bandwidth estimation for radio based train control and communication ...

BANDWIDTH ESTIMATION FOR RADIO BASED TRAIN CONTROL AND COMMUNICATION SYSTEM Byungsik Yoon1, Min-woo Jung1, Sook-jin Lee1, Keun-hong Min2, and Young-Kyu Kim2 1 Wireless Convergence Research Team, ETRI, Daejeon, Republic of KOREA 2 Train Control & Communication Research Department, KRRI, Uiwang, Republic of KOREA

Abstract - The candidates of frequency bands and mobile communication systems for radio based train control and railway communication are introduced in this paper. The strength and weakness of frequency bands and mobile communication systems are also addressed for developing optimum frequency utilization and cost efficient railway infrastructure. A new calculation method of frequency spectrum bandwidth is proposed for radio based train control and railway communication system. Proposed method would be used to request new frequency band for dedicated railway communication system in Korea. Keywords: Railway communication, Train control, CBTC, GSM-R.

and standard based technology. Modern mobile communication systems provide very high data rate and very high mobility with secure IP equipment interoperability. These key features are required for modernized railway communication architecture. In this paper, we introduce the analysis result of frequency bands and mobile communication systems for Korean Radio based Train Control System (KRTCS) [3]. The candidates of licensed frequency for KRTCS are addressed in section 2. In section 3, we describe various candidates of mobile communication system for railway infrastructure. The estimation results of spectrum bandwidth for each system are shown in section 4, and conclusions are drawn in section 5.

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Introduction

During the last two decades, overall railroad infrastructures have been rapidly developed with increased demand for railway services. Especially, railway signaling and communication are key factors to maintain reliable, safe and secure operation. In the past, the wired communication systems were used for train signaling and communications in railway industry. Nowadays, wireless communication systems have emerged with a variety of advantages. The radio based train control and communication system is consider as cost efficient digital replacement for existing wired and analogue old railway system. North America and European countries have developed the radio based train control system to improve interoperability, safety, and cost efficiency. Unlike, a U.S. communications-based train control (CBTC) uses unlicensed frequency band, European GSM-R (GSM-Railway) uses the licensed frequency band with 8 MHz bandwidth (876 ~ 880 MHz for uplink and 921 ~ 925 MHz for downlink) [1][2]. Although, there are a lot of difficulty to be given dedicated licensed frequency band from the government, licensed frequency has many advantages over train control system. Licensed frequencies has no interference with other frequency band and it allows higher transmit power to increase radio coverage. Therefore, the reliability for secure communication can be increased, whereas infrastructure efforts along the railway track will be reduced. Especially, mobile communication based train control system has cost-effective deployment based on open

Frequency candidates for KRTCS

Since current Korean railway communication infrastructure is based on analog based radio system, railway communication services are restricted by poor quality of services, limited applications, large number of staff using the same channel and limited local connectivity. Because current radio system for railway infrastructure doesn’t meet the requirement of future railway communication, Korean government prepares new railway communication system. KRTCS is next generation train control and communication system which will replace current railway infrastructure in 2015. KRTCS use the dedicated radio spectrum for train control and railway communication. However, there are lots of challenges to develop and commercialize KRTCS. The biggest obstacle of implementing KRTCS is the acquisition of radio frequency. Unfortunately, only limited license frequency bands remain for public use. 700MHz spectrum bands (698 ~ 806 MHz) which are supposed to be freed up with analog to digital TV transition can be used for KRTCS [4]. Frequencies below 1 GHz are more attractive for radio based train control than those above 1 GHz. Lower path loss for greater radio coverage and lower Doppler shift for high speed train operation are the key features in 700MHz frequency. However, most country already has a plan to use this frequency band for next generation mobile communications, public safety, and ITS. New frequency allocation for railway infrastructures seems to be very competitive.

Fig. 1. Frequency candidates for KRTCS White spaces refer to frequencies (54 ~ 698 MHz) allocated to a broadcasting service but not used locally [5]. A train control system which has very sophisticate location information can use these frequency bands. Even if white spaces have superior frequency properties and less competitive than 700 MHz, there are unlicensed spectrum allocation tendency for public usage. Because licensed spectrum provides very high reliability with minimum interferences, locally licensed white space frequency should be given for train control and communication system. Some technical difficulty also should be solved for implementing seamless frequency alternation by location information. Exiting railroad infrastructures have their own radio frequencies for railway operation. 150 MHz, 440 MHz analog VHF and UHF bands are used for train driver and control center communication in Korea. Re-farming of the existing frequency has no need to allocate new frequency band and it can use superior frequency properties as well. However, new technologies for utilizing limited spectrum bandwidth should be needed and they increase the complexity of system and implementation cost. Frequency re-farming is heavily bounded by a very small amount of frequency bands which are currently used in Korea railway communication. In conclusion, although there are very competitive to get frequency license, some frequency area of 700MHz spectrum bands is the most suitable candidate for KRTCS. This frequency area not only has good frequency spectral properties but also could be allocated up to 10MHz bandwidth for KRTCS. After careful estimation and analysis of required bandwidth for KRTCS, Korean railway companies and organizations have schedule to apply 700MHz frequency distribution process in 2011

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Mobile communication for KRTCS

candidates

The KRTCS technologies shall be based on international standardization frame work to enhance the economic feasibility. It should support for high data rate and high speed train operation. High data rate with low latency

are needed to support high capacity network utilization and real time operation in full mobility. Many different mobile services are currently provided by various mobile telecommunication service companies in Korea. 2G mobile systems are ruled out due to the fact that it is not suitable for future railway communication requirement. Wideband Code Division Multiple Access (WCDMA), Wireless Broadband (WiBro, WiMax) and 3GPP Long Term Evolution (LTE) is 3G or beyond 3G mobile system. These systems currently provide high quality mobile services and these are strong candidate for KRTCS. WCDMA is 3G mobile communication network which densely deploys across the nation. Existing WCDMA network can be used as fallback system when dedicated WCDMA based train control fail to operation. Meanwhile, high cost equipments (Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN)) for packet transmission increase the implementation cost. Moreover, WCDMA is relatively old school telecommunication technology compare to WiBro and 3GPP LTE. WiBro (WiMAX) is wireless broadband internet technology developed by Korea [6]. Although it offers a high data throughput and wide area of coverage with relatively low cost network equipments, the major telecom companies tend to adopt their next generation mobile system as 3GPP LTE technology. In 2008, awareness was created on a new technology, with a promising name 3GPP LTE. This would impact GSMR life cycle. 3GPP LTE is latest mobile network technology which evolved from WCDMA system. It provides high speed data rate (100 Mbps for downlink) and high mobility (up to 500 Km/h) over the all IP-based network. Recently, International Union of Railways (UIC) prepares to develop the LTE-Railway for next generation of GSM-R [7]. The popularity and globalization is very important for the interoperation and longtime maintenance of railroad infrastructure. However, the preparation of commercialized LTE chip sets and test equipments for developing the radio based railway communication system needs more time.

Most prominent candidate mobile system is 3GPP LTE system. Although 3GPP LTE is not fully commercialized at this moment, it satisfies future railway communication requirement and will evolve to global railway standardization. The public hearing and discussion will be scheduled for railway organizations to decide mobile system for KRTCS in 2011.

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[bps/Hz/Cell] for WCDMA, WiBro, and LTE system respectively. Based on these parameters, we can deliver required spectrum bandwidth for establishing the KRTCS. For the highest reliability of train operation, fully duplicated network structure with overlaid radio cells is usually used in railway network planning. The duplicated network structure is shown in fig 2. As a result, total bandwidth should be double compare to single cell based system.

Spectrum bandwidth calculation

Spectrum bandwidth estimation method varies from one area to another. Determining how many frequency bands are needed is vital to frequency band request to government. In this paper, we estimate the frequency spectrum bandwidth based on ITU-R M.1390 for train communication system. ITU-R M.1390 is the general methodology to estimate spectrum bandwidth for various mobile communication services using 12 parameters [8]. These 12 parameters are analyzed to apply radio based train control system. The estimation of total data rate which are used during the train operation is important to calculate bandwidth estimation. To estimate maximum transmission data rate, we assumed that Seoul station is the highest data traffic transmission place and diameter of cell coverage is 6 Km. As a result, the data rate for automatic train control (ATC) is induced about 530 kbps. To calculate the voice communication data rate, we should consider the point to point call, group call, broadcast call and emergence call from the railway many difference communication scenarios. The total voice data rate is calculated as 224 kbps with 30 second average conversation time. 1.1 Mbps data rate is required for advanced railway data service like on-line ticketing, arrival and transit information. The surveillance video service is needed to improve the security of railway facilities and passenger monitoring. Video data rate is calculated as 2 Mbps respectively. Overall data rate is shown in table 1. Table 1. Estimation of data rate for KRTCS Function

Data rate

ATC

Up link 530 kbps

Down link 530 kbps

Voice call

224 kbps

224 kbps

Data service

1,100 kbps

BTS

BTS

MSC/VLR HLR

MSC/VLR HLR

TRAU A

TRAU B

BSC A

BSC B

BTS

BTS

BTS

BTS

BTS

Cell A1

Cell A2

Cell A3

Cell A4

Cell B1

Cell B2

Cell B3

Cell B4

Fig. 2. Overlayed radio cells for railway communication

WCDMA is Frequency Division Duplexing (FDD) system and offers 5 MHz bandwidth for up and down link respectively. For constructing overlaid radio cell, 40 MHz bandwidth is needed for implementing KRTCS. WiBro uses Time Division Duplexing (TDD) system and has 5 MHz, 8.75 MHz, 10 MHz bandwidth profile in Korea. TDD can share the same bandwidth for up and down link connection. So, 17.5 MHz bandwidth can be used for overlaid radio cell. 3GPP LTE uses FDD and has 1.3 MHz, 3 MHz, 5 MHz, 10 MHz bandwidth profile in Korea. To implement overlaid radio cell, 12 MHz bandwidth are required for KRTCS. Consequently, the required spectrum bandwidth of WCDMA, WiBro, and LTE are shown in Table 2. Table 2. Required spectrum bandwidth for KRTCS

2,304 kbps

2, 048 kbps

Mobile System

Up link

Video service Total

3,058 kbps

3,902 kbps

WCDMA

5.86 MHz

Down link

Total (Overlayed Cell)

7.47 MHz

40 MHz (10 MHz x 4)

WiBro One of the critical parameter for estimating bandwidth is net system capability of mobile communication system. It is a measure of how much data are transmitted to given frequency bandwidth in a specific condition of mobile communication radio cell. The net system capabilities for train control environment are induced as 0.51, 1.30, and 1.70

BTS

2.30 MHz

2.93 MHz

17.5 MHz (8.75 MHz x 2)

3GPP LTE

1.70 MHz

2.24 MHz

12 MHz (3 MHz x 4)

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Conclusions

In this paper, we describe candidates of frequency band and mobile communication system for Korean radio based train control system. Each frequency bands and mobile systems are analyzed to construct efficient train control system. Required frequency bandwidth is calculated using ITU-R recommendation under the railway environment. The analysis results of each mobile communication system will be used to select the dedicated mobile communication system for the KRTCS. The strong candidate of frequency band and mobile system for KRTCS is 700MHz spectrum band and 3GPP LTE mobile system respectively. As a result of bandwidth estimation, 3GPP LTE mobile system requires 12 MHz bandwidth for dedicated radio based train control and railway communication. The calculation result of required bandwidth for KRTCS will be used to apply frequency bandwidth request process of 700MHz frequency distribution in 2011.

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

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References

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