Using Fast Frequency Hopping Technique to Improve Reliability of Underwater Communication System

Acoustic underwater communication systems designed to work reliably in shallow coastal waters must overcome major limitations such as multipath propagation and the Doppler effect. These restrictions are the reason for the complexity of receivers being built, whose task is to decode a symbol on the basis of the received signal. Additional complications are caused by the low propagation speed of the acoustic wave in the water and the relatively narrow bandwidth. Despite the continuous development of communication systems using coherent modulations, they are still not as reliable as is desirable for reliable data transmission applications. This article presents an acoustic underwater communication system that uses one of the varieties of the spread spectrum technique i.e., the fast frequency hopping technique (FFH). This technique takes advantage of binary frequency-shift keying (BFSK) with an incoherent detection method to ensure the implementation of a system whose main priority is reliable data transmission and secondary priority is the transmission rate. The compromised choice of parameters consisted of the selection between the narrow band of the hydroacoustic transducer and the maximum number of carrier frequency hops, which results from the need to take into account the effects of the Doppler effect. In turn, the number of hops and the symbol duration were selected adequately for the occurrence of multipath propagations of an acoustic wave. In addition, this article describes experimental communication tests carried out using a laboratory model of the FFH-BFSK data transmission system in the shallow water environment of Lake Wdzydze/Poland. The test results obtained for three channels of different lengths are discussed.

METHOD OF SIGNAL PARAMETERS ESTIMATION OF RADIOSTATIONS WITH FAST FREQUENCY HOPPING SPREAD SPECTRUM

Modern special-purpose radios utilize ultra-short burst mode with frequency hopping spread spectrum. Such digital radios operate, as a rule, in the fast frequency hopping spread spectrum mode and use a considerable number of frequency channels with frequency separation between adjacent channels, which is significantly less than the spectrum width of the frequency element. The main modulation in such radios is frequency manipulation. The identification of signals with fast frequency hopping spread spectrum and frequency manipulation is performed by estimating the speed of frequency tuning, the speed of information transmission, the number of frequency channels and the step of the frequency grid and comparing the obtained values with the corresponding characteristics of known radio stations with fast frequency hopping spread spectrum. The time limits and the duration of the frequency elements are calculated by the envelope of the received signal. For this purpose, a suitable method has been developed, which consists in filtering the envelope by means of a moving average window for suppression of the noise component and threshold processing. The threshold value is selected by the given probability of its exceeding by noise sample. The proposed approach to determining the time limits of frequency elements does not require any a priori signal information. To determine the frequencies of the frequency elements periodogram estimates are used. An exponential extrapolation of the discrete power spectrum was used to increase the frequency resolution at a fixed window length of the fast Fourier transform. It is shown that in order to uniquely determine the step of the frequency grid and the diversity of frequency manipulation frequencies, it is necessary to analyze the number of frequency elements not less than the number of frequency channels. By analyzing the histogram of the differences of sorted frequency vector of the frequency elements, determine the frequency grid, frequency diversity and the multiplicity of frequency manipulation. An approximate estimate of the number of frequency channels is calculated as the ratio of the magnitude of the frequency vector of the frequency elements to the step of the frequency grid. As a rule, symbol transmission at frequency manipulation is carried out at orthogonal frequencies, so it can be roughly assumed that the symbolic speed is equal to the frequency diversity of the frequency manipulation. The baud rate can be set only after signal demodulation and bit stream analysis, which can determine the frame structure and the number of service and information bits. The proposed method provides the estimation of the values of the parameters necessary for the identification of radio stations of signal parameters with a relative error not exceeding 0.3%, with values of signal to noise ratio not lower than 5 dB.