Hyeonsu Kim

Performance Analysis of Underwater Acoustic Communication Systems Using Underwater Channel Simulation Tool

The performance of underwater acoustic communication system is sensitive to the Doppler shift and ISI(Inter-Symbol Interference). Therefore, the simulation algorithm needs to consider time-spread due to multipath arrivals which cause the ISI, and time-varying Doppler shift along with moving source and receiver. For this purpose, VirTEX(Virtual Time series EXperiment) based on Ray model has been developed. In this paper, VirTEX is used to compare the characteristics of ocean waveguide from the experimental data and illustrate the performance. The CIR(Channel Impulse Response) that characterizes the multipath arrivals with representative time-spread due to multipath arrivals is compared between numerically simulated and experimental probe signal. Also, the communication performance analysis for BER(Bit Error Rate) is compared between numerically simulated and experimental data signal. As a result, VirTEX can be useful as a simulation tool for evaluating the performance of underwater acoustic communication system.

Equalizer Mode Selection Method for Improving Bit Error Performance of Underwater Acoustic Communication Systems

The linear and decision-feedback equalization can mitigate time-varying intersymbol interference (ISI) caused by time-varying multipath propagation for underwater acoustic channels. The perfect elimination of interference components, however, is difficult using the linear equalization and the decision feedback equalizer has an error propagation problem. To overcome these shortcomings, this paper proposes an equalizer mode selection method using training sequences. The proposed method selects an equalization mode corresponding to the signal-to-noise ratio (SNR). If the SNR is low, the proposed system operates the linear equalizer for preventing the error propagation and if the SNR is high, the decision feedback equalizer for eliminating the residual ISI. Therefore, the proposed method can improve the error performance compared to the conventional equalizers. The computer simulation shows the proposed method improves the bit error performance using practical underwater channels responses acquired from the sea experiment.

A non-interfering cognitive radio system for spectrum sharing

We present a cognitive radio beamformer for spectrum sharing with primary users. The proposed method generates orthogonal multiple beams for cognitive users using orthogonal subspaces and opportunistic beamforming methods to increase the sum-rate of cognitive radio systems. Simulations demonstrate that the proposed scheme does not cause interference for the primary users and provides a sum-rate capacity of 2.72bps/Hz for up to 20 cognitive users with two transmit antennas.

Macrocell Protection Interference Alignment in Two-Tier Downlink Heterogeneous Networks

Conventional interference alignment (IA) has been developed to mitigate interference problems for the coexistence of picocells and macrocells. This paper proposes a macrocell protection interference alignment (MCP-IA) in two-tier MIMO downlink heterogeneous networks. The proposed method aligns the interference of the macro user equipment (UE) and mitigates the interference of the pico-UEs with a minimum mean squared error interference rejection combining (MMSE-IRC) receiver. Compared to the conventional IA, the proposed MCP-IA provides an additional array gain obtained by the precoder design of the macro BS and a diversity gain achieved by signal space selections. The degrees of freedom (DoF) of the proposed MCP-IA are equal to or greater than that of the conventional IA and are derived theoretically. Link level simulations show the link capacity and the DoF of the macro UE, and also exhibit the proposed MCP-IA attaining additional array gain and diversity gain. The system level simulation illustrates that the proposed method prevents the interference of the macro UE completely and preserves the throughput of the pico-UE irrespective of the number of picocells. For antenna configuration, the system level simulation demonstrates that the proposed MCP-IA throughput of the macro UE is not affected by the number of picocells and that the proposed MCP-IA throughput of the picocells approaches that of single-user MIMO (SU-MIMO) with a 3% loss.

Low collision random access using grouping nodes in underwater communications

This paper proposes a random access method using grouping nodes which are located closely to decrease collisions of nodes by long delay propagation time in underwater acoustic environments. To reduce collision probability, each node selects its group, which is predetermined by a distance from the fusion center based on RSSI level, and aligns its transmission time to the farthest node in the group. To reduce collision in a node group, nodes choose their subcarriers to load their information based on the RSSI level. Computer simulations demonstrate that Reception Success Rate (RSR) of the proposed scheme has five times larger than that of conventional PN sequence random access. Maximum throughput of the proposed method also exhibits two times larger than the conventional PN codes.

Underwater acoustic communication system using diversity based on ray modeled underwater acoustic channel in Yellow Sea

This paper proposes an adequate UWA (Underwater Acoustic) communication system of underwater communication network in the Yellow Sea. UWA channel is obtained from Bellhop ray tracing method with Yellow Sea environments. Based on this channel, communication parameters for CDMA (Code Division Multiple Access) and SC-FDM (Single Carrier-Frequency Division Multiplexing) using diversity techniques are calculated. In order to prove the proposed methods, BER (Bit Error Rate) and data rate are obtained using computer simulations and the adequate communication system for long rms delay spread and low Eb/No environments is proposed from the simulation.

Superimposed pilots aided estimation of phase varying channels for underwater acoustic communication

A channel estimation methodology that exploits superimposed pilot sequences for underwater acoustic communication is proposed and evaluated. The proposed scheme first estimates the time-varying channel from the received signal which includes the known superimposed pilot sequence. In a later stage fits the channel response in then fitted to a parametric model that characterizes each channel tap through linear phase progression. In order to evaluate the performance of the proposed scheme, measured channel responses are utilized. Simulation results indicate that the proposed scheme improve upon conventional schemes, particularly in the low signal-to-noise ratio region.

MMSE estimation with CFAR detection for underwater communication channel

Various channel estimation methods have been proposed for shallow underwater communication channel with sparse channel type. A matching pursuit (MP), which is one of typical sparse channel estimation methods, can detect dominant components of sparse channel sequentially as a portion of matching. Mean squared error of MP, however, converges to fixed value even though signal-to-noise ratio (SNR) increases. This paper proposes a minimum mean squared error (MMSE) estimation method with constant false alarm (CFAR) detection for sparse channel estimation. Unlike MP, the mean squared error of the proposed method decreases as SNR increases and is lower than that of conventional MMSE estimators on the sparse channel. Experimental results show that the proposed method estimates sparse underwater communication channel efficiently.

Extraction of Time Coherence Using Detection of Dominant Components for Underwater Acoustic Communication Channels at East Sea

In this paper, we propose a method that estimates the channel response from underwater communication signals with MMSE (Minimun Mean Squared Error) and detects dominant components automatically based on power of response components using CFAR (Constant False Alarm Rate). Statistical characteristics are analyzed with variation of magnitude and phase and time coherence via experimental data obtained by drifting transmitter and receiver. We show that bit error rate has small difference, 1.2 times, compared with the case using every channel information estimated within data period when estimation and equalization is performed with extracted characteristic obtained by the proposed method.