Abstract
In an underwater acoustic cellular (UAC) system, underwater equipment or sensor nodes need to detect the identity of an underwater base station (UBS) and synchronise it with a serving UBS. It is known that, in an underwater acoustic channel, the temporal variability of the ocean coupled with the low speed of sound in water may induce a significant Doppler shift. In this paper, two different types of cell search techniques (CSTs) are proposed to detect the cell ID and correct timing of the UBS in UAC systems with a Doppler shift: CST based on linear frequency modulation with full bandwidth in the time domain (LFM-FT) and CST based on linear frequency modulation in the frequency domain (LFM-FF). The performances (auto-correlation, cross-correlation, ambiguity function, and cross ambiguity function) of the proposed techniques are analysed and compared with simulation results. It is demonstrated by simulation that the proposed techniques perform better than previous techniques in both AWGN and multipath channels when a Doppler shift exists. It is also shown that the LFM-FF-CST achieves the best performance in the presence of a Doppler shift and is suitable for mobile UAC systems.
Highlights
With the tremendous scientific and technological advances over recent decades, a wide range of underwater exploration applications have emerged
In order to overcome these limitations, two different types of cell search techniques (CSTs) are proposed in this paper: CST based on linear frequency modulation with full bandwidth in the time domain (LFM-FT) and CST
The LFM-FT-CST was proposed by modifying the previous LFM-based CST (LFM-CST) in the time domain so that full bandwidth can be used in the design of preambles with different frequency sweeping parameters
Summary
With the tremendous scientific and technological advances over recent decades, a wide range of underwater exploration applications have emerged. Many synchronisation techniques such as signal detection, timing/frequency acquisition, and Doppler scale estimation/compensation have been developed [21]. The conventional terrestrial cellular system’s CSTs cannot be applied to the UAC system because the slow propagation of acoustic waves produces a large Doppler spread and/or shift. In the LFM-CST, full bandwidth cannot be assigned to all preambles since the CID is mapped to its frequency sweeping parameter. In order to overcome these limitations, two different types of CST are proposed in this paper: CST based on linear frequency modulation with full bandwidth in the time domain (LFM-FT) and CST based on linear frequency modulation in the frequency domain (LFM-FF) In both the LFM-FT-CST and LFM-FF-CST, multiple LFM preambles are generated by changing the frequency sweeping parameters of the LFM signal as in the previous LFM-CST.
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