Gang Qiao

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.

Kite-type passive acoustic detection system for underwater small targets

Potential underwater threats from some small intruders may pose a significant risk to security of some important waterside infrastructures, coastal facilities, subsea pipelines and ships by terrorists or hostile forces. Detection and tracking technologies in farther range are required to counter such threats by taking an appropriate response in time. The limitations of active sonar in high cost, negative effects on marine life, low covertness and high reverberation have stimulated researches in passive detection methods. The Kite-type Passive Acoustic Detection System (KPADS) was developed to provide alarms in the area of underwater security and protection against terrorism intruders and threats from the waterside, including unmanned underwater vehicles, small submarines, surface vessels, divers and underwater robots etc. This paper reports on the detection and tracking of stationary noise source and moving targets in Fuxianhu Lake of China. A series of technologies is applied to a passive triangular array of 18 hydrophones in a uniformly-spaced linear arrangement. Experimental results are presented demonstrating the system performance of detection range up to 8000m, tracking better not less than 4000m, bearing accuracy up to 1°, and are verified by GPS ground truth.

Experimental Study of Long-Range Shallow Water Acoustic Communication Based on OFDM-Modem

In this paper, we focus on the need for remote and robust underwater acoustic(UWA) communication in shallow sea. A set of OFDM underwater acoustic communication algorithm is designed including the techniques of Doppler estimation and compensation, channel equalization and frequency diversity. This algorithm is realized in the DSP and successfully applied on the OFDM-Modem platform. In Da- Lian shallow sea trial, the horizontal communication distance is 36 kilometers under the condition of QPSK modulation, 1/2 rate convolution code, bandwidth between 4 and 8kHz. The communication rate of the OFDM-Modem can reach 426bps and the ultimate BER is less than 10-3.

OFDM demodulation using virtual time reversal processing in underwater acoustic communication

The extremely long underwater channel delay spread causes severe inter-symbol interference (ISI) for underwater acoustic communications. Passive time reversal processing (PTRP) can effectively reduce the channel time dispersion in a simple way via convolving the received packet with a time reversed probe signal. However the probe signal itself may introduce extra noise and interference (self-correlation of the probe signal). In this paper, we propose a virtual time reversal processing (VTRP) for single input single output (SISO) Orthogonal Frequency Division Multiplexing (OFDM) systems. It convolves the received packet with the reversed estimated channel, instead of the probe signal to reduce the interference. Two sparse channel estimation methods, matching pursuit (MP), and basis pursuit de-noising (BPDN), are adopted to estimate the channel impulse response (CIR). We compare the performance of VTRP with the PTRP and without any time reversal processing through MATLAB simulations and the pool experiments. The results reveal that VTRP has outstanding performance over time-invariant channels.

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.

Underwater cylindrical shell in different thickness recognition using biomimetic dolphin clicks

For the traditional narrowband sonar signal, recognizing underwater small targets is an extremely difficult task. Therefore, a series of biomimetic dolphin clicks, a kind of broadband and transient-like sonar signal, were used as the transmitting waveform for recognition tasks, and the tasks were presented from experiments in an anechoic pool. In this experiment, three biomimetic dolphin clicks, differing in the energy distribution in the time-frequency domain, were used to recognize three copper cylindrical shells which have a diameter of 10 centimeters, 40 centimeters high and different thickness. The echoes from three targets were collected by receiving system, and were analyzed in the time domain and frequency domain. In addition using wigner-ville distribution, the echoes were projected to the time-frequency space for feature observation. Then the time-frequency features of echoes were extracted by singular value decomposition, and these features were classified by a support vector machine for recognizing target echoes. Experimental results indicated that these cylindrical shells in different thickness can be recognized by the biomimetic dolphin clicks, and different clicks obtain different echo responses and different identification results. Therefore, the echo responses have strong dependence with the time-frequency distribution of transmitting waveform. Furthermore the results show a promising way to improve underwater acoustical recognition in an intelligent waveform design manner by a dynamic closed-loop feedback sonar system.

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.

Full-duplex, Relative Clock Based and Collision Free Protocol for Underwater Acoustic Networks

This paper presents some experimental results of underwater acoustic communication networks both in a lake trial and a sea trial. Three specially designed MAC protocols are introduced and tested: full-duplex based protocol (FD-MAC), relative clock based protocol (RC-MAC) and collision free protocol (CF-MAC). The FD-MAC protocol was implemented on a full-duplex modem; a vector hydrophone was used to improve the receiving gain, and dual-mode communication and acoustic baffle technology were applied to reduce the local emission interference. Experimental results show that the exposed/hidden terminal problems can be well resolved. The RC-MAC protocol was an improved TDMA protocol suited for centralized networks in the underwater environment; neither global clock synchronization nor the periodical broadcast of a synchronized signal is needed. The net schedule was arranged by the gateway according to the propagation delays, and the sub nodes were working on their local clock by mapping the net schedule to their local time line separately. The working slot for each sub node can be customized according to the information flow to improve the network throughput, and the main node and sub node can be reversed to prolong the network lifetime. Experimental results show that this protocol was easy to implement and worked well. The CF-MAC protocol was designed for the regional underwater observation networks. This protocol avoids channel contentions by using the top-down channel assignment. In this scheme, temporary channel access is assigned by the gateway/AUV using a require-data-send signal (RDS). RDS is used to wake up the specific node and cannot be heard by the other nodes. Two types of acknowledgement signal were designed to shorten the poll duration and to save energy. The practicability of this protocol was validated by experiments. All modems used in these trials were designed and implemented by our lab.