• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.12A
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• pp.941-949
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• 2011

In a cooperative communication system with a source node and multiple relays equipping single antenna and a destination node equipping multiple antennas, the selective cooperative spatial multiplexing scheme can obtain spatial multiplexing gain and additional selection diversity gain. But it can degrade a bit error rate performance because some received symbols forwarded from particular relays may be lost by attenuation due to path-loss. We propose a relay and transmission mode selection scheme which selects minimum number of multiple relays having the channel capacity larger than a given data rate and transmission mode which switches spatial multiplexing and spatial diversity mode in cooperation phase to enhance the bit error rate performance. The proposed scheme achieves 1.5~2dB gain at the low SNR range compared with the conventional scheme by obtaining additional spatial diversity gain.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.5C
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• pp.376-383
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• 2012

In this paper, we provide a general expression of spatial multiplexing gain (SMG) for two mutually interfering multiple-input multiple-output (MIMO) broadcast channels, referred to as MIMO-IBC, when some of user messages are made available to base stations through a common noiseless backbone line. The MIMO-IBC has two base stations and multiple users, each equipped with multiple antennas, where independent messages are transmitted over fixed channels. From the derived results, we observe the variation of the SMG with respect to the presence of a coordination as well as various antenna distributions, and compare the derived result to the SMG of the case with full cooperation among users.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.8B
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• pp.904-909
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• 2011

Cell zooming adjusts the cell range of base stations depending on traffic condition. The cell zooming can be implemented by the adjustment of antenna angles, the clustering of the base stations, and the cell relay. The base stations can adjust the cell range in term of energy efficiency, which can then reduce the overall energy consumption of cellular networks. There is, however, a trade-off between the energy savings and the blocking probability of user calls. A periodic scheme that manages the cell zooming of the base stations was proposed but it was inadequate for dealing with the dynamic nature of traffic patterns. This paper proposes a semi-periodic cell zooming scheme along with the algorithms that select such base stations and define the operation procedure. Simulation results show that the proposed method outperforms the existing scheme in terms of the energy savings without the degradation of the blocking probability.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.1A
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• pp.50-58
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• 2008

The use of space-division multiple access(SDMA) in the downlink of multiuser multi-input/multi-output(MIMO) wireless transmission systems can provide substantial gains in system throughput. When the channel state information(CSI) is available at the transmitter, a considerable performance improvement can be attained by adapting the transmission rates to the reported CSI. In addition, to combat frequency selective fadings in wideband wireless channels, bit-interleaved coded OFDM(BIC-OFDM) modulation schemes are employed to provide reliable packet delivery by utilizing frequency diversity through channel coding. In this paper, we propose an adaptive modulation and coding(AMC) scheme combined with an opportunistic scheduling technique for the MIMO BIC-OFDM with bandwidth-limited feedback channels. The proposed scheme enhances the link performance by exploiting both the frequency diversity and the multiuser diversity. To reduce the feedback information, the proposed AMC scheme employs rate adaptation methods based on an OFDM symbol rather than on the whole subchannels. Simulation results show that the proposed scheme exhibits a substantial performance gain with a reasonable complexity over single antenna systems.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.6A
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• pp.642-647
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• 2007

HAP(High Altitude Platform) systems have been proposed due to their unique advantages over terrestrial and satellite systems as the alternative wireless communication system to deliver the third generation IMT-2000 wireless services. It has been required to study for the power control in W-CDMA HAP system as well as the terrestrial mobile system in order to mitigate interference and increase the capacity. In this paper, a new power control has been proposed for HAP system considering the interference profile into the DB(distributed balancing) SIR(signal to interference ratio)-based algorithm which has been considered in terrestrial system, and estimated by the outage performance of the proposed DB algorithm is better remarkably than DBPA(distance-based power allocation) which is proposed for HAP system, and it is the same regardless of the antenna maximum gain and its sidelobe characteristics.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.3A
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• pp.319-326
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• 2008

All over the world more than 2,000 VORs(VHF Omni-directional Ranges) including 39 in Korea are being operated for formulating air routes. But VOR reveals some difficulties in cost and time when relocating already an installed one due to its poor performance from the interference by its geometrically surrounding obstacles. DVOR(Doppler VOR) which is more popular than CVOR(Conventional VOR) has huge antennas and make it very difficult to assume a sufficient number of proper site candidates for permanent installation. This paper develops an efficient method for predicting proper site of DVOR installation by isolating transmitter from sideband antennas and counterpoise, making the transmitter easily moveable and verifying the performance of DVOR only with a transmitter and an antenna. The method was tested with a real flight measurement for DVOR performance around Gwangju Airport in Korea.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.3A
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• pp.327-334
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• 2008

Conventional on-channel-repeaters (OCRs) have a crucial problem that the power of a re-transmitted signal is highly limited by a feedback signal due to antenna coupling. The power limitation problem in OCRs has been solved by incorporating a demodulation-type feedback canceller which eliminates unwanted feedback signals by estimating a feedback channel. In applying the demodulation-type feedback canceller to T-DMB repeaters, there is a troublesome problem of unfrequent known pilot symbols, resulting in poor convergence performance of channel estimation. To solve this problem and enhance the accuracy of estimation, we propose a partitioning method of the Phase Reference Symbol (PRS) transformed in time domain. Since filter coefficients are updated every one partitioned subgroup, the number of updates is increased by the number of partitioned subgroups and thus the convergence speed is enhanced. The improved performance of feedback-channel estimation is directly connected with the feedback-cancellation performance. Simulation result shows that the feedback canceller incorporating the proposed partitioning method has a good performance in terms of residual feedback power.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39B no.3
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• pp.151-161
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• 2014

3GPP LTE-Advanced (Release 10) system specifies carrier aggregation (CA) to enable high data rate on using multiple frequency bands, including the variout CA-specific deployment scenarios. Considering one of those scenarios in which the different directional sector antenna is employed by each frequency band, we propose a per-carrier cell selection scheme that can improve the average throughput of the cell-edge users by allowing each user equipment (UE) to select the frequency band of the adjacent cell. Furthermore, a distributed algorithm for inter-cell copperative scheduling in this scheme is proposed to support proportional fairness among the cells. It has been shown that the proposed scheduling algorithm for the per-carrier cell selection scheme improves the cell-edge user throughput roughly by 50% over that of the conventional scheme.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.53 no.6
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• pp.330-336
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• 2004

In this work, in order to obtain the detailed information about lightning electromagnetic pulses, the electric fields radiated from multiple lightning return strokes were measured and analyzed statistically. The electric field measuring system consists of a hemisphere antenna of 30cm in diameter, integrator and data acquisition device, and its frequency bandwidth ranges from 200Hz to 1.56MHz, and the sensitivity is 0.96㎷/V/m. The electric field signals are digitized every 200ns with the transient signal analyzer having the resolution of 12-bit and the recording length of 5 kilowords and are registered at personal computer. As a result, the electric fields produced by the first return stroke begin with a slow initial part or front, which starts just after or during the last stepped leader. On the average the rise times of the electric fields for the positive first, second and third strokes are 4.21${\mu}\textrm{s}$, 3.94${\mu}\textrm{s}$ and 2.75${\mu}\textrm{s}$, respectively, and those for the negative first, second and third strokes are 3.46${\mu}\textrm{s}$, 3.15${\mu}\textrm{s}$ and 2.79${\mu}\textrm{s}$, respectively. The zero-crossing times of the electric fields for first return strokes range from about 10 to 80${\mu}\textrm{s}$. The mean zero-crossing times for subsequent return strokes are shorter than those for first lightning return strokes.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.10 s.89
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• pp.969-976
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• 2004

The wide and growing use of cellular phones has raised the question about the possible health risks associated with radio-frequency electromagnetic fields. It would be helpful for phone users to blow the exposure levels during cellular phone use. But it is very difficult to recognize the amount of exposure, because measuring accurate level of RF is a difficult matter. In this study, we developed a model to estimate the exposure level and the individual risk of exposure by utilizing the available informations that we can get. We used such parameters as usage time a day, total using period, distance between cellular phone and head, slope of cellular phone, hands-free usage, antenna pulled out or not SAR(Specific Absorption Rate) of cellular phone, and flip or folder type. We proposed a model presenting individual risk of RF exposure from level 0 to 10 by using a neural network.