Young Jun Hong

A Practical Cooperative Multicell MIMO-OFDMA Network Based on Rank Coordination

An important challenge of wireless networks is to boost the cell edge performance and enable multi-stream transmissions to cell edge users. Interference mitigation techniques relying on multiple antennas and coordination among cells are nowadays heavily studied in the literature. Typical strategies in OFDMA networks include coordinated scheduling, beamforming and power control. In this paper, we propose a novel and practical type of coordination for OFDMA downlink networks relying on multiple antennas at the transmitter and the receiver. The transmission ranks, i.e. the number of transmitted streams, and the user scheduling in all cells are jointly optimized in order to maximize a network utility function accounting for fairness among users. A distributed coordinated scheduler motivated by an interference pricing mechanism and relying on a master-slave architecture is introduced. The proposed scheme is operated based on the user report of a recommended rank for the interfering cells accounting for the receiver interference suppression capability. It incurs a very low feedback and backhaul overhead and enables efficient link adaptation. It is moreover robust to channel measurement errors and applicable to both open-loop and closed-loop MIMO operations. A 20% cell edge performance gain over uncoordinated LTE-A system is shown through system level simulations.

Long-Term Channel Information-Based CoMP Beamforming in LTE-Advanced Systems

Coordinated multi-point (CoMP) transmission and reception is a network multiple-input multiple- output (MIMO) technology considered in 3GPP LTE- Advanced systems. In order to improve reliability and capacity of the services for the user equipments (UEs) at the cell edges, CoMP utilizes cooperation among neighboring enhanced node Bs (eNBs). Accordingly, backhaul delay for sharing control signals among eNBs should be carefully handled as UE mobility increases in a fading channel environment; otherwise, CoMP operations derived from inaccurate channel state information (CSI) of neighboring eNBs can severely degrade the system performance. We propose CoMP beamforming schemes using long-term CSI such as spatial correlation matrices instead of instantaneous CSI of neighboring cells. Since long- term CSI varies relatively slowly, the proposed scheme is inherently robust even when a UE moves at a high speed and/or the backhaul delay is large. Using a multi-cell simulator which reflects realistic CoMP system environments, the performance gain of the proposed schemes is evaluated and discussed.

Explicit vs. Implicit Feedback for SU and MU-MIMO

SU and MU-MIMO performance relies on accurate link adaptation in order to benefit from multi-user scheduling, beamforming, adaptive coding and modulation. Such accuracy highly depends on the type of the channel state information feedback. LTE-Advanced has defined two major types of feedback, i.e. implicit and explicit feedback. Implicit feedback makes some assumptions on the transmit precoding and receiver processing at the time of CSI and CQI feedback. The CSI is expressed in terms of a recommended precoder, commonly denoted as PMI. Explicit feedback refers to the feedback of channel information without making any assumption on the transmit and receiver processing. In this paper, we discuss pros and cons of such feedback mechanisms for both SU and MU-MIMO and compare performance of both approaches using system level simulations compliant with LTE-A system. It is shown that implicit feedback is the preferred feedback framework for both SU and MU-MIMO.

Extremely Low-Profile Antenna for Attachable Bio-Sensors

In this paper, an extremely low-profile patch-type slot antenna for on-body wireless bio-sensor is presented in the medical body area network (MBAN) band. By locating the proposed antenna at the top of the sensor as a sensor cover layer, it is able to maintain compact and flexible sensor structure as well as to enhance its radiation efficiency. The proposed antenna consists of a rectangular loop for balanced feeding and patch with slots for radiating element, and it is designed on flexible printed circuit board (FPCB). To further improve the performance of the antenna with extremely low-profile sensor, air substrate is employed to satisfy the bandwidth, gain, and efficiency requirements. The proposed antenna is measured on a human phantom model with permittivity and conductivity close to those of a human body. From the measured results, it is found that the 3-dB bandwidth of the antenna is sufficient to cover the entire MBAN band (2.36-2.39 GHz) and has a relatively reasonable peak gain of -04 dBi on the human body model.

Low profile patch antenna for on-body wireless sensor application in MBAN band

A low profile patch antenna design for on-body wireless sensor in the medical body area network (MBAN) band is presented. By locating the proposed antenna at the top of the sensor, i.e., farthest from a human body, we can maximize the radiation efficiency of low profile sensor. The antenna consists of a rectangular loop and patch with slots for size reduction, employing printed flexible printed circuit board (FPCB). The proposed antenna is measured on a lossy medium with permittivity and conductivity close to those of the human body. The 3-dB bandwidth of antenna is sufficient to cover the MBAN band (2.360-2.395 GHz) and has a relatively good peak gain with -0.4 dBi for 1 mm separation from the lossy medium, especially considering its low profile characteristic (0.008λ).