• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.9A
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• pp.871-877
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• 2007

In this paper, we propose a new transmit diversity scheme using window functions in orthogonal frequency division multiplexing (OFDM) system. Transmit diversity of the scheme is varied with window functions and the condition of the window function to maximize transmit diversity is derived. The proposed scheme can be considered as a generalization of the diversity schemes such as cyclic delay diversity (CDD), orthogonal transmit diversity (OTD), and frequency switched transmit diversity (FSTD).

• Journal of Communications and Networks
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• v.9 no.1
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• pp.75-83
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• 2007

Orthogonal frequency division multiplexing (OFDM) systems with multiple transmit antennas can exploit space-time block coding on each subchannel for reliable data transmission. Spacetime coded OFDM systems, however, are very sensitive to time variant channels because the channels need to be static over multiple OFDM symbol periods. In this paper, we propose to mitigate the channel variations in the frequency domain using a linear filter in the frequency domain that exploits the sparse structure of the system matrix in the frequency domain. Our approach has reduced complexity compared with alternative approaches based on time domain block-linear filters. Simulation results demonstrate that our proposed frequency domain block-linear filter reduces computational complexity by more than a factor of ten at the cost of small performance degradation, compared with a time domain block-linear filter.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.15 no.5
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• pp.444-450
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• 2004

In mobile communications environment, STBC-OFDM(Space Time Block Code-Orthogonal Frequency Division Multiplexing) system with transmit diversity obtains the MRRC(Maximal Ratio Receiver Combining) diversity gain in time-invariant channel between two received symbols. But in time-variant channel, due to the interference between received symbols, MRRC diversity gain cant be obtained. So, when the mobile device with transmit diversity moves in high speed, the scheme to reduce the performance degradation due to the interference is needed. In this paper, we propose the receiver architecture with advanced transmit diversity, which improves the performance of STBC-OFDM system. The proposed architecture obtains the diversity gain without the change of transmit bandwidth at the receiver with the interference canceller using ZF(Zero Forcing) algorithm. Simulation results show performance improvement as doppler shift is increasing.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.7B
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• pp.695-705
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• 2002

In this paper, we propose a simple open loop transmit diversity (TD) scheme for the wideband code division multiple access (W-CDMA) systems in Rician multipath fading channels such as rural area or satellite channels where line-of-sight (LOS) paths are in presence. The proposed scheme does not require any pre-processing of transmit data, resulting in simpler structure as compared to conventional closed loop transmit adaptive array (TxAA) and open loop space-time transmit diversity (STTD). We analytically derive the probability density function of signal-to-noise ratio at the Rake receiver output and the uncoded bit error rate performance of the proposed scheme. Extensive simulation is Performed to verify the analytical performance of the proposed scheme under typical Rician multipath fading channel environments. Moreover, comparative results with the conventional TxAA and STTD are also provided. Simulation results show that the proposed scheme shows slightly better performance than the conventional open loop STTD under the channels with very weak LOS components, however, it significantly outperforms the STTD under the channels with dominant LOS components, and achieves a close performance of ideal closed loop TxAA.

• Proceedings of the Korean Information Science Society Conference
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• 2003.10c
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• pp.172-174
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• 2003

송신단에 다중안테나를 사용하는 송신 다이버시티 기법에 대한 연구가 최근 활발히 이루어지고 있다. 본 논문에서는 CDMA(Code Division Multiple Access)2000과 3GPP(The third generation partnership project)에서 고려한 개루프 송신 다이버시티 기법인 STTD(Space time transmit diversify), OTD (Orthogonal transmit diversity), STS(Space time spreading)를 다수 반송파 CDMA 시스템에 적용시킬 경우 하향링크 (Downlink)에서의 성능을 평가한다. STS와 STTD방식은 동일한 성능을 보이지만 STS방식은 복잡도와 높은 PAR(Peak to average ratio)의 단점을 보이며, OTD는 이 두 방식에 비해 성능이 떨어지는 것을 보여준다.

• Journal of Communications and Networks
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• v.14 no.2
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• pp.206-212
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• 2012

In this paper, in order to effectively apply unitary space-time modulation to the direct-sequence spread-spectrum multiple-access (DSSS-MA) networks, we propose a low-rate, noncoherent, unitary, and space-time modulated DSSS system supporting any number of transmit antennas based on Walsh matrices. The proposed scheme simultaneously performs bandwidth spreading and space-time coding and outperforms those using high-rate, conventional unitary space-time constellations. Furthermore, the proposed scheme allows for a simple detector structure based on fast Walsh transforms.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.26 no.5
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• pp.435-444
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• 2015

This paper specifies on a virtual array design of a 77 GHz vehicle radar for detecting a moving target at a time division transmit and a near range. The virtual array designed reduces a hardware complexity, weight and price. However, a synthesized beam of the virtual array has a high side lobe by a phase distortion of receive signals when the moving target is detected at the time division transmit. For this, a subarray receive signal with a same phase is used and the side lobe of the synthesized beam is suppressed above at least 10 dB. Also the virtual array has a beam distortion by a spherical wave when the vehicle radar operates at near range. So a boresight receive signal of each target range is compensated at each receive signal. Therefore the synthesized beam with compensation recovers a normal main lobe and improves the side lobe about 10~15 dB.

• Journal of Sensor Science and Technology
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• v.23 no.5
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• pp.349-355
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• 2014

In this paper, we developed a new method to transmit multiple visible light channels in time division mode using the AC power line frequency in order to prevent the crosstalk between adjacent optical signals. Synchronizing pulses are generated from the 220 V power line, and one pulse period is subdivided into several time slots for visible light channels. Each channel transmits data in a predefined time slot without interfering adjacent channels. In experiments, synchronizing pulses with a repetition rate of 240 Hz were generated from the 60 Hz power line, and three VLC channels with a bit rate of 9.6 kbps transmitted data independently using the time slots between synchronizing pulses. This configuration is very useful in constructing time division VLC networks for multiple sensors.

• Journal of Communications and Networks
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• v.15 no.1
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• pp.102-110
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• 2013

This paper considers a cooperative orthogonal frequency division multiple access (OFDMA)-based cognitive radio network where the primary system leases some of its subchannels to the secondary system for a fraction of time in exchange for the secondary users (SUs) assisting the transmission of primary users (PUs) as relays. Our aim is to determine the cooperation strategies among the primary and secondary systems so as to maximize the sum-rate of SUs while maintaining quality-of-service (QoS) requirements of PUs. We formulate a joint optimization problem of PU transmission mode selection, SU (or relay) selection, subcarrier assignment, power control, and time allocation. By applying dual method, this mixed integer programming problem is decomposed into parallel per-subcarrier subproblems, with each determining the cooperation strategy between one PU and one SU. We show that, on each leased subcarrier, the optimal strategy is to let a SU exclusively act as a relay or transmit for itself. This result is fundamentally different from the conventional spectrum leasing in single-channel systems where a SU must transmit a fraction of time for itself if it helps the PU's transmission. We then propose a subgradient-based algorithm to find the asymptotically optimal solution to the primal problem in polynomial time. Simulation results demonstrate that the proposed algorithm can significantly enhance the network performance.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.4A
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• pp.360-370
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• 2010

In this paper, a distributed beamforming problem is considered in an amplify-and-forward (AF) wireless relay network consist of multiple source-destination pairs and relaying nodes. To exploit degree of freedom of the number of beamformers, in the first step, we proposed that the sources transmit their signals through orthogonal channels. During the second step, the relays transmit their received signals multiplied by complex weights to amplify and compensate for phase changes introduced by the backward channels through one common channel. The optimal beamforming vectors are obtained through minimization of the total relay transmit power while the signal-to-interference-plus-noise ratios (SINRs) at the destinations are above certain thresholds to meet a quality of services (QoSs) level. In the numerical example, it is shown that the proposed scheme needs less transmit power for moderate network data rates than other schemes, such as space division multiplexing or time-division multiplexing scheme.