Hyung-Deug Bae

Analysis of handover failures in LTE femtocell systems

In this paper, we evaluate handover (HO) performance when user equipment (UE) moves between femto cell and macro cell in long term evolution (LTE) systems. The focus is on the HO performance for inbound and outbound mobility which corresponds to handoff between the femtocell and the macro cell. Due to the severe signal-to-interference noise ratio (SINR) degradation near the edge area of femtocell, HO triggering for inbound and outbound mobility needs to be carefully selected than that of the macro cell. Too late HO triggering leads to radio link failures (RLF), whereas too early HO triggering causes ping-pong HO. In this paper, we take the neighbouring cell configurations into account to keep the allowable RLF rate and low ping-pong rates. We propose a cell-types adaptive handover margins algorithm, which assigns different handover margins in line with neighbouring cell types. Simulation results indicate that the low ping-pong rate and the low RLF rate are achieved simultaneously by using the proposed algorithm.

Network architectures for packet classification in wireless mobile network

In IEEE 802.16e, one of mechanisms for service differentiation over the air interface relies on packet classification in the mobile subscriber station (MSS) and the access point (AP). Classification involves matching multiple packet header fields against a possibly large set of classifier rules identifying the different service classes. In the Internet router, two important classification properties are fast lookup time and low memory consumption. In a wireless mobile network, there are frequent dynamic updates of classifier rules whenever MSS move among cells. Fast handoffs demand fast update time. Frequent updates between MSS and AP degrade the efficiency of network resources and classification performance. The paper considers distributed and centralized network architectures for classification, and analyzes the signaling loads for updating classifier rules during handoffs and their classification performance. We show that the distributed architecture has a better classification performance but generates more signaling load. The paper concludes that, for fast handoffs, a tradeoff between classification performance and the efficiency of network resources is needed.

Mode Switching for Device-to-Device Communication in LTE-A Network

Device-to-device (D2D) communication enables nearby user equipments (UEs) to communicate directly without passing it through a base station (BS). When UE wish to communicate over direct link it is required to know the channel condition of its D2D link and its cellular link to select a link providing the best quality of service, so-called mode selection. UEs participating D2D communication need to conduct measurement over its UE-UE link and its eNB-UE link as well, and BS makes decision on mode switching based on the measurement report of UE. In this paper, we study the measurement procedures and reporting algorithms required for D2D communication. Frequent measurement report can cause the ping-pong mode switching whereas infrequent measurement report results in the delay of mode switching, thereby, leading to losing opportunity to offload traffic from the network. Wasteful frequent ping-pong mode switching is observed in some cases. We show that our proposal prevents frequent ping-pong mode switching without significant degradation in offloading performance achievable from D2D communication.

Analysis of service interruption time due to system information measurement in 3GPP LTE femtocell

Femtocell is one of promising cellular network technologies to enhance both of cell coverage and capacity. However, User Equipments (UEs) have to measure system information of Home evolved NodeBs (HeNBs) because most of femto-cells are Closed Subscriber Group (CSG) cell. This paper introduces four measurement methods of system information in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system. And, we also analyzed performance of the proposed measurement methods in aspect of service interruption time and measurement delay. We find out that the autonoumous methods have small measurement delay and the methods which use several small gaps have small service inturruption time.

Measurement of Inter-Frequency Small Cell in Heterogeneous Networks

Inter-frequency small cell deployments on hotspot locations are seen as a potential solution to offload macro user traffic to small cells. In order to discover inter-frequency small cells, macro user equipments (UE) are required to perform gap-assisted measurement. The currently existing short measurement periods can increase offloading, but at low UE speeds, it leads to unnecessarily measurement to scan neighbor cells and results in waste of UE battery. Long measurement periods are discussed in 3GPP to reduce UE power consumption. However, due to long measurement periods and consequently delayed cell discovery, inter-frequency mobility and offloading performance are degraded, especially at UE high speeds. In this paper, it is shown using simulation that mobility performance and the small cell dwelling time are degraded at high UE speeds when adopting the long measurement periods. Based the simulation results, it is proposed that applying the long measurement period is effective for low mobility UE to reduce the UE power consumption while keeping the acceptable handover failure rate and the offloading performance.