Anti-Multipath Orthogonal Chirp Division Multiplexing for Underwater Acoustic Communication

Peibin Zhu,Xiaomei Xu,Xingbin Tu,Yougan Chen,Yi Tao

doi:10.1109/access.2020.2966072

Peibin Zhu, Xiaomei Xu + Show 3 more

Open Access

https://doi.org/10.1109/access.2020.2966072

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Abstract

Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new anti-multipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of $10^{-6}$ , while the OFDM has a BER floor of $10^{-3}$ . The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

Highlights

Research on underwater acoustic communication (UWAC) technology is important for both military and civilian applications
The receiver scheme of the DP-Rake orthogonal chirp division multiplexing (OCDM) system is shown in Fig. 9(b): at first, after conversion to the baseband signal, the received signal is cross-correlated with the local dual hyperbolic frequency modulation (HFM) signal, the correlation peak is determined to complete the frame synchronization [22], and the Doppler shift caused by relative motion is compensated by Doppler estimation and phase correction; the obtained data are transferred to the data pick-based rake receiver, and each data block is subjected to OCDM demodulation under the setting of each rake finger; the demodulated data is subjected to cyclic redundancy check (CRC) check and data pick to obtain the final information data
The bit error rate (BER) performance comparison of SC-frequency domain equalization (FDE), orthogonal frequency division multiplexing (OFDM), OCDM, and DP-Rake OCDM systems is shown in Fig. 10(a): the BER performance of OFDM and SC-FDE is equivalent, which is due to the use of forward error correction (FEC) coding, Doppler estimation, and phase correction techniques in these systems, as these techniques eliminate the Doppler shift introduced by channel variations; the BER performance of OCDM is better than OFDM because OCDM provides both spreading and diversity gain through its chirp-based signal; DP-Rake OCDM has the best BER performance, but the improvement over OCDM is not significant