Measurement Analysis for Handover Decision Procedure in a High-Speed Railway LTE System of High-

Speed Railway

Jungjin Choi1, Longzhe Han1, Byungsik Yoon2 and Hoh Peter In1,*

1 College of Information and Communications, Korea University

Seoul, Republic of Korea {04jinny, lzhan, hoh_in}@korea.ac.kr

2Mobile Convergence Service Research Team, Electronics and Telecommunications

Research Institute, Deajeon, Republic of Korea bsyoon@etri.re.kr

Abstract. Because of their speed and cost-effectiveness, high-speed trains are becoming a popular mode of transportation. To meet increasing passenger demand with limited capacity, a reliable real-time train control system is indispensable. Due to its excellent performance and low maintenance cost, the Long Term Evolution (LTE) system is under consideration for adoption by train control systems as the wireless communication infrastructure. However, because of the high speed of trains, handovers occur frequently, increasing the probability of handover failure and link interruption. Handover trigger threshold values are particularly important for the handover procedures in the high-speed railway control area. In this paper, through the extensive experiments in the OPNET simulator, we find that the existing handover trigger threshold values are inappropriate for the high-speed railway environment. The necessary changes are proposed to improve the handover procedure.

Keywords: LTE, Handover, High-speed railway

1 Introduction

High-speed trains are an energy-efficient and reliable mode of public transportation. They are also more eco-friendly than modes of transportation than such as cars and airplanes which use fossil fuel. Several countries have been developing high-speed railways because they are a promising transportation system. To meet increasing passenger demand with the limited transport capacity of a high-speed railway, train operating times and distance intervals need to be tightly controlled. In order to increase the passenger throughput and reduce the time interval between consecutive trains, a reliable real-time railway control system is essential.

* Corresponding Author

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The most important requirements for a railway control system are reliability and real-time communication. Recently, the stability of wireless technology has increased. In particular, key features have been developed such as low latency and broad bandwidth, for the railway monitoring information exchange like video conferencing and Voice over Internet Protocol (VoIP). Considering these requirements and the current technology, the LTE standard is an appropriate technology for a real-time control system. Hence, LTE-based railway control systems are under test in high-speed railway control areas.

However, the faster a train moves, the more frequently handover occurs. Handover trigger timing is an important issue, as frequently delayed handovers decrease the quality of service. In this paper, OPNET [1] simulation results prove that existing handover algorithms are inappropriate for high-speed railways. The changes necessary to improve the handover timing are also proposed.

The rest of this paper is organized as follows. The handover trigger algorithm of LTE and the high-speed railway environment are introduced in Section 2. Section 3 presents OPNET simulation results and analysis. Finally, conclusions and future work are presented in Section 4.

2 Handover Algorithm and High-Speed Railway Characteristics

The Intra LTE handover process consists of several phases. This paper focuses on the initial measurement report trigger phase because the handover decision time of the serving Evolved Node B (eNB) varies by vendor and pure handover execution time is small and relatively constant. In the measurement reporting phase, Event A3 is a typical measurement report triggering criteria. The criteria in [2] is summarized briefly as

Mt > Ms + Hys . (1)

where Mt is the measurement result of the target cell including offsets, Ms is the measurement result of the serving cell including offsets, and Hys is the hysteresis parameter for the event.

If the criteria in (1) is continuously met during TTT (Time to Trigger), a measurement report will be triggered. The parameters TTT and Hys are control parameters that block a ping-pong handover, which happens when the direction and speed of user equipment (UE) are not constant and the propagation environment is unstable. In other words, Event A3 is an algorithm that is designed with the assumption that ping-pong handovers occur frequently.

A high-speed railway has two characteristics that are important for the handover decision. Firstly, railroad tracks for the high-speed railway are installed in a relatively straight line in flat and open land for safety and cost-efficiency. Hence, eNBs are deployed close to the railroad line to maximize the coverage area and for cost-effectiveness. Secondly, the speed of the train constantly changes for the same reasons as above.

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It is known that the UE will travel straight towards the target eNB after passing the serving eNB at a constant speed. This inferred fact affects the measurement confidence. If the measurement is reliable enough, the current Event A3 algorithm could be improved to trigger the handover without the help of Hys and TTT.

3 Simulation Results and Analysis

Simulations were performed on the OPNET network simulator. Serving and target eNBs were deployed on a straight railway line. The distance between the serving and target eNBs was 2000 m. The UE (i.e., train) moved on the straight railway line at a speed of 300 Km/h. The UE measured the reference signal from the surrounding eNBs.

The relationship between the measurements and the distance between the UE and an eNB was evaluated. Several path loss models are supported by the OPNET simulator; The Vehicular Environment model and the Hata Extension Suburban/Rural model were applied to the same scenario to simulate different propagation path loss environments. The simulation results are shown in Fig. 1.

The analysis focuses on the measurements when the UE approaches the target eNB after it has passed the serving eNB, because the measurements at this point affect the handover trigger time. In Fig. 1, the x-axis of the graphs indicates the distance in meters between the UE and the target eNB. The y-axis indicates Md, given by

Md = Mt - Ms . (2)

where Md is the measured difference between the target and serving eNB reference signals, and a positive Md indicates that the signal of the target eNB is better than the serving eNB. Graphs (a) and (c) give Md of the Reference Signal Received Power (RSRP) measurement in dBm, and graphs (b) and (d) give Md of the Reference Signal Received Quality (RSRQ) measurement in dB.

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Fig. 1. Scatter graphs of distance versus Md.

In Fig. 1, there are differences in the smoothness of the signal change according to the type of path loss model and the measurement type. However, as the UE approaches the target eNB, Md increases relatively constantly, and a high correlation is inferred. Correlation coefficients, calculated from the results using Pearson’s r in [4], are given in Table 1, and they show that the two measurements are highly correlated, as every correlation coefficient is very close to -1. This high correlation is a result of the characteristics of the high-speed railway described in Section 2. In particular, the static mobility characteristics of a high-speed train lead to high correlation. Hence, the dynamic mobility characteristics of the UE assumed by the Event A3 algorithm are inappropriate for a high-speed railway, and Hys and TTT are derived from an inappropriate assumption. This is critical because the handover decision time can be delayed in a high-speed railway environment due to of the behavior of Hys and TTT.

Table. 1. Measurement correlation coefficients.

Applied Path-Loss Model Y-Axis Measurement Type Correlation Coefficient

Vehicular Environment RSRP -0.966 RSRQ -0.993

Hata Extension Suburban/Rural RSRP -0.973 RSRQ -0.978

4 Conclusions and Future Work

The analysis of the simulation results showed that the correlation between distance and Md is very high, proving that the assumption behind the parameters Hys and TTT is wrong. This result brings into question the necessity of Hys and TTT. If Hys and TTT are removed from the Event A3 algorithm, the time delays caused by them will also be removed, but the reliability of the resulting measurements will no longer be guaranteed.

Hence, as future work, we plan to simulate a measurement evaluation model using a linear regression analysis to replace Hys and TTT. The model evaluates the reliability of the latest measurement while preventing time delays. The improved algorithm will trigger handover according to the measurement evaluation result.

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Acknowledgments. This research was supported by a grant (10PURTB056851-01) from the “Future Urban Railway Development Program” funded by the Minster of Land, Transport and Maritime Affairs of the Korea Government.

References

1. OPNET Technologies, Inc., OPNET Modeler v.17.5., http://www.opnet.com 2. 3GPP: Radio Resource Control (RRC) Protocol Specification, Specification 36.331, Section

5.5.4, http://www.3gpp.org 3. Aguado, M., Onandi, O., Agustin, P., Higuero, M., Taquet, E.: Wimax on Rails. IEEE

Vehicular Technology Magazine, vol. 3, no. 3, pp. 47--56 (2008) 4. Pearson Product-moment Correlation Coefficient, http://en.wikipedia.org/wiki/ 5. LTE EMM Procedure, http:// www.netmanias.com/bbs/zboard.php?id=techdocs

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