• Journal of the Korea Academia-Industrial cooperation Society
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• v.7 no.2
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• pp.195-199
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• 2006

A wideband power amplifier for a frequency hopping radio unit is designed. To obtain higher efficiency, it is designed for the Class B mode overdrived. The broadband transformer and feedback circuits are adapted to obtain broadband characteristics. The designed amplifier is simulated using the ADS, which is a CAD software from HPEEsof, Simulation results of the designed amplifier are well suited for the design specifications. The designed amplifier are fabricated. Measured results of the fabricated amplifier well agreed with the simulation results and are in good agreement with the predicted performance.

• Korean Journal of Materials Research
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• v.18 no.1
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• pp.26-31
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• 2008

The dc resistivity and thermoelectric power of bilayered perovskite $La_{1.4}(Sr_{0.2}Ca_{1.4})Mn_2O_7$ were measured as a function of the temperature. In the ferromagnetic phase, ${\rho}(T)$ was accurately predicted by $a_0+a_2T^2+a_{4.5}T^{4.5}$ with and without an applied field. At high temperatures, a significant difference between the activation energy deduced from the electrical resistivity and thermoelectric power, a characteristic of small polarons, was observed. All of the experimental data can be feasibly explained on the basis of the small polaron.

• Proceedings of the Korean Institute of Communication Sciences Conference
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• 1987.04a
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• pp.201-205
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• 1987

In FH-SS systems when the bopping band is enough wide to onerlap with conventional band limited communication cethod (CBM). The portion of suchacts as an interfering signal. Thus it is gard to use them all together. This paper presents how the frequency gopping systems can simultaneously share the same band with CBM. The proposed mithod is that the frequency gopping band can arbitraily controlled by setting the specific input bith of digital frequency synthesizer to logical zero state We realized this by putting the hopping band Controller between pseudeo random generater and frequency synthesizer.

• Journal of information and communication convergence engineering
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• v.6 no.1
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• pp.47-50
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• 2008

A novel high channel efficiency transceiver based on a fast acquisition frequency synthesizer has been designed. The direct digital synthesis (DDS) technique is applied and a simple memory look-up table is incorporated to expedite channel acquisition. The technique simplifies the frequency control process in the transceiver and thus reduces the channel switching time. As a result, the channel efficiency is improved. The designed transceiver is ideal for frequency hopping mobile communication applications.

• Proceedings of the IEEK Conference
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• 2002.06a
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• pp.137-140
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• 2002

2.4Ghz ISM 대역에서 동작하는 Bluetooth는 동일한 대역에서 동작하는 WLAN(802.11.b), Cordless phone, microwave oven등의 신호가 interference로 작용하여, 성능이 크게 저하된다. 특히 Bluetooth과 WLAN은 상호간 interference로 작용하여 상호 성능 저하를 초래한다. 따라서, 두 장치간의 coexistence를 위한 방안으로 adaptive hopping frequency(AFH)이 제안되었다. 이 논문에서는 3 channel status를 이용한 AFH 방식을 제안하고, 그 성능을 검증하였다.

• Proceedings of the Korea Institute of Convergence Signal Processing
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• 2003.06a
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• pp.213-216
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• 2003

UWB 통신 기술은 초광대역 특성 때문에 높은 전송 속도와 저전력 통신이 가능하여 의료장비에 응용이 가능하며, 특히 유선이라는 제약성으로 큰 불편을 겪고 있는 ECG시스템에 적용되어질 수 있다. 따라서 근거리 무선 통신 기술로 급부상하고 있는 UWB 통신 기술의 응용을 위해 UWB 신호 특성 및 변조 방식의 연구가 행하여져야 한다. 이에 본 논문에서는 UWB에 사용가능한 각종 펄스의 특성과 적용하고자 하는 시스템 환경에서 발생할 수 있는 UWB 신호의 간섭 영향을 분석하였으며, 간단한 UWB 송수신 시스템을 시뮬레이션 하였다.

• Journal of the Institute of Convergence Signal Processing
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• v.2 no.3
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• pp.39-44
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• 2001

In frequency hopping spread spectrum(FHSS) communication systems, exact frequency synchronization is required due to the random hopping of carrier frequencies between transmitter and receiver even under harsh channel conditions. For synchronization of FHSS communications, multi-frequency hopping synchronization(MFHS) method has been used in which a small set of frequencies are repeatedly sent several times for long duration. But this long duration resulted in being easily detected by the unauthorized users as well as long duration of acquisition time. In this paper, motivated by these problems, an adaptive synchronization method(ASM) is proposed. ASM is technics to reduce the synchronization time where the number of synchronization frequencies and repetition numbers is adaptively changed (increased or decreased) according to the channel conditions. The performance analysis showed that the time duration of synchronization was reduced to 0.2sec, and the influence of jamming or interference was decreased to 46% in ASM.

• Journal of the Korean Ceramic Society
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• v.39 no.3
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• pp.294-298
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• 2002

Polycrystalline $La_{2.1}Sr_{1.9}Mn_3O_{10}$ with layered perovskite structure have been successfully synthesized and investigated with respect to their thermoelectric, electric and magnetic properties. The large magnetoresistance (MR) effect with $-{\Delta}{\rho}/{\rho}_0$ of ∼120% at 0.85T was observed in a wide temperature range below a cusp temperature in resistivity of about 120K, which is well below the magnetic $T_C$. At high temperature, a singnificant difference between the activation energy deduced from the electrical resistivity and thermoelectric power, a characteristic of small polaron, is observed. All of the experimental data can be well explained on the basis of the small polaron model.

• IEIE Transactions on Smart Processing and Computing
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• v.4 no.5
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• pp.371-378
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• 2015

In this paper, a different type of pulse width modulation (PWM) control scheme for a buck converter is introduced. The proposed buck converter uses PWM with frequency hopping and a low quiescent.current low dropout (LDO) voltage regulator with a power supply rejection ratio enhancer to reduce high spurs, harmonics and output voltage ripples. The low quiescent.current LDO voltage regulator is not described in this paper. A three-bit binary-to-thermometer decoder scheme and voltage ripple controller (VRC) is implemented to achieve low voltage ripple less than 3mV to increase the efficiency of the buck converter. An internal clock that is synchronized to the internal switching frequency is used to set the hopping rate. A center frequency of 2.5MHz was chosen because of the bluetooth low energy (BLE) application. This proposed DC-DC buck converter is available for low-current noise-sensitive loads such as BLE and radio frequency loads in portable communications devices. Thus, a high-efficiency and low-voltage ripple is required. This results in a less than 2% drop in the regulator's efficiency, and a less than 3mV voltage ripple, with -26 dBm peak spur reduction operating in the buck converter.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.15 no.12
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• pp.2534-2542
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• 2011

In this paper, we show that performance can be improved by using multiple antennas in the conventional orthogonal code hopping multiplexing (OCHM) scheme, which was proposed for accommodating a larger number of users with low channel activities than the number of orthogonal codewords used in code division multiple access (CDMA)-based communication systems through downlink statistical multiplexing. First, we introduce two different types of OCHM systems together with orthogonal codeword allocation strategies, and then derive their mathematical expression for log-likelihood ratio (LLR) values according to the two different schemes. Next, when a turbo encoder based on the LLR computation is used, we evaluate performance on the frame error rate (FER) for the aformentioned OCHM system. For comparison, we also show performance for the existing symbol mapping method using multiple antennas, which was used in 3GPP standards. As a result, it is shown that our OCHM system with multiple antennas based on the proposed orthogonal codeword allocation strategy leads to performance gain over the conventional system---energy required to satisfy a target FER is significantly reduced.