Hyukmin Son

Soft Load Balancing Over Heterogeneous Wireless Networks

Ongoing next-generation networks are expected to be deployed over current existing networks in the form of overlayed heterogeneous networks, particularly in hot-spot areas. Therefore, it will be necessary to develop an interworking technique, such as load balancing, to achieve increased overall resource utilization in the various heterogeneous networks. In this paper, we present a new load-balancing mechanism termed ldquosoftrdquo load balancing, where the Internet Protocol (IP) traffic of a user is divided into subflows, each of which flows into a different access network. The terminology of soft load balancing involves the use of both load-sharing and handover techniques. Through a numerical analysis, we obtain an optimal load-balancing ratio (LBR) to determine the volume of traffic delivered to each network over an overlayed multicell environment. Using the optimal LBR, a more reliable channel transmission can be achieved by efficiently reducing the outage probability for given user traffic.

CoMP-CSB for ICI Nulling with User Selection

The capacity of downlink multiple-input multiple-output (MIMO) cellular networks is significantly limited by inter-cell interference (ICI), particularly at cell boundaries. Recently, two types of coordinated multiple point transmission (CoMP) technologies, joint processing and transmission (JPT) and coordinated scheduling and beamforming (CSB), were proposed. These technologies are intended for the latest cellular communication standard in order to improve the performance of cell-edge users who suffer from significant ICI. In this paper, we propose an ICI cancellation technique based on a user selection algorithm for CoMP-CSB. Under partial channel state information (CSI) and no data sharing condition, each base station (BS) concentrates more on the direction of interference to the adjacent cells users, during the user selection process. Unlike prior concepts for a single-cell environment, in which each BS generates a precoding matrix for selected users to be served, the proposed technique considers the effects of interference to users located in adjacent cells. Although there are obvious trade-offs between ICI mitigation and the number of simultaneous scheduled users in terms of system capacity, the simulation results demonstrate that our proposed algorithm achieves higher sector throughput and is more robust against ICI if the system is limited by interference. Furthermore, through simulation we are able to obtain the preferred option for the coordination distance (R) and the number of degrees of freedom for ICI nulling (ξ).

Semisoft Handover Gain Analysis Over OFDM-Based Broadband Systems

Various approaches to analyze handover have been developed to guarantee the quality of service of multimedia services over mobile communication networks. However, no framework for multicarrier-based broadband systems, e.g., multicarrier code-division multiple access and orthogonal frequency-division multiple access, is available based on the perspective of link capacity. This paper presents a handover technique referred to as semisoft-handover-utilizing macroscopic diversity, which permits both hard and soft handover advantages for services over multicarrier-based broadband networks to be retained. Theoretical analysis is then performed to measure the handover gain over the forward link in terms of an outage probability. Simulation data verify that semisoft handover outperforms other traditional handover techniques, in particular for high-data-rate services.

Forward-Link Capacity Analysis for MC-CDMA

OFDM-based networks utilizing the frequency reuse factor of 1 may produce the severe ICI (intercell interference) at the cell boundary even though overall cell capacity is increased and network deployment is facilitated. In the forward-link, the ICI may rise above a QoS (quality of service) threshold beyond some distance from BSs (base stations). In this paper, we analyze the forward-link capacity of an MC-CDMA system as a function of the ICI according to the distance from a cell. To achieve this goal, a closed form of the outage probability is derived and utilized to obtain the accommodated number of users and system parameters.

Frequency Reuse Power Allocation for Broadband Cellular Networks

An FRPA (frequency reuse power allocation) technique by employing the frequency reuse notion as a strategy for overcoming the ICI (intercell interference) and maintaining the QoS (quality of service) at the cell boundary is described for broadband cellular networks. In the scheme, the total bandwidth is divided into sub-bands and two different power levels are then allocated to sub-bands based on the frequency reuse for forward-link cell planning. In order to prove the effectiveness of the proposed algorithm, a Monte Carlo simulation was performed based on the Chemoff upper bound. The simulation shows that this technique can achieve a high channel throughput while maintaining the required QoS at the cell boundary.

Bandwidth and region division for broadband multi-cell networks

A cell planning technique termed BRD (bandwidth and region division) is presented for overcoming interference, maintaining QoS (quality of service) and improving channel capacity over OFDM (orthogonal frequency division multiplexing)-based broadband cellular networks. Through an optimal combination of sectorization and zero padding, bandwidth and region division is achieved that minimizes the outage probability for forward-link cell planning. In order to verify the effectiveness of the proposed algorithm, a Monte Carlo simulation is performed over multi-cell environments.

Iterative Best Beam Selection for Random Unitary Beamforming

Due to multi-user diversity, the performance of a random unitary beamforming (RUB) can be improved upon in proportion to the number of users at the expense of an increase in feedback. It is essential to identify a mechanism to determine a set of orthonormal beams for improving the channel capacity. This paper proposes a unique scheme, named iterative best beam selection-based RUB (I-RUB), in which the base station selects the best beamforming vectors from unitary matrices generated by the proposed iterative algorithm. Using the selected beamforming vector at each stage, the beam set can ultimately be constructed for high sum-rate capacity of downlink, which leads to a reduction in feedback compared to those of other RUB schemes based on the beam set selection.

Throughput and QoS improvement via fixed relay station cooperated beam-forming

Throughput and quality-of-service (QoS) over multicell environments are two of the most challenging issues that must be addressed when developing next generation wireless network standards. Currently, multiple-input/multiple-output (MIMO), inter-cell coordination and multi-hop relay technologies are viable options for improving channel capacity or coverage extension. Nevertheless, severe QoS degradation occurs in the outer region of multi-cells due to significant interference from neighboring cells or relay stations, thereby limiting overall performance. This paper describes an effective technique, fixed relay station cooperated beam-forming (FCBF), which combines MIMO, multi-hop relay and multi-cell coordination. Simulated testing of FCBF demonstrates an increase of 10% in the average sum-rate and a decrease of 25% in the outage probability compared with conventional techniques. In particular, throughput at the cell boundary is remarkably increased with FCBF compared with traditional beam-forming technologies.

OFDM-Based Semi-Soft Handover for High Data Rate Services

Various approaches for analyzing handover have been developed to guarantee the QoS (Quality of Service) of multimedia services over mobile communication networks. However, no framework for multicarrier-based broadband systems, such as MC-CDMA (Multi-Carrier Code Division Multiple Access) or OFDMA (Orthogonal Frequency Division Multiple Access) is available, from the perspective of link capacity. This paper presents a handover technique, referred to as semi-soft handover utilizing macro diversity, which permits both hard and soft handover advantages for services over multicarrier-based broadband networks to be retained. A theoretical analysis is then performed to measure the handover gain over the forward link in terms of an outage probability. The simulation data verifies that the semi-soft handover outperforms other traditional handover techniques, in particular, for high data rate services.

A Multi-User MIMO Downlink Receiver and Quantizer Design Based on SINR Optimization

In a quantization based multiuser multiple-input/multiple-output (MU-MIMO) broadcast channel, the effective channel gain (i.e., the norm of a channel vector obtained from the receive combiner) needs to be considered in addition to a reduction of quantization errors for maximizing the signal-to-interference plus noise ratio (SINR). In this work, we prove that the effective channel gains in the space limited by an N_R x M_T MIMO channel form a min(M_T,N_R)-dimensional ellipsoid in the channel space. Utilizing the geometric proof, the achievable effective channel gain is derived as a function of its direction for a given MIMO channel. Based on the derivation, we finally propose a quantization vector selection criterion and an optimal receive combiner to maximize the SINR. The proposed maximum SINR based combining (MSC) is proven to be a better solution for maximizing the SINR compared to quantization based combining (QBC) and maximum gain based combining (MGC), each of which attempts to minimize quantization errors and maximize the effective channel gain.