• ETRI Journal
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• v.38 no.1
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• pp.90-99
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• 2016

In this work, we investigate differentially encoded blind transceiver design in low signal-to-noise ratio (SNR) regimes for orthogonal frequency-division multiplexing (OFDM) signaling. Owing to the fact that acquisition of channel state information is not viable for short coherence times or in low SNR regimes, we propose a time-spread frequency-encoded method under OFDM modulation. The repetition (spreading) of differentially encoded symbols allows us to achieve a target energy per bit to noise ratio and higher diversity. Based on the channel order, we optimize subcarrier assignment for spreading (along time) to achieve frequency diversity of an OFDM modulated signal. We present the performance of our proposed transceiver design and investigate the impact of Doppler frequency on the performance of the proposed differentially encoded transceiver design. To further improve reliability of the decoded data, we employ capacity-achieving low-density parity-check forward error correction encoding to the information bits.

• Proceedings of the KIEE Conference
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• 2000.11b
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• pp.361-364
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• 2000

In this study, the system which can make the generator's output voltage more stable by using the inverter in terms of PWM method, is designed It is one of the method reducing velocity of the wind in the process of the wind power generation. Thus, in this system, it is necessary to use a excellent current control inverter. So pulse with modulation method with a high-speed switching element is introduced to control the output current. And also, in order to get a fast response when the standard current generated by the vector control algorithm is supplied with the generator, the output control system with the fast response character and the best current control character is suggested. In this way, the result from the introduction of the control system is that a response character to the changable velocity of the wind is excellent, causing the remarkable reduction of the percentage of the harmonic and the outstanding stability of the variation of the output voltage.

• International Journal of Aeronautical and Space Sciences
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• v.13 no.1
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• pp.64-73
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• 2012

Pointing stability of high precision observation satellites must satisfy the stringent requirements to perform at a designed level. As even a small vibrational disturbance can result in severe degradation of the optical performance, the effects of inorbit vibrational environment on the performance of optical payload must be predicted and analyzed in the design phase in order to ensure that the requirements imposed on the payload are fully met. In this paper, an integrated framework for the evaluation of the performance of optical payloads is developed. The developed simulation tool comprises of the reaction wheel induced disturbance model, state space model of a structure in modal form and Cassegrain reflector model. The performance degradation of the optical system due to jitter is expressed by using modulation transfer function (MTF) and image simulation. Moreover, vibration isolator model is also added to show the effectiveness of using a vibration isolator for the elimination of the effects of jitter in the acquisition of an image.

• Journal of Electrical Engineering and Technology
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• v.13 no.3
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• pp.1185-1201
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• 2018

This paper presents a sensorless direct torque control (DTC) combined with sliding mode approach (SM) and space vector modulation (SVM) to achieve mainly a high performance and reduce torque and flux ripples of a parallel-connected two five-phase permanent magnet synchronous machine (PMSM) drive system. In order to increase the proposed drive robustness and decrease its complexity and cost, the rotor speeds, rotor positions, fluxes as well as torques are estimated by using a sliding mode observer (SMO) scheme. The effectiveness of the proposed sliding mode observer in conjunction with the sliding mode control based DTC is confirmed through the application of different load torques for wide speed range operation. Comparison between sliding mode control and proportional integral (PI) control based DTC of the proposed two-motor drive is provided. The obtained speeds, torques and fluxes responses follow their references; even in low and reverse speed operations, load torques changes, and machines parameters variations. Simulation results confirm also that, the ripples of the torques and fluxes are reduced more than 3.33% and 16.66 %, respectively, and the speed overshoots and speed drops are reduced about 99.85% and 92.24%, respectively.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.5 no.2
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• pp.330-343
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• 2011

In this paper, we propose two schemes to reduce the complexity of fixed-complexity sphere decoder (FSD) algorithm in the ordering and tree-search stages, respectively, while achieving quasi-ML performance. In the ordering stage, we propose a QR-decomposition-based FSD signal ordering based on the zero-forcing criterion (FSD-ZF-SQRD) that requires only a few number of additional complex flops compared to the unsorted QRD. Also, the proposed ordering algorithm is extended using the minimum mean square error (MMSE) criterion to achieve better performance. In the tree-search stage, we introduce a threshold-based complexity reduction approach for the FSD depending on the reliability of the signal with the largest noise amplification. Numerical results show that in $8{\times}8$ MIMO system, the proposed FSD-ZF-SQRD and FSD-MMSE-SQRD only require 19.5% and 26.3% of the computational efforts required by Hassibi’s scheme, respectively. Moreover, a third threshold vector is outlined which can be used for high order modulation schemes. In $4{\times}4$ MIMO system using 16-QAM and 64-QAM, simulation results show that when the proposed threshold-based approach is employed, FSD requires only 62.86% and 53.67% of its full complexity, respectively.

• Korean Journal of Metals and Materials
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• v.48 no.1
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• pp.1-7
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• 2010

Experiments have demonstrated that the addition of a moderate amount of V to $Fe_{52}Co_{(20-x)}B_{20}Si_4Nb_4V_x$ amorphous alloy enhances the plasticity of the alloy. In particular, $Fe_{52}Co_{17.5}B_{20}Si_4Nb_4V_{2.5}$ alloy withstood a maximum of 8.3% strain prior to fracture along with a strength exceeding 4.7 GPa. Energy dispersive x-ray spectroscopy conducted on the $Fe_{52}Co_{17.5}B_{20}Si_4Nb_4V_{2.5}$ alloy exhibited evidence of compositional modulation, indicating that nm-scale phase separation had occurred at local regions. In this study, the role played by nm-scale phase separation on the plasticity was investigated in terms of structural disordering and shear localization in order to better understand the structural origin of the enhanced plasticity shown by the developed alloy.

• Current Optics and Photonics
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• v.2 no.2
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• pp.134-139
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• 2018

A novel approach for frequency-tripling vector signal generation via balanced detection without precoding and optical filter is proposed. The scheme is mainly utilizing an integrated dual-polarization quadrature phase shift keying (DPQPSK) modulator. In the DPQPSK modulator, one QPSK modulator is driven by an RF signal to generate high-order optical sidebands, while the other QPSK modulator is modulated by I/Q data streams to produce baseband vector signal as an optical carrier. After that, a frequency-tripling 16-quadrature-amplitude-modulation (16QAM) vector millimeter-wave (mm-wave) signal can be obtained by balanced detection. The proposed scheme can reduce the complexity of transmitter digital signal processing. The results show that, a 4 Gbaud baseband 16QAM vector signal can be generated at 30 GHz by frequency-tripling. After 10 km single-mode fiber (SMF) transmission, the constellation and eye diagrams of the generated vector signal perform well and a bit-error-rate (BER) below than 1e-3 can be achieved.

• Journal of Power Electronics
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• v.19 no.1
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• pp.44-57
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• 2019

Recently, the use of LLCL filters for grid inverters has been suggested to give better harmonic attenuation than the commonly used L and LCL filters, particularly around the switching frequency. Nevertheless, this filter is mainly designed for constant switching frequency pulse width modulation (CSF PWM) methods. In variable switching frequency PWM (VSF PWM), the harmonic components are distributed across a wide frequency band which complicates the use of a high order filter, including LCL and LLCL filters. Recently, a confined band variable switching frequency (CB-VSF) PWM method has been proposed and demonstrated to be superior to the conventional constant switching frequency (CSF) PWM in terms of switching losses. However, the applicability of LLCL filters for this type of CB-VSF PWM has not been discussed. In this paper, the authors study the suitability of an LLCL filter for CB-VSF PWM and propose design guidelines for the filter parameters. Using simulation and experimental results, it is demonstrated that the effectiveness of an LLCL filter with CB-VSF PWM depends on the parameters of the filters as well as the designed variable frequency band of the PWM. Simulation results confirm the performance of the suggested LLCL design, which is further validated using a lab scale prototype.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.15 no.12
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• pp.4584-4599
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• 2021

In IEEE 802.11p-based wireless access in vehicular environments (WAVE) communication systems, channel estimation (CE) is one of the important issues to provide stable communication service. It is hard to apply conventional CE schemes based on data pilot to real systems, because error propagation occurs in high mobility and modulation order environments, resulting in degrading the CE performance. In this paper, we propose one data pilot using multiple receive antennas (ODPM) CE scheme based on the weighted sum using update matrix (WSUM) with time-domain averaging (TDA) to overcome this problem. Within the process of WSUM-TDA in the proposed scheme, the maximum ratio combining (MRC) technique is applied so as to create more accurate one data pilot. Moreover, a new reliability test criterion is proposed as the fashion of utilizing MRC, which makes it possible to apply selective TDA that guarantees performance improvement. In simulation results, the packet error rate (PER) performance of the proposed ODPM is compared with that of conventional CE methods and its superiority is demonstrated.

• Journal of Satellite, Information and Communications
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• v.4 no.2
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• pp.34-38
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• 2009

This paper is related with optimum burst structure design for high efficient TDMA satellite return link transmission. In general, some typical burst structure for data transmission is composed of a pair of preamble and traffic data in the DVB-RCS (Digital Video Broadcasting. Return Channel via Satellite) and IPOS (IP over Satellite) standard. This structure has some difficulties to increase spectral efficiency that it requires a large of preamble length, high SNR environment, or receiver complexity. To cope with them, burst structure with distributed pilot symbol can be used to alleviate the residual frequency offset effect by calculating accurate frequency offset than conventional one. In particular, we investigate some relevant to proposed distributed pilot structure, previously and analyze their strong points/drawbacks in terms of synchronization to draw the most appropriate one.