Songzuo Liu

Superposition Coding for Downlink Underwater Acoustic OFDM

Superposition coding (SC) is a nonorthogonal scheme for downlink communications, in which all users are allowed to use the full degrees of freedom of the channel and successive interference cancellation (SIC) is adopted for user decoding. Combining SC with orthogonal frequency-division multiplexing (OFDM) modulation, the optimal resource allocation depends on the perfect channel state information (CSI) of all users at the transmitter, which is hard to obtain for underwater acoustic (UWA) channels. In this paper, we propose a practical OFDM-modulated SC scheme for downlink UWA communications, where the transmitter splits the power between two users based on statistical CSI. The expressions to characterize the boundary of the ergodic rate region achievable by the proposed scheme over long codewords are presented first, followed by the analysis of outage probability when coding is applied within one OFDM block. Then we examine the performance of SC in an OFDM-modulated system with practical coding and modulation pairs. Simulation results show that the OFDM-modulated SC scheme outperforms the orthogonal frequency-division multiple access (OFDMA) in performance of both block error rate (BLER) and spectral efficiencies under different data rate pairs. Recorded data from both medium-range and short-range sea tests verify that channel statistics are stable over a long period of time and can be used to assist resource allocation for the proposed scheme. Compared with OFDMA, considerable increase of spectral efficiencies has been found for SC based on experimental data when both users with disparate channels have nonzero data rates.

Dolphin Sounds-Inspired Covert Underwater Acoustic Communication and Micro-Modem

A novel portable underwater acoustic modem is proposed in this paper for covert communication between divers or underwater unmanned vehicles (UUVs) and divers at a short distance. For the first time, real dolphin calls are used in the modem to realize biologically inspired Covert Underwater Acoustic Communication (CUAC). A variety of dolphin whistles and clicks stored in an SD card inside the modem helps to realize different biomimetic CUAC algorithms based on the specified covert scenario. In this paper, the information is conveyed during the time interval between dolphin clicks. TMS320C6748 and TLV320AIC3106 are the core processors used in our unique modem for fast digital processing and interconnection with other terminals or sensors. Simulation results show that the bit error rate (BER) of the CUAC algorithm is less than 10−5 when the signal to noise ratio is over ‒5 dB. The modem was tested in an underwater pool, and a data rate of 27.1 bits per second at a distance of 10 m was achieved.

MIMO-OFDM underwater acoustic communication systemsA review

The ever increasing demand for bandwidth, efficiency, spatial diversity and performance of underwater acoustic (UWA) communication has opened doors for the use of Multi-Input Multi-Output (MIMO). A combination of MIMO and Orthogonal Frequency Division Multiplexing (OFDM) has proved to be a promising solution for many scenarios in UWA communication; on the contrary, it also amplifies the design challenges for implementing such schemes to acquire the required bandwidth efficiency. The goal of this study is to provide a comprehensive survey of the latest researches in the field of UWA MIMO-OFDM communication. The previous works are summarized, reviewed and compared according to their years of publication while problems faced by UWA MIMO-OFDM communication are highlighted. The articles are classified according to the focused techniques like channel estimation, equalization, coding and detection. Furthermore the works are compared based on the complexity and performance of the algorithms while some future research issues are identified.

HEU OFDM-modem for Underwater Acoustic Communication and Networking

In view of the requirements for the applications of ocean development, environmental monitoring, disaster prevention etc., the HEU OFDM-modem for underwater acoustic communication and networking is presented in this paper. Orthogonal frequency division multiplexing (OFDM) technology is utilized in the physical layer, and protocols of ALOHA, MACAW are adopted in the data link layer. We conducted a mobile point-to-point communication experiment in May 2014, in the South Sea, China, where the bit error rate was less than 1.8x10-5, with bandwidth of 4 kHz, data rate of 3.03 kbps, and horizontal communication distance of 5 km. A small-scale underwater networking experiment was also carried out in the South Sea during the period from June 2nd to July 15th, 2014, where each nodes depth and the water temperature around the node measured by TD (Temperature and Depth) profiling instruments were sent back to the gateway every 25 minutes through underwater acoustic channel.

Erratum: A Combined Doppler Scale Estimation Scheme for Underwater Acoustic OFDM System

Since the performance of orthogonal frequency division multiplexing (OFDM) system is severely limited by frequency-dependent Doppler shifts over underwater acoustic (UWA) channels, a combined Doppler scale estimation scheme is proposed, containing two steps: (1) the initial Doppler scale estimation via transmitting two identical short OFDM symbols preceded by a cyclic prefix (CP) as a preamble prior to the data transmission; and (2) fine synchronization and Doppler scale estimation for each CP-OFDM block are achieved by the CP-based method, and eventually the resampling methodology is adopted for nonuniform Doppler shifts compensation. The proposed scheme has been tested with real data from the Songhua Lake experiment where the transmitter is moving at a speed of about 2.5 m/s, and the Yellow sea trial in which the transmitter and receiver are floating freely with waves and currents. Excellent performance results are obtained by block-by-block processing, without buffering the whole packet, which is suitable for real-time receiver processing over time-varying UWA channels.

Digital Self-Interference Cancellation for Asynchronous In-Band Full-Duplex Underwater Acoustic Communication

To improve the throughput of underwater acoustic (UWA) networking, the In-band full-duplex (IBFD) communication is one of the most vital pieces of research. The major drawback of IBFD-UWA communication is Self-Interference (SI). This paper presents a digital SI cancellation algorithm for asynchronous IBFD-UWA communication system. We focus on two issues: one is asynchronous communication dissimilar to IBFD radio communication, the other is nonlinear distortion caused by power amplifier (PA). First, we discuss asynchronous IBFD-UWA signal model with the nonlinear distortion of PA. Then, we design a scheme for asynchronous IBFD-UWA communication utilizing the non-overlapping region between SI and intended signal to estimate the nonlinear SI channel. To cancel the nonlinear distortion caused by PA, we propose an Over-Parameterization based Recursive Least Squares (RLS) algorithm (OPRLS) to estimate the nonlinear SI channel. Furthermore, we present the OPRLS with a sparse constraint to estimate the SI channel, which reduces the requirement of the length of the non-overlapping region. Finally, we verify our concept through simulation and the pool experiment. Results demonstrate that the proposed digital SI cancellation scheme can cancel SI efficiently.

Biologically inspired covert underwater acoustic communication—A review

Abstract With the increasing demand of military and marine applications, the requirement for covert underwater acoustic communication is drastically increasing recently. Innovative and inimitable ways are being explored pushing back the traditional methods, augmenting covert characteristics. Our research focuses on biologically inspired covert underwater acoustic communication mimicking marine mammal sounds. The communication signal could be detected by the eavesdropper but it is excluded from the process of recognition due to similarity with cetacean vocals achieving excellent Low Probability of Interception characteristics. The purpose of this study is to provide a comprehensive survey on bionic covert underwater acoustic communication using cetacean vocals since its inception. The works are summarized, compared and future research trends are highlighted.

A slide match and iteration approach for mobile underwater acoustic communication

This paper focusses on the requirements for mobile underwater acoustic communication. In this paper, we adopt a chirp signal, which has a Doppler insensitive property, as the communication carrier, and a slide match and propose an iteration based approach to estimate the sequenced Doppler coefficient. We first estimate the initial Doppler coefficient of the synchronization signal using Fractional Fourier transformation. Then, based on this initial coefficient, a slide match and iteration approach is applied to adjacent chirp codes which produces information to estimate the new Doppler coefficient and demodulation information at the same time. Compared with other methods, the proposed algorithm could obtain a higher precision of the Doppler coefficient with low computational burden. Simulation results show that the proposed method outperforms the traditional constant Doppler compensation approach and it could be easily realized in practical application.