• Journal of the Korea Institute of Information and Communication Engineering
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• v.24 no.3
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• pp.405-411
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• 2020

Recently, studies on underwater communication, related to the development of underwater resources, disaster monitoring and defense, have been actively carried out. In the design of wireless communication systems, path loss is the most important information to derive a link budget that is required to guarantee communication reliability by calculating received power level for the given communication link. The underwater acoustic channel have different characteristics according to geographical location and relevant environmental factors such as water temperature, depth, wave height, algae, and turbidity. Subsequently, many research institutes aiming to develop underwater acoustic communication systems are researching actively on the underwater acoustic channels in various sea areas. In Korea, however, studies on the path loss of the acoustic channel are still insufficient. Therefore, in this study, the path loss of the acoustic channel are studied based on measurement data of the at-sea experiment conducted at Geohae-do, southern sea of Korea.

• New & Renewable Energy
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• v.3 no.3
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• pp.14-19
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• 2007

A flow channel model of a bipolar plate with varying cross-sectional area was newly designed for improving performance and efficiency of a PEM fuel cell stack. As a result, the varying cross-sectional area model showed poor uniformity in velocity distribution, however, maximum velocity in the flow path is about 30% faster than that of the uniform cross-sectional area model. The proposed varying cross-sectional area model is expected to diffuse operating fluids more easily into diffusion layer because it has relatively higher values in pressure distribution compared with other flow channel models. It is expected that the implementation of the varying cross-sectional area model can reduce not only the mass transport loss but also the activation loss in a PEM fuel cell, and open circuit voltage of a fuel cell can thus be increased slightly.

• Journal of the Korean Institute of Telematics and Electronics S
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• v.35S no.7
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• pp.6-13
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• 1998

To constoruct a multi-channel system like Optical Frequency Division Multiplexing (OFDM) with the vast amount of transmission capability, a frequency stabilization is essential technique for getting narrower channel spacing and for minimizing fluctuation of oscillation frquency of each channel. This paper proposes a novel multi-channel optical frequency stabilization scheme that uses wavelength crossover properties of a Arrayed Waveguide Grating(AWG). The proposed scheme includes an effective control algorithm that carries out frequency stabilization of all channels through a simple control circuit, simulaneously. And also, a design method of AWG is included which has higher resolutio in consideration of system loss.

• International Journal of Advanced Culture Technology
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• v.9 no.4
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• pp.307-314
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• 2021

Recently, positively asymmetric binary pulse amplitude modulation (2PAM) has been proposed to improve the bit error rate (BER) performance of the weak channel gain user, with a tolerable BER loss of the strong channel gain user, for non-orthogonal multiple access (NOMA). However, the BER loss of the stronger channel gain user is inevitable in such positively asymmetric 2PAM NOMA scheme. Thus, we propose the negatively asymmetric 2PAM NOMA scheme. First, we derive closed-form expressions for the BERs of the negatively asymmetric 2PAM NOMA. Then, simulations demonstrate that for the stronger channel gain user, the BER of the proposed negatively asymmetric 2PAM NOMA improves, compared to that of the conventional positively asymmetric 2PAM NOMA. Moreover, we also show that for the weaker channel gain user, the BER of the proposed negatively asymmetric 2PAM NOMA is comparable to that of the conventional positively asymmetric 2PAM NOMA, over the power allocation range less than about 10 %.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.12 no.12
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• pp.5917-5935
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• 2018

Adaptive Multi-Rate Codec is one of the codecs which is used for making voice calls. It helps to connect people who are scattered in various geographical areas. It adjusts its bit-rate according to the user's channel conditions. It plays a vital role in providing an improved speech quality of voice connection in Long Term Evolution (LTE). There are some constraints which need to be addressed in providing this service profitably. Quality of Service (QoS) is the dominant mechanism which determines the quality of the speech in communication. On several occasions, number of users are trying to access the same channel simultaneously by standing in a particular region for a longer period of time. It refers to Multi-user channel sharing problem which leads to resource loss very often. The main aim of this paper is to develop a novel RSA - QoS scheduling algorithm for reducing the Resource Loss Ratio. Eventually, it increases the throughput.The simulation result shows that the RSA - QoS increases the number of users for accessing the resources better than the existing algorithms in terms of resource loss and throughput. Ultimately, it enhances the QoS in Mobile Networks.

• Journal of the Optical Society of Korea
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• v.11 no.2
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• pp.51-54
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• 2007

A holographic demultiplexer aimed at producing low polarization dependence loss using a long period volumetric diffraction grating inside photopolymer has been reported and experimentally demonstrated. To obtain the long period gratings, two kinds of gratings are recorded by angle of $4^{\circ}\;and\;6^{\circ}$ corresponding to the polarization dependence property of below 0.5 dB. From the experimental results, we demonstrate the 0.8 nm-spaced 21 channel holographic demultiplexer with the polarization dependence loss of less than -0.38 dB, the channel uniformity of 0.495 dB, and the channel crosstalk of -13 dB.

• ETRI Journal
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• v.42 no.6
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• pp.827-836
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• 2020

Measuring the diffraction loss for high frequencies, long distances, and large diffraction angles is difficult because of the high path loss. Securing a well-controlled environment to avoid reflected waves also makes long-range diffraction measurements challenging. Thus, the prediction of diffraction loss at millimeter-wave frequency bands relies on theoretical models, such as the knife-edge diffraction (KED) and geometrical theory of diffraction (GTD) models; however, these models produce different diffraction losses even under the same environment. Our observations revealed that the KED model underestimated the diffraction loss in a large Fresnel-Kirchhoff diffraction parameter environment. We collected power-delay profiles when millimeter waves propagated over a building rooftop at millimeter-wave frequency bands and calculated the diffraction losses from the measurements while eliminating the multipath effects. Comparisons between the measurements and the KED and GTD diffraction-loss models are shown. Based on the measurements, an approximation model is also proposed that provides a simple method for calculating the diffraction loss using geometrical parameters.

• Journal of Communications and Networks
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• v.16 no.3
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• pp.322-334
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• 2014

Multimedia content is very sensitive to packet loss and therefore multimedia streams are typically protected against packet loss, either by supporting retransmission requests or by adding redundant forward error correction (FEC) data. However, the redundant FEC information introduces significant additional bandwidth requirements, as compared to the bitrate of the original video stream. Especially on wireless and mobile networks, bandwidth availability is limited and variable. In this article, an adaptive FEC (A-FEC) system is presented whereby the redundancy rate is dynamically adjusted to the packet loss, based on feedback messages from the client. We present a statistical model of our A-FEC system and validate the proposed system under different packet loss conditions and loss probabilities. The experimental results show that 57-95%bandwidth gain can be achieved compared with a static FEC approach.

• Journal of electromagnetic engineering and science
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• v.15 no.2
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• pp.97-103
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• 2015

This paper investigates the effect of absorption loss by a human body to the received signal strength with respect to on-body transmitting antenna positions in on-to-off wireless body area networks. This investigation is based on measurement results obtained from experiments performed on human bodies (male and female) using planar inverted-F antennas in an anechoic chamber. The total absorption loss by the human body is also presented through the SEMCAD-X simulations. Our investigation showed that the received signal strength becomes lower when the transmitting antenna is mounted at a specific position where more absorption loss is experienced. The statistical analyses of on-to-off body channel characteristics based on the measurement results are presented.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.9A
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• pp.1306-1312
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• 2000

The quality loss of a X-band transmitter has been derived by means of MC simulation. The transmitter as a payload of LEO(Low Earth Orbit) satellite is capable of the down transmission the image data of hundreds Mbps generated from the Electro-Optical Instrument in real time. The parameters such as data asymmetry amplitude unbalance,phase unbalance, wave shaping and channel interference are included in the quality loss simulation Assuming that normally distributed gaussian noise is simply added to the channel, the quality loss of 0.7 dB has been obtained through this simulation based on a 95% confidence interval. The obtained quality loss can be applied to the link budgets as an additional loss item.