Hyunwook Yang

Future of wireless communication: RadioApps and related security and radio computer framework

This article introduces a novel flexible mobile device reconfiguration classes (MDRCs) framework which is expected to be applicable to future generation mass market radio devices. This framework sets the scene for a vision of a smooth evolution of reconfigurability features in mobile devices, paving the way from current, mainly static implementation choices toward a fully flexible device platform environment. For this flexible mobile device framework, baseband interfaces are introduced as they are currently discussed in ETSI reconfigurable radio systems standardization. Furthermore, this technical solution is positioned with respect to security requirements and the basic regulatory framework which is currently under revision in Europe. Indeed, the future revised Radio Equipment and Telecommunications Terminal Equipment Directive is expected to allow for such advanced reconfiguration, enabling users to acquire and install so-called RadioApps software components which may affect the compliance of a mobile device to the essential requirements of the Directive.

Implementation of a Zero-Forcing Precoding Algorithm Combined with Adaptive Beamforming Based on WiMAX System

We propose a novel precoding algorithm that is a zero-forcing (ZF) method combined with adaptive beamforming in the Worldwide Interoperability for Microwave Access (WiMAX) system. In a Multiuser Multiple-Input Multiple-Output (MU-MIMO) system, ZF is used to eliminate the Multiple Access Interference (MAI) in order to allow several users to share a common resource. The adaptive beamforming algorithm is used to achieve the desired SNR gain. The experimental system consists of a WiMAX base station that has 2 MIMO elements, each of which is composed of three-array antennas and two mobile terminals, each of which has a single antenna. Through computer simulations, we verified that the proposed method outperforms the conventional ZF method by at least 2.4 dB when the BER is 0.1%, or 1.7 dB when the FER is 1%, in terms of the SNR. Through a hardware implementation of the proposed method, we verified the feasibility of the proposed method for realizing a practical WiMAX base station to utilize the channel resources as efficiently as possible.

Online Calibration for LTE-Based Antenna Array System

This paper presents a novel calibration method that equalizes the impulse responses of all the Radio Frequency (RF) modules of an antenna array system operating in Long-Term Evolution (LTE) evolved NodeB (eNB). The proposed technique utilizes the Zadoff-Chu (Z-C) sequence of the Primary Synchronization Signal (PSS) and Sounding Reference Signal (SRS) that are available in every LTE data frame for downlink and uplink, respectively, for estimating and compensating the differences in the impulse responses among the RF modules. The proposed calibration method is suitable for wide bandwidth signal environments of LTE because it equalizes the impulse response of each RF module, which is ultimately equivalent to compensate the phase and amplitude differences among RF modules for the entire frequency band. In addition, the proposed method is applicable while the target eNB is transmitting or receiving a data stream. From various experimental tests obtained from a test-bed implemented with 2 RF modules, it has been verified that the proposed method provides a reliable calibration for Release 10 Time Division Duplex (TDD) LTE signals. Phase errors after the calibration in our test-bed have been found to be about 2.418° and 2.983° for downlink and uplink, respectively.

Implementation of a WiBro system using GPU

This paper presents an implementation of a communication system which operates on a realtime basis by using a graphic processing unit (GPU). In order to cope with an extremely highspeed data rate in modern communication systems, system processor should support the high system throughput and high operation complexity. GPU can satisfy the requirement of high complexity by processing a lot of data in parallel. Also, GPU provides a convenient operation system which enables relatively easy implementation of various waveforms. In this paper, we implement WiBro system and demonstrate the performance in realtime.

Performance analysis of a MU-MIMO system with limited feedback and user selection

This paper proposes a novel user selection algorithm to enhance the system performance of a multi-user, multiple-input, multiple-output (MU-MIMO) system. The proposed algorithm first selects a primary user set using a predetermined threshold, and selects sub-users for each primary user to define user groups that maximize the determinant of the channel matrix consisting of selected user(s) and a candidate user. The user group that provides the maximum sum rate is then determined. According to computer simulation results, the proposed algorithm improves the system sum rate, compared to the conventional algorithm, by approximately 0.5bps/Hz.

Calibration method using the Zadoff-Chu sequence

A novel calibration method using the Zadoff-Chu (ZC) sequence as a test signal is proposed. Using this technique, the timing information of the received test signal that contains the phase characteristics of each antenna path can be accurately estimated. Performance of the proposed method is first analyzed through computer simulation. Then, the proposed calibration algorithm is implemented on a Field Programmable Gate Array (FPGA) board to verify its excellent performance in terms of the estimation accuracy for both the amplitude characteristics and the phase characteristics of each antenna path of a given array system.

A Novel Lattice Reduction Precoding Method

In this work, a novel lattice reduction (LR) precoding method is proposed. The technique combines conventional LR precoding with a method of reducing the singular value coefficients of the LR-reduced basis matrix. The performance of the new technique was comparable to that of sphere encoding, while its complexity was lower than that of other sub-optimal methods.