Real Underwater Acoustic Channel Research Articles

Analytical method of parameter determination for OFDM system over measurement-based underwater acoustic channel modeling

In underwater acoustic (UWA)-OFDM systems, the major problem in implementation of complex equalization algorithms at receiver can be avoided by finding appropriate system parameters. This paper proposes an analytical method for determining the UWA-OFDM system parameters in order to maximize the achievable system capacity and to limit interference power. Unlike most existing research studies that analyzed the system performance without consideration of the inter-carrier interference (ICI), we focus on evaluating the UWA-OFDM system performance under both the ambient noise and ICI effect. The signal-to-interference-plus-noise ratio and the achievable system capacity are formulated by using the simulation UWA channel model, which is derived from the measurement data of the real UWA channel from a very shallow water environment in West Lake, Hanoi, Vietnam. Using the proposed method, the UWA-OFDM system parameters including the symbol duration, the number of subcarriers, the system bandwidth, and the transmit power are determined to achieve the maximal spectral efficiency of 3.2bits/sec/Hz. The proposed analytical method can be used as a practical guideline to determine appropriate transmission parameters for the design of UWA-OFDM systems.

Methods of Designing Shallow Underwater Acoustic Channel Simulators

In this paper, two typical methods of designing underwater acoustic (UWA) channel simulators, the geometry based and the measurement based, are investigated. Then, the performance of each method is analyzed by comparing the statistical properties of the simulated channels with those of the real shallow UWA channel measured in Halong bay, Vietnam, in June 2015. The results show that the measurement-based channel simulator provides the simulated channel which matches well with the real UWA channel, but it requires application of complex optimization computation methods to estimate a larger number of channel parameters. The geometry-based simulator has a lower complexity than the measurement-based simulator. However, the statistical properties obtained by this simulator do not fit with those of the real UWA channel. This is our motivation to propose an effective channel simulator, which is not only simple in computation but also in good agreement with the real UWA channel. The parameters of the proposed UWA channel simulator can be directly exploited from the measurement data without applying any optimization computation method. Moreover, the simulation results show that the channel statistical properties obtained by the proposed method match well with those of the real UWA channel.

CSI feedback-based CS for underwater acoustic adaptive modulation OFDM system with channel prediction

In this paper, we investigate the performance of adaptive modulation (AM) orthogonal frequency division multiplexing (OFDM) system in underwater acoustic (UWA) communications. The aim is to solve the problem of large feedback overhead for channel state information (CSI) in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing (CS). We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate (BER) and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying (QPSK) modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform (DCT) method.

수중 통신 환경을 위한 MIMO-OFDM 시스템 설계

In this paper, we first analyze the differences of underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems and conventional terrestrial OFDM system, and give a simple introduction of the backgrounds. By considering the real UWA channel environments, the measured channel data is used to generate the UWA channel model and calculate the relative parameters for underwater OFDM systems. Practical least square (LS) based channel estimation with linear interpolation are adopted to obtain the channel state information (CSI) at receiver side. As multi-input multi-output (MIMO) processing techniques, Alamouti code is implemented and evaluated to perform for space time block coding (STBC) and space frequency block coding (SFBC) for UWA OFDM systems with the MIMO configuration of , at the same time, maximum ratio combining (MRC) is performed for the purpose of comparison. The simulation results show that, with perfect channel estimation, SFBC failed to work duo to the serious frequency selectivity of UWA channel environments. When the practical channel estimation is applied, in the case of STBC, the proposed 4-column pilot pattern gives better performance about 7dB than SISO system.

An Underwater Acoustic Implementation of DFT-Spread OFDM

This paper presents a design of DFT-spread OFDM system applied to an underwater acoustic channel. It does not only combine all the advantages of a conventional OFDM system but also reduces the peak-to-average power ratio of the transmit signal. Besides, the scheme spreads the information over several subcarriers as a result of the application of an additional DFT operation and leads to a diversity gain in a frequency-selective fading channel, which is one of the many challenges of communicating data through an underwater acoustic channel. Simulation results show that our proposal possesses good bit-error-rate performance. The system has been tested in a real underwater acoustic channel—the experimental pool in Xiamen University. The experimental results show that the DFT-spread OFDM system can achieve better results than a simple OFDM system in a benign underwater channel.