the frequency standard signal. After a fixed time delay is introduced through photoelectric conversion-fiber transmission-electro-optical conversion, the carrier is separated in the electrical domain after carrier recovery, and does not affect data transmission.

[0027] (2) The local and remote ends use photodetectors to receive the optical signal from the other end, recover the radio frequency signal, and use carrier tracking technology in a high-speed demodulator to recover the radio frequency carrier. Then, the local radio frequency carrier is mixed, low-pass filtered, and integrated in the digital domain to obtain the frequency and phase discrimination value.

[0028] like Figure 3 As shown, the processing procedure of the high-speed demodulator is as follows:

[0029] The local and remote ends receive optical signals from each other, recover the radio frequency (RF) signals, and perform quadrature down-conversion on the RF signals. The down-converted I and Q signals are then digitally AGC-enabled. The I and Q baseband signals obtained from the digital AGC are noncoherently accumulated and then processed by FFT. The resulting FFT values ​​are used for frequency estimation to determine the frequency Δf. Simultaneously, carrier error extraction is performed to extract the tracking error, resulting in the error frequency control word cw_err. Δf and cw_err are used for carrier phase compensation, and the phase-compensated carrier signal is used for phase detection to obtain the frequency and phase discrimination values. Complex multiplication is performed on the I and Q baseband signals obtained from the digital AGC and the phase-compensated carrier signal. The resulting real part is used as the I data, and the resulting imaginary part is used as the Q data. The carrier error extraction is adjusted based on the I and Q data after complex multiplication, and soft decision and error correction decoding are performed to recover the original data stream.

[0030] (3) The local end frames the local frequency and phase discrimination values ​​through a high-speed modulator and modulates them onto the radio frequency carrier. Then, it sends them to the remote end after electro-optic modulation. The remote end uses the frequency and phase discrimination values ​​of the remote end and the received frequency and phase discrimination values ​​of the peer end to perform comprehensive processing in a joint phase detector to obtain the phase difference between the two signals. According to the system requirements, the filtering rate and loop bandwidth of the loop filter are designed to obtain the specific loop parameters. The voltage control signal of the voltage-controlled crystal oscillator is obtained by using the phase difference and loop parameters through related calculations.

[0031] (4) The remote end uses voltage control signal to regulate the voltage-controlled crystal oscillator to achieve frequency synchronization between the remote end and the local end. The local end uses the joint phase detection result to detect the frequency synchronization status.

[0032] like Figure 4 As shown, the single-fiber bidirectional transmission link is designed as follows:

[0033] The master-end DA unit outputs a radio frequency (RF) signal, which is then converted from electro-optical signal to optical signal. This optical signal is then output to the long-distance optical fiber through the bidirectional transmission port of the master optical circulator. The optical signal transmitted to the slave end is output to the slave optical receiving unit through the bidirectional transmission port of the slave optical circulator. The slave optical receiving unit then converts the electro-optical signal back to RF signal and outputs it to the slave AD unit. Conversely, the RF signal output from the slave DA unit passes through the slave optical transmitting unit, slave optical circulator, long-distance optical cable, master optical circulator, and master optical receiving unit, and is converted back to RF signal before being output to the master AD unit.

[0034] The entire fiber optic transmission link uses optical circulators configured at the master and slave ends to achieve bidirectional transmission of radio frequency signals over a single fiber, ensuring the symmetry of the fiber optic transmission link.

[0035] Frequency synchronization accuracy test results are as follows:

[0036] During testing, the master and slave devices were placed in the same location using a long-distance fiber optic cable. A 10MHz standard frequency signal generated by a rubidium clock and a 10MHz regenerated frequency signal generated by a voltage-controlled VCXO were connected to a frequency synchronization accuracy testing instrument for measurement, obtaining the Allan variance curve. In this embodiment, fiber optic cable was not used; a direct RF cable connection was employed. The frequency synchronization accuracy is shown in the table below.

[0037] Table 1

[0038] time Frequency stability 1s 3.790e-13 10s 1.353e-13 100s 1.58e-14 1000s 1.74e-15

[0039] The above description is merely a specific implementation of the present invention in the embodiments, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the protection scope of the present invention.

Claims

1. A frequency synchronization method for a coherent clock system in high-speed data transmission, characterized by, The method comprises the following steps: (1) generating wideband intermediate frequency signals at the local end and the remote end respectively, up-converting and modulating the intermediate frequency signals to radio frequency carriers, and then modulating the radio frequency signals to optical signals and coupling the optical signals into an optical fiber channel to transmit to the opposite end; (2) receiving the optical signals from the opposite end at the local end and the remote end, recovering the radio frequency signals, and using carrier tracking technology to recover the radio frequency carriers, and then performing frequency mixing, low-pass filtering and integral cleaning in the digital domain with the local radio frequency carriers to obtain frequency and phase discrimination values; (3) framing the local frequency and phase discrimination values at the local end and modulating the frequency and phase discrimination values to radio frequency carriers, sending the radio frequency carriers to the remote end, and using the frequency and phase discrimination values at the remote end and the received frequency and phase discrimination values from the opposite end to jointly phase-discriminate at the remote end, and then performing loop filtering on the joint phase discrimination values at the remote end to obtain a voltage control signal of a voltage-controlled crystal oscillator; (4) using the voltage control signal to regulate the voltage-controlled crystal oscillator at the remote end to realize frequency synchronization between the remote end and the local end, and using the joint phase discrimination results to detect the frequency synchronization state at the local end; The step (2) comprises the following steps: The local end and the remote end receive the optical signal of the opposite end, recover the radio frequency signal, quadrature down-convert the radio frequency signal, digitally AGC the I and Q signals after down-conversion, non-coherently accumulate the I and Q baseband signals after digital AGC, then perform FFT processing, use the generated FFT operation value signal to perform frequency estimation, estimate the frequency ; simultaneously extract the carrier error, extract the tracking error, obtain the error frequency control word cw_err, use and cw_err to perform carrier phase compensation, use the carrier signal after phase compensation to perform phase discrimination to obtain the frequency discrimination phase discrimination value, simultaneously perform complex multiplication operation on the I and Q baseband signals after digital AGC and the carrier signal after phase compensation, use the generated real part data as the I channel data and the generated imaginary part data as the Q channel data; and adjust the carrier error extraction according to the I and Q channel data after complex multiplication operation, simultaneously perform soft decision and error correction decoding to recover the original data stream.

2. The frequency synchronization method using a high-speed data transmission intermediate phase coherent clock system according to claim 1, characterized in that, In the step (3), the remote end uses the frequency and phase discrimination values at the remote end and the received frequency and phase discrimination values from the opposite end to jointly phase-discriminate, and then performs loop filtering on the joint phase discrimination values at the remote end to obtain a voltage control signal of a voltage-controlled crystal oscillator; specifically comprising the following steps: The remote end uses the frequency and phase discrimination values at the remote end and the received frequency and phase discrimination values from the opposite end to comprehensively process the phase difference between the signals at the two ends, and designs the filter rate and the loop bandwidth according to the system requirements to obtain specific loop parameters, and uses the phase difference and the loop parameters to obtain the voltage control signal of the voltage-controlled crystal oscillator through correlation operation.

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