# A Method for Underwater Wireless Data Transmission in a Hydroacoustic Channel under NLOS Conditions
## 2 Proposed Method of Data Transmission  
The data transmission set used in this work consists of two parts, transmitting and receiving. It operates in the single input multiple output (SIMO) mode, i.e., the transmitter uses a single transmitting hydrophone as a projector, and the receiver has a set of three receiving hydrophones in fixed spatial configuration. The basic parameters of the data transmission system are as follows:  
\(N_{sf}\)--the number of symbol in a frame. \(N_{bs}\)--the number of bits in one symbol. \(F_{d}\)--the lower frequency of the bandwidth (in Hz). \(\Delta f_{t}\)--the frequency resolution of the spectrum in the transmitter (in Hz). \(\Delta f_{bits}\)--the frequency interval between bits (in Hz). \(\Delta f_{r}\)--the desired frequency resolution at the receiver (in Hz). \(\tau_{guard}\)--the guard interval (in sec). \(F_{cpenoulle}\)--the preamble carrier frequency. \(N_{preamble}\)--the preamble sequence length. \(S_{preamble}\)--the preamble modulation rate. These parameters defining the communication conditions, are set prior to transmission. They are available to both the transmitter and the receiver. In addition, their change allows the system to be adapted to the need of the use and the conditions of the sound wave propagation in water. The transmitter works in accordance with the block diagram shown in Figure 1.  
The binary data is divided into individual symbols depending on the number of symbols in the frame and the number of bits transmitted in one symbol. After being divided into symbols, each symbol is mapped to the frequencies. Before the mapping begins, a frequency grid is generated based on the configuration data, i.e., the lower frequency of the bandwidth, the frequency resolution of the spectrum at the transmitter, the number of bits transmitted in the symbol, and the frequency interval between bits. In this work, the MFSK modulation in which each bit is mapped to two frequencies was used. In the used MFSK modulation technique a detection threshold at the receiver is not required, moreover the transmitted signal power does not depend on the symbol form. For a pair of frequencies assigned to a bit, the signal is emitted on one of them, depending on the value of the bit. Forcing the emission is carried out by entering the value 1 or 0 on the appropriate spectrum component occupied by a given bit. This produces a vector that maps data to frequencies. In the next step, an inverse fast Fourier transform (IFFT) is performed on the vectors previously formed for each symbol, with a random phase assumed for each spectral component. The phase randomization aims to evenly distribute the instantaneous values of signal in the time domain and simultaneous reducing the probability of periodic signal amplification. The next step is forming the output signal, which consists in the appropriate time setting of the individual symbols in relation to each other, as well as adding the so-called preamble. The preamble is generated based on a known pseudo-random binary sequence (PRBS) of a predetermined length that modulates the phase of a sinusoidal signal with the predetermined carrier frequency. The final form of the signal is shown in Figure 2.  
Figure 1: Block diagram of the transmitter. Figure 2: Signal form \(\tau_{premble}\)—duration of the preamble, \(\tau_{guard}\) —guard interval, \(\tau_{symbol}\) —duration of the symbol. After the preamble, and between the individual symbols, a defined waiting time \(\tau_{guard}\) is used to enable the just-transmitted signal to fade out in the propagation environment. This is to prevent any interference between the individual symbols. The presence of the previous signal in the propagation environment during the transmission of the next one may lead to inter-symbol interference and ultimately errors in reception. In the presented case it was assumed that the guard interval is longer than the channel memory time. In the receiver, the process is opposite of that in the transmitter. Figure 3 shows the block diagram of the receiver.