Abstract: Any information needs to be transmitted through sound, light, and electrical signals. Due to the severe attenuation of light and electrical signals in seawater, sound waves are the only known energy form that can propagate over long distances in water. Therefore, in recent years, research and development of underwater acoustic communication systems in the ocean have become a hot topic. Underwater acoustic communication refers to a communication system that uses underwater acoustic channels to transmit data between both parties. The composition of underwater acoustic communication systems is very similar to that of traditional wireless communication systems. However, underwater acoustic communication systems convert electrical signals into acoustic signals, and the acoustic signals carrying information propagate in the water to complete the system’s data transmission

Any information needs to be transmitted through sound, light, and electrical signals. Due to the severe attenuation of light and electrical signals in seawater, and the fact that sound waves are the only known energy form that can propagate over long distances in water, research and development of underwater acoustic communication systems in the ocean have become a hot topic in recent years. Underwater acoustic communication refers to a communication system that uses underwater acoustic channels to transmit data between two parties. The composition of underwater acoustic communication systems is very similar to that of traditional wireless communication systems. However, underwater acoustic communication systems convert electrical signals into sound signals, and the sound signals carrying information propagate in the water to complete the data transmission of the system.

The overall structure of an underwater acoustic communication system

The design diagram of a CDMA based underwater acoustic communication modulation/demodulation system is shown in Figure 1. The entire design system mainly realizes signal CDMA modulation/demodulation, controls DAC and ADC for digital acquisition, and analog-to-digital conversion and analog-to-digital conversion are implemented by dedicated integrated chips. The function of a power amplifier is to amplify modulated signals, and signal amplification and conditioning are the inverse process of power amplification; The transmitting underwater acoustic transducer converts the electromagnetic energy generated by the amplifier into sound energy, while the receiving underwater acoustic transducer converts the received sound signal into an electrical signal.

 

Figure 1 Basic model of underwater acoustic communication system

The schematic diagram of the designed underwater acoustic communication system circuit is shown in Figure 2. The main control chip of the system is EP3C10E144C8N from Altera’s Cyclone III series, which mainly includes communication module, spread spectrum module, BPSK modulation module, and corresponding demodulation module; The peripheral circuits include the power supply circuit of the entire system, ADS7800 chip for A/D conversion, TY5639 chip for D/A conversion, crystal oscillator circuit for providing clock signals for the entire system, 74HC245A level conversion chip for compatibility between TTL level and CMOS level, JTAG interface for burning target programs, and data transmission circuit.

 

Figure 2 Circuit Design Block Diagram of Underwater Acoustic Communication System

The working process of the system: Firstly, the upper computer simulates the transmitting end and sends the digital signal to the FPGA chip through the serial port. The communication module receives the digital information and sequentially transmits it to the spread spectrum module BPSK modulation module. At this point, the received digital information is modulated and the generated signal is converted into an analog electrical signal through a D/A converter. Then, the electrical signal is converted into an acoustic signal through an underwater acoustic transducer and sent out, carrying the acoustic signal sent by the sender for propagation in the underwater environment. Next is the receiving end, which also has an underwater acoustic transducer responsible for converting the received acoustic signal into an electrical signal. After being converted by an A/D converter, the obtained data signal is synchronized and subjected to BPSK demodulation. Finally, the demodulated digital signal is transmitted to the serial port through the communication module, and then sent to the receiving end. The underwater acoustic communication process is completed in one go.

FPGA Implementation of 2 Systems

CDMA, also known as Code Division Multiple Access, is a modulation and multiple access method based on spread spectrum communication. Spread spectrum communication technology is an information transmission method that requires the bandwidth occupied by the signal to be far greater than the minimum bandwidth required for the transmitted information; The broadening of the frequency band is achieved through encoding and modulation methods, and is independent of the transmitted information data; At the receiving end, the same spreading code is used for correlation demodulation to despread and recover the transmitted information data. Its theoretical basis is the Shannon formula in information theory:

C=B log2 (1+S/N) (1)

In the formula: C represents the maximum information rate that the channel may transmit, and 3500-15 114M5330-01 represents the channel capacity; B represents channel bandwidth; S represents the average power of the signal; N represents noise power.

From equation (1), it can be seen that in the case of low signal-to-noise ratio, increasing bandwidth can be used to improve the anti-interference performance of the system, ensuring that the channel capacity remains unchanged. In other words, under the same channel capacity, broadband systems have better anti-interference performance than narrowband systems. Therefore, when the signal-to-noise ratio is too small and communication quality cannot be guaranteed, increasing bandwidth can be used to improve communication quality.

Figures 3 and 4 show the working principle of a direct spread spectrum system. The information code output by the signal source is added or multiplied with the pseudo-random code generated by the pseudo-random code generator to generate a spread spectrum sequence with the same rate as the pseudo-random code. Then, the carrier is used to modulate the spread spectrum sequence to obtain the RF signal that has been spread spectrum modulated. The process of despreading at the receiving end is the same as spreading, using local pseudo-random sequences to perform correlation despreading on the received signal and then demodulating it.

 

Figure 3 Schematic diagram of transmission unit

 

Figure 4 Schematic diagram of receiving unit

2.1 Launch Unit Design

The transmission unit mainly includes a pseudo-random sequence code module (PN code generator), a spreading module, and a BPSK modulation module.

2.1.1 PN code generator

The PN code generator adopts the principle of an m-sequence generator. The 3500-15 114M5330-01m sequence type longest linear shift register is formed by adding feedback to the shift register. The necessary and sufficient condition for a linear feedback memory shifter to generate an m-sequence is that the period feature polynomial is the original polynomial. This design features a 7-stage generator with a cycle of 127, using a primitive polynomial f (x)=1+x+x2+x6, written in VHDL language.

2.1.2 Spread Spectrum Module

XOR the m-sequence generated by the PN code generator with the input digital signal to complete the spread spectrum function. The RTL diagram of the spread spectrum module is shown in Figure 5.

 

Figure 5 Spread Spectrum Module RTL Diagram

2.1.3 BPSK modulation module

The modulation module selects BPSK modulation with constant envelope characteristics, which is a modulation method that controls the phase of the carrier through baseband signals, causing the carrier phase to jump. When the code element is’ 1 ‘, the modulated phase changes to 180 °. When the code element is’ 0′, the modulated phase changes to 0 °. Therefore, a BPSK modulation module was designed, which exemplifies two ROMs and is generated through Matlab. The mif file is used to store data at 0 ° and 180 °, as well as an address selector and data selector.

The simulation diagram of the entire transmission end is shown in Figure 6, where clk is the system clock, clk_bpsk is the clock modulated by BPSK, datain is the input data, m1out is the generated m-sequence, spre_out is the spread spectrum waveform, and bpsk_out is the output modulated by BPSK. It can be clearly seen from the results that the output signal has two phase changes, one is a jump from 0 ° to 180 °, and the other is a jump from 180 ° to 0 °, indicating that the data has been correctly modulated.

 

Figure 6 Simulation diagram of transmission unit

2.2 Receiver Unit Design

In order to verify the feasibility of the designed system, a BPSK demodulation and despreading module was designed in the system, and the modulated data at the transmitting end was directly used as input data for the receiving end. In the BPSK demodulation module, a ROM is also instantiated, which stores data with a phase of 0 °. The data synchronized through the carrier is multiplied by the output data of the ROM, and then a sampling decision is made. The decision result is shown in Figure 7. In the figure, sprieout is the data that has been spread spectrum at the transmitting end, and sameout is the data that has been sampled and delayed by 70 clk_bpsk. The purpose is to place the data exactly on the rising edge of the data, and p-out represents the start of demodulation output. From the figure, it can be seen that the delayed data after the decision is exactly the same as the spread spectrum data, with only a delay of a certain time indicating the demodulation time.

 

Figure 7 Results of BPSK Demodulation Module

Assuming that PN code synchronization has already been performed in the despreading module, only a certain time delay has been applied here to make it exactly the same as the PN code at the transmitting end, and then XOR with the demodulated data from BPSK to obtain the output data. The result is shown in Figure 8, where sp_end is the despreading completed data, p-end is the flag bit indicating the start of despreading, and datain is the input raw data. From the figure, it can be seen that the original input data in the despreading data domain is the same, with only a period of delay, indicating that the system has performed the correct demodulation.

 

Figure 8 Demodulation simulation diagram

3 Conclusion

This article designs a modulation/demodulation system for underwater acoustic communication based on FPGA direct sequence spread spectrum system, with the aim of enhancing anti-interference and confidentiality in underwater wireless communication. The system includes signal spread spectrum, BPSK modulation, and corresponding demodulation modules, which have been successfully verified on Modelsim simulation software. Although BPSK modulation has the advantages of narrow bandwidth, high frequency, and strong anti-interference ability compared to 2FSK, 2ASK is widely used in medium to high speed communication. However, in higher speed communication systems, BPSK modulation can no longer meet the requirements of frequency band utilization and system effectiveness, so a multi band phase modulation system is basically adopted. In addition, absolute phase modulation systems can cause phase inversion, so relative phase modulation systems should be considered. Based on this idea, underwater wireless communication will definitely have better application prospects.