Hyun-Seo Park

TCP performance issues in LTE networks

We describe issues on the TCP performance in LTE networks. TCP throughput is determined by various factors such as TCP window size, loss probability and RTT. We present measurement results of TCP throughput in a real LTE testbed with various factors and TCP auto-tuning enabled. As expected, the test results show that the AM bearer outperforms the UM bearer due to less loss probability in the former. However, test results show that delay spikes incurred by the RLC ARQ increase TCP RTT and degrade TCP performance. We propose some enhancements for improving TCP throughput in LTE networks.

Precise nonselective chemically assisted ion-beam etching of AlGaAs/GaAs Bragg reflectors byin situ laser reflectometry

Precision etch-depth control is realized by a chemically assisted ion-beam etching system incorporated within situ laser reflectometry. By counting the number of interference fringes, etch-depth control better than a quarter-wave thickness is easily obtained. Optimized etching conditions for highly anisotropic etching of bulk GaAs and AlGaAs/GaAs distributed Bragg reflectors are obtained. With the ability to etch-stop just below the active region by thein situ monitoring, InGaAs vertical-cavity surface-emitting lasers with CW threshold current density as low as 380 A cm-2 with output power >11 mW are fabricated. Spatial uniformity is 5% over a 1-cm2 sample, which corresponds to one pair over 20 pairs of quarter-wave stacks of AlGaAs/GaAs distributed Bragg reflectors.

Two-Step Handover for LTE HetNet mobility enhancements

Through the study on “HetNet mobility enhancements for LTE”, it was observed that handover (HO) performance in heterogeneous networks (HetNets) deployments is not as good as in pure macro deployments. It also showed that high speed user equipment (UE) suffers much higher handover failure (HOF) rate than low speed UE. The major cause of HOF is the failure of handover command transmission from source eNodeB (eNB) to UE. There is the tradeoff between optimizing HO parameters to reduce HOF and increased ping-pongs. This makes HO performance optimization an intricate problem. In this paper, we propose a solution that improves HO performance with regard to HOF rate and ping-pong at the same time. It consists of early HO preparation and ping-pong avoidance. The early HO preparation helps to relieve HOF problems with expedited transmission of HO command message. But this premature HO command causes increased ping-pong rate. The ping-pong avoidance delays HO execution to suppress unnecessary HOs. This solution also improves to help with recovery from radio link failure (RLF).

Mobile service provision system based on sensor and SIP event notification mechanism

As the users need to take a mobile service while roaming around increases, many solutions are needed to figure out the users current location and service information and prepare subscribed mobile service without users intervention. We propose a mobile service provision system, which is based on sensor and SIP-specific event notification mechanism. In this system, dynamically subscribed service terminal is selected and used to provide mobile service to a roaming user by a sensor-equipped user agent which is also interworking with 3GPP IMS for getting extended user profile stored in HSS. And this user profile, capability report and command script which are used in our system are all XML-based documents, so these can also be easily modified and applied to any other services.

An efficient handover scheme for providing the inter-beam mobility

This paper proposes an inter-beam handover (IBH) scheme in millimeter wave (mmWave) communication systems. Since the width of a beam transmitted by a base station (BS) is very narrow, the IBH can frequently occur within a BS. For this reason, the previous handover scheme based on the radio resource control (RRC) message can cause the increase of the signaling overhead. In addition, the performance of the IBH can be dramatically deteriorated because the strength of the received signal can be rapidly decreased at the edge of a beam. To solve these problems, the proposed scheme performs the IBH by exploiting the medium access control (MAC) control element (CE) to reduce the signaling overhead, and it applies the inter-beam coordinated scheduling (IBCS) to improve the strength of the received signal at the edge of a beam.

Taking Advantage of Multiple Handover Preparations to Improve Handover Performance in LTE Networks

LTE handover procedure supports multiple preparations feature. But the feature is of help to successful re-establishment after radio link failure during handover procedure. The recently announced results from a field test conducted in a downtown in North America show handover failure rate over 21% in dense LTE networks. In this paper, we propose Early Handover Preparation with Ping-Pong Avoidance (EHOPPPA) handover which utilizes multiple preparations feature for improving handover performance. With EHOPPPA handover, the UE can receive multiple Handover Command messages of multiple prepared target eNBs and select the best eNB by cell selection. Simulation results show that the proposed scheme can achieve about 60% reduction in handover failure rate and nearly 100% successful recovery rate from radio link failure during handover without increasing ping-pong rate.

System coverage of Mm wave based 5G mobile communication system

To meet avalanche of traffic volume, the mmWave band in the 30-300GHz range is being considered as a candidate for new radio bands for 5G mobile communication systems. ETRI is developing mmWave based 5G mobile communication system in the Giga KOREA 5G (GK-5G) project. This paper introduces overview and features of the GK-5G mobile communication system and evaluates system coverage of the GK-5G mobile communication system through the system level simulation considering real geospatial information.