Abstract
Due to the complexity and variability of the underwater acoustic channel, the communication signal is affected by multi-path, time delay, and Doppler frequency shift. Based on the advantageous characteristics of fractional Fourier transform on chirp signal processing, a fractional Fourier transform–based algorithm using combined linear frequency–modulated signal is proposed, which can estimate parameters of underwater acoustic channel and has a better performance than the existing methods. To distinguish multi-user in underwater acoustic communication system, a single-carrier direct sequence code division multiple access communication system combined with the fractional Fourier transform–based algorithm is proposed. Thus, a preliminary study on underwater multi-target identification is carried out. The simulation and experimental results show that the fractional Fourier transform–based algorithm is simple and effective, and the energy can be focused at the “best” fractional order, which can directly determine the multi-path number and complete the channel estimation. The proposed single-carrier direct sequence code division multiple access communication system has good performance on bit error rate when we use corresponding spreading code to distinguish multi-user.
Highlights
To improve the performance of underwater acoustic communication for sensor network,[1,2,3] the estimation of underwater acoustic channel parameters in real time is needed, which mainly includes the transmitted signal design and its detection algorithm.[4,5,6] In general, the time delay and Doppler frequency shift joint estimation can be achieved by means of resampling and timedomain correlation,[7] which can be only applied to same Doppler frequency shift for multi-path
After the combined linear frequency– modulated (LFM) signal passed through the underwater acoustic channel, affected by the effects of frequency shift and time delay, the peak coordinates in the u-domain of the fractional Fourier transform (FRFT)-processed signal are changed
In this article, targeting the need of channel parameters’ estimation and multi-user distinction in underwater acoustic communication, an SC-DS-code division multiple access (CDMA) system combined with the FRFT-based algorithm is proposed
Summary
To improve the performance of underwater acoustic communication for sensor network,[1,2,3] the estimation of underwater acoustic channel parameters in real time is needed, which mainly includes the transmitted signal design and its detection algorithm.[4,5,6] In general, the time delay and Doppler frequency shift joint estimation can be achieved by means of resampling and timedomain correlation,[7] which can be only applied to same Doppler frequency shift for multi-path. From equation (2), when the ‘‘best’’ fractional order is taken, the FRFT amplitude of the combined LFM signal will form a series of peaks at the corresponding u-domain, which is determined by the chirp basis decomposition characteristic of FRFT.[22] After the combined LFM signal passed through the underwater acoustic channel, affected by the effects of frequency shift and time delay, the peak coordinates in the u-domain of the FRFT-processed signal are changed. After performing the FRFT for the received combined LFM signal, the corresponding u-domain interval for each pair of peaks can be obtained according to its u-domain figure and equation (6), which can be shown in equation (8)
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