Byong Ok Lee

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.

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.

Design of Non-Regenerative MIMO-Relay System with Partial Channel State Information

Design strategy for non-regenerative multiple-input multiple-output (MIMO) relay system with partial channel state information (CSI) is developed. We assume that the CSI of the base station-relay link is fully known to the relay, while the CSI of the relay-mobile station link is not known except its channel statistics. Based on this model, weighting matrices to increase ergodic capacity is proposed for downlink and uplink MIMO-relay system, respectively, with the approximation for both the high and low signal-to-noise ratio (SNR) region. We also propose a switching scheme to cover the intermediate SNR region. Numerical results show that the proposed scheme outperforms the conventional one especially when the SNR becomes high.

Uplink User Selection Scheme for Multiuser MIMO Systems in a Multicell Environment

We propose an interference-aware user selection scheme for uplink multiuser multiple-input multiple-output (MU-MIMO) systems in a multicell environment. The proposed scheme works in a distributed manner. Each mobile station (MS) determines its transmit beamforming vector based on the locally available channel state information (CSI), and informs the associated base station (BS) of the amount of potential interference caused to adjacent cells along with the resulting beamforming vector. Then, the BS selects a set of users to be served simultaneously with consideration of intercell interference. The user selection scheme is devised either to maximize the sum rate or to achieve proportional fairness among users. For each case, we derive an optimal user selection criterion and propose a suboptimal distributed user selection algorithm with low complexity. Simulation results confirm that the proposed scheme offers significant throughput enhancement due to reduction of the intercell interference in a multicell environment.

Enhanced Unitary Beamforming Scheme for Limited-Feedback Multiuser MIMO Systems

In this letter, we consider practical downlink multiuser multiple-input multiple-output systems where each user has multiple antennas and codebook-based limited channel feedback is available at base station. We propose a preprocessing scheme for downlink transmission where a unitary beamforming matrix is used. Firstly, we propose how to construct the unitary matrix that maximizes sum rate. Secondly, we propose a codebook-based channel feedback method for the proposed beamforming method. Numerical results show that the proposed scheme outperforms conventional zero-forcing beamforming scheme in terms of sum rate.

Distributed MIMO Precoding Strategies in a Multicell Environment

We investigate distributed multiple-input multiple-output (MIMO) precoding schemes for coping with the intercell interference in a multicell environment. In particular, we introduce the effective channel matrix for two different precoding strategies, referred to as received interference-aware precoding (RIAP) and generated interference-aware precoding (GIAP). Then, we propose a hybrid precoding scheme and a switching scheme. Simulation results are provided to compare the performance in a multicell environment.

Balancing Uplink and Downlink under Asymmetric Traffic Environments Using Distributed Receive Antennas

Recently, multimedia services are increasing with the widespread use of various wireless applications such as web browsers, real-time video, and interactive games, which results in traffic asymmetry between the uplink and downlink. Hence, time division duplex (TDD) systems which provide advantages in efficient bandwidth utilization under asymmetric traffic environments have become one of the most important issues in future mobile cellular systems. It is known that two types of intercell interference, referred to as crossed-slot interference, additionally arise in TDD systems; the performances of the uplink and downlink transmissions are degraded by BS-to-BS crossed-slot interference and MS-to-MS crossed-slot interference, respectively. The resulting performance unbalance between the uplink and downlink makes network deployment severely inefficient. Previous works have proposed intelligent time slot allocation algorithms to mitigate the crossed-slot interference problem. However, they require centralized control, which causes large signaling overhead in the network. In this paper, we propose to change the shape of the cellular structure itself. The conventional cellular structure is easily transformed into the proposed cellular structure with distributed receive antennas (DRAs). We set up statistical Markov chain traffic model and analyze the bit error performances of the conventional cellular structure and proposed cellular structure under asymmetric traffic environments. Numerical results show that the uplink and downlink performances of the proposed cellular structure become balanced with the proper number of DRAs and thus the proposed cellular structure is notably cost-effective in network deployment compared to the conventional cellular structure. As a result, extending the conventional cellular structure into the proposed cellular structure with DRAs is a remarkably cost-effective solution to support asymmetric traffic environments in future mobile cellular systems.

Distributed user selection scheme for uplink multiuser MIMO systems in a multicell environment

We propose an interference-aware user selection scheme for uplink multiuser multiple-input multiple-output systems in a multicell environment. The proposed scheme works in a distributed manner. Each mobile station determines its transmit beamforming vector based on the locally available channel state information, and informs the associated base station (BS) of the amount of potential interference caused to adjacent cells along with the resulting beamforming vector. Then, the BS selects a set of users to be served simultaneously with consideration of intercell interference. The user selection scheme is devised either to maximize the sum rate or to achieve proportional fairness among users. For each case, we derive an optimal user selection criterion and propose a suboptimal distributed user selection algorithm with low complexity. Simulation results confirm that the proposed scheme offers significant throughput enhancement due to reduction of the intercell interference in a multicell environment.

Transmit antenna selection for MIMO systems in a multicell environment

Transmit antenna selection scheme is developed for multiple-input multiple-output (MIMO) systems in a multicell environment. We first formulate an optimal transmit antenna selection criterion to maximize the total achievable rate. The optimal scheme, however, works in the centralized manner, requiring global channel state information and signaling overhead between cells. As a practical solution, we derive a decentralized transmit antenna selection criterion which consider not only the desired channel but also interference channel to adjacent cells. The proposed scheme works in the decentralized manner and thus eliminates the need for feedback and signaling overhead between cells. The simulation results show that the proposed scheme outperforms the conventional scheme and provides a close performance to the centralized optimal scheme in terms of the achievable rate.

Load-aware beamforming and association for system-wide utility maximization

In the cellular systems, the performance can be severely degraded if the loads are imbalanced among cells. We consider a joint optimization of association control and beamforming to maximize a system-wide utility function as a means to balance the load across multiple cells. The beamforming weight vector can be adapted as the channel status varies, but it is not practical to change the user association from a cell to another at the same frequency. Accordingly, we propose algorithms to adapt the beamforming weight vectors according to the channel status variation during the runtime while the association control and the beamforming policy, affecting the runtime beamforming weight vector determination, are selected in a longer time scale. Simulation results demonstrate that the proposed algorithms significantly improve the system-wide performance compared with legacy schemes by balancing the load.