Youngchol Choi

A digital acoustic transceiver for underwater communication without PLL and DLL

This paper proposes a phase coherent all-digital transceiver for underwater acoustic communication, which allows the system to reduce complexity and increase robustness in time variant underwater environments. It is designed in the digital domain except for amplifiers and implemented by using a multiple digital signal processors (DSPs) system. For phase coherent reception, conventional systems employed phase-locked loop (PLL) and delay-locked loop (DLL) for synchronization, but this paper suggests a new frame synchronization scheme based on the quadrature receiver structure without using them. DSP implementation is based on block data parallel architecture (BDPA). We show experimental results in the underwater anechoic basin at KRISO. The results indicate that the frame synchronization is performed without PLL. Also, we show that the adaptive equalizer can compensate frame synchronization error and the correction capability is dependent on the length of equalizer.

A phase coherent all-digital transmitter and receiver for underwater acoustic communication systems

This paper proposes a phase coherent all-digital transmitter and receiver system for underwater communication, which allows the system to reduce complexity and increase robustness in time variant underwater environments. It is designed in the digital domain except for amplifiers and implemented by using a multiple digital signal processors (DSPs) system. For phase coherent reception, conventional systems employed phase-locked loop (PLL) and delay-locked loop (DLL) for synchronization, but this paper suggests a new frame synchronization scheme based on the quadrature receiver structure without using them. We show experimental results in the underwater anechoic basin at KRISO. The results indicate that the frame synchronization is performed without PLL. Also, we show that the adaptive equalizer can compensate frame synchronization error and the correction capability is dependent on the length of equalizer.

An experimental model of surface reflection in the underwater anechoic basin at KRISO

The behavior of propagating waves in the ocean is highly dependent on the surface geometry of the ocean as well as the spatial distributions of temperature, pressure, and salinity in the media. Waves are refracted due to the spatial variation of the media properties, reflected by free surfaces and bottoms, and scattered by small floating particles. These phenomena make it difficult to identify original signals during acoustic measurements. Among them reflection has the most significant effect on signal distortion by building up multi-paths between transmitters and receivers. In this research, a surface reflection model is investigated by an experimental approach. The experiments were performed in the underwater anechoic basin at KRISO/KORDI. It is found that a reflected wave is composed of three parts: negative and positive peaks and a ringing component. Among them the negative peak plays a dominant role in the surface reflection.

Broadband FIR Beamformer for Underwater Aoustic Communication

Beamforming for underwater acoustic communication (UAC) is affected by the broadband feature of UAC signal, which has relatively low center frequency compared with the signal bandwidth. The narrow-band assumption does not hold good in the UAC. (Unrealistic assumption) In this paper, we present computer simulation results of a broadband FIR beamformer for UAC using the baseband equivalent array signal model for phase-coherent UAC. We consider a vertical uniform linear array with 8 omni-directional elements and UAC system with carrier frequency 25 KHz and symbol rate 5 KHz. Pulse shaping filter (PSF) is the raised cosine filter with roll-off factor 0.25. Hence bandwidth to carrier frequency ratio is 0.25. The sensor spacing is half of the wavelength corresponds to carrier frequency. For simplicity, we assume that there is no signal fading and phase synchronization is perfect. We have considered two propagating paths: direct and surface reflected ones. The surface reflected path is regarded as interference signal. The signal to noise ratio (SNR) and signal to interference ratio (SIR) are fixed to 6 dB and OdB, respectively. The results show that the broadband FIR beamformer achieves optimum signal to interference and noise ratio (SINR) and outperforms the conventional narrowband beamformer by 0.5 dB of SINR when the inter-tap delay is a quarter of the symbol interval and the order of FIR filter is one. The broadband FIR beamformer performance is degraded when the FIR filter order is increased above the optimum value. The inter-tap delay should be small enough and it is related to the optimum FIR filter order.

Design of Underwater Ad-hoc Communication Protocol for Underwater Acoustic Networks

In this paper a cross layer protocol, referred to as an underwater ad-hoc communication (UAC) protocol, is proposed for underwater acoustic networks (UANets). An underwater node (UN), which tries to transfer data to another UN or a buoy in ad-hoc manner, can access channel as well as determine routing path by employing the UAC protocol. The channel access, route determination, and reliable data transfer are designed being adaptive to underwater environments. In addition, we propose both UN and packet architectures in order to efficiently implement the UAC protocol for UANets.

Phase tracking loop in the underwater QPSK transmission

In this paper, we propose a design method for phase tracking loop with 2nd order loop filter, which can trace a small frequency variation as well as a phase variation. The loop parameter totally governs the traceable frequency range, the transient period and the phase fluctuating bound at the steady state. To determine the proper loop parameter, the proposed algorithm iteratively computes the loop parameter sets using the loop characteristics at the transient state and the steady state until the fluctuating bound of tracking phase error is less than the desired bound. After sequentially applying the computed parameter sets to the loop, the desired performance of phase tracker can be obtained when the symbol error rate is reasonably small.

Applications of beamforming and acoustic holography in an anechoic tank with surface reflections

Recently, our institute, KRISO/KORDI, constructed a small anechoic tank which has an anechoic lining at the four walls and bottom. Because surface reflections still occur, special care must be taken for acoustic measurements and array signal processing, for example, source localization and acoustic holography. In this paper the effects of the surface reflections to the array signal processing methods are investigated. First, estimated errors by both methods are analyzed for various measurement parameters. Beamforming experiments with vertical line arrays are performed. Holographic reconstruction of simple and complex acoustic sources is also performed to compare the true and estimated results.