Hak-Lim Ko

Performance Analysis of the Spatial Correlation for Underwater Channel Environments

In this paper, we have analyzed the performance of space diversity technologies using experimental data in order to analyze the usability of space diversity techniques in underwater channel environments. After analyzing the experimental data we found out that high diversity gain should not be expected when using a stationary transmitter with a stationary receivers in swallow underwater channel. And we also find out that the distance between sensors should be at least 8 wavelengths apart to take advantage of diversity gain in underwater moving channel environments.

A switched beamforming system with multiuser detectors

We propose a switched beamforming system combined with multiuser detectors (MUD) in order to suppress the multiple access interference that is coming from the directions close to the desired users signal. Since a multiuser detector, in the proposed system, works only for one beam signal, the required processing delay is significantly reduced compared to the conventional MUD. We have simulated the proposed system and our simulation results show that the proposed interference canceller achieves high SINR in a heavy interference channel, which a conventional spatial filtering system cannot achieve.

Performance analysis of beamforming techniques in ad-hoc communication between moving vehicles

In this paper we have analyzed the performance of adapting beamforming techniques to the Ad-hoc communication between moving vehicles. For the analysis we measured the wireless channel characteristics while moving in high speeds with two vehicles, a transmitter and a receiver. After analyzing the measurement data we found out that the effect of fading can be reduced significantly (9 dB with 99% reliability). Also, the reliable communication range can be increased from not only the beamforming gain but also the gain obtained by reducing the effects of fading. Additionally, in this paper we have analyzed the effects of beamforming to the RMS delay spread using our collected data. Through our analysis we conclude that with beamforming techniques, the adaptive equalizer can be simplified in moving channels.

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.

The performance analysis of diversity technologies for underwater moving channel environments using experimental data

In this paper, we have analyzed the performance of diversity technologies using experimental data recorded from mobile underwater channel environments in order to analyze the usability of MIMO techniques in underwater channel environ-ments. After analyzing the experimental data we found out that high diversity gain should not be expected when using a stationary transmitter with a stationary array of receivers in swallow underwater channels.

The smart antenna module for RF repeater

The signals received at the receiving antenna are consist of signals from base station (or mobile terminal) and transmitted signals from the other side of antenna in a RF repeater. And the amplification rate of the RF repeater is limited by the feedbacked signals from the same repeater. Therefore, a receiving antenna has to be isolated from a transmitting antenna in order to reduce the effect of the feedbacked signal in the RF repeater. Or, recently, ICS (interference cancellation system) techniques have been studied to estimate and to remove the feedback signals. However, it requires lots of hardware complexity and computational delay. To solve these problems, we have studied the implementation and adaptation of smart antenna system for RF repeaters. We have designed a smart antenna module with switching beam structure in order to reduce the hardware and computational complexity. After analyzing the proposed smart antenna module, we found out that the proposed repeater increases the amplification rate 15dB compare to the conventional RF repeater.

Virtual Mobile Backbones for Wireless Ad-hoc Networks with the support of Directional Array Antennas

Ad-hoc network techniques are in the center of attention to many scientists and engineers in the fact that wireless ad-hoc networks will bring high speed communication to the users with no given fixed infrastructure such as the base station or the AP (access point). However, the infrastructure-less topology brings new limitations such as small connectivity of nodes in the network caused by the short communication range of nodes. In this work, to solve these problems we propose the adaptation of directional array antennas in a virtual mobile backbone ad-hoc network. This adaptation can partially solve the complexity problem by activating directional array antennas only at backbone nodes and by performing beamforming techniques it will bring better connectivity of the nodes in the network resulting higher throughput performance

The Performance Analysis of Diversity Technologies for Mobile Ad Hoc Communications Between Moving Cars

In this paper, we have applied diversity technologies to the IEEE 802.11g to use the IEEE 802.11g in moving channel environments. After analyzing the measurement data, it is shown that the diversity gain is 14 dB at the probability of 1%. The reliable communication range is 63 m when transmitting power is 10 dBm, while the range increases to 163 m when the diversity technique is used. Also the communication range increases from 122 m to 306 m by using diversity technologies when the transmitting power is 20 dBm

The performance analysis of STTD combined with angle diversity scheme

Diversity is essential in achieving good performance in a fading channel. In this paper we have analyzed the performance of the STTD combined with angle diversity scheme. We have simulated the proposed scheme under the assumption that the received signals from different angles have different signal powers. Our simulation results show that the proposed scheme has better performance than the space diversity system when one of the received path signal power is about 0.2 times of the power of the other received path signal.

Smart antenna for multiuser detector

We propose a switched beamforming system with a multiuser detector in order to suppress the multiple access interference that is coming from the desired users direction. Since a multiuser detector in the proposed system work only for one beam signal, the required processing delay and the hardware complexity are significantly reduced. We have simulated the proposed system and our simulation results show that the proposed interference canceller achieves a low BER in a heavy interference channel, which conventional spatial filtering system cannot achieve.