# Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System
## 2 Features of a Reliable Underwater Communication System  
Spread spectrum techniques were originally developed for applications in military systems due to their low probability of intercept and high resistance to jamming signals, i.e., they make it difficult for the opponent to eavesdrop on transmissions and unauthorized sources transmitting false information--interfering with communication through interference with the receiver system. In addition, it is possible to share the frequency band with many types of conventional modulations with or without minimal interference.  
This technique is based on the statement: The more the noise dominates the signal, the wider the band the signal must occupy to receive it correctly. It results from [PERSON]'s theorem on channel bandwidth with a specified bandwidth \(B\) and signal-to-noise ratio (SNR) [16]. This technique significantly expands the bandwidth used, while allowing you to work with a much smaller ratio of signal strength to noise power, which is done by converting the narrowband information signal to a signal with a much wider band [17; 18].  
There are many varieties of spread spectrum technique, among which the most typical are [19]:  
* Direct Sequence Spread Spectrum technique (DSSS) [20],
* Frequency Hopping Spread Spectrum technique (FHSS),
* Time Hopping Spread Spectrum technique (THSS),
* Chirp Spread Spectrum technique (CSS) [21].  
One of these varieties is considered in the article, i.e., FHSS, as a technique that will ensure reliable operation of an underwater communication system. The next part of the chapter is devoted to discussing the principle of the system with this technique, and then the implementation of the receiver and the assumptions of this technique in the case of counteracting the limitation of the communication channel.  
Communication systems utilizing a frequency hopping technique most often use frequency-shift keying (FSK) with an incoherent detection method [19]. A diagram of a typical system with the details of the modulator and demodulator is shown in Figure 1.  
In the FHSS technique, all available channel bandwidth are divided into adjacent frequency subchannels. The carrier frequency between the subchannels is switched using a selected code sequence, which is known in both the transmitter and the receiver. The \(L\)-length code sequence controls the frequency synthesizer in each symbol interval \(T_{S}\). In the transmitter, the sent symbol is subjected to channel coding and interleaving, and then goes to the FSK modulator, which sends the symbol with duration \(T_{S}\), and gives the signal the appropriate frequency for the subchannel depending on the FSK modulation order used. The signal thus generated is then placed in the appropriate frequency subchannel by means of a frequency conversion system for a time equal to the time of single _hop time_\(T_{H}\) (or _dwell time_). Then, after amplification, such a signal goes through the hydroacoustic transducer to the underwater channel. On the receiving side, the amplified signal in the preamplifier goes to the receiver. In the receiver, the signal is reduced to the baseband in which FSK demodulation occurs. The synchronization signal necessary to maintain the synchronization of the code sequence generator is obtained from the received signal by the code sequence synchronizer system [19].  
Figure 1: Scheme of data transmission system with frequency hopping technique with details of the modulator and demodulator.  
When the carrier frequency changes every few bits of data stream, it is a system with _slow frequency hopping technique_ (SFH).