Jin-Up Kim

Dynamic load balancing architecture in heterogeneous wireless network environment

This paper discusses load balancing architecture among heterogeneous wireless networks (HWNs). Load balancing is an important issue for efficient use of radio resources not only in a single network but among different Radio Access Technologies (RATs) in HWNs. Cooperative management of radio resources among heterogeneous RATs is considered to reduce imbalance of utilization of radio resources and enhance availability of the system. For effective load balancing, we define a “community” concept, in which radio resources of various RATs are managed together. Radio resource management functional entities of RATs cooperatively work in the community. We present a load balancing architecture applied to 3GPP LTE, WiBro and WLAN and describe architectural considerations. We also present a straightforward load balancing algorithm that takes account of bandwidth usage and received signal to noise ratio. The load balancing algorithm of community radio resource management enhances utilization of radio resources and reduces call blocking probability of individual RATs in the community and further enhances average performance of the whole community.

The Design Method and Performance Analysis of RF Subsampling Frontend for SDR/CR Receivers

RF subsampling can be used by radio receivers to directly down convert and digitize RF signals. The goal of software-defined radio (SDR) design is to place analog-to-digital converter (ADC) as near the antenna as possible. Based on this, an RF subsampling frontend (FE) for SDR is designed and verified by a hardware platform. The effects of timing jitter, ADC resolution, and folding noise dominating SNR degradation sources in the digital FE were considered. We present an efficient method of SNR measurement and an analysis of its performance. The experimental results indicate that the three degradation sources are sufficient to estimate the performance of the RF subsampling FE, and this conclusion matches the theoretical analysis results.

Efficient SNR estimation in OFDM system

It is very important to estimate the signal to noise ratio (SNR) of received signal and to transmit the signal effectively for the modern communication system. The performance of existing non-data-aided (NDA) SNR estimation methods are substantially degraded for high level modulation scheme such as M-ary amplitude and phase shift keying (APSK) or quadrature amplitude modulation (QAM). In this paper, we propose a SNR estimation method which uses zero point auto-correlation of received signal per block and auto/cross- correlation of decision feedback signal in orthogonal frequency division multiplexing (OFDM) system. Proposed method can be studied into two types; Type 1 can estimate SNR by zero point auto-correlation of decision feedback signal based on the second moment property. Type 2 uses both zero point auto-correlation and cross-correlation based on the fourth moment property. In block-by-block reception of OFDM system, these two SNR estimation methods can be possible for the practical implementation due to correlation based the estimation method and they show more stable estimation performance than the previous SNR estimation methods.

A New Spectral Correlation Approach to Spectrum Sensing for 802.22 WRAN System

rapidly, much attention has been paid on the use of spectrum resources. Recently, a new technology called Cognitive Radio (CR) was developed for using the unused spectrum band efficiently. In the CR systems, one possible approach is to allocate the unused spectrum band to the usage spectrum band provided by spectrum sensing technique. In this paper, we propose a new spectral correlation based signal feature detection method for spectrum sensing. To improve spectrum sensing sensitivity, the proposed method computes the autocorrelation of received signal before the spectral correlation detection. Simulation results show that the proposed method provides better performance of spectrum sensing than conventional spectral correlation detection method even in the low SNR environment. Index Terms--Cognitive radio, Cyclostationary, Spectrum sensing.

R2²SDF FFT Implementation with Coefficient Memory Reduction Scheme

Fast Fourier transform (FFT) is a key building block for orthogonal frequency division multiplexing (OFDM) systems. Due to the development of wireless portable devices, it is important to minimize the size and power of a FFT processor. One of the methods to satisfy such demands is reducing the size of twiddle coefficient memory. This paper presents an effective coefficient memory reduction scheme for a R22SDF FFT implementation. When applying a conventional method to an N- point R22SDF FFT, the number of twiddle coefficients is 3N/4. However, the proposed scheme requires only (N/8+1) coefficients and its additional hardware architecture is very simple. The effectiveness of the proposed method is verified by implementation results on a FPGA.

Bandpass Sampling Digital Frontend Architecture for Multi-Band Access Cognitive Radio

Bandpass sampling (BPS) theory enables the conversion of RF frequency directly down to baseband, which is a significant technique in Cognitive radio (CR) systems because they require flexibility in RF frontend. Based on this concept, second-order BPS frontends for CR systems are designed and verified by using a hardware platform. The methods adopted to manipulate spectrums to remove aliasing and images are described, and the equations employed to design interpolants are provided. The results of experiment indicate that the proposed second-order BPS frontends can realize universal access and environment sensing functionalities in CR systems.

Dynamic Spectrum Allocation with Efficient SINR-Based Interference Management

This paper considers a dynamic spectrum allocation (DSA) model that a spectrum broker (SB) coordinates the allocation of the spectrum with the regional license inside the region of responsibility. It has a potential to leverage the spectrum utilization but requires a sophisticated approach to manage the allocation-dependent interference effect. Thus, the SB is responsible to manage the wireless interference between base stations (BSs) within the region for quality of service (QoS) provisioning over the allocated channels. In this paper, we address the interference constrained DSA problem and propose an interference management scheme that collaboratively works with the spectrum allocation algorithm to implement the DSA. By the allocation-aware interference management based on the received signal-to-interference-plus-noise-ratio (SINR), the proposed scheme can reflect the context of the SBs decisions at the allocation process. Simulation results demonstrate that the proposed scheme efficiently distributes the spectrum resource while guaranteeing the QoS requirements of all BSs with allocated channels.

Reverse effect of ground electrode on the signal loss of human body communication

In this paper, we analyzed the effects of the ground electrode, especially the reverse effect in which the signal loss increases by the ground electrode. We measured how the signal loss varies in relation to the ground electrode. Furthermore, we simulated the current distribution to analyze the effects of the ground electrode. In other papers, the body has an artificial posture in which two arms and a shoulder formed a straight line. In our approach, we measured the signal loss of a person with a natural posture for actual human body communication; that is, the persons two arms and shoulder did not form a straight line. Also, we reflected the modified posture of the body in the simulation model.

Local breakout of mobile access network traffic by mobile edge computing

Rapid growth of mobile traffic can be load of mobile operators core network. Local breakout is a offloading solution of access network to save core network load and to reduce end-to-end latency. This paper proposes local breakout system in base station with Mobile Edge Computing(MEC) infrastructure. We present the service framework and interfaces for MEC server and also present architecture based on Network Function Virtualization (NFV). By running on the commercial LTE based test-bed, we can show that the system works properly and gives us about 58 percent local breakout ratio.

Suppression of synchronization errors in OFDM based carrier aggregation system

OFDM (Orthogonal Frequency Division Multiplexing) has been chosen as the air interface technique for LTE (Long Term Evolution) standard. Furthermore, CA (Carrier Aggregation) is defined by 3GPP (the 3rd Generation Partnership Project) to support wide-bandwidth transmission. However, OFDM system is sensitive to synchronization errors generated by multipath delay, Doppler shift and oscillator instability. In this paper, we address the synchronization problem in non-continuous CA scenario and propose a block type pilot based synchronization algorithm to suppress the STO (Sampling Timing Offset) and CFO (Carrier Frequency Offset). The pilot pattern used here is CAZAC (Constant Amplitude Zero Autocorrelation) sequence, which is also exploited in LTE-Advanced standard. Since the proposed method is a parallel processing, different bands could experience different synchronization errors. Simulation results show the proposed method can significantly improve system performance and has smooth performance when synchronization errors vary.