Water communication is very difficult, mainly due to the multipath effect of the channel, the time variable effect, the frequency wide and narrow, the signal attenuation is serious, especially in long distance transmission middle. Underwater communication compared to wired communication, the rate is very low because underwater communication is an acoustic wave rather than radio waves. Common water sound communication methods are spreading communication technologies such as CDMA, etc. Supplement: The development of water sound communication technology has been more mature, and many foreign institutions have developed the sound communication modem, and the communication methods are mainly: OFDM, spread spectrum, and other modulation methods. In addition, the water sound communication technology has developed to the network, and the network technology (Ad hoc) in the radio can be applied to the water voice network, which can achieve all-round and stereochemical communication in the ocean (can be with AUV, UUV, etc. Unmanned equipment is used in combination), but only a few national tests are successful.
Working principle is first information such as text, voice, image, etc., converted to an electrical signal by the electric transmitter, and is directly digitally processed by the encoder, The electrical signal is converted into an acoustic signal. The acoustic signal passes the information to the receiving transducer through water, and the sound signal is converted into an electrical signal. After deciphering the digital information, the electrical receiver turns the information into sound, text and pictures.
This is an international high-level technology, which clearly receives the voice signal in a distance of water, and there is only a very few military power in the world to do. The aerobic communication machine uses analog signal, but the waves, fish, ships in the ocean produce noise, so that the sound field in the ocean is extremely confusing, and the sound wave generates "multi-channel interference signal" when transmitting in seawater. The problem, causing the received signal to be blurred.
Since half of the century, experts in the field of water have been helping this problem, and the old-fashioned simulated water acoustic communication machine has been used. Due to the generation of digital communications, the signal interference on the land was successfully solved, and experts in the field of water have also begun to explore in this area.
They carefully analyze several methods of anti-multi-dried interference, and finally the topic group consistently believes in the means of electromagnetic wave resistance - frequency hopping communication, it can both resist multiple ways and can guarantee information security.
Because the seawater is complex, it is absorbed in part, and the higher the frequency, the higher the absorption, and there is less absorption for the high frequency of high frequency. Experts measure, the acoustic wave frequency is the optimal frequency transmitted by 4000 Hz, and the frequency of frequency hopping communication is achieved with the frequency of 4000 Hz, and the distance between the frequency point and the frequency point is small.
my country Xiamen University has completed the 863 project handed over to them by Professor Xu Keping, has successfully solved it in 10 kilometers. The signals are cleaned, and their system has reached practical requirements. They seriously analyze several methods of anti-multi-distance interference in the world, and finally the topic group consistently believes in the means of electromagnetic wave anti-interference - frequency hopping communication, which can both resist multi-channel interference and guarantee information security.
If the frequency of frequency hopping techniques of electromagnetic waves transmit a set of signals in the sea, when the frequency resources are sufficient, the frequency is different, and when the circuit is processed, 1 and 0 are represented by two different frequencies. It is equivalent to the difference in color, such as: Red, orange, yellow, green, green, blue, and purple represents a text. Although the time is inconsistent, although the time arrives is inconsistent, because the color difference is large, the frequency is high. The recipient is easily identified, which solves the signal interference problem. After the research they develop a new frequency hopping technology, I finally successfully solved the multi-channel interference problem. Because voice transmission is the bottleneck problem of the most difficult to overcome the sound communication, the accuracy is extremely high, the difficulty is the largest, and the successful voice transmission is complete, so that this project is completely successful, they did it.
The topic group has also welcomed a new round of challenges, and puts a distance of 50 kilometers of digital voice and image delivery, as well as digital color image delivery.
Water communication is the most important and key technology in the current marine military, the research direction exerts the advantages and characteristics of the relevant disciplines of electronic and oceans in Xiamen University, topics The group completed the "Water Image Water Transmission Practical Motor", "863" related topic projects such as "video image water transmission test", "underwater image transmission system" project passed the national "863" expert group inspection. The system can achieve full direction in the shallow seas area, transmit a frame (160 * 100 pixels, 16 gradation) every 8 seconds, 10 km away. 99 years China International High-tech Achievements Trade Exhibit Project. The "300-meter deep saturated diving clock water channel" project reached the advanced level of international similar products, the received voice is clear, and the communication has been installed in 300 meters deep saturation diving clock, successful in the South China Sea. This topic group is close to the research results in this direction or reaches the international advanced level. The developed voice water sound communication machine, image transmission prototype and water sound data telemetry device, is expected to form a water multimedia information transmission system, and in the near future, it is desirable to form three-dimensional information network for underwater, land and space.
Ocean Acoustics is a rapidly developed discipline, and the water sound multimedia communications is a goal of the marine science and technology community for many years. People hope to be in underwater. Voice, images, text, and data are transmitted just like land. We believe that with the in-depth development of research, the active efforts of the science and technology workers, this goal can be achieved soon.