Kyungjun Lee

Analysis of multi-level threshold handoff algorithm

A PCS system which consists of microcells is considered, and it is assumed that the high mobility calls and low mobility calls coexist. To enhance the quality of service the multi-level threshold handoff algorithm is used. The velocity adaptive handoff (VAH) algorithm is analyzed as one of the multi-level threshold handoff algorithm. In the VAH algorithm, the handoff threshold value is assigned according to the velocity of the mobile in order to lower the handoff failure probability and to maintain the carried traffic constantly in the handoff area. Since it is necessary to estimate the velocity of the mobile for the VAH algorithm, a velocity estimation method that use a velocity codebook is analyzed. A prioritized handoff procedure is used to evaluate the VAH algorithm in respect of the traffic characteristics.

Anomalous X-ray reflectivity study of metal oxide thin films

Anomalous X-ray reflectivity measurements have been performed to extract electron density profile as a function of depth. Using a model independent analysis scheme based on distorted wave Born approximation, we have demonstrated that element-specific density profiles in a film can be obtained from reflectivity measurements done at two different X-ray energies, one at an absorption edge of the corresponding metal and another one away from it. The merit of this technique has been demonstrated with the results on high dielectric constant metal oxide Ta 2 O 5 films on Si(001). Our results show different Ta profiles near interfaces for Ta 2 O 5 /Si interface and Ta 2 O 5 /SiO 2 interface, implying different kinetics at these interfaces during annealing process.

Self-assembled plasmonic nanoparticles on vertically aligned carbon nanotube electrodes via thermal evaporation.

This study details the development of a large-area, three-dimensional (3D), plasmonic integrated electrode (PIE) system. Vertically aligned multiwalled carbon nanotube (VA-MWNT) electrodes are grown and populated with self-assembling silver nanoparticles via thermal evaporation. Due to the geometric and surface characteristics of VA-MWNTs, evaporated silver atoms form nanoparticles approximately 15-20 nm in diameter. The nanoparticles are well distributed on VA-MWNTs, with a 5-10 nm gap between particles. The size and gap of the self-assembled plasmonic nanoparticles is dependent upon both the length of the MWNT and the thickness of the evaporated silver. The wetting properties of water of the VA-MWNT electrodes change from hydrophilic (∼70°) to hydrophobic (∼120°) as a result of the evaporated silver. This effect is particularly pronounced on the VA-MWNT electrodes with a length of 1 μm, where the contact angle is altered from an initial 8° to 124°. Based on UV-visible spectroscopic analysis, plasmonic resonance of the PIE systems occurs at a wavelength of approximately 400 nm. The optical behavior was found to vary as a function of MWNT length, with the exception of MWNT with a length of 1 μm. Using our PIE systems, we were able to obtain clear surface-enhanced Raman scattering (SERS) spectra with a detection limit of ∼10 nM and an enhancement factor of ∼10(6). This PIE system shows promise for use as a novel electrode system in next-generation optoelectronics such as photovoltaics, light-emitting diodes, and solar water splitting.

Compressive Random Access Using Multiple Resource Blocks for MTC

In this paper, we propose a compressive random access scheme using multiple resource blocks (RBs) to support massive connections for machine type communications (MTC) in 5G systems. The main advantage of the proposed scheme over conventional compressive random access schemes is that there can be more devices than preambles as each active device is to randomly choose a preamble for connection to an access point (AP) at the cost of preamble collisions. To mitigate the performance degradation due to preamble collisions, a block of subcarriers is divided into multiple RBs and an active device is to randomly choose an RB and a preamble within the chosen RB. For multiple signal detection, we consider a low- complexity compressive sensing (CS) algorithm. We also demonstrate that using multiple RBs in random access can offer the benefits of reducing the computation time with a multicore processor as well as improving the throughput.

Pilot Signal Design for Compressive Sensing Based Random Access in Machine-Type Communications

In machine-type communications (MTC), a number of devices are present in a cell, but only a few of them attempt to access by sending preambles or pilots. For the sparse activity of devices, compressive sensing (CS) can be applied for joint detection of active devices and estimation of channel profiles. In this paper, a theoretical connection between pilots and the coherence of a CS measurement matrix is revealed for CS-based detection and estimation. Then, the Zadoff-Chu (ZC) and the power residue (PR) sequences are studied as potential candidates for pilot signals in CS-based random access. It is also shown that the CS measurement matrices from the ZC- and the PR-based pilots have theoretically bounded low coherence, which presents a theoretical guarantee of reliable recovery performance. Simulation results demonstrate that the proposed deterministic pilots outperform random binary and complex-valued pilots for CS-based random access in MTC.

Secure compressive random access for meter reading in smart grid using multi-antenna access point

In this paper, we study the advantages of multiple antennas at an access point (AP) for random access by smart meters in smart utility network (SUN) in terms of access delay and security. For random access with the capability of multiple signal recovery, we consider compressive random access and show that multiple antennas can help improve the access delay. In addition, we consider updating keys to generate spreading codes at smart meters using a physical layer security scheme that is available when the AP is equipped with multiple antennas. We show that the probability of successful attack by an eavesdropper can be low by updating keys. Consequently, in this paper, we demonstrate that multiple antennas at the AP in SUN is indispensable for random access with a low latency and security.