Kwonhyung Lee

Indoor Channel Characteristics for Visible Light Communications

In this letter, we present indoor multipath dispersion characteristics for visible light communications (VLC). Since the VLC uses a wide spectrum between 380 nm and 780 nm, the conventional narrowband model for infrared may not apply. We generalize the Barrys model by including wavelength-dependent white LED characteristics and spectral reflectance of indoor reflectors. We perform a computer simulation to compare the power delay profile of the VLC with that of infrared communications. From our studies, we show that the VLC provides a larger transmission bandwidth than infrared communications.

Modulations for Visible Light Communications With Dimming Control

Visible light communication (VLC) uses solid-state lightings to transmit information; therefore, it is necessary that modulation schemes for VLC provide dimming control. In this letter, we propose a multiple pulse position modulation (MPPM) to offer both functions of modulating data-stream and controlling the brightness at the same time. According to the dimming level, we control the number of pulses of MPPM in one symbol duration. We analyze communication performance in terms of the normalized power requirement and spectral efficiency. From our studies, we show that MPPM is superior to variable on-off keying (VOOK) and variable pulse position modulation (VPPM) proposed in the IEEE 802.15 VLC task group.

Channel model and modulation schemes for visible light communications

We present indoor multipath dispersion characteristics for visible light communications (VLC). We generalize the conventional ray-tracing model by including wavelength-dependent white LED characteristics and spectral reflectance of indoor reflectors. We perform a computer simulation to compare the power delay profile of the VLC with that of infrared communications. The VLC uses solid-state lightings to transmit information; therefore it is necessary that modulation schemes for the VLC provide dimming control. We consider several modulation schemes to offer both functions of modulating data-stream and controlling the brightness at the same time. We analyze communication performance in terms of power requirement and spectral efficiency.

Power efficient visible light communication systems under dimming constraint

Visible light communication (VLC) is a short range optical wireless communication using the light emitting diode (LED) lights, which can provide both illumination and communication. In this paper, we characterize the indoor multi-path dispersion for VLC and propose multiple pulse position modulation (MPPM) as a power efficient modulation. Due to the reflections in indoor environment, equalization process is essential to mitigate inter-symbol interference (ISI). Frequency domain equalizer (FDE) is implemented to reduce computational complexity. We evaluate communication performance in terms of bit error rate (BER) and the spectral efficiency. Simulation results show that the proposed system yields a comparable BER performance to other equalization systems with lower complexity.

A Chaotic UWB System for Home networks

Emerging wireless technologies will play an important role in home network applications. Chaotic UWB technology has several inherent properties that are suited to home networks. In particular, applications have been focused such as low power consumption, low complexity, and low cost. In this paper, we describe the chaotic signal based UWB systems and the optimum sample number to minimize the bit error rate of the chaotic on-off keying. Moreover, we propose new scheme using the optical orthogonal code based chaotic on-off keying in an asynchronous channel for simultaneously operating piconets environment: all users are allowed to transmit their signal at any time, and moreover there is usually no network synchronization or coordinator among users.

Magnetic, magneto-optical and dynamic characteristics of exchanged-coupled NdTbFeCo/TbFeCo films for 532 nm recording media

A magnetic, magneto-optical and structural study was performed on exchanged-coupled (EC) layers between NdTbFeCo and TbFeCo films for 532 nm recording media. The saturation magnetization (Ms) of separate read-out (RO) and storage (ST) films varied from 177 to 269 emu/cm/sup 2/ and from 73 to 87 emu/cm/sup 2/, respectively, in the sputtering power range 0.4 to 0.73 kW, at a fixed Ar gas flow rate. Coercivity (Hc) and Kerr angle (/spl theta//sub k/) of RO were in the range 0.46 kOe to 0.65 kOe and 0.34/spl deg/ to 0.38/spl deg/, respectively, for Ar gas flow rate between 15 to 40 sccm, and no great changes were found in the surface morphologies, as observed with atomic force microscopy (AFM). Hc and squareness rose from 2 kOe to 6 kOe and 0.6 to 0.98 with increasing R from 1.5 to 8, where R is the thickness ratio of NdTbFeCo to TbFeCo (total thickness 200 /spl Aring/, 250 /spl Aring/) in the five layered system SiN/sub x//NdTbFeCo/TbFeCo/SiN/sub x//Al-Ti. The carrier-to-noise ratio (CNR) increased by about 8 dB, as R was raised from 1.5 to 3. The films were prepared by magnetron rf-dc continuous sputtering. No atomic mixing between NdTbFeCo and TbFeCo layer occurred, as confirmed by TEM cross sectional observations.

Comparision of giant magneto-resistance behavior with conductance change based on the bulk-scattering in pseudo spin-valve

In this study, the effects of post annealing temperature ranging from 250 degC to 350 degC on the giant magnetoresistance (GMR) ratio and conductance change (DeltaG) have been investigated. The structure of the GMR spin-valve consists of Cu (10 A) / IrMn (80 A) / CoFe (15 A) / NOL / CoFe (20 A) / Cu (28 A) / CoFe (tf) / NOL / Cu (10 A) / Al2O3 where tf is varied from 10 A to 200 A and NOL is a nano-oxide layer.