• Journal of the Korean Institute of Telematics and Electronics D
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• v.36D no.10
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• pp.61-70
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• 1999

In this paper, we analyze several exposure time schedules to obtain uniform multiple hologram and measure the diffraction efficiencies of holograms stored in $LiNbO_3$:Fe(0.015wt%) crystal through experiments. The minimum angular selectivity and the intensity ration of two incident beams are investigated precisely by the high resolution system(minimum angle : $0.0028^{\circ}$). Considering initial recording time, total exposure time and time constants, the efficiency distribution(0.014 ${\pm}$ 0.005%) of 100 holograms is measured and the 330 analog images are recorded in the same place. Also, total 500 holograms are retrieved by spatial multiplexing.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.4A
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• pp.418-425
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• 2008

In this paper, we propose two different types of cooperative transmission protocols, referred to as spatial multiplexing with receive diversity (SMRD), that are bandwidth-efficient. We show that the BER performance can be significantly improved with a proper design of SMRD protocol under the AF (Amplify-and-Forward) and the DF (Decode-and-Forward) modes of relaying, when there is no interference among all symbols transmitted in the same time slot. BER analysis and our simulation result show that the proposed transmission protocol achieves a significant gain over no-cooperation (direct transmission) without any bandwidth expansion.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.9 no.5
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• pp.905-911
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• 2005

We propose an efficient adaptive subcarrier selection scheme, in which the active subcarriers and their modulation and coding schemes (MCSs) are selected at the receiver, and subsequently conveyed to the transmitter using limited feedback We theoretically show that capacity maximization can be achieved by selecting subcarriers with highest signal-to-noise ratios (SNRs) and adapting the number of active subcarriers according to channel environments. Furthermore, an ordering based adaptive subcarrier selection algorithm is proposed to select the optimal active subcarriers with low complexity. Numerical results show that the proposed adaptive subcarrier selection scheme provides higher capacity than that obtained by water-filling approaches, even with limited feedback.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.9C
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• pp.734-742
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• 2008

In spatially multiplexed MIMO systems that enable high data rate transmission over wireless communication channels, the spatial demultiplexing at the receiver is a challenging task and various demultiplexing methods have been developed. Among the previous methods, maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD), sphere decoding (SD), QOC, and MOC schemes have been reported to achieve a (near) maximum likelihood (ML) hard decision performance. In general, however, the reliability of soft output of these schemes is not satisfactory. In this paper, we propose a method which enhances the reliability of soft output. By computer simulations, we demonstrate the improved performance by the proposed method.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.19 no.8
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• pp.1433-1445
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• 1994

In this paper, an coherent optical correlator system based on the off-axis projection synthetic discriminant funtion (OA-pSDF) was analyzed and implemented optically. The filter was synthesized by combining conventional pSDF with single reference plane wave multiplexing. Synthesized pSDF were transformed to binary phase only filters (BPOFs) and fabricated as computer generated holograms(CGHs), which was used in the real time optical correlator system instead of using expensive spatial light modulators(SLMs). From the characteristic test, it was found that OA-pSDF showed distortion invariance and good performances in discrminating subset images. The proposed OA-pSDF BPOF could overcome the limitations of conventional BPOFs : that is distortion variance such as acale and rotation, especially out of plane variance.

• Journal of Communications and Networks
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• v.9 no.4
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• pp.473-481
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• 2007

Energy efficiency is one of the most critical concerns for wireless sensor networks. By allowing sensor nodes in close proximity to cooperate in transmission to form a virtual multiple-input multiple-output(MIMO) system, recent progress in wireless MIMO communications can be exploited to boost the system throughput, or equivalently reduce the energy consumption for the same throughput and BER target. However, these cooperative transmission strategies may incur additional energy cost and system overhead. In this paper, assuming that data collectors are equipped with antenna arrays and superior processing capability, energy efficiency of relevant traditional and cooperative transmission strategies: Single-input-multiple-output(SIMO), space-time block coding(STBC), and spatial multiplexing(SM) are studied. Analysis in the wideband regime reveals that, while receive diversity introduces significant improvement in both energy efficiency and spectral efficiency, further improvement due to the transmit diversity of STBC is limited, as opposed to the superiority of the SM scheme especially for non-trivial spectral efficiency. These observations are further confirmed in our analysis of more realistic systems with limited bandwidth, finite constellation sizes, and a target error rate. Based on this analysis, general guidelines are presented for optimal transmission strategy selection in system level and link level, aiming at minimum energy consumption while meeting different requirements. The proposed selection rules, especially those based on system-level metrics, are easy to implement for sensor applications. The framework provided here may also be readily extended to other scenarios or applications.

• Korean Journal of Optics and Photonics
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• v.9 no.6
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• pp.385-389
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• 1998

In this paper, we propose an optical correlator system using volume holograms for database of matched filters. Optical correlator has high speed and parallel processing characteristics of optics. Matched filters are recorded into a volume hologram that can store data with high density, transfer them with high speed, and select a randomly chosen data element. The multiple reference images of database are prerecorded in a photorefractive crystal in the form of Fourier transform images, simply by passing the image displayed in a spatial light modulator through a Fourier transform lens. The angular multiplexing method for multiple holograms of database can be achieved by rotating the crystal by use of a step motor. Experimental results show that the proposed system can be used for the fingerprint recognition.

• Korean Journal of Optics and Photonics
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• v.11 no.5
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• pp.371-376
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• 2000

For multi-hologram recording especially in a dIsk-shaped storage medium. we had studied a simple cost-effective method to unplement the rotational, angular, and ,patial multiplexmg techniques together in order to enhance the area storage density. Holographic storage with both rotational and angular multIplexing was realized by controlling the rderence beam directly with a pair of wedge prisms. whlie the storage WIth spatIal multlplexmg by shIfting the storage medium. In thIS paper \ve show that lhe area storage density of our system is strongly dependent on f numbers of the lenses in the srgnal and reference anns, and also show that the area storage densrly becollles maximal when the f number of the lens in the signal arm is approximately twice as long for a given f number of the lens in the reference emn. e emn.

• Journal of Communications and Networks
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• v.11 no.3
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• pp.287-296
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• 2009

Although asymptotic bounds of wireless network capacity have been heavily pursued, the answers to the following questions are still critical for network planning, protocol and architecture design: Given a three-dimensional (3D) network space with the number of active users randomly located in the space and using the wireless communication technology, what are the expected per-flow throughput, network capacity, and network transport capacity? In addition, how can the protocol parameters be tuned to enhance network performance? In this paper, we focus on the ultra wideband (UWB) based wireless personal area networks (WPANs) and provide answers to these questions, considering the salient features of UWB communications, i.e., low transmission/interference power level, accurate ranging capability, etc. Specifically, we demonstrate how to explore the spatial multiplexing gain of UWB networks by allowing appropriate concurrent transmissions. Given 3D space and the number of active users, we derive the expected number of concurrent transmissions, network capacity and transport capacity of the UWB network. The results reveal the main factors affecting network (transport) capacity, and how to determine the best protocol parameters, e.g., exclusive region size, in order to maximize the capacity. Extensive simulation results are given to validate the analytical results.

• ETRI Journal
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• v.44 no.4
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• pp.686-694
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• 2022

Fog screens offer a great advantage to be used as scattering screens in three-dimensional displays. In the absence of a fixed rigid form, fog screens work as screens without any physical obstruction. The density of water droplets can be regulated to adjust the screen's transparency and scattering level. This study proposes a method for crosstalk reduction between adjacent viewing windows by decreasing the divergence of view of the projector, with concentration lenses. By applying the Scheimpflug principle, we also reduce the keystone effect, which occurs when the fog screen is slanted with respect to the optical axis. In this study, we have realized a tabletop multiview fog display without any structures on the table and established that the proposed method is feasible for decreasing crosstalk effectively.