Oh-Soon Shin

Design of an OFDM Cooperative Space-Time Diversity System

In this paper, we propose a wireless system that realizes theoretical benefits of space-time cooperation. Specifically, we design a space-time cooperative system based on orthogonal frequency division multiplexing (OFDM), which we refer to as a cooperative (CO)-OFDM system. Our design includes a two-phase space-time cooperation protocol, as well as a transmitter and receiver architecture that facilitates cooperation. Furthermore, we devise a frame structure, on which we build practical timing and frequency synchronization algorithms and a channel estimation algorithm. In particular, the proposed frequency synchronization algorithm utilizes the underlying structure of the cooperation protocol, and the proposed channel estimation algorithm is based on a pairwise orthogonal construction of two sequences. We validate the performance of the proposed synchronization and channel estimation algorithms through simulations. We then present simulation results that demonstrate the overall performance advantage of the CO-OFDM system over an OFDM system without cooperation, not only under idealistic assumptions but also under realistic situations where the proposed algorithms are employed.

Antenna-assisted round robin scheduling for MIMO cellular systems

Packet scheduling is investigated for the downlink of a multiple-input multiple-output (MIMO) cellular system. We propose an antenna-assisted round robin scheduling (AA-RRS) scheme to improve the conventional RRS scheme. The AA-RRS scheme provides fair channel access chance to users as the RRS scheme, whereas it increases the system capacity through the effective use of multiple antennas in achieving a diversity effect from multiple users. Simulation results show that the AA-RRS scheme achieves a significant capacity gain over the RRS scheme. We also investigate the effects of long-term power control on the system capacities for the proposed scheduling scheme.

Overview of enabling technologies for 3GPP LTE-advanced

As the long term evolution (LTE) standard comes to an end, 3rd Generation Partnership Project is discussing further evolution of the LTE to meet the international mobile telecommunications advanced requirements, which is referred to as LTE-Advanced (LTE release 10 and beyond). This article first presents the network infrastructure of the LTE-Advanced, and then provides an in-depth overview of enabling technologies from the physical layer aspects, including carrier aggregation, advanced multiple-input multiple-output (MIMO) techniques, wireless relays, enhanced inter-cell interference coordination (eICIC), and coordinated multipoint (CoMP) transmission/reception. In particular, we describe concept and principle of each technology and elaborate important technical details. Moreover, we discuss promising study items of the LTE-Advanced for further enhancement.

Transmit power allocation for a modified V-BLAST system

In this letter, we present a modification of Vertical Bell Laboratories Layered Space-Time (V-BLAST), and propose an effective transmit power allocation (TPA) scheme for the modified system. The proposed TPA scheme minimizes the uncoded bit-error rate (BER) averaged over all detection stages, and requires small feedback overhead. Simulation results show that the modified V-BLAST system with the proposed TPA scheme provides a significant reduction in the uncoded BER compared with the conventional V-BLAST system. When the minimum mean-square error nulling is adopted, the modified V-BLAST system is found to achieve the uncoded BER performance comparable to that of the maximum-likelihood detection for the conventional V-BLAST architecture.

Utilization of multipaths for spread-spectrum code acquisition in frequency-selective Rayleigh fading channels

A novel acquisition scheme that utilizes multipaths to improve the acquisition performance is proposed for frequency-selective fading channels. The proposed acquisition scheme employs nonconsecutive search and joint triple-cell detection. The performance is analyzed in frequency-selective Rayleigh fading channels. Equations for the probabilities of detection and false alarm are derived, and an expression for the mean acquisition time is developed. The mean acquisition time performance of the proposed and conventional acquisition schemes is evaluated and compared. It is found that the proposed acquisition scheme significantly outperforms the conventional one. The effects of various channel parameters such as the number of resolvable paths, the shape of the multipath intensity profile (MIP) and the signal-to-interference ratio (SIR) on acquisition performance are also investigated.

A novel uplink MIMO transmission scheme in a multicell environment

Uplink multiple-input multiple-output (MIMO) transmission scheme is developed for time division duplex (TDD) systems in a multicell environment. We propose a precoding scheme that maximizes the total achievable rate and works in the decentralized manner with only locally available channel state information (CSI) at each transmitter. We first establish and solve a decentralized optimization problem for the case of multiple-input single-output (MISO) channels, introducing a new precoding design metric called signal to generated interference plus noise ratio (SGINR). By extending the result to general MIMO channels, we propose an SGINR-based precoding scheme where the number of transmit streams is selected adaptively to the surrounding environments. Simulation results confirm that the proposed precoding scheme offers significant throughput enhancement in multicell environments.

A novel multiple access scheme for uplink cellular systems

A novel multiple access scheme for the uplink cellular systems, referred to as FH/SS-OFDMA, is proposed. The proposed scheme employs both frequency hopping and code multiplexing on the frequency domain to acquire frequency diversity and suppress intercell interference. The performance of the proposed scheme is investigated and evaluated in cellular environments and compared with that of other multiple access schemes. It has been found that the proposed one outperforms other multiple access schemes. Simulation results for various parameters, such as system load, modulation scheme, spreading factor, and interference level are also presented.

Interference-Aware Decentralized Precoding for Multicell MIMO TDD Systems

Multiple-input multiple-output (MIMO) precoding scheme is developed for time division duplex (TDD) systems in a multicell environment. The proposed scheme is designed to maximize the total achievable rate, and to work in the decentralized manner with only locally available channel state information (CSI) at each transmitter. We first establish and solve a decentralized optimization problem for the case of multiple-input single-output (MISO) channels. We introduce a new precoding design metric called signal to generating interference plus noise ratio (SGINR). Based on the SGINR metric, the MISO precoding scheme is extended to general MIMO channels. Simulation results confirm that proposed precoding scheme offers significant throughput enhancement in multicell environments.

Proportional fair scheduling for wireless communication with multiple transmit and receive antennas

The effective applications of the proportional fair scheduling (PFS) scheme are investigated for the downlink of a cellular system with multiple transmit and receive antennas. We propose an improved PFS scheme based on spatial multiplexing (SM), referred to as P-PFS/SM, after reviewing the conventional PFS scheme based on spatial diversity (C-PFS/SD) and that based on spatial multiplexing (C-PFS/SM). The proposed PFS scheme exploits the space domain as well as the multiuser domain in scheduling, improving the system throughput. The performance of the P-PFS/SM scheme is compared with that of the C-PFS/SD and C-PFS/SM schemes in terms of the system throughput and user throughput. It is shown that the P-PFS/SM scheme is not much advantageous compared to the C-PFS/SM scheme without power control. However, the P-PFS/SM scheme is found to provide significant throughput improvement over the C-PFS/SD and C-PFS/SM schemes, when long-term power control is adopted.

Performance evaluation of MB-OFDM and DS-UWB systems for wireless personal area networks

Ultra-wideband (UWB) radio has been proposed for physical layer standard of the future high-speed wireless personal area networks (WPANs). One proposal is referred to as multi-band orthogonal frequency division multiplexing (MB-OFDM), and the other is direct-sequence ultra-wideband (DS-UWB). In this paper, we evaluate the performance of these physical layers as proposed to the IEEE 802.15 Task Group 3a. Specifically, we evaluate and compare the 10% outage frame error rate (FER) versus transmit-receive distance for the two systems using standard UWB channel models. In an AWGN channel, the two approaches yield essentially the same performance. In fading channels, the MB-OFDM system is shown to outperform the DS-UWB system for the assumed receiver structures. However, the relative performance ranking turns primarily on the signal processing complexity of the receiver, especially at high rates. Furthermore, the DS-UWB system is found to be more robust to imperfect channel estimation than the MB-OFDM system.