Study on the inductively coupled high rate system and long-distance transmission

The inductively coupled transmission chain can carry multiple Marine sensors, which is an important means to obtain Marine data, but the current transmission rate of inductively coupled data is low, which can not meet the needs of future Marine data transmission. The induction coupling system designed in this paper adopts the structure of double magnetic loop and a double-winding, and adopts the new single frequency different code algorithm to realize the data transmission rate of 150kbps. Different water environments were set up in the laboratory for testing, and the lower bit error rate was found in the water environment with the ion concentration of 10%-35%, which met the requirements of inductive coupling data transmission in a multi-water environment. It is of great significance to establish the research of underwater inductively coupled long-distance transmission for the subsequent design of a new inductively coupled transmission chain and the improvement of the data transmission rate.


Introduction
The development of Marine science cannot be separated from Marine monitoring technology.In underwater data transmission, underwater acoustic transmission is not suitable for long-distance transmission environments due to the easy attenuation and interference of sound waves [1], while cable transmission needs to consider the problems of water tightness.The inductively coupled transmission chain is composed of a magnetic ring and a mooring cable.The mooring cable passes through the magnetic ring to realize data transmission, this method does not need to consider the problem of watertightness, and is suitable for a variety of complex and harsh sea conditions.It can form a vertical structure of underwater multi-acquisition nodes by carrying a variety of Marine sensors on a mooring cable [2].
At present, the data transmission rate of the existing inductive coupling system is 9600bps.As the number of mounted underwater sensors increases, the data volume becomes larger, and higher rates are required for data transmission.To achieve higher data transmission rates, higher frequency signals are needed to build [3] , [4].The following work is done in this paper: the finite element simulation software is used to simulate the magnetic ring, and the single frequency different code algorithm is designed according to the working frequency of the magnetic ring for signal transmission.The double-winding structure is designed to meet the needs of signal sending and receiving.Simulate different water environments in the laboratory and analyze the corresponding bit error rate of data.Study the data condition of the system during long-distance transmission.The results show that the system has a low bit error rate and achieves a data transmission rate of 150kbps in a multi-water environment.

Circuit principle
The induction-coupled chain is mainly composed of a mooring cable and magnetic ring, and the seawater and mooring cable form a signal loop, which is equivalent to a coil with 1 turn.Figure 1 shows the schematic diagram of the coupled transmission system and the equivalent circuit model [5].

Figure 1. Transmission schematic diagram and equivalent circuit model
The voltage signal Ei is input into the sending magnetic loop, the mooring cable and the seawater to generate the current in a single-turn loop.Eo electromotive force is generated when the current passes through the receiving magnetic ring.The calculation process is as follows: (2-3) Among them, 1 is the primary winding inductance,  2 is the single-turn loop inductance, and  3 is the secondary winding inductance. is the resistance composed of the transmission cable and seawater.According to the formula (2-1)-(2-3), the output voltage Eo can be derived as: According to the formula (2-4), the magnitude of the induced electromotive force Eo is related to the signal frequency, the inductance ratio, and the resistance formed by the seawater and the mooring cable.In the case of constant resistance, the induced electromotive force Eo is mainly related to the ratio of signal frequency and inductance.

Principle of algorithm
Generally, the operating frequency range of the magnetic ring is small.To use the operating frequency signal to represent and transmit code element data more accurately, a new single frequency different code algorithm is adopted in this paper.Sinusoidal waveforms with 2/F cycle time and 1/F cycle time are sent respectively to represent code elements 1 and 0, and F is the signal frequency.As shown in Figure 2. The higher the frequency of the sine wave, although it can meet the transmission requirements, it will cause insufficient utilization of resources.The lower the frequency, the wrong representation of the symbol signal will be caused, and the correct data transmission cannot be achieved.The algorithm requires that the following formula be satisfied between the signal frequency F and the data transmission rate N.

Magnetic simulation
At the operating frequency, the magnetic permeability of the magnetic ring is larger and the loss in the transmission link is lower, and the inductance of the magnetic ring is larger.It is necessary to divide the magnetic ring into two halves for installation, but the air gap between the magnetic ring will cause the inductance to decrease.
To explore the effect of the air gap structure on the magnetic ring, a nanocrystalline magnetic ring with a size of 30*15*20(mm) and several turns of 10 was selected for simulation.The result is shown in Figure 3.The figure above shows that the magnetic induction intensity gradually decreases from inside the ring to outside the ring, and the magnetic induction intensity at the gap is larger, indicating that there is magnetic leakage.With the increase of the air gap distance, the inductance of the magnetic ring decreases gradually, which seriously affects the transmission performance of the magnetic ring.

Experimental test
The test platform is shown in Figure 4, including the signal generator (RIGOL DG2102), LCR bridge (TH2827A), oscilloscope (Tektronix TBS2000B) and constant temperature water tank (RKT-302).The measured operating frequencies of Mn-Zn ferrite, Permalloy, and nanocrystalline magnetic ring are 600kHz~720kHz, 1kHz~20kHz and 10kHz~200kHz, respectively.When the required data transmission rate is 150kbps, it indicates that the transmission time of one bit is 6.7μs, and according to formula (2-5), the signal frequency must meet 540kHz~750kHz.Finally, a manganese zinc ferrite magnetic ring and 700kHz sine wave signal were selected to construct the algorithm.
To verify the influence of the air gap structure on data transmission, the Mn-Zn ferrite magnetic ring was cut into two halves along the middle.The signal generator generated sinusoidal signals of 5V and 700kHz and measured the inductance of the magnetic ring and the received signal amplitude.The results are shown in the table 1: Table 1 The relationship between air gap and inductance and amplitude As can be seen from the above table, the magnetic ring air gap will cause attenuation of the transmitted signal and inductance.When the air gap increases to 1mm, the received signal attenuates by 73.46% compared with the seamless magnetic ring.The inductance is attenuated by 87.85%.The above conclusions are consistent with the simulation results.

Double-winding structure design
Due to the half-duplex nature of the inductive coupling transmission, the above-water platform and the underwater node complete the data-sending and receiving tasks respectively at the same time.At present, the double magnetic loop structure can be used in two ways: single-winding and double-winding.The structure principle diagram is shown in Figure 6:  The signals received by the single-winding structure and the double-winding are 296mV and 735mV respectively.Since the signal amplitude is related to the inductance ratio, the inductance of the receiving end of the magnetic ring is greater than that of the transmitting end, and the receiving end winding is required to be more than the transmitting end winding when the magnetic ring parameters are the same.
The above results show that the single-winding structure of the magnetic ring sharing the same winding can not satisfy the transmission characteristics of inductive coupling.The double-winding method is more appropriate for the transformer structure of coupled transmission.The double-winding structure will be used in the final design.

Transmission algorithm design
The algorithm uses a 700kHz sine wave signal to design, and the corresponding 10011100 code element signal waveform and the output signal after the receiving end analyzes the waveform are shown in Figure 7.

Figure 7. Code signal waveform
The figure above shows that the algorithm designed in this paper can be correctly analyzed after transmission by mooring cable to complete data transmission.

Bit error rate analysis
The water environment with different ion concentrations is set up in the constant temperature tank.Using the magnetic ring and mooring cable selected above, immerse both ends of the mooring cable in the solution.Measure the data reception of the floating platform.Figure 8 shows the bit-error rate results of data received by the system under different water environments.

Figure 8. Bit error rate results of multi-waters
The figure shows that The ion concentration is related to the bit error rate of data .When the ion concentration is 10%, the bit error rate of the system is 0.001, and when the ion concentration increases to 15%-35%, the bit error rate is 0. This is because the higher the ion concentration, the lower the resistance of water, and the less attenuation the signal is subjected to during transmission.The appeal results show that the system can meet the transmission requirements of each water area, and effectively improve the reliability of inductively coupled channel transmission.

Long-distance transmission study
With the development of underwater inductive coupling technology, long-distance transmission is an important direction for the development of inductive coupling systems [6] , [7] , [8].To study the transmission of inductive coupling systems at different transmission distances, an experimental environment was set up to study the data transmission of inductive coupling systems when the length of mooring cables was 5m-100m.Both ends of the mooring cables were immersed in seawater, and the obtained data curves were fitted the results are shown in Figure 9: The above experiments investigated the relationship between the length of the mooring cable and the received signal under the excitation of 20V.As can be seen from the figure above, as the length of the mooring cable increases, the signal gradually becomes smaller.This is due to the increase in length, the resistance of the mooring cable and the sea water becomes larger, and the signal attenuation is greater.Therefore, when the long-distance transmission is carried out, to ensure that the receiving end can correctly analyze the waveform, the signal needs to be sent through the necessary processing circuit.

Conclusion
This paper analyzes the principle of induction-coupled transmission, establishes the double-winding structure, and designs a new single frequency different code transmission algorithm, which encodes and sends the data with the typical operating frequency signal of the magnetic ring.The algorithm adopts the frequency signal of 700kHz to achieve a transmission rate of 150kbps.The system has been tested in different water environments, and long-distance underwater transmission has been studied.The experiment shows that the system is suitable for the transmission environment below 100 meters, and the data transmission rate of 150kbps is achieved successfully, and the reliability of the inductive coupling system is improved.

Figure 3 .
(a) Magnetic induction diagram (b) Air gap and inductance diagram

Figure 4 .
Figure 4. Experimental test platform Nanocrystalline, Permalloy, and Mn-Zn ferrite magnetic rings with sizes of 30*15*20(mm) were selected to study the effects of different materials on the performance of signal transmission.The mooring cable length is 20m.The number of turns of the magnetic ring at the sending end and the receiving end is 10 turns and 30 turns respectively, and the sine wave signal of 5V and 1kHz~1MHz is excited.The amplitude-frequency characteristic curve is shown in Figure 5.

Figure 5 .
Figure 5. Frequency characteristic diagramThe measured operating frequencies of Mn-Zn ferrite, Permalloy, and nanocrystalline magnetic ring are 600kHz~720kHz, 1kHz~20kHz and 10kHz~200kHz, respectively.When the required data transmission rate is 150kbps, it indicates that the transmission time of one bit is 6.7μs, and according to formula (2-5), the signal frequency must meet 540kHz~750kHz.Finally, a manganese zinc ferrite magnetic ring and 700kHz sine wave signal were selected to construct the algorithm.To verify the influence of the air gap structure on data transmission, the Mn-Zn ferrite magnetic ring was cut into two halves along the middle.The signal generator generated sinusoidal signals of 5V and 700kHz and measured the inductance of the magnetic ring and the received signal amplitude.The results are shown in the table 1:Table1The relationship between air gap and inductance and amplitude

Figure 6 .
Figure 6.(a) Single-winding structure (b) Double-winding structure In the figure above, (a) indicates the single-winding structure of the magnetic ring, receiving and sending share the same winding, (b) indicates the double-winding structure of the magnetic ring, receiving and sending share different windings, the input excitation is 5V signal, and the resistance of 20m plastic-coated steel cable is 2.3Ω.The measurement results are shown in the table 2:

Figure 9 .
Figure 9. Cable length and signal conditionThe above experiments investigated the relationship between the length of the mooring cable and the received signal under the excitation of 20V.As can be seen from the figure above, as the length of the mooring cable increases, the signal gradually becomes smaller.This is due to the increase in length, the resistance of the mooring cable and the sea water becomes larger, and the signal attenuation is greater.Therefore, when the long-distance transmission is carried out, to ensure that the receiving end can correctly analyze the waveform, the signal needs to be sent through the necessary processing circuit.

Table 2
Relation between winding and signal