Spread spectrum and time-digital conversion coordinated underwater communication measurement system and method

Abstract

The invention relates to a spread spectrum and time-digital conversion cooperative underwater communication measurement system and method, belongs to the technical field of underwater wireless optical communication, and solves the technical problems of limited communication rate and insufficient positioning precision in an underwater complex environment. The measurement system comprises a master-end underwater wireless optical communication machine and a slave-end underwater wireless optical communication machine, each of the master-end underwater wireless optical communication machine and the slave-end underwater wireless optical communication machine comprises an optical transmitting unit, an optical receiving unit, a signal processing unit, an upper computer and a shell. And the optical transmitting unit, the optical receiving unit and the upper computer are respectively connected with the signal processing unit. The optical transmitting unit, the optical receiving unit and the signal processing unit are arranged in the shell, and the upper computer is arranged outside the shell. The measurement method comprises the steps of system initialization and link establishment, communication and time service synchronization, distance measurement signal triggering and transmitting, and distance calculation and display. The method is used for underwater robot collaborative operation, ocean monitoring and deep sea detection.

Description

Technical Field

[0001] This invention belongs to the field of underwater wireless optical communication technology, specifically relating to an underwater communication measurement system and method that combines spread spectrum and time-to-digital conversion. Background Technology

[0002] The rise of underwater wireless optical communication ranging and timing technology stems from the inherent bottlenecks of traditional underwater acoustic communication in terms of speed, latency, and anti-interference capabilities. With the development of underwater unmanned swarm collaboration and high-precision monitoring, the demand for high-speed, low-latency, and high-precision ranging, positioning, and timing is becoming increasingly urgent. However, existing acoustic systems suffer from insufficient underwater acoustic communication speeds due to bandwidth limitations, propagation delays, and multipath effects. This leads to a mismatch between communication, positioning, and timing references, causing a chain reaction of spatiotemporal errors and failing to guarantee ranging and timing accuracy. While direct optical ranging offers speed advantages, it is limited by insufficient accuracy in measuring time intervals. This not only fails to guarantee simultaneous ranging and timing during communication but also results in excessively large ranging and timing errors, making it difficult to meet current high-precision requirements.

[0003] The applicant argues that, against this backdrop, underwater wireless optical communication (UWOC) technology, with its high bandwidth, low latency, high precision, and strong anti-interference capabilities, has become a breakthrough direction. However, the complex underwater environment, such as high attenuation coefficients, turbulence, and multipath effects, leads to severe attenuation and distortion of optical signals. This necessitates the integration of spread spectrum coding, high-precision time interval measurement, and dynamic error compensation calculations to achieve integrated communication, ranging, and timing functions, propelling this technology from the laboratory to real-world deep-sea scenarios. Summary of the Invention

[0004] To overcome the limitations of communication rate, positioning accuracy, and anti-interference in complex underwater environments, this invention proposes an underwater communication measurement system and method that combines spread spectrum and time-to-digital conversion.

[0005] The technical solution adopted by this invention to solve its technical problem is:

[0006] An underwater communication and measurement system that combines spread spectrum and time-to-digital conversion includes a master underwater wireless optical communication unit and a slave underwater wireless optical communication unit.

[0007] The master underwater wireless optical communication unit is located at the master node, and the slave underwater wireless optical communication unit is located at the slave node. The optical axes of the master and slave underwater wireless optical communication units are aligned.

[0008] The master underwater wireless optical communication unit and the slave underwater wireless optical communication unit have the same structure, both including an optical transmitting unit, an optical receiving unit, a signal processing unit, a host computer, and a shell;

[0009] The optical transmitting unit, optical receiving unit, and signal processing unit are housed inside the outer casing, while the host computer is located outside the outer casing. The optical transmitting unit, optical receiving unit, and host computer are each connected to the signal processing unit.

[0010] The optical transmitting unit receives communication, ranging, and timing signals from the signal processing unit, and generates and transmits optical signals carrying communication and ranging information.

[0011] The optical receiving unit converts the received optical signal into an electrical signal, which is then amplified, filtered, and transmitted to the signal processing unit.

[0012] The signal processing unit is used for signal processing.

[0013] The host computer connects to the signal processing unit through a data interface, sends system parameters, displays communication content, distance measurement values ​​and time synchronization errors in real time, and triggers ranging commands and working mode switching.

[0014] The outer casing is waterproof and sealed, but light signals can pass through it.

[0015] The underwater communication and measurement system described above includes a signal processing unit comprising a communication processing module, a time synchronization module, a ranging calculation module, and a data interaction module.

[0016] The communication processing module generates a spread spectrum modulation signal carrying communication data and a timestamp and outputs it to the optical transmitting unit; it performs decoding, tracking and despreading on the received signal to recover the communication data and timestamp.

[0017] The time synchronization module parses the timestamp information of the bidirectional transmission at the local end, calculates the clock deviation between the master underwater wireless optical communication device and the slave underwater wireless optical communication device, and generates a time synchronization signal to calibrate the local clock.

[0018] The ranging calculation module combines the pseudo-random spreading code autocorrelation peak offset with the timestamp and spreading code phase offset, and refines the propagation delay measurement through frequency control parameters to calculate the precise position. The data interaction module is used for time-division multiplexing of communication, ranging, and timing data transmission channels, and interacts with the host computer to exchange communication data, distance measurement values, and timing status.

[0019] The aforementioned underwater communication and measurement system includes an optical transmitting unit comprising a light source driving circuit and a light source. The light source driving circuit receives communication, ranging, and timing signals from the signal processing unit, generates a modulation driving current, and drives the light source to convert the current into an optical signal carrying spread spectrum coding, which is then emitted in a time-division multiplexing manner.

[0020] The optical receiving unit includes a photodetector and a signal amplification circuit. The photodetector receives optical signals and converts them into electrical signals. The converted electrical signals are amplified and filtered by the signal amplification circuit and then transmitted to the signal processing unit.

[0021] The outer casing includes a housing and an optical window. The housing is a waterproof, sealed, and pressure-resistant housing made of high-strength, pressure-resistant metal material, capable of withstanding the hydrostatic pressure of the mission area; the optical window is located on the light transmission path of the housing, made of pressure-resistant glass, and integrates an optical lens to focus the light signal.

[0022] The system parameters are communication data and communication rate.

[0023] In the aforementioned underwater communication and measurement system, the light source of the optical transmitting unit is a light source array, using LEDs or LDs with wavelengths of 450nm or 520nm.

[0024] The aforementioned underwater communication and measurement system comprises an optical receiving unit consisting of a filter, a PMT detector, a transimpedance amplifier, and a bandpass filter.

[0025] The signal processing unit of the aforementioned underwater communication and measurement system employs an FPGA based on a high-performance programmable logic device, and includes a spread spectrum module, a pseudo-random code joint modulation module, and a time-to-digital converter.

[0026] The spread spectrum module is used to spread communication and ranging signals with time frames.

[0027] The pseudo-random code joint modulation module generates and manages multi-order pseudo-random codes, which are m-sequence spreading codes that are synchronously modulated with communication data, timestamps, and ranging signals. The order of the multi-order pseudo-random code can be manually adjusted.

[0028] The time-to-digital converter is used for timestamp generation and measurement, providing a time scale.

[0029] The host computer is connected to the signal processing unit via Ethernet through a watertight cable, providing parameter configuration, system monitoring, and real-time data display.

[0030] The underwater communication and measurement system described above, the signal processing unit also includes a BeiDou / GNSS clock calibration interface.

[0031] The BeiDou / GNSS clock calibration interface supports access to external clock sources from BeiDou or GPS satellite timing systems for clock discipline and calibration of the local crystal oscillator. In the absence of an external clock signal, it automatically switches to an operating mode based on the master node clock.

[0032] The time-to-digital converter uses the TDC-GPX2 chip, providing ps-level time accuracy and 4-channel measurement.

[0033] The high-performance programmable logic device is the Xilinx Zynq platform.

[0034] An underwater communication measurement method combining spread spectrum and time-to-digital conversion includes the following steps:

[0035] Step 1: System Initialization and Link Establishment

[0036] The first step is optical axis alignment: using mechanical or optical adjustments, the optical axes of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit are aligned, and link detection is performed. The received signal strength, signal-to-noise ratio, and bit error rate are monitored and compared with preset thresholds until the link quality meets the requirements for link establishment.

[0037] The second step involves the host computer sending out the initial system configuration parameters: both the host computer of the master underwater wireless optical communication unit and the host computer of the slave underwater wireless optical communication unit send out the initial system configuration parameters.

[0038] The third step is local clock initialization: the local clocks of both the master underwater wireless optical communication device and the slave underwater wireless optical communication device are initialized and self-calibrated, or connected to an external high-precision clock source for time synchronization, and the initial calibration coefficients are recorded.

[0039] Step 2, Communication and Time Synchronization

[0040] The first step is the generation and transmission of communication data frames: the master underwater wireless optical communication unit generates communication data frames according to a predetermined frame structure. The communication data frames embed local timestamps and are transmitted through the optical transmission unit of the master underwater wireless optical communication unit.

[0041] The second step is response frame generation and transmission: the underwater wireless optical communication unit receives and parses the communication data frame, extracts the valid communication data and the master's timestamp information, and then generates a response frame with its own timestamp embedded and transmits it back to the master underwater wireless optical communication unit.

[0042] The third step is local clock closed-loop correction: Based on the bidirectionally exchanged timestamp information, the master and slave underwater wireless optical communication devices jointly estimate the clock deviation and frequency offset. A dynamic error compensation algorithm is used to perform closed-loop correction of the local clock until the clock synchronization error between the master and slave underwater wireless optical communication devices reaches the preset target.

[0043] Step 3, ranging signal triggering and transmission

[0044] The first step is to issue a ranging command: the host computer of the main underwater wireless optical communication device issues a ranging command and enters the ranging working state.

[0045] The second step involves the main underwater wireless optical communication unit generating and transmitting a spread spectrum ranging signal: the main underwater wireless optical communication unit generates and transmits a spread spectrum ranging signal containing a ranging preamble, and simultaneously records the transmission time stamp t1 and the corresponding spreading code phase.

[0046] Step 4, Distance Calculation and Display

[0047] The first step is to detect and capture the preamble code from the underwater wireless optical communication device at the end: After the underwater wireless optical communication device at the end detects the preamble code, it completes the capture and tracking, and records the second timestamp t2 of the reception time.

[0048] The second step is to send the ranging signal from the underwater wireless optical communication device at the slave end: The ranging signal from the underwater wireless optical communication device at the slave end is embedded with t2 and the current third timestamp t3, and then sent back to the master underwater wireless optical communication device.

[0049] The third step is for the master underwater wireless optical communication unit to obtain the phase offset: the master underwater wireless optical communication unit receives the response signal and records the arrival time, the fourth timestamp t4, completes the code phase and correlation peak calculation, and obtains the spreading code phase offset Δτ.

[0050] The fourth step is to calculate the distance between the master node and the slave node: a bidirectional round-trip delay model is used to calculate the distance d between the master node and the slave node.

[0051] Step 5: Real-time display of ranging results and quality metrics: The host computers of both the master and slave underwater wireless optical communication units display the ranging results in real time. Quality metrics for the ranging process are generated and output, and ranging time, link status, and synchronization status are recorded.

[0052] The above-mentioned underwater communication measurement method involves continuous ranging, that is, repeating steps 3 and 4 several times.

[0053] Kalman filtering or least squares criterion algorithms are used to remove outliers in the distance, and a weighted average is performed on the valid distance measurement samples to output the distance value.

[0054] In the above-mentioned underwater communication measurement method, the spreading code parameters include order and length.

[0055] The initial configuration parameters include spreading code type and parameters, communication rate, frame structure format, master / slave roles, synchronization and ranging accuracy thresholds.

[0056] The clock synchronization error of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit is ≤1ms.

[0057] The formula for calculating the distance d between the master node and the slave node is as follows:

[0058]

[0059] In the above formula, d is the distance between the master node and the slave node, c is the equivalent propagation velocity in the water medium, and Δτ is the phase offset of the spreading code.

[0060] The quality metrics include peak signal-to-noise ratio (SNR), tracking loop residual, and synchronization residual error.

[0061] Measurement data reporting involves packaging and reporting the measurement data or archiving it.

[0062] The beneficial effects of this invention are:

[0063] An underwater communication and measurement system that combines spread spectrum and time-to-digital conversion utilizes a pseudo-random code joint signal modulation unit to fuse communication data, timestamps, and ranging codes into a single optical signal. By correcting clock deviations through dynamic error compensation, it features deep coordination of communication, ranging, and timing, significantly reducing system complexity and energy consumption. It solves the pain points of spectrum competition and spatiotemporal reference mismatch in traditional split systems, and significantly improves communication rate and ranging and timing accuracy.

[0064] An underwater communication and measurement system that combines spread spectrum and time-to-digital conversion employs a collaborative processing architecture to replace the separate design of traditional acoustic communication and inertial navigation systems. Within a high-speed optical communication link, this system can perform high-precision time interval measurement and clock calibration in a time-division multiplexing manner, significantly reducing system power consumption, effectively compressing system size, and improving integration and efficiency.

[0065] An underwater communication and measurement system that combines spread spectrum and time-to-digital conversion employs an adaptive spreading factor control unit to monitor channel attenuation intensity in real time and dynamically adjust the pseudo-random code length. This mechanism enables the system to flexibly respond to communication needs under different water quality conditions, while simultaneously balancing high-speed communication with high-precision ranging and timing functions, further enhancing the system's adaptability and stability. Attached Figure Description

[0066] Figure 1 This is a schematic diagram of the main underwater wireless optical communication device structure according to Embodiment 1 of the present invention;

[0067] Figure 2 This is a schematic diagram of the autocorrelation peak of the pseudo-random code in Embodiment 2 of the present invention;

[0068] Figure 3 This is a schematic diagram of the ranging and positioning page of the host computer software in Embodiment 2 of the present invention.

[0069] Reference numerals: 1. Optical transmitting unit, 2. Optical receiving unit, 3. Signal processing unit, 4. Host computer, 5. Housing, 51. Housing, 52. Optical window. Detailed Implementation

[0070] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0071] Example 1

[0072] An underwater communication and measurement system coordinating spread spectrum and time-to-digital conversion is disclosed. The core of the system is a processing and timing unit comprised of a programmable control unit and a high-precision time conversion device. Data communication, distance measurement, and time synchronization are coordinated on the same optical link. Spread spectrum coding and time-division multiplexing are used to transmit communication data and ranging signals. Bidirectional timestamp exchange is used to estimate and compensate for clock deviations between master and slave nodes. Combined with correlation peak interpolation and dynamic tracking, timing drift caused by underwater channel distortion is suppressed, improving the accuracy of time interval measurement. The system includes two structurally identical or similar underwater wireless optical communication units, respectively located at the master and slave nodes. The structure of the master underwater wireless optical communication unit is as follows: Figure 1 As shown, the underwater wireless optical communication device includes an optical transmitting unit 1, an optical receiving unit 2, a signal processing unit 3, a pressure-resistant structural housing 51, an optical window 52, ​​and a host computer 4;

[0073] The optical transmitting unit 1 includes a light source driving circuit and a light source. The light source driving circuit is configured to receive communication, ranging, and timing signals from the signal processing unit 3 and generate a modulation driving current to drive the light source to convert the current into an optical signal carrying spread spectrum coding and emit it in a time-division multiplexing manner.

[0074] The optical receiving unit 2 includes a photodetector and a signal amplification circuit, configured to convert the received weak light signal into an electrical signal, which is then amplified, filtered, and transmitted to the signal processing unit 3.

[0075] The signal processing unit 3 includes:

[0076] (a) Communication processing module, configured as follows:

[0077] Generate a spread spectrum modulation signal carrying communication data and a timestamp and output it to optical transmitting unit 1;

[0078] Decode, track, and despread the received signal to recover communication data and timestamps;

[0079] (b) Time synchronization module, configured as follows:

[0080] Analyze the timestamp information transmitted bidirectionally at this end to calculate the system clock offset;

[0081] Generate a time synchronization signal to calibrate the local clock;

[0082] (c) Ranging calculation module, configured as follows:

[0083] Combination such as Figure 2 The pseudo-random spreading code autocorrelation peak offset shown, combined with the timestamp and spreading code phase offset, is used to refine the propagation delay measurement through frequency control parameters, thereby calculating the precise position.

[0084] (d) Data interaction module, configured as follows:

[0085] Time-division multiplexing of communication, ranging and timing data transmission channels;

[0086] It interacts with the host computer 4 to exchange communication data, distance measurement values, and time synchronization status;

[0087] The pressure-resistant structural housing 51 is a sealed housing made of pressure-resistant material, configured to withstand the static water pressure of the mission area; the optical window 52 is made of pressure-resistant glass, set on the light transmission path of the housing 51 and integrated with an optical lens to achieve signal focusing, together forming the underwater wireless optical communication device housing 5.

[0088] The host computer 4 is connected to the signal processing unit 3 via a data interface and is configured as follows:

[0089] Send communication data and system parameters such as communication rate;

[0090] Real-time display such as Figure 3 The communication content, distance measurement value, and time synchronization error are shown.

[0091] Trigger ranging command and switch working mode;

[0092] Example 2

[0093] An underwater communication and measurement system coordinating spread spectrum and time-to-digital conversion includes two identical underwater wireless optical communication devices, installed at a master node and a slave node, respectively. Each underwater wireless optical communication device includes the following modules:

[0094] Optical Transmitting Unit 1: This unit includes a light source array and a light source driving circuit, responsible for generating and transmitting optical signals carrying communication and ranging information. In this embodiment, the wavelengths of the master and slave device light sources are physically isolated, using 450nm and 520nm blue and green light sources (such as LEDs or LDs) respectively. These light source wavelengths are set as underwater optical window wavelengths, effectively improving the propagation distance of underwater optical signals and ensuring the stability of long-distance transmission in complex underwater environments. Optical Receiving Unit 2: This unit consists of a filter, a photodetector (such as a PMT detector), a transimpedance amplifier, and a bandpass filter. It is responsible for receiving weak optical signals from the other end and converting them into electrical signals. This structure ensures high-sensitivity optical signal detection and can reliably recover signals under strong underwater light scattering and absorption conditions.

[0095] Signal Processing Unit 3: This unit utilizes an FPGA based on high-performance programmable logic devices, such as the Xilinx Zynq platform, and integrates multiple modules to achieve efficient signal processing. These include:

[0096] Spread spectrum module: Used to spread communication and ranging signals with time frames. It uses a multi-order pseudo-random code with an adjustable order according to the site conditions. The higher the order, the higher the ranging accuracy, but the lower the communication rate. It can be manually adjusted according to the water quality conditions on site.

[0097] Pseudo-random code joint modulation module: generates and manages multi-order pseudo-random codes, such as m-sequence spreading codes, and modulates them synchronously with communication data, timestamps, and ranging signals to ensure effective differentiation of signals during transmission.

[0098] Time-to-digital converter: performs high-precision timestamp generation and measurement, provides high-precision time scale for the system, ensures time synchronization and accurate distance measurement, and can use time-to-digital converters, such as the TDC-GPX2 chip, to provide ps-level time accuracy and 4-channel measurement.

[0099] BeiDou / GNSS Clock Calibration Interface (Optional): This module supports access to external high-precision clock sources, such as BeiDou and GPS satellite timing systems, to perform clock discipline and calibration on the local crystal oscillator, significantly improving the long-term timing stability and synchronization accuracy of the entire system. In the absence of an external clock signal, the system automatically switches to an operating mode based on the master node clock, ensuring continuous availability of the timing function.

[0100] Pressure-resistant structural housing 51: Made of high-strength pressure-resistant metal material, it can withstand the hydrostatic pressure at extreme water depths and together with the optical window 52, ​​it forms a waterproof and sealed housing 5, ensuring the safety and stability of the system in the underwater environment.

[0101] Optical window 52: Made of pressure-resistant glass, it has high pressure resistance and excellent light transmittance, while also having a light-focusing function, effectively focusing the incoming light signal and improving reception efficiency. Together with the pressure-resistant structural unit, it forms the outer shell 5 of the entire device, ensuring reliable operation of the device in harsh environments such as deep sea.

[0102] Host computer 4: Connected to signal processing unit 3 via Ethernet through a watertight cable, providing parameter configuration, system monitoring, and real-time data display. The software on host computer 4 features remote control, communication content viewing, distance measurement result display, and time synchronization error monitoring, supporting operator management and maintenance of the equipment.

[0103] An underwater communication measurement method that combines spread spectrum and time-to-digital conversion achieves its core function through a three-stage collaborative mechanism. Its working principle is as follows:

[0104] Communication period:

[0105] In this stage, the FPGA control signal processing unit 3 modulates the raw data acquired from the sensor using NRZ-OOK (non-return-to-zero) modulation, generating optical pulse signals that are transmitted to the underwater channel via an LED array. The optical receiving unit 2 at the receiving end captures the weak optical signal and recovers the communication data and timestamp information through filtering, amplification, and signal demodulation. This process employs spread spectrum technology, effectively improving the system's anti-interference capability, especially in complex underwater environments, ensuring accurate data transmission.

[0106] Time synchronization period:

[0107] The master and slave devices exchange timestamp information to calculate the link clock deviation and correct it through a dynamic error compensation mechanism. Based on the calculation results, the system outputs a compensation voltage to the voltage-controlled crystal oscillator to adjust the local clock, achieving millisecond-level clock synchronization. This mechanism ensures the accuracy of time synchronization among all nodes in the system, avoids synchronization errors caused by clock drift, and meets the requirements for high-precision timekeeping.

[0108] Distance measurement period:

[0109] During the ranging process, the ranging signal and the local timestamp are modulated into an m-sequence using spread spectrum technology. After underwater propagation, this signal is measured by a TDC (Transient Data Center). Figure 2 The propagation delay of the signal autocorrelation peak is shown, and a fine delay measurement is performed by combining the phase offset of the spreading code. Through frequency control parameter adjustment, the system further improves ranging accuracy and calculates the precise distance to the target position. The host computer displays the result in real time. Figure 3 The distance values ​​shown allow users to perform further operations and analyses based on the distance measurement results.

[0110] Through the coordinated operation of these three stages, the system of this invention can simultaneously achieve high-speed and reliable communication, accurate ranging, and stable timing functions in an underwater environment, and can be widely used in underwater robots, marine monitoring, and deep-sea exploration.

[0111] An underwater communication measurement method combining spread spectrum and time-to-digital conversion includes the following steps:

[0112] Step 1: System Initialization and Link Establishment

[0113] 1.1 The optical axis alignment of the master and slave ends is completed through mechanical or optical adjustment mechanisms to perform the link detection process. The system monitors indicators such as received signal strength, signal-to-noise ratio, and bit error rate, and compares them with preset thresholds to determine whether the link quality meets the link establishment requirements.

[0114] 1.2 The host computer 4 issues the initial system configuration parameters, including but not limited to: spreading code type and parameters (such as order and length), communication rate, frame structure format, master and slave roles, synchronization and ranging accuracy thresholds, etc.

[0115] 1.3 Perform local clock initialization and self-calibration, and optionally connect an external high-precision clock source for time synchronization, record the initial calibration coefficients for subsequent dynamic compensation.

[0116] Note: When the link quality is below the threshold, the system can automatically trigger an adaptive mechanism, such as by reducing the communication code rate, extending the preamble sequence length, or increasing the duty cycle of the transmitted signal, to prioritize the reliable establishment of the initial link.

[0117] Step 2, Communication and Time Synchronization

[0118] 2.1 The master end generates a communication data frame according to a predetermined frame structure. The frame embeds a local high-precision timestamp and is transmitted through optical transmission unit 1.

[0119] 2.2 The slave receives and parses the data frame, extracts the valid communication data and the master's timestamp information, and then generates a response frame with its own timestamp embedded and sends it back to the master.

[0120] 2.3 Based on the bidirectionally exchanged timestamp information, the master and slave sides jointly estimate the clock deviation and frequency offset, and use a dynamic error compensation algorithm to perform closed-loop correction on the local clock until the synchronization accuracy of the two clocks reaches the preset target, for example, the synchronization error can be configured to ≤1ms.

[0121] Step 3, ranging signal triggering and transmission

[0122] 3.1 The host computer issues a ranging command, and the main terminal enters the ranging working state.

[0123] 3.2 The master end generates and transmits a spread spectrum ranging signal containing a ranging preamble, and simultaneously records the transmission time timestamp t1 and the corresponding spreading code phase.

[0124] Step 4: Distance Calculation and Result Display

[0125] 4.1 After the preamble is detected by the slave device, the acquisition and tracking are completed, and the second timestamp t2 of the reception time is recorded.

[0126] 4.2 The slave end constructs a response ranging signal, embeds t2 and the current third timestamp t3 in it, and sends it back to the master end.

[0127] 4.3 The master end receives the response signal and records the arrival time, the fourth timestamp t4, and completes the fine calculation of the code phase and correlation peak to obtain the spreading code phase offset Δτ.

[0128] 4.4 Distance calculation is performed using a two-way round-trip time delay model:

[0129]

[0130] Where c is the equivalent propagation velocity in the water medium, which can be set on the host computer or corrected in real time according to temperature, salinity and pressure, and Δτ is the spreading code phase offset, which is a fine time compensation amount obtained by interpolating the spreading code phase / correlation peak, and is used to improve resolution and suppress code phase quantization error.

[0131] The host computer displays the current distance value.

[0132] 4.5 To suppress random errors, the system can continuously execute multiple ranging processes and employ algorithms to remove outliers, such as Kalman filtering or the least squares criterion. A weighted average of the valid measurement samples is then used to output the final distance value. Simultaneously, the system can generate and output quality metrics for this ranging process, such as the correlation peak signal-to-noise ratio (SNR), tracking loop residual, and synchronization residual error.

[0133] 4.6 The host computer displays in real time, such as Figure 3 The ranging results and quality metrics shown are displayed. The ranging time, link status and synchronization status are recorded, and the measurement data can be packaged and reported or archived.

Claims

1. An underwater communication and measurement system that combines spread spectrum and time-to-digital conversion, characterized in that, Includes a master underwater wireless optical communication unit and a slave underwater wireless optical communication unit; The master underwater wireless optical communication device is located at the master node, and the slave underwater wireless optical communication device is located at the slave node. The optical axes of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit are aligned. The main underwater wireless optical communication unit and the slave underwater wireless optical communication unit have the same structure, both including an optical transmitting unit (1), an optical receiving unit (2), a signal processing unit (3), a host computer (4), and a shell (5); The optical transmitting unit (1), optical receiving unit (2), and signal processing unit (3) are placed inside the housing (5), and the host computer (4) is placed outside the housing (5); the optical transmitting unit (1), optical receiving unit (2), and host computer (4) are respectively connected to the signal processing unit (3); The optical transmitting unit (1) receives communication, ranging, and timing signals from the signal processing unit (3), and generates and transmits optical signals carrying communication and ranging information. The optical receiving unit (2) converts the received optical signal into an electrical signal, which is then amplified, filtered, and transmitted to the signal processing unit (3). The signal processing unit (3) is used for signal processing; The host computer (4) is connected to the signal processing unit (3) through the data interface, sends out system parameters, displays communication content, distance measurement value and time synchronization error in real time, and triggers distance measurement command and working mode switching; The outer casing (5) is waterproof and sealed, and light signals can pass through the outer casing (5).

2. The underwater communication and measurement system according to claim 1, characterized in that, The signal processing unit (3) includes a communication processing module, a time synchronization module, a ranging calculation module, and a data interaction module; The communication processing module generates a spread spectrum modulation signal carrying communication data and a timestamp and outputs it to the optical transmitting unit; it performs decoding, tracking, and despreading on the received signal to recover the communication data and timestamp. The time synchronization module parses the timestamp information of the bidirectional transmission at the local end, calculates the clock deviation between the master underwater wireless optical communication device and the slave underwater wireless optical communication device, and generates a time synchronization signal to calibrate the local clock. The ranging calculation module combines the pseudo-random spreading code autocorrelation peak offset with the timestamp and spreading code phase offset, and refines the propagation delay measurement through frequency control parameters to calculate the precise position. The data interaction module is used for time-division multiplexing of the transmission channels for communication, ranging and timing data, and interacts with the host computer (4) to exchange communication data, distance measurement values ​​and timing status.

3. The underwater communication and measurement system according to claim 1, characterized in that, The optical transmitting unit (1) includes a light source driving circuit and a light source; the light source driving circuit receives communication, ranging, and timing signals from the signal processing unit (3), generates a modulation driving current, and drives the light source to convert the current into an optical signal carrying spread spectrum coding and emit it in time division. The optical receiving unit (2) includes a photodetector and a signal amplification circuit. The photodetector receives optical signals and converts the received optical signals into electrical signals. The converted electrical signals are amplified and filtered by the signal amplification circuit and transmitted to the signal processing unit (3). The outer shell (5) includes a shell (51) and an optical window (52); the shell (51) is a waterproof, sealed, and pressure-resistant shell made of high-strength pressure-resistant metal material, which can withstand the hydrostatic pressure of the mission area; the optical window (52) is set on the light transmission path of the shell (51), made of pressure-resistant glass, and integrates an optical lens to realize the focusing of light signals; The system parameters are communication data and communication rate.

4. The underwater communication and measurement system according to claim 3, characterized in that, The light source of the optical emission unit (1) is a light source array, which uses LEDs or LDs with wavelengths of 450nm and 520nm.

5. The underwater communication and measurement system according to claim 1, characterized in that, The optical receiving unit (2) consists of a filter, a PMT detector, a transimpedance amplifier, and a bandpass filter.

6. The underwater communication and measurement system according to claim 1, characterized in that, The signal processing unit (3) adopts an FPGA based on a high-performance programmable logic device, including a spread spectrum module, a pseudo-random code joint modulation module, and a time-to-digital converter. The spread spectrum module is used to spread communication and ranging signals with time frames. The pseudo-random code joint modulation module generates and manages multi-order pseudo-random codes, which are m-sequence spreading codes that are synchronously modulated with communication data, timestamps, and ranging signals; the order of the multi-order pseudo-random codes can be manually adjusted. The time-to-digital conversion device is used for timestamp generation and measurement, and provides a time scale; The host computer (4) is connected to the signal processing unit via Ethernet through a watertight cable, providing parameter configuration, system monitoring and real-time data display.

7. The underwater communication and measurement system according to claim 6, characterized in that, The signal processing unit (3) also includes a BeiDou / GNSS clock calibration interface; The BeiDou / GNSS clock calibration interface supports access to external clock sources from BeiDou or GPS satellite timing systems to perform clock discipline and calibration on the local crystal oscillator. In the absence of an external clock signal, it automatically switches to an operating mode based on the master node clock. The time-to-digital converter uses the TDC-GPX2 chip, providing ps-level time accuracy and 4-channel measurement; the high-performance programmable logic device is the Xilinx Zynq platform.

8. A method for underwater communication measurement using spread spectrum and time-to-digital conversion (TDC) coordination, employing the underwater communication measurement system using spread spectrum and TDC coordination as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1, System Initialization and Link Establishment: The first step is optical axis alignment: mechanical or optical adjustment is used to align the optical axes of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit, and link detection is performed; the received signal strength, signal-to-noise ratio and bit error rate are monitored and compared with preset thresholds until the link quality meets the link establishment requirements; The second step is for the host computer to send the initial configuration parameters of the system: both the host computer (4) of the master underwater wireless optical communication device and the host computer (4) of the slave underwater wireless optical communication device send the initial configuration parameters of the system. The third step is local clock initialization: the local clocks of the master underwater wireless optical communication device and the slave underwater wireless optical communication device are both initialized and self-calibrated, or connected to an external high-precision clock source for time synchronization, and the initial calibration coefficients are recorded. Step 2, Communication and Time Synchronization: Step 1, communication data frame generation and transmission: The master underwater wireless optical communication unit generates communication data frames according to a predetermined frame structure. The communication data frames embed local timestamps and are transmitted through the optical transmission unit (1) of the master underwater wireless optical communication unit. The second step is response frame generation and transmission: the underwater wireless optical communication device at the slave end receives and parses the communication data frame, extracts the valid communication data and the master end timestamp information, and then generates a response frame with its own timestamp embedded and transmits it back to the master end underwater wireless optical communication device. The third step is local clock closed-loop correction: the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit jointly estimate the clock deviation and frequency offset based on the bidirectionally exchanged timestamp information; a dynamic error compensation algorithm is used to perform closed-loop correction of the local clock until the clock synchronization error of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit reaches the preset target. Step 3, ranging signal triggering and transmission: The first step is to issue a ranging command: the host computer (4) of the main underwater wireless optical communication device issues a ranging command and enters the ranging working state; The second step is for the main underwater wireless optical communication unit to generate and transmit spread spectrum ranging signals: the main underwater wireless optical communication unit generates and transmits spread spectrum ranging signals containing ranging preambles, and records the transmission time timestamp t1 and the corresponding spreading code phase. Step 4, Distance Calculation and Display: The first step is to detect and capture the preamble code from the underwater wireless optical communication device at the end: After the underwater wireless optical communication device at the end detects the preamble code, it completes the capture and tracking, and records the second timestamp t2 of the reception time; The second step is to transmit the ranging signal from the underwater wireless optical communication device at the slave end: the ranging signal from the underwater wireless optical communication device at the slave end is embedded with t2 and the current third timestamp t3, and then transmitted back to the master underwater wireless optical communication device. The third step is for the main underwater wireless optical communication unit to obtain the phase offset: the main underwater wireless optical communication unit receives the response signal and records the arrival time fourth timestamp t4, completes the code phase and correlation peak calculation, and obtains the spreading code phase offset Δτ. The fourth step is to calculate the distance between the master node and the slave node: using a bidirectional round-trip delay model, the distance d between the master node and the slave node is calculated. Fifth step, real-time display of ranging results and quality measurement indicators: The host computer (4) of the master underwater wireless optical communication device and the host computer (4) of the slave underwater wireless optical communication device both display the ranging results in real time; Generate and output quality metrics for the ranging process, and record ranging time, link status, and synchronization status.

9. The underwater communication measurement method according to claim 8, characterized in that, Continuous ranging means repeating steps 3 and 4 several times in succession; Kalman filtering or least squares criterion algorithms are used to remove outliers in the distance, and a weighted average is performed on the valid distance measurement samples to output the distance value.

10. The underwater communication measurement method according to claim 8, characterized in that, The spreading code parameters include order and length; The initial configuration parameters include spreading code type and parameters, communication rate, frame structure format, master / slave roles, synchronization and ranging accuracy thresholds; The clock synchronization error of the master underwater wireless optical communication unit and the slave underwater wireless optical communication unit is ≤1ms; The formula for calculating the distance d between the master node and the slave node is as follows: In the above formula, d is the distance between the master node and the slave node, c is the equivalent propagation velocity in the water medium, and Δτ is the phase offset of the spreading code. The quality metrics include peak signal-to-noise ratio (SNR), tracking loop residual, and synchronization residual error. Measurement data reporting involves packaging and reporting the measurement data or archiving it.

Images