Research Of Underwater Radio Frequency Communication Modem

There has been a great interest in developing underwater wireless communication systems using electromagnetic waves. Researchers have turned their attention to underwater radio frequency communication technology. Such products could be used for the hydrological environment pollution monitoring sensor network, marine resources exploration, military underwater AUV, and so forth. But as a result of underwater communication channel conditions are very harsh that ordinary medium for free space of communications technology cannot very well satisfy the requirements of underwater wireless communication. At present, it is not easy to find out the specialized products for underwater radio frequency communication in the domestic market. It is basically monopolized by the research institute of Europe and the United States. Therefore, it is very necessary to strengthen the research in this field.Modem is an indispensable part of the RF communication system. But the existing of modem products on the market are mostly designed for air medium communication scenarios, which the carrier frequency is used in the 433 MHZ to 2.4 GHz ISM band. If we directly used this kind of radio frequency electromagnetic signal in the underwater environment. The signal attenuation would be very severely and even can't implement the normal communication. Therefore, we need to specially develop the underwater modem system of radio frequency communication scenarios.Compared with underwater acoustic communication technology and underwater optical communication technology. There are many outstanding advantages of the radio frequency signal of MHz such as a good environmental noise impact resistance and high transmission rate and the simple structure of system and so forth. According to the design experience of overseas research institute in underwater radio frequency communication modem. On one hand, we need to meet the signal at close range to communicate with high transmission rate. On the other side, it also can avoid the severe attenuation of high frequency RF signal underwater.In this paper, we study the modem with the earner frequency of 1MHz. We introduced the spread spectrum communication modulation technology in order to enhance the ability of underwater communication system. Because the spread spectrum communication modulation technology can resist complex background environment crosstalk noise influence. We also design the active filter circuit that can filter the useless stray noise signal and ensure the purity of useful signal spectrum. We use the Cyclone ? E series FPGA chip and realize the modem of digital signal processing in the way of Verilog HDL logic encoding.