Underwater image transmission method based on inductive coupling anchor mooring chain

Abstract

On the basis of the inductive coupling anchor mooring chain of an overwater buoy fixed-point observation platform, the underwater image transmission method based on the marine inductive coupling anchor mooring chain is provided, the signal transmission frequency is relatively low due to the influence of the marine water body environment and the modulation modes of low-bandwidth ASK and the like, and the signal transmission efficiency is high. And the requirements of big data and high-speed transmission of marine environment images, videos and the like cannot be met. According to the invention, the UDP protocol and the OFDM technology are combined, the image data are segmented and packaged and are transmitted to the sending node by using the UDP protocol, the image data are coded by using a simplified data frame, the transmission bandwidth utilization rate is improved by using the OFDM modulation technology, and the MAC address is added to each underwater node, so that the reliability of data transmission among multiple nodes is enhanced; the optimal image transmission quality is determined under the conditions that the mapping mode is 16QAM, the coding rate is 1 / 2 and the OFDM emission center frequency is 800kHz, and technical reference is provided for solving the underwater image transmission technology.

Description

Technical Field

[0001] The invention is based on the inductive coupling anchor chain of an above-water buoy fixed-point observation platform and proposes an underwater image transmission method based on the inductive coupling anchor chain. Background Art

[0002] In marine ecological monitoring, underwater images can truly reflect the underwater natural environment and biological community conditions and their changes, among which long-distance, high-speed underwater communication is an indispensable part. At present, underwater communication is traditionally achieved through acoustic, radio frequency and optical channels. The performance of acoustic communication is limited by low bandwidth, high transmission loss, time-varying multipath propagation, high delay and Doppler spread. Currently available underwater acoustic communication can support data rates of up to tens of kbps and up to hundreds of kbps for short distances (several meters). However, its propagation rate is greatly affected by underwater noise, and factors such as seawater salinity, temperature, and pressure will also limit its propagation rate. The transmission of electromagnetic wave signals in seawater and air is very different. Due to the high conductivity of seawater, the propagation range of high-frequency electromagnetic waves is limited by fundamental wave attenuation and noise factors. When the operating frequency is 2.4GHz, the electromagnetic wave attenuates by about 1600dB / m in seawater. In this case, only short-distance communication can be achieved. Underwater optical wireless communication (UOWC) has the ability to increase the data transmission rate to Gbps or even Mbps within a range of 100 meters. However, due to the propagation characteristics of light, it is easily affected by underwater suspended particles and marine plankton, which will limit the realization of underwater long-distance communication. Although radio frequency electromagnetic waves or UOWC can use mobile platforms such as autonomous underwater vehicles (AUVs) to increase the transmission distance, it is necessary to solve complex problems such as the watertightness of cables or optical cables, and the position cannot be changed at will, which greatly reduces the number of nodes.

[0003] Underwater inductive coupling transmission technology uses seawater and anchor cables of fixed observation platforms as transmission media to achieve long-distance real-time transmission of underwater observation data. The transmission distance can reach more than several kilometers, and it has the advantages of long-term, continuous, and all-weather automatic observation. In recent years, research on underwater inductive coupling transmission technology has been continuously promoted. In 1997, a drag-and-drop CTD inductive coupling chain was developed to achieve mixed transmission of data and energy. Since energy and data transmission use time-division multiplexing to share channels, the data transmission rate is only 9600bps. The dual energy system for non-contact power supply and data transmission of moored buoy underwater sensors meets the two-way signal transmission between underwater sensors and surface buoys, and its signal transmission rate is 9600bps. Using a two-terminal network model with compensation capacitors, DPSK modulation is used at the resonant frequency to enable the underwater non-contact monitoring system to achieve a transmission rate of 1200bps in a 1000-meter deep sea test.

[0004] At present, the data transmission rate based on inductive coupling is low, and the channel modulation is mostly single-carrier mode, which cannot meet the needs of high-speed transmission of big data such as images and videos in multi-dimensional monitoring of marine environment. At present, there is a lack of relevant research on image transmission using inductive coupling transmission structure, and there is no report on the transmission mode combining multi-carrier modulation technology OFDM and MAC frame in inductive coupling transmission structure. Therefore, an underwater image transmission method based on marine inductive coupling anchor chain is proposed, and the optimal data transmission parameters are explored, and the signal transmission center frequency, signal bandwidth, data modulation mapping method and coding mode are determined, so as to provide a reference for underwater image transmission technology based on inductive coupling anchor chain. Summary of the invention

[0005] The main purpose of the present invention is to solve the image data transmission rate problem of multiple underwater node users of the marine inductive coupling anchor chain. The present invention establishes an inductive coupling anchor chain structure for image transmission and proposes an OFDM+UDP technology-based method to split and package the image data of each user into data blocks, which are transmitted to the sending node using the UDP protocol. The image data is encoded at the sending node to form an image transmission data frame, which is modulated and up-converted to the selected carrier using the OFDM technology. The transmission signal transmits the image to the receiving end through the inductive coupling anchor chain. The inductive coupling principle is used at the receiving end to collect the sent data, and the data is demodulated into image data blocks. The image data is restored to the original image through a splicing algorithm. The invention can increase the image transmission rate in the inductive coupling anchor chain to 0.84Mbit / s.

[0006] The technical solution of the present invention:

[0007] According to the characteristics of the seawater channel, the present invention first establishes an inductively coupled anchor chain structure for underwater image transmission, and uses UDP technology combined with OFDM technology to realize image transmission on the inductively coupled channel. First, an image segmentation algorithm is used to segment the original image into the maximum transmittable size, and then the UDP protocol is used to transmit the image from the acquisition device to the sending device. The OFDM technology is used at the sending device to perform transmission work. First, a simplified transmission frame is constructed and the length of each part of the frame is determined, and the image data is placed in the frame. Then, mapping coding is selected, and OFDM modulation parameters, channel estimation algorithms, and OFDM transmission parameters are determined according to the current bandwidth of the inductively coupled channel. OFDM demodulation and image splicing are completed in the receiving device, and OFDM technology is combined with UDP technology, which greatly improves the transmission rate of image data. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 The structure diagram of underwater image transmission based on inductively coupled anchor chain;

[0009] Figure 2Figure 1. Method for splitting and combining transmission images of inductive coupling channels for ocean use;

[0010] Figure 3 The original image sent;

[0011] Figure 4 Received image. DETAILED DESCRIPTION

[0012] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0013] Step 1: Image transmission method based on inductive coupling anchor chain

[0014] The inductive coupling transmission structure designed by the present invention is as follows Figure 1 As shown in the figure, the inductive coupling anchor chain used is composed of a transmitting magnetic ring, a receiving magnetic ring and a steel cable, wherein the transmitting magnetic ring and the receiving magnetic ring are both made of manganese-zinc ferrite, and the dimensions are 38.16mm in outer diameter, 18.18mm in inner diameter and 13.58mm in thickness; 6 turns of insulating cable are wound around the transmitting magnetic ring and the receiving magnetic ring, and the winding diameter is 2.7mm. The steel cable passes through the transmitting magnetic ring and the receiving magnetic ring, and the two ends of the steel cable are immersed in seawater respectively, thus forming a closed transmission channel for underwater images; the collected underwater images are segmented and modulated into OFDM symbols, and the OFDM symbols are sent out by the sending device through the transmitting magnetic ring, coupled to the receiving magnetic ring through the closed transmission channel of the inductive coupling anchor chain, and the receiving device completes the collection, demodulation and image stitching tasks.

[0015] Step 2: Image splitting and packaging

[0016] In order to facilitate underwater image acquisition and transmission, the transmitted image format uses the JPEG format with a very high compression rate, and the storage occupancy of this format file is relatively small. Considering the balance between the effectiveness and efficiency of data transmission, due to the simple frame structure used in the present invention, the maximum number of bytes that can be carried is 1500 bytes, so the picture is split and numbered before transmission. The image uses a color RGB image with a pixel size of 1920*1080 and a horizontal and vertical resolution of 96dpi, with a total size of 502334bytes. The splitting and reorganization method adopted by the present invention is to divide the original experimental image into 1620 sub-images according to the size of 128*10, thereby ensuring that the storage occupancy of each sub-image is less than 1500 bytes, and the sub-images after splitting are numbered, and the sub-image data are transmitted to the modulation end of the sending device of the underwater node in sequence using the UDP protocol with low transmission delay and no need to wait for confirmation, and the subsequent data frame construction and modulation and sending tasks are performed.

[0017] Step 3: Construct image transmission frame

[0018] In order to ensure the systematic and reliable data transmission between the above-water host and multiple underwater image information acquisition nodes, the present invention constructs a relatively complete set of simple frame structures for inductively coupled underwater image data transmission, and assigns a unique MAC address to each above-water and underwater node. The overall structure of the data frame is as follows: Figure 2 As shown in (b), it consists of two parts: the preamble and the data. The preamble consists of the short training field STF, the long training field LTF and the signal information field SIGNAL. The data code consists of the MAC frame and is used to carry image data and routing information. The structure of the MAC frame is as follows: Figure 2 As shown in (a), the routing information is carried by the frame header composed of BSS, SA, and DA. BSS represents the identification address of an inductively coupled transmission system as a whole, SA is the MAC address of the data sending device, and DA is the MAC address of the data receiving device. The above three addresses are all composed of 6-byte hexadecimal numbers, thereby achieving accurate data collection and command transmission between the surface host and each underwater node. FrameBody is used to carry image data, and can transmit up to 1500 bytes of image data at a time. FCS is a frame check sequence, which uses CRC check to check the integrity of the MAC frame at the receiving end. After the MAC frame is encapsulated, the data transmission stage is carried out.

[0019] The short training field in the preamble is used for coarse time and frequency synchronization at the receiving end, and the long training field is used for fine time synchronization and channel estimation. Both are training sequences. The signal information field is used to record the data length of the entire MAC frame, the convolutional coding and mapping modulation used. Its structure diagram is shown in the figure below. Figure 2 As shown in (c), the Rate field is used to record the convolutional coding and mapping modulation scheme used by the MAC frame, the Length field is used to record the data length in the current transmission frame, and the Tail is used for zero calibration of the convolutional coding, setting logic 0. The Reserved and Parity bytes are used for even parity check. The valid information contained in the signal information field is used as a label to annotate the sample stream after passing the parity check, carrying the coding mapping modulation information and the MAC frame length for subsequent decoding.

[0020] Step 4: OFDM modulation parameter selection

[0021] The present invention uses OFDM modulation to transmit images, and determines that each conventional OFDM symbol is composed of 64 subcarriers, of which 48 subcarriers carry data, 4 carriers carry pilot signals, and the center carrier and the remaining carriers are zeroed. The carrier spacing of each short OFDM symbol is 4 times that of the conventional OFDM symbol. The short training sequence is composed of 10 repeated short OFDM symbols. From the frequency domain, the short training sequence occupies 12 subcarriers among 52 non-zero subcarriers in each OFDM symbol, and the carrier spacing is 4 times that of the conventional OFDM symbol. The subcarrier sequence number is: {-24, -20, -16, -12, -8, -4, 4, 8, 12, 16, 20, 24}. The long training sequence includes two conventional OFDM symbols and a long protection interval. From the frequency domain, the OFDM symbol of the long training sequence is the same as the regular OFDM symbol, consisting of 53 subcarriers, with the DC carrier being 0, occupying the subcarrier numbers from -26 to 26, and the remaining carriers being zeroed. The signal field SIGNAL consists of a regular OFDM symbol.

[0022] Step 5: Channel Estimation Algorithm

[0023] According to the characteristics of high transmission stability, small delay, and no Doppler effect in seawater medium, the present invention regards the inductively coupled seawater channel as a slow fading channel. Therefore, the pilot-based LS channel estimation algorithm suitable for the slow fading channel is selected to compensate the phase offset of the received data of the system. LS estimation is to calculate the optimal value of the estimated data by minimizing the least squares error between the estimated data and the real data. The pilot signal is inserted before the transmitter performs IFFT, and the pilot signal is separated after the receiver performs FFT. The receiving end uses the LS channel estimation algorithm to estimate the frequency response of each subchannel based on the received pilot, and compensates the received frequency domain signal according to the frequency response.

[0024] Step 6: OFDM transmission parameter selection

[0025] The present invention first combines the characteristics of the marine inductive coupling channel and uses the following parameters for image transmission. The bandwidth range of the transmitted signal is selected within 300kHz-700kHz, and the channel coding uses convolution coding and interleaving coding, wherein the convolution coding uses a coding method with a code rate of 1 / 2, the mapping modulation method is 16QAM, and the OFDM signal is up-converted to a center frequency of 800kHz, and then sent using a transmitting magnetic ring. In this mode, the image quality is highest when the transmission rate is 0.51Mbit / s, and the fastest transmission speed is 0.84Mbit / s, which can meet the needs in different scenarios.

[0026] Step 7: Demodulation and image stitching at the receiving end

[0027] After the signal is collected by the receiving magnetic ring, it is down-converted in the receiving device to restore the received signal to a baseband signal and complete OFDM demodulation, thereby obtaining a sub-image of the sent image data. The images are spliced ​​in sequence according to the order of the received sub-images to obtain a complete image that was sent.

[0028] Step 8: Feasibility verification of the experimental platform based on USRP

[0029] In order to simulate the working process of the marine inductive coupling anchor chain, the present invention built an experimental platform based on the inductive coupling theoretical model. The underwater image transmission method based on the marine inductive coupling anchor chain proposed by the present invention was verified, and the optimal parameters of the OFDM algorithm that meet the underwater image transmission quality were determined.

[0030] Step 9: Comparison of actual image transmission effects

[0031] The present invention combines OFDM+UDP technology to provide a new method for underwater image transmission technology based on marine inductive coupling anchor chain. By comparing the actual image transmission at various bandwidths under the 16QAM 1 / 2 800kHz transmission center frequency with the original image, it can be found that the image transmission effect is best when the bandwidth is 423kHz, and the image transmission quality is second at 400kHz, 450kHz, and 700kHz bandwidths. The image quality at 200kHz is the worst. The original image is Figure 3 As shown, the 700kHz bandwidth transmission image is as follows Figure 4 Therefore, when the bandwidth range is 300kHz-700kHz, the maximum number of sub-image information lost is 5, that is, the packet loss rate is less than 0.3086%, which can be used for underwater image information acquisition and transmission, laying the foundation for future underwater massive data transmission.

Claims

1. A method for underwater image transmission based on inductively coupled anchor chain, characterized in that: include: Image transmission method, image segmentation method, image transmission data frame design and OFDM symbol design based on inductively coupled anchor tether; Step 1: Image transmission method based on inductive coupling anchor chain The inductive coupling anchor chain used is composed of a transmitting magnetic ring, a receiving magnetic ring, and a steel cable. The transmitting magnetic ring and the receiving magnetic ring are both made of manganese-zinc ferrite, with dimensions of 38.16mm in outer diameter, 18.18mm in inner diameter, and 13.58mm in thickness. Six turns of insulating cable are wound around the transmitting magnetic ring and the receiving magnetic ring, with a winding diameter of 2.7mm. The steel cable passes through the transmitting magnetic ring and the receiving magnetic ring, and both ends of the steel cable are immersed in seawater, thus forming a closed transmission channel for underwater images. After the collected underwater images are segmented, they are modulated into OFDM symbols, and the OFDM symbols are sent out by the transmitting device through the transmitting magnetic ring, coupled to the receiving magnetic ring through the closed transmission channel of the inductive coupling anchor chain, and the receiving device completes the collection, demodulation and image stitching tasks. Step 2: Image segmentation method The underwater images transmitted are all in JPEG format. The images are segmented into 128*10 sizes to ensure that the storage capacity of all segmented sub-images is less than 1500 bytes. At the same time, all sub-images are numbered from left to right and from top to bottom. All sub-images are transmitted to the sending device using the UDP protocol. The sub-image transmission order is output in the order of segmentation numbers. Step 3: Image transmission data frame design The data frame of image transmission consists of two parts: a preamble and a data code. The preamble consists of a short training field STF, a long training field LTF and a signal field SIGNAL. The data code consists of a MAC frame and is used to carry image data and routing information. At the same time, a unique MAC address is assigned to the surface host and multiple underwater image acquisition nodes. The routing information is located in the frame header of the MAC address and consists of three parts: BSS, SA and DA. Among them, BSS represents the MAC address of the inductive coupling anchor chain used, SA is the MAC address of the sending device, and DA is the MAC address of the receiving device. The above three addresses are all composed of hexadecimal numbers with a length of 6 bytes. Step 4: OFDM symbol design Each conventional OFDM symbol consists of 64 subcarriers, of which 48 subcarriers carry data, 4 subcarriers carry pilot signals, the center carrier and the remaining carriers are set to zero, the carrier spacing of each short OFDM symbol is 4 times that of the conventional OFDM symbol, the short training field STF consists of 10 repeated short OFDM symbols, the long training field LTF includes two conventional OFDM symbols and a long guard interval, the signal field SIGNAL consists of a conventional OFDM symbol, the bandwidth range is selected as 300kHz-700kHz, the channel coding uses convolution coding and interleaving coding, where the convolution coding uses a coding method with a code rate of 1 / 2, the mapping modulation method is 16QAM, and the OFDM signal is up-converted to a center frequency of 800kHz.

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