Chengbing He

A Variable-Rate Spread-Spectrum System for Underwater Acoustic Communications

A key research area in underwater acoustic (UWA) communication is the development of advanced modulation and detection schemes for improved performance and range-rate product. In this communication, we propose a variable-rate underwater data transmission system based on direct sequence spread spectrum (DSSS) and complementary code keying (CCK), particularly for shallow-water acoustic channels with severe multipath propagation. We provide a suboptimum receiver that consists of a bidirectional decision feedback equalizer (BiDFE) to cancel both postcursor and precursor intersymbol interference (ISI). We also develop iterative signal processing and time-reversal (TR) diversity processing to mitigate the effect of error propagation in BiDFE. We present performance analysis on bit error rate (BER) for different data rates. Our works show that this new variable-data-rate DSSS-CCK is a suitable candidate for UWA communications over varying channel conditions and distance.

Improving Passive Time Reversal Underwater Acoustic Communications Using Subarray Processing

Multichannel receivers are usually employed in high-rate underwater acoustic communication to achieve spatial diversity. In the context of multichannel underwater acoustic communications, passive time reversal (TR) combined with a single-channel adaptive decision feedback equalizer (TR-DFE) is a low-complexity solution to achieve both spatial and temporal focusing. In this paper, we present a novel receiver structure to combine passive time reversal with a low-order multichannel adaptive decision feedback equalizer (TR-MC-DFE) to improve the performance of the conventional TR-DFE. First, the proposed method divides the whole received array into several subarrays. Second, we conduct passive time reversal processing in each subarray. Third, the multiple subarray outputs are equalized with a low-order multichannel DFE. We also investigated different channel estimation methods, including least squares (LS), orthogonal matching pursuit (OMP), and improved proportionate normalized least mean squares (IPNLMS). The bit error rate (BER) and output signal-to-noise ratio (SNR) performances of the receiver algorithms are evaluated using simulation and real data collected in a lake experiment. The source-receiver range is 7.4 km, and the data rate with quadrature phase shift keying (QPSK) signal is 8 kbits/s. The uncoded BER of the single input multiple output (SIMO) systems varies between 1×10−1 and 2×10−2 for the conventional TR-DFE, and between 1×10−2 and 1×10−3 for the proposed TR-MC-DFE when eight hydrophones are utilized. Compared to conventional TR-DFE, the average output SNR of the experimental data is enhanced by 3 dB.

Single carrier with multi-channel time-frequency domain equalization for underwater acoustic communications

Single-carrier with frequency domain equalization (SC-FDE) has been considered for bandwidth efficiency underwater acoustic (UWA) communication recently due to its reduced computational complexity and low peak-to-average power ratio. A multi-channel time-frequency domain equalization method for pseudurandom noise (PN) based SC-FDE is proposed in this paper. The proposed equalizer includes a multi-channel frequency domain equalizer followed by a low order multi-channel adaptive time domain decision feedback equalizer (DFE). The proposed algorithm is applied to the real data receiving from a lake test conducted in November 2011. It is demonstrated that the uncoded error-free data rates of around 1500 and 3000 bps are achieved using one transmitter and six-channel receiving hydrophone array at a distance of 1.8 km. Experiment results shows that the performance can be enhanced by 4.5-5.5 dB in terms of output signal-to-noise ratio (SNR).

Energy Management and Power Allocation for Underwater Acoustic Sensor Network

This paper investigates energy allocation in underwater acoustic nodes powered by energy harvesting. Our goal is to maximize the expected total amount of delivered data over a finite time slots. Two scenarios are considered for different knowledge levels of channel state information (CSI). In one scenario, the transmitter receives CSI at the end of each data transmission epoch and we consider the energy allocation problem for sensing and transmission. In the second scenario, the transmitter receives delayed CSI after multiple time slots, while only the energy allocation for transmission is considered. The underwater acoustic channel is modeled as a finite-state Markov chain to characterize its time varying nature. We employ a stochastic dynamic programming (DP) approach to derive the optimal allocation policy for both scenarios. To reduce the inherent computation complexity of DP approach, we also present a suboptimal algorithm by analyzing the structure of DP solution. Our results show the DP approach achieves substantial performance improvement that is preserved substantially by the suboptimal algorithm to provide a good performance-complexity tradeoff.

Multiuser underwater acoustic communication using cyclic shift keying

A novel code division multiple access (CDMA) using cyclic shift keying (CSK) signaling, namely CSK-CDMA, has been proposed for multiuser underwater acoustic communication. The proposed method uses the cyclic correlation characteristic of spread spectrum signals and time reversal processing to increase the data rate each user and to reduce the complexity of the receiver. In CSK-CDMA system, multiple bits information is represented by the cyclically shifted version of the basic spread spectrum waveform at the transmitter. Therefore, the CSK scheme provides a higher data rate than conventional direct sequence spread spectrum. At the receiver, the peak position of the cyclic correlator output is estimated to recovery information. Multiuser communications are demonstrated with lake experimental recorded data. The data rate is 40 bits/s/user for 4 users with zeros error bits and 20 bits/s/user for 8 users with BER around 10-2 at low signal-to-noise ratio (-3 dB), given 4 kHz bandwidth, respectively.

Multi-channel iterative FDE for single carrier block transmission over underwater acoustic channels

Recently, single carrier block transmission (SCBT) has received much attention in high-rate phase-coherent underwater acoustic communication. However, minimum-mean-square-error (MMSE) linear FDE may suffer performance loss in the severely time dispersive underwater acoustic channel. To combat the channel distortion, a novel multi-channel receiver with maximum ratio combining and a low complex T/4 fractional iterative frequency domain equalization (FDE) is investigated to improve diversity gain and the bit error rate (BER) performance. The proposed method has been verified by the real data from a lake underwater acoustic communication test in November 2011. At 1.8 km, the useful data rates are around 1500 and 3000 bits/ s for BPSK and QPSK respectively. The results show the improvements of system performance. Compared with MMSE FDE system, the output SNR improvement is 6.9 dB, and the BER is from 10-3 to no error bits for BPSK. The output SNR improvement is 5.3 dB, and the BER is from 1.91×10-2 to 2.2×10-4 for QPSK.

Single Carrier with Frequency Domain Equalization for Synthetic Aperture Underwater Acoustic Communications

Phase-coherent underwater acoustic (UWA) communication systems typically employ multiple hydrophones in the receiver to achieve spatial diversity gain. However, small underwater platforms can only carry a single transducer which can not provide spatial diversity gain. In this paper, we propose single-carrier with frequency domain equalization (SC-FDE) for phase-coherent synthetic aperture acoustic communications in which a virtual array is generated by the relative motion between the transmitter and the receiver. This paper presents synthetic aperture acoustic communication results using SC-FDE through data collected during a lake experiment in January 2016. The performance of two receiver algorithms is analyzed and compared, including the frequency domain equalizer (FDE) and the hybrid time frequency domain equalizer (HTFDE). The distances between the transmitter and the receiver in the experiment were about 5 km. The bit error rate (BER) and output signal-to-noise ratio (SNR) performances with different receiver elements and transmission numbers were presented. After combining multiple transmissions, error-free reception using a convolution code with a data rate of 8 kbps was demonstrated.

Joint channel estimation and detection using Markov chain Monte Carlo method over sparse underwater acoustic channels

This study proposes a novel approach to joint channel estimation and detection of orthogonal frequency division multiplexing transmission over underwater acoustic (UWA) multipath channels exhibiting cluster sparsity. Unlike most sparse channel estimations, the authors exploit the cluster-sparsity characteristic of UWA channels without additional prior information. They adopt a modified spike-and-slab prior model in their non-parametric Bayesian learning framework. To avoid the need for a closed-form Bayesian estimate, they apply the Markov chain Monte Carlo technique to joint achieve channel estimation and signal detection. The proposed solution is amenable to being integrated with soft-input soft-output decoding to improve the performance through turbo iteration. Simulation results demonstrate improved bit error rate of the proposed algorithm over existing algorithms.

Spatial CCK Modulation and Iterative Detection Over Frequency-Selective Fading Channels

In this letter, we present a novel spatial modulation (SM) transmission with multiple antennas by applying non-orthogonal complementary code keying (CCK). We propose a high-performance turbo receiver for this CCK-SM signaling over frequency-selective fading channels. Compared with conventional SM, our proposed CCK-SM transceiver utilizes the advantages of CCK to achieve substantial coding gain without the cost of high complexity. Our novel receiver incorporates iterative block decision feedback equalization with a CCK soft decoder to achieve clear performance gain. Simulation test shows substantial performance gain by the proposed CCK-SM transceiver over the conventional SM.

Fast MUSIC algorithm for joint DOD-DOA estimation based on Gibbs Sampling in MIMO array

Multiple signal classification (MUSIC) algorithm has been used for joint Direction of Departure (DOD) and Direction of Arrival (DOA) estimation in Multiple Input Multiple Output (MIMO) array. It has shown excellent angle estimation performance. However, the two dimensional angle peaks searching of MIMO MUSIC algorithm renders high computational complexity. In order to resolve this problem, a fast MIMO MUSIC algorithm for joint DOD-DOA Estimation based on Gibbs Sampling is proposed. The proposed algorithm combines Markov Monte Carlo method with MIMO MUSIC estimator. Firstly, the power of the MIMO MUSIC spectrum function is regarded as the target distribution up to a constant scalar, then the Gibbs sampler sample from the scalar, and the joint DOD-DOA estimation can be achieved from the linger position. Simulation results show that the proposed method not only keeps the similar DOA estimation performance which is achieved by the MIMO MUSIC algorithm, but also reduces the computational complexity.