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 1Wireless Convergence Research Team, ETRI, Daejeon, Republic of KOREA 2 Train Control & Communication Research Department, KRRI, Uiwang, Republic of KOREA

and standard based technology. Modern mobile Abstract - The candidates of frequency bands and mobile communication systems provide very high data rate and very communication systems for radio based train control and high mobility with secure IP equipment interoperability. railway communication are introduced in this paper. The These key features are required for modernized railway strength and weakness of frequency bands and mobile communication architecture. communication systems are also addressed for developing optimum frequency utilization and cost efficient railway In this paper, we introduce the analysis result of infrastructure. A new calculation method of frequency frequency bands and mobile communication systems for spectrum bandwidth is proposed for radio based train control Korean Radio based Train Control System (KRTCS) [3]. The and railway communication system. Proposed method would candidates of licensed frequency for KRTCS are addressed in be used to request new frequency band for dedicated railway section 2. In section 3, we describe various candidates of communication system in Korea. mobile communication system for railway infrastructure. The estimation results of spectrum bandwidth for each system are Keywords: Railway communication, Train control, CBTC, shown in section 4, and conclusions are drawn in section 5.

GSM-R.

2 Frequency candidates for KRTCS 1 Introduction Since current Korean railway communication During the last two decades, overall railroad infrastructure is based on analog based radio system, railway infrastructures have been rapidly developed with increased communication services are restricted by poor quality of demand for railway services. Especially, railway signaling services, limited applications, large number of staff using the and communication are key factors to maintain reliable, safe same channel and limited local connectivity. Because current and secure operation. In the past, the wired communication radio system for railway infrastructure doesn’t meet the systems were used for train signaling and communications in requirement of future railway communication, Korean railway industry. Nowadays, wireless communication government prepares new railway communication system. systems have emerged with a variety of advantages. The KRTCS is next generation train control and communication radio based train control and communication system is system which will replace current railway infrastructure in consider as cost efficient digital replacement for existing 2015. KRTCS use the dedicated radio spectrum for train wired and analogue old railway system. North America and control and railway communication. However, there are lots European countries have developed the radio based train of challenges to develop and commercialize KRTCS. The control system to improve interoperability, safety, and cost biggest obstacle of implementing KRTCS is the acquisition of efficiency. Unlike, a U.S. communications-based train radio frequency. Unfortunately, only limited license control (CBTC) uses unlicensed frequency band, European frequency bands remain for public use. GSM-R (GSM-Railway) uses the licensed frequency band with 8 MHz bandwidth (876 ~ 880 MHz for uplink and 921 ~ 700MHz spectrum bands (698 ~ 806 MHz) which are 925 MHz for downlink) [1][2]. Although, there are a lot of supposed to be freed up with analog to digital TV transition difficulty to be given dedicated licensed frequency band from can be used for KRTCS [4]. Frequencies below 1 GHz are the government, licensed frequency has many advantages more attractive for radio based train control than those above over train control system. Licensed frequencies has no 1 GHz. Lower path loss for greater radio coverage and lower interference with other frequency band and it allows higher Doppler shift for high speed train operation are the key transmit power to increase radio coverage. Therefore, the features in 700MHz frequency. However, most country reliability for secure communication can be increased, already has a plan to use this frequency band for next whereas infrastructure efforts along the railway track will be generation mobile communications, public safety, and ITS. reduced. Especially, mobile communication based train New frequency allocation for railway infrastructures seems to control system has cost-effective deployment based on open be very competitive.

Fig. 1. Frequency candidates for KRTCS

White spaces refer to frequencies (54 ~ 698 MHz) are needed to support high capacity network utilization and allocated to a broadcasting service but not used locally [5]. A real time operation in full mobility. Many different mobile train control system which has very sophisticate location services are currently provided by various mobile information can use these frequency bands. Even if white telecommunication service companies in Korea. 2G mobile spaces have superior frequency properties and less systems are ruled out due to the fact that it is not suitable for competitive than 700 MHz, there are unlicensed spectrum future railway communication requirement. Wideband Code allocation tendency for public usage. Because licensed Division Multiple Access (WCDMA), Wireless Broadband spectrum provides very high reliability with minimum (WiBro, WiMax) and 3GPP Long Term Evolution (LTE) is interferences, locally licensed white space frequency should 3G or beyond 3G mobile system. These systems currently be given for train control and communication system. Some provide high quality mobile services and these are strong technical difficulty also should be solved for implementing candidate for KRTCS. seamless frequency alternation by location information. WCDMA is 3G mobile communication network which Exiting railroad infrastructures have their own radio densely deploys across the nation. Existing WCDMA frequencies for railway operation. 150 MHz, 440 MHz network can be used as fallback system when dedicated analog VHF and UHF bands are used for train driver and WCDMA based train control fail to operation. Meanwhile, control center communication in Korea. Re-farming of the high cost equipments (Serving GPRS Support Node (SGSN), existing frequency has no need to allocate new frequency Gateway GPRS Support Node (GGSN)) for packet band and it can use superior frequency properties as well. transmission increase the implementation cost. Moreover, However, new technologies for utilizing limited spectrum WCDMA is relatively old school telecommunication bandwidth should be needed and they increase the complexity technology compare to WiBro and 3GPP LTE. of system and implementation cost. Frequency re-farming is heavily bounded by a very small amount of frequency bands WiBro (WiMAX) is wireless broadband internet which are currently used in Korea railway communication. technology developed by Korea [6]. Although it offers a high data throughput and wide area of coverage with relatively In conclusion, although there are very competitive low cost network equipments, the major telecom companies to get frequency license, some frequency area of 700MHz tend to adopt their next generation mobile system as 3GPP spectrum bands is the most suitable candidate for KRTCS. LTE technology. This frequency area not only has good frequency spectral properties but also could be allocated up to 10MHz In 2008, awareness was created on a new technology, bandwidth for KRTCS. After careful estimation and analysis with a promising name 3GPP LTE. This would impact GSM- of required bandwidth for KRTCS, Korean railway R life cycle. 3GPP LTE is latest mobile network technology companies and organizations have schedule to apply 700MHz which evolved from WCDMA system. It provides high speed frequency distribution process in 2011 data rate (100 Mbps for downlink) and high mobility (up to 500 Km/h) over the all IP-based network. Recently, 3 Mobile communication candidates International Union of Railways (UIC) prepares to develop the LTE-Railway for next generation of GSM-R [7]. The for KRTCS popularity and globalization is very important for the The KRTCS technologies shall be based on interoperation and longtime maintenance of railroad international standardization frame work to enhance the infrastructure. However, the preparation of commercialized economic feasibility. It should support for high data rate and LTE chip sets and test equipments for developing the radio high speed train operation. High data rate with low latency based railway communication system needs more time. Most prominent candidate mobile system is 3GPP LTE [bps/Hz/Cell] for WCDMA, WiBro, and LTE system system. Although 3GPP LTE is not fully commercialized at respectively. Based on these parameters, we can deliver this moment, it satisfies future railway communication required spectrum bandwidth for establishing the KRTCS. requirement and will evolve to global railway standardization. For the highest reliability of train operation, fully duplicated The public hearing and discussion will be scheduled for network structure with overlaid radio cells is usually used in railway organizations to decide mobile system for KRTCS in railway network planning. The duplicated network structure 2011. is shown in fig 2. As a result, total bandwidth should be double compare to single cell based system. 4 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 MSC/VLR MSC/VLR this paper, we estimate the frequency spectrum bandwidth HLR HLR based on ITU-R M.1390 for train communication system. TRAU A TRAU B ITU-R M.1390 is the general methodology to estimate spectrum bandwidth for various mobile communication BSC A BSC B 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 BTS BTS BTS BTS BTS BTS BTS BTS 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 Cell A1 Cell A2 Cell A3 Cell A4 Cell B1 Cell B2 Cell B3 Cell B4 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 Fig. 2. Overlayed radio cells for railway communication emergence call from the railway many difference communication scenarios. The total voice data rate is calculated as 224 kbps with 30 second average conversation WCDMA is Frequency Division Duplexing (FDD) system time. 1.1 Mbps data rate is required for advanced railway data and offers 5 MHz bandwidth for up and down link service like on-line ticketing, arrival and transit information. respectively. For constructing overlaid radio cell, 40 MHz The surveillance video service is needed to improve the bandwidth is needed for implementing KRTCS. WiBro uses security of railway facilities and passenger monitoring. Video Time Division Duplexing (TDD) system and has 5 MHz, 8.75 data rate is calculated as 2 Mbps respectively. Overall data MHz, 10 MHz bandwidth profile in Korea. TDD can share rate is shown in table 1. 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 Table 1. Estimation of data rate for KRTCS MHz bandwidth profile in Korea. To implement overlaid radio cell, 12 MHz bandwidth are required for KRTCS. Function Data rate Consequently, the required spectrum bandwidth of WCDMA, Up link Down link WiBro, and LTE are shown in Table 2. ATC 530 kbps 530 kbps Voice call 224 kbps 224 kbps Table 2. Required spectrum bandwidth for KRTCS Data service 1,100 kbps Mobile Up link Down link Total Video service 2,304 kbps 2, 048 kbps System (Overlayed Cell) Total 3,058 kbps 3,902 kbps WCDMA 5.86 MHz 7.47 MHz 40 MHz (10 MHz x 4)

WiBro 2.30 MHz 2.93 MHz 17.5 MHz (8.75 MHz x 2) One of the critical parameter for estimating bandwidth is net system capability of mobile communication system. It is a 3GPP 1.70 MHz 2.24 MHz 12 MHz measure of how much data are transmitted to given frequency LTE (3 MHz x 4) 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 5 Conclusions [7] G. Tingting, and S. Bin, “A high-speed railway mobile communication system based on LTE,” ICEIE, vol. 1, pp. In this paper, we describe candidates of frequency band 414-417, Aug. 2010. and mobile communication system for Korean radio based train control system. Each frequency bands and mobile [8] Recommendation ITU-R M.1390. “Methodology for the systems are analyzed to construct efficient train control calculation of IMT-2000 terrestrial spectrum requirements.” 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 (10PURT- B056851-01) from “Future Urban Railway Development Program” funded by Minster of Land, Transport and Maritime Affairs of Korea government.

6 References

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