Abstract
The performance of Underwater Acoustic Communication (UAC) systems are strongly related to the specific propagation conditions of the underwater channel. Horizontal, shallow-water channels are characterised by extremely disadvantageous transmission properties, due to strong multipath propagation and refraction phenomena. The paper presents the results of communication tests performed during a shallow, inland-water experiment with the use of a laboratory model of a UAC system implementing the Orthogonal Frequency-Division Multiplexing (OFDM) technique. The physical layer of data transmission is partially configurable, enabling adaptation of the modulation and channel coding parameters to the specific propagation conditions. The communication tests were preceded by measurement of the UAC channel transmission properties. Based on the estimated transmission parameters, four configurations of OFDM modulation parameters were selected, and for each of them, communication tests were performed with the use of two Error-Correction Coding (ECC) techniques. In each case, the minimum coding rate was determined for which reliable data transmission with a Bit Error Rate (BER) of less than 10 − 4 is possible.
Highlights
The designers of shallow-water communication systems try to implement the techniques of modern radiocommunications, but both the Bit Error Rate (BER) and data transmission rates achieved are much lower in the case of Underwater Acoustic Communication (UAC) systems
The system enables the Orthogonal Frequency-Division Multiplexing (OFDM) modulation parameters and Error-Correction Coding (ECC) rate to be adjusted to the propagation conditions as to obtain the desired Bit Error Rate, which is less than 10−4
The tested UAC channel was about 1 km long and the depth was variable along this distance from 20 to 40 m
Summary
The designers of shallow-water communication systems try to implement the techniques of modern radiocommunications, but both the BER and data transmission rates achieved are much lower in the case of UAC systems. Differences in attenuation of frequency components of the transmitted signal due to growth in the range have degrading influence on the shape of the signal spectrum, and the time-domain waveform is distorted To avoid these distortions, the bandwidth should be reduced as the system’s range increases. The differences in attenuation have a limiting effect on the bandwidth of the system and reduces its throughput Another phenomenon that strongly impacts the transmission properties of the UAC channel consists of reflections from the sea-bottom and the water’s surface, as well as other objects present in the water. This causes multipath propagation, which goes hand-in-hand with strong refraction, caused by a significant change in sound velocity as a function of depth.
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