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Abstract： Wireless communication technologies play an essential role in supporting railway operation and control. The current Global System for Mobile Communications?Railway （GSM?R） system offers a rich set of voice services and data services related with train control， but it has very limited multimedia service bearer capability. With the development of commercial wireless industry， Long?Term Evolution （LTE） mobile broadband technology is becoming the prevalent technology in most of commercial mobile networks. LTE is also a promising technology of future railway mobile communication systems. The 3rd Generation Partner Project （3GPP） and China Communications Standards Association （CCSA） have proposed two feasible LTE based broadband trunking communication solutions： the 3GPP Mission Critical Push to Talk （MCPTT） solution and B?TrunC solution. In this paper， we first introduce the development of railway mobile communications and LTE technology. The user requirements of future railway mobile communication system （FRMCS） are then discussed. We also analyze the suitability of the two LTE?based solutions for LTE based Next?Generation Railway Mobile Communication System （LTE?R） from different aspects.
Keywords： 3GPP MCPTT; B?TrunC; FRMCS; LTE?R
DOI： 10.12142/ZTECOM.201901009
http：//kns.cnki.net/kcms/detail/34.1294.TN.20190314.1717.004.html， published online March 14， 2019
Manuscript received： 20180101
1 Introduction
Railway mobile communications originated from the incompatibility of track cables and analogue railway radio networks. Due to their limited bearing capability， less anti?interference capability， and lack of encryption， the analogue railway radio networks were replaced by the Global System for Mobile Communications?Railways （GSM?R） systems. GSM?R bears the railway trunking dispatching voice services and the train control data services as well， which fulfills the mobile communication service requirements of railway systems. However， the limited capacity， higher cost and development cycle inherited in GSM has led to the situation that the market of GSM?R is moving inexorably towards its end.
In 2012， International Union of Railways （UIC） started to evaluate the actual situation of global railway market and to study the needs of railway operators and passengers. The second version of “User Requirements Specification” for Future Railway Mobile Communication System （FRMCS） was released in March 2016， which is an indispensable step towards the introduction of a successor of GSM?R. Besides the existing GSM?R services， several potential new application requirements would also be added into the FRMCS， such as multimedia dispatching communications and real time video monitoring [1]. GSM?R cannot satisfy the users’ requirements of FRMCS and GSM industry will be terminated in 2025， which are motivating the design of GSM?R successor. Long?Term Evolution （LTE） mobile broadband technology is currently the most successful commercial broadband mobile communication system. LTE provides higher data capacity than GSM and presents a flat architecture to reduce deployment and maintenance costs as long as standard entities are used. In this sense， two feasible LTE based broadband trunking communication solutions， 3GPP Mission Critical Push to Talk （MCPTT） [2] and LTE?based broadband trunking communication （B?TrunC） [3]， have been proposed by the 3rd Generation Partner Project （3GPP） and China Communications Standards Association （CCSA）. MCPTT is a new global standard for replacing the conventional group communication systems， such as Trans European Trunked Radio （TETRA）， P25 and others. MCPTT is based on the LTE architecture and will be available for governments/public safety or railway operators. 3GPP is focusing on the network infrastructure design， while the client?side is left up to manufacturers to design and implement according to the requirements of users. Developed by CCSA， B?TrunC is designed based on LTE Release 9 [4]. The B?TrunC standard has been developed to address the evolving needs driven by the emergence of new trunking communication requirements， such as multimedia dispatch applications.
However， neither MCPTT nor B?TrunC is specially designed for railway mobile communications. In this context， the suitability of the two LTE?based solutions for LTE based FRMCS （LTE?R） will be analyzed in this paper from the following six aspects： architecture， system functionality and performance， standardization level， interoperability， high mobility support capability， and backward compatibility with GSM?R.
The content of this paper is organized as follows. First， a brief introduction of the railway mobile communications and LTE systems are presented in Section 2. Then， the user requirements of FRMCS are discussed in Section 3， where the MCPTT and B?TrunC are introduced. In Section 4， we analyze the suitability of the two feasible solutions for LTE?R. Finally， conclusions are drawn in Section 5.
2 Development of Railway Mobile
Communications
The high?speed railway （HSR） is becoming the major mode of transportation for a journey and has been developed rapidly worldwide， benefitting from its advantages of safety， reliability， convenience， comfortableness， and low energy consumption. To guarantee the reliability， availability and safety of the train?ground data transmission， railway mobile communication systems play a key role in the HSR operation [1]. In the early stage of railway mobile communications， several analogue radio networks supported mobile communication applications for drivers and trackside workers. For example， British Rail developed the National Radio Network （NRN） specifically used for the operational railways， which provides radio coverage for 98% of the rail network through base stations and radio exchanges. NRN could provide dedicated open channels on talk?through mode for incident management and an override priority facility， in order to ensure that all emergency calls could be immediately connected to the railway’s train control offices and electrical control rooms [5].
The analog radio network has reached to its limits due to the rapid growth of communications traffic volume and the increasing demands for security， economy， efficiency and safety of railway traffic.
In 1994， the GSM standard of European Telecommunication Standards Institute （ETSI） was selected by UIC as the first Digital Railways Radio Communication System standard， because it is the sole system in commercial operation and it is already proven with off?the?shelf products available， requiring the minimum modifications. However， GSM could not fulfill all the necessary requirements of the efficient railway services. Therefore it was necessary that the advanced voice call features should be identified and added to the standard GSM. UIC， together with the railway operators， launched the European Integrated Radio Enhanced Network （EIRENE） to specify the requirements for mobile networks to fulfill the needs of railways including the features of additional group and broadcast calls [6]. To validate whether these EIRENE functional specifications could be transferred into technical implementations， three prototypes were developed by manufacturers and three pilot?lines were planned and realized in France， Italy and Germany. These three pilot?line systems were built to test different aspects of operation， such as railway station environment， complex radio coverage topology with tunnels and bends， and high speed lines at speeds up to 300 km/h [7].
In 1997， 32 railway operators all over Europe signed a Memorandum of Understanding （MoU） to terminate the investment in the analogue radio systems and start to invest the implementation of GSM?R. As of today， the number of signatories has increased to 38， including railway operators outside Europe. Today， over 100 000 km of railway lines are operated by GSM?R systems and this amount is still growing. In addition， the industry has given commitment to support the GSM?R technology until at least 2030 [1]. 3 User Requirements of Next?Generation
Railway Mobile Communication System
As telecommunication standards are evolving and it usually takes a long time to realize the application of any technology specifications， it is urgent to start the research work on the successor of GSM?R. Thus UIC decided to set up the FRMCS project to prepare the necessary steps towards the introduction of a successor of GSM?R in 2012. The project started with the situation evaluation of actual railway systems and investigation on needs of users， and ended with the delivery of the first specification for user requirements of the next?generation railway mobile communication system.
In 2016， a new version of FRMCS user requirement specification was published [1]， in which the traffic requirements are classified into two categories including communication applications and supporting applications. Besides， the users are classified into three groups： critical users， performance users， and business users. The critical users refer to the applications that can enhance the reliability， availability， maintainability and safety （RAMS） of railway systems. The performance users refer to the applications that can improve the performance of railway operation， such as train departure and telemetry. The business users refer to the applications that can support the railway business operation in general.
Comparing with the functional requirement specification of GSM?R system， the new version has some new broadband mobile services that are high demanded， such as real time video and wireless Internet on?train for passengers. Furthermore， more train?ground data services are included， such as monitoring and control of non?critical infrastructure and trackside maintenance communications.
All the above mentioned traffic requirements cannot be met by narrow?band mobile communication systems， and the industry support of GSM?R will be ended in 2030. Thus， it is urgent to develop a dedicated broadband mobile communication system as the successor of GSM?R.
Besides the traffic requirements， the fundamental design principles of FRMCS are proposed for user requirements， which include application decoupled with system architecture， interoperability， reuse of the existing infrastructure， high mobility support， backward compatibility with GSM?R， etc. [1]. These principles will guide the design of LTE?R systems.
4 Two Feasible Solutions for LTE?R
To provide a wide range of data?centric services， such as video sharing， multimedia dispatching， and ubiquitous Internet and intranet access for governments and organizations involved in public safety and security， two LTE based broadband trunking communication systems， B?TrunC system and 3GPP MCPTT system， are designed. These two systems are considered as the candidates for LTE?R because the user requirements of railway systems are similar to public safety communication services and LTE is the currently most popular 4G mobile broadband systemthere. 4.1 LTE Based Broadband Trunking Communication
system （B?TrunC）
To accomplish broadband multimedia trunking service requirements， i.e.， group communication demand for voice， data， and video， CCSA initially started to develop a Broadband Trunking Communication （B?TrunC） system in 2012， which has been admitted by ITU?R as the Public Protection and Disaster Relief （PPDR） Recommendation Standard.
The B?TrunC system is designed based on the TD?LTE system with 3GPP Release 9. The single?cell point?to?multipoint （SC?PTM） is the feature of air interface of B?TrunC system， which is designed to realize the group communications. The SC?PTM is a new type of radio access method dedicated to multicast through the Physical Downlink Shared Channel （PDSCH） in a single cell. For SC?PTM transmission， user equipment in a group receives the group data through a common radio resource region in the PDSCH. This concept naturally allows the group data to be multiplexed with the normal unicast data within a PDSCH subframe and thus does not cause the problem of radio resource granularity.
Besides the SC?PTM technology， two trunking communication entities are involved in System Architecture Evolution （SAE） of LTE system， which are Trunking Control Function （TCF） and Trunking Media Function （TMF）. TCF is responsible for trunking service scheduling， call setup and release， session management， authentication， registration， and cancellation. TMF is responsible for trunking user plane management， routing， data forwarding， encoding， etc. The interface between terminals and the system， Uu?T， and the interface between the core network and the dispatcher， D， are designed. To enhance the latency performance of B?TrunC， the Multimedia Broadcast/Multicast Service （MBMS） and Push to Talk over Cellular （PoC） technologies of LTE are carried out. Now CCSA has completed the general technical requirements and air interface standards for B?TrunC.
4.2 LTE and MCPTT Standardization Roundup
In this section， the 3GPP LTE standardization process related with MCPTT is introduced.
The fully commercial operation of LTE systems started from the 3GPP Release 8 specification， which was finalized in 2008. The latest version of LTE， Release14 had been frozen by mid?2017.
LTE Release 8 specified one primary broadband technology based on Orthogonal Frequency Division Multiple Access （OFDM）. LTE Release 8 is mainly deployed in a macro/microcell layout and can provide improved system capacity and coverage， high peak data rates， low latency， reduced operating costs， multi?antenna support， flexible bandwidth operation and seamless integration with existing systems [8]. LTE Release 9 provides some minor enhancements to LTE Release 8 with respect to the air interface. These features include dual?layer beamforming and time difference of arrival based location techniques. To support the video on demand， video conference and other multimedia services， MBMS architecture is included in the Evolved Packet Core （EPC） of LTE. LTE Release 10 realizes the following features： bandwidth extension via carrier aggregation to support deployment bandwidth up to 100 MHz， downlink spatial multiplexing including single?cell multi?user MIMO transmission and uplink spatial multiplexing， and heterogeneous networks with emphasis on Type 1 and Type 2 relays [9]. For this release， LTE technology refers to LTE?A as a formal 4G system. As the capacity and performance of the LTE traffic channels are progressively improved， the downlink control channels may become a bottleneck. To address this issue， an enhanced physical downlink control channel （EPDCCH） has been introduced in 3GPP LTE Release 11 [10].
The core specification item of Release 12 in terms of group communication is Group Communication System Enabler （GCSE）. The conventional physical channels in LTE can be good media for providing group communications in some basic scenarios [11]. GCSE defines the requirements for group communications and proposes system architecture on top of the existing physical channels. The LTE system of Release 12 has two fundamental physical channels for transferring the data： the PDSCH， which is commonly used for normal unicast data， and the physical multicast channel （PMCH）， which is designed for evolved MBMS （eMBMS）. Furthermore， direct device?to?device （D2D） communication is the other feature of Release 12.
Releases 13 and 14 focus on the air interface aspects. They are a part of the continued evolution of LTE?Advanced and play a role as a bridge from 4G to 5G. To meet the user requirements of PPDR， the MCPTT services and system architecture are defined， whose technology enhancement and realization has been completed in Release14 [12]， [13]. Other techniques in this release include enhancement of D2D proximity services， indoor positioning enhancements， and the single?cell point?to?multipoint （SC?PTM） [14].
5 Analysis of Feasible Architecture
Solutions of LTE for Railway
Neither 3GPP MCPTT nor B?TrunC is specially designed for Railway communications. They cannot fully fulfill the user requirements of LTE?R. Next， the suitability of the two LTE?based solutions for FRMCS are analyzed from six aspects， including architecture， system functionality and performance， standardization， interoperability， high mobility support capability， and backward compatibility with GSM?R.
5.1 System Architecture
The functionality and commoditization of future railway system determine the design principles of future railway mobile communications system. The FRMCS architecture design should satisfy the services and architecture decoupling principle. As GSM?R is a modified off?the?shelf technology system based on GSM offering， the enhancement to deliver specific “R” （railway） functionality has proven expensive for the railways. For future railway communication system， the specific “R” （railway） functionality provisioning should be decoupled with communication system. B?TrunC provides the fundamental solution for group communication. To provide the group communication services， several logical channels are designed including Trunking Control Channel （TCCH）， Trunking Traffic Channel （TTCH）， Trunking Paging Channel （TPCH）， and Trunking Paging Control Channel （TPCCH）. These channels are mapped to the PDSCH. To implement the trunking service management， two logical function entities， i.e. TCF and TMF， are included in EPC， and the non?access stratum （NAS） signaling messages related with Trunking Service Management （TSM） are designed. The B?TrunC system architecture is shown in Fig. 1.
The 3GPP designs the GCSE architecture to realize group communications through the PMCH in MCPTT system based on the eMBMS architecture. The 3GPP MCPTT system architecture is shown in Fig. 2. The PMCH allocation is fixed in different uplink?downlink configurations and the trunking applications are provided by the MCPTT server in the application layer.
In summary， B?TrunC redesigns the LTE bearer layer and service layer of Uu and NAS interfaces in LTE system to support a variety of trunking services. 3GPP MCPTT enhances the bearing capability for trunking services based on the mature commercial LTE product; all the special applications are realized in the application layer， which is independent with the system architecture. Therefore， the 3GPP MCPTT architecture is similar to the decoupled design version of LTE?R.
Because the 3GPP scheme can support the trunking services on the application layer of LTE system， the implementation of railway functional is flexible. GCSE realizes the group communication via fix allocated PMCH， while the PDSCH is used to implement the group communication， so the resource utilization of B?TrunC system is better.
5.2 System Functionality and Performance
Besides the narrow band trunking communication services borne in the TETRA system， B?TrunC is designed to bear more multimedia trunking services， such as video group call， video individual call， video forwarding， and delivery to group.
3GPP MCPTT system collects the trunking service requirements of PPDR in MCPTT over LTE stage 1， which includes group call， broadband， late call entry， dynamic group call， call prioritization， etc.
For railway trunking communications， functional addressing and location dependent addressing are two typical services. The functionality and technical realization of the two services in the B?TrunC and 3GPP are being standardized. Due to the decoupling design of MCPTT system， more extra undefined new service requirements can be developed in the application layer of communication system according 3GPP specifications， the new service requirements of FRMCS can simply be supported by the 3GPP solution. The system performance is also an important aspect to evaluate the system suitability. The key performance metrics for trunking communication of B?TrunC and MCPTT system are listed in Tables 1 and 2. From the tables we can see that the QoS requirements of B?TrunC are stricter than those of MCPTT system. This is because that B?TurnC has simplified system architecture and technology realization.
5.3 Standardization
The standardization process of B?TrunC system is shown in Fig. 3. B?TrunC Release 1 was finished in 2014. At this stage， the broadband trunking functionality including group multimedia services is enhanced in a local network system， and the radio interface and interface between core network and dispatcher， i.e. Uu and D， are standardized respectively. The network interoperability and roaming architecture of B?TrunC is designed in Release 2， which was finished in the beginning of 2017. Besides that， more technical realizations for rail transportation communication service requirements would be considered at this stage. In Release 3， the additional functionalities including device direct communication and cognitive radio had been considered by the end of 2017.
The standardization process of 3GPP MCPTT system is also shown in Fig. 3. 3GPP Release 13 was frozen in 2016; the functional architecture and signaling flows to support MCPTT services were finished and eMBMS functionality were enhanced. In Release 14， more mission critical video and data service requirements has been considered. The roaming， preemption， service continuity of MCPTT services were enhanced in this version.
In summary， in terms of multimedia trunking communication functionality， B?TrunC has realized the multimedia group communication at stage 1， so B?TrunC is developed ahead of 3GPP MCPTT. However， by the end of 2017， the two systems had similar functionalities.
5.4 Interoperability
In most of countries and regions， such as Europe and China， different rail lines are operated by different railway operators. For example， two railway operators， i.e. Guangzhou Railway and Hong Kong Railway Corporation Limited （MTR）， manage the rails from Guangzhou to Shenzhen and to Hong Kong. The radio access subsystem in the Hong Kong section has to interconnect with the core network layer in the Guangzhou?Shenzhen section. Because the signaling of the two sections is provided by different vendors， the device interoperability and interface openness are mandatory requirements for the railway mobile communications. Therefore， GSM?R requires the interoperability as a mandatory feature， which should be inherited by LTE?R. For 3GPP R13 specifications， besides the standardized interfaces， the interfaces of group communication entities are also standardized， such as the MB2 interface between the MCPTT server and Broadcast Multicast?Service Centre， M1 interface between the Evolved UMTS Terrestrial Radio Access Network （E?UTRAN） and multimedia broadcast/multicast service gateway （MBMS?GW）， and GC1 interface between MCPTT client and MCPTT server.
For the B?TrunC system， the Uu?T interface between the Trunking UE and eNode?B and the D interface between Trunking core network and dispatcher are defined in Release 1. The enhanced S1 interface and the interfaces of Trunking core network are standardized in Release 2.
In summary， as the 3GPP is a global mobile telecommunications standardization organization， MCPTT specifications have better interoperability and interface openness.
5.5 High Mobility Support Capability
High speed trains will be operated up to 500 km/h in Europe and China in the near future. The severe Doppler shift of uplink and downlink signals and frequently handover have great impact on the reliability of services transmission， so LTE?R requires the mandatory high mobility support.
For LTE system， in terms of mobility， 3GPP Release 9 is designed aiming primarily at low mobility applications in the range of 0 to 15 km/h to show the highest system performance. The system is capable of working well at higher speeds from 15 km/h to 120 km/h and providing the basic functional support from 120 km/h to 350 km/h. In Release 14， the high speed railway is considered as a typical scenario， so the high mobility support is required to achieve up to 500 km/h. In Release 14， the frequency offset correction algorithm and multi?RRU co?cell sharing technology are adopted to improve the system performance with high speed railway scenarios.
To evaluate the effects of high mobility on the B?TrunC system and MCPTT system， we set up the hard?in?the?loop simulation system in the lab. Because there is no MCPTT products currently， the MCPTT is designed based on the Frequency Division Duplex （FDD） system. Therefore， the FDD LTE system is used to illustrate the performance evaluation results of MCPTT and the simulation results are shown in Table 3. The simulation results show that the throughput and block error rate of FDD LTE are a little better than B?TrunC， and the two systems have similar performance in terms of ping delay. Therefore， the MCPTT system has better performance in a high mobility environment. 5.6 Backward Compatibility with GSM?R
LTE?R requires backward compatibility with GSM?R. On one hand， it means that the borne services can be automatic transferred between GSM?R network and LTE?R. On the other hand， it should be enabled to reuse the existing equipment of GSM?R which has not reached the end of its lifecycle， such as the towers， hardware of base stations， and on?board devices.
The two feasible solutions are designed based on the commercial LTE system， so the backward compatibility with GSM system can be guaranteed. GSM?R was designed by the ETSI but involved based on 3GPP specifications， so the backward compatibility of MCPTT with GSM?R is considered in 3GPP Release 14 and 15 specifications. For the B?TrunC system， there is no plan to investigate the backward compatibility with GSM?R. Besides， the most GSM?R macro base station products support a seamless upgrade to LTE eNode B that satisfies 3GPP specifications.
6 Conclusions
The development of LTE technology offers an excellent opportunity to improve both the performance and capabilities of railway mobile communication systems. In this work， we analyzed the suitability of the B?TrunC and 3GPP MCPTT on LTE?R. In terms of system functionality， performance， and standardization process， the B?TrunC system keeps ahead of MCPTT system. In terms of system architecture， interoperability， high mobility support capability， and backward compatibility with GSM?R， the MCPTT system performs better than B?TrunC. Besides， 3GPP MCPTT is an international standard， so LTE?R is designed based on the 3GPP MCPTT which is benefit for globalization of railways.
References
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[4] China Academy of Telecommunication Research （CATR）. The Progress of B?TrunC Broadband Trunking Standard Based on TD?LTE [EB/OL]. （2014）. http：//enterprise.huawei.com/ilink/enenterprise/download/HW_328557 [5] Ian Allan Publishing. Rail Radio Revolution [J]. Modern Railways， 2004， 61（673）： 65-67
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Biographies
SUN Bin received the B.S. degree in electronic engineering and the M.S. degree in electronic engineering from Beijing Jiaotong University， China in 2004 and 2007， respectively. From 2007 to 2015， he was a R&D manager with BeiJing LiuJie Technology Co.， Ltd. He is currently an assistant researcher with National Research Center of Railway Safety Assessment， Beijing Jiaotong University. His main research interest is the interconnection and interworking of core network for dedicated railway mobile communication system.
DING Jianwen （[email protected]） received his B.S. and M.S. degrees from Beijing Jiaotong University， China in 2002 and 2005， respectively. He is currently an associate researcher with National Research Center of Railway Safety Assessment， Beijing Jiaotong University. He received the second prize of progress in science and technology of the Chinese Railway Society. His research interests are broadband mobile communication and personal communication， dedicated mobile communication system for railway， and safety communication technology for train control system. LIN Siyu received the B.S. degree in electronic engineering and Ph.D. degree in electronic engineering from Beijing Jiaotong University， China in 2007 and 2013， respectively. From 2009 to 2010， he was an exchange student with the Universidad Politécnica de Madrid， Spain. From 2011 to 2012， he was a visiting student with the University of Victoria， Canada. He is currently an associate professor with Beijing Engineering Research Center of High?speed Railway Broadband Mobile Communications， Beijing Jiaotong University. His main research interest is performance analysis and channel modeling for wireless communication networks， dedicated railway mobile communication system.
WANG Wei is the LTE?R technical director and a railway wireless communication system expert of ZTE Corporation， with rich experience of the GSM?R system design. He has a deep understanding of GSM?R and LTE?R and has undertaken several major railway?related projects on wireless communication systems.
CHEN Qiang is a LTE?R product manager and railway wireless communication system expert of ZTE Corporation， with rich experience of railway communications. He is familiar with international and domestic railway standards， service applications， and service processes.
ZHONG Zhangdui is a professor and supervisor of Ph.D. candidates at Beijing Jiaotong University， China. He is now a Chief Scientist of the State Key Laboratory of Rail Traffic Control and Safety， Beijing Jiaotong University. He is also a director of the Innovative Research Team of Ministry of Education of China and a Chief Scientist of Ministry of Railways of China. He is an executive council member of the Radio Association of China and a deputy director of Radio Association of Beijing， China. His research interests are wireless communications for railways， control theory and techniques for railways， and GSM?R system. He has authored/co?authored seven books， five invention patents， and over 200 scientific research papers in his research areas. He received the MaoYiSheng Scientific Award of China， ZhanTianYou Railway Honorary Award of China， and Top 10 Science/Technology Achievements Award of Chinese Universities.
Keywords： 3GPP MCPTT; B?TrunC; FRMCS; LTE?R
DOI： 10.12142/ZTECOM.201901009
http：//kns.cnki.net/kcms/detail/34.1294.TN.20190314.1717.004.html， published online March 14， 2019
Manuscript received： 20180101
1 Introduction
Railway mobile communications originated from the incompatibility of track cables and analogue railway radio networks. Due to their limited bearing capability， less anti?interference capability， and lack of encryption， the analogue railway radio networks were replaced by the Global System for Mobile Communications?Railways （GSM?R） systems. GSM?R bears the railway trunking dispatching voice services and the train control data services as well， which fulfills the mobile communication service requirements of railway systems. However， the limited capacity， higher cost and development cycle inherited in GSM has led to the situation that the market of GSM?R is moving inexorably towards its end.
In 2012， International Union of Railways （UIC） started to evaluate the actual situation of global railway market and to study the needs of railway operators and passengers. The second version of “User Requirements Specification” for Future Railway Mobile Communication System （FRMCS） was released in March 2016， which is an indispensable step towards the introduction of a successor of GSM?R. Besides the existing GSM?R services， several potential new application requirements would also be added into the FRMCS， such as multimedia dispatching communications and real time video monitoring [1]. GSM?R cannot satisfy the users’ requirements of FRMCS and GSM industry will be terminated in 2025， which are motivating the design of GSM?R successor. Long?Term Evolution （LTE） mobile broadband technology is currently the most successful commercial broadband mobile communication system. LTE provides higher data capacity than GSM and presents a flat architecture to reduce deployment and maintenance costs as long as standard entities are used. In this sense， two feasible LTE based broadband trunking communication solutions， 3GPP Mission Critical Push to Talk （MCPTT） [2] and LTE?based broadband trunking communication （B?TrunC） [3]， have been proposed by the 3rd Generation Partner Project （3GPP） and China Communications Standards Association （CCSA）. MCPTT is a new global standard for replacing the conventional group communication systems， such as Trans European Trunked Radio （TETRA）， P25 and others. MCPTT is based on the LTE architecture and will be available for governments/public safety or railway operators. 3GPP is focusing on the network infrastructure design， while the client?side is left up to manufacturers to design and implement according to the requirements of users. Developed by CCSA， B?TrunC is designed based on LTE Release 9 [4]. The B?TrunC standard has been developed to address the evolving needs driven by the emergence of new trunking communication requirements， such as multimedia dispatch applications.
However， neither MCPTT nor B?TrunC is specially designed for railway mobile communications. In this context， the suitability of the two LTE?based solutions for LTE based FRMCS （LTE?R） will be analyzed in this paper from the following six aspects： architecture， system functionality and performance， standardization level， interoperability， high mobility support capability， and backward compatibility with GSM?R.
The content of this paper is organized as follows. First， a brief introduction of the railway mobile communications and LTE systems are presented in Section 2. Then， the user requirements of FRMCS are discussed in Section 3， where the MCPTT and B?TrunC are introduced. In Section 4， we analyze the suitability of the two feasible solutions for LTE?R. Finally， conclusions are drawn in Section 5.
2 Development of Railway Mobile
Communications
The high?speed railway （HSR） is becoming the major mode of transportation for a journey and has been developed rapidly worldwide， benefitting from its advantages of safety， reliability， convenience， comfortableness， and low energy consumption. To guarantee the reliability， availability and safety of the train?ground data transmission， railway mobile communication systems play a key role in the HSR operation [1]. In the early stage of railway mobile communications， several analogue radio networks supported mobile communication applications for drivers and trackside workers. For example， British Rail developed the National Radio Network （NRN） specifically used for the operational railways， which provides radio coverage for 98% of the rail network through base stations and radio exchanges. NRN could provide dedicated open channels on talk?through mode for incident management and an override priority facility， in order to ensure that all emergency calls could be immediately connected to the railway’s train control offices and electrical control rooms [5].
The analog radio network has reached to its limits due to the rapid growth of communications traffic volume and the increasing demands for security， economy， efficiency and safety of railway traffic.
In 1994， the GSM standard of European Telecommunication Standards Institute （ETSI） was selected by UIC as the first Digital Railways Radio Communication System standard， because it is the sole system in commercial operation and it is already proven with off?the?shelf products available， requiring the minimum modifications. However， GSM could not fulfill all the necessary requirements of the efficient railway services. Therefore it was necessary that the advanced voice call features should be identified and added to the standard GSM. UIC， together with the railway operators， launched the European Integrated Radio Enhanced Network （EIRENE） to specify the requirements for mobile networks to fulfill the needs of railways including the features of additional group and broadcast calls [6]. To validate whether these EIRENE functional specifications could be transferred into technical implementations， three prototypes were developed by manufacturers and three pilot?lines were planned and realized in France， Italy and Germany. These three pilot?line systems were built to test different aspects of operation， such as railway station environment， complex radio coverage topology with tunnels and bends， and high speed lines at speeds up to 300 km/h [7].
In 1997， 32 railway operators all over Europe signed a Memorandum of Understanding （MoU） to terminate the investment in the analogue radio systems and start to invest the implementation of GSM?R. As of today， the number of signatories has increased to 38， including railway operators outside Europe. Today， over 100 000 km of railway lines are operated by GSM?R systems and this amount is still growing. In addition， the industry has given commitment to support the GSM?R technology until at least 2030 [1]. 3 User Requirements of Next?Generation
Railway Mobile Communication System
As telecommunication standards are evolving and it usually takes a long time to realize the application of any technology specifications， it is urgent to start the research work on the successor of GSM?R. Thus UIC decided to set up the FRMCS project to prepare the necessary steps towards the introduction of a successor of GSM?R in 2012. The project started with the situation evaluation of actual railway systems and investigation on needs of users， and ended with the delivery of the first specification for user requirements of the next?generation railway mobile communication system.
In 2016， a new version of FRMCS user requirement specification was published [1]， in which the traffic requirements are classified into two categories including communication applications and supporting applications. Besides， the users are classified into three groups： critical users， performance users， and business users. The critical users refer to the applications that can enhance the reliability， availability， maintainability and safety （RAMS） of railway systems. The performance users refer to the applications that can improve the performance of railway operation， such as train departure and telemetry. The business users refer to the applications that can support the railway business operation in general.
Comparing with the functional requirement specification of GSM?R system， the new version has some new broadband mobile services that are high demanded， such as real time video and wireless Internet on?train for passengers. Furthermore， more train?ground data services are included， such as monitoring and control of non?critical infrastructure and trackside maintenance communications.
All the above mentioned traffic requirements cannot be met by narrow?band mobile communication systems， and the industry support of GSM?R will be ended in 2030. Thus， it is urgent to develop a dedicated broadband mobile communication system as the successor of GSM?R.
Besides the traffic requirements， the fundamental design principles of FRMCS are proposed for user requirements， which include application decoupled with system architecture， interoperability， reuse of the existing infrastructure， high mobility support， backward compatibility with GSM?R， etc. [1]. These principles will guide the design of LTE?R systems.
4 Two Feasible Solutions for LTE?R
To provide a wide range of data?centric services， such as video sharing， multimedia dispatching， and ubiquitous Internet and intranet access for governments and organizations involved in public safety and security， two LTE based broadband trunking communication systems， B?TrunC system and 3GPP MCPTT system， are designed. These two systems are considered as the candidates for LTE?R because the user requirements of railway systems are similar to public safety communication services and LTE is the currently most popular 4G mobile broadband systemthere. 4.1 LTE Based Broadband Trunking Communication
system （B?TrunC）
To accomplish broadband multimedia trunking service requirements， i.e.， group communication demand for voice， data， and video， CCSA initially started to develop a Broadband Trunking Communication （B?TrunC） system in 2012， which has been admitted by ITU?R as the Public Protection and Disaster Relief （PPDR） Recommendation Standard.
The B?TrunC system is designed based on the TD?LTE system with 3GPP Release 9. The single?cell point?to?multipoint （SC?PTM） is the feature of air interface of B?TrunC system， which is designed to realize the group communications. The SC?PTM is a new type of radio access method dedicated to multicast through the Physical Downlink Shared Channel （PDSCH） in a single cell. For SC?PTM transmission， user equipment in a group receives the group data through a common radio resource region in the PDSCH. This concept naturally allows the group data to be multiplexed with the normal unicast data within a PDSCH subframe and thus does not cause the problem of radio resource granularity.
Besides the SC?PTM technology， two trunking communication entities are involved in System Architecture Evolution （SAE） of LTE system， which are Trunking Control Function （TCF） and Trunking Media Function （TMF）. TCF is responsible for trunking service scheduling， call setup and release， session management， authentication， registration， and cancellation. TMF is responsible for trunking user plane management， routing， data forwarding， encoding， etc. The interface between terminals and the system， Uu?T， and the interface between the core network and the dispatcher， D， are designed. To enhance the latency performance of B?TrunC， the Multimedia Broadcast/Multicast Service （MBMS） and Push to Talk over Cellular （PoC） technologies of LTE are carried out. Now CCSA has completed the general technical requirements and air interface standards for B?TrunC.
4.2 LTE and MCPTT Standardization Roundup
In this section， the 3GPP LTE standardization process related with MCPTT is introduced.
The fully commercial operation of LTE systems started from the 3GPP Release 8 specification， which was finalized in 2008. The latest version of LTE， Release14 had been frozen by mid?2017.
LTE Release 8 specified one primary broadband technology based on Orthogonal Frequency Division Multiple Access （OFDM）. LTE Release 8 is mainly deployed in a macro/microcell layout and can provide improved system capacity and coverage， high peak data rates， low latency， reduced operating costs， multi?antenna support， flexible bandwidth operation and seamless integration with existing systems [8]. LTE Release 9 provides some minor enhancements to LTE Release 8 with respect to the air interface. These features include dual?layer beamforming and time difference of arrival based location techniques. To support the video on demand， video conference and other multimedia services， MBMS architecture is included in the Evolved Packet Core （EPC） of LTE. LTE Release 10 realizes the following features： bandwidth extension via carrier aggregation to support deployment bandwidth up to 100 MHz， downlink spatial multiplexing including single?cell multi?user MIMO transmission and uplink spatial multiplexing， and heterogeneous networks with emphasis on Type 1 and Type 2 relays [9]. For this release， LTE technology refers to LTE?A as a formal 4G system. As the capacity and performance of the LTE traffic channels are progressively improved， the downlink control channels may become a bottleneck. To address this issue， an enhanced physical downlink control channel （EPDCCH） has been introduced in 3GPP LTE Release 11 [10].
The core specification item of Release 12 in terms of group communication is Group Communication System Enabler （GCSE）. The conventional physical channels in LTE can be good media for providing group communications in some basic scenarios [11]. GCSE defines the requirements for group communications and proposes system architecture on top of the existing physical channels. The LTE system of Release 12 has two fundamental physical channels for transferring the data： the PDSCH， which is commonly used for normal unicast data， and the physical multicast channel （PMCH）， which is designed for evolved MBMS （eMBMS）. Furthermore， direct device?to?device （D2D） communication is the other feature of Release 12.
Releases 13 and 14 focus on the air interface aspects. They are a part of the continued evolution of LTE?Advanced and play a role as a bridge from 4G to 5G. To meet the user requirements of PPDR， the MCPTT services and system architecture are defined， whose technology enhancement and realization has been completed in Release14 [12]， [13]. Other techniques in this release include enhancement of D2D proximity services， indoor positioning enhancements， and the single?cell point?to?multipoint （SC?PTM） [14].
5 Analysis of Feasible Architecture
Solutions of LTE for Railway
Neither 3GPP MCPTT nor B?TrunC is specially designed for Railway communications. They cannot fully fulfill the user requirements of LTE?R. Next， the suitability of the two LTE?based solutions for FRMCS are analyzed from six aspects， including architecture， system functionality and performance， standardization， interoperability， high mobility support capability， and backward compatibility with GSM?R.
5.1 System Architecture
The functionality and commoditization of future railway system determine the design principles of future railway mobile communications system. The FRMCS architecture design should satisfy the services and architecture decoupling principle. As GSM?R is a modified off?the?shelf technology system based on GSM offering， the enhancement to deliver specific “R” （railway） functionality has proven expensive for the railways. For future railway communication system， the specific “R” （railway） functionality provisioning should be decoupled with communication system. B?TrunC provides the fundamental solution for group communication. To provide the group communication services， several logical channels are designed including Trunking Control Channel （TCCH）， Trunking Traffic Channel （TTCH）， Trunking Paging Channel （TPCH）， and Trunking Paging Control Channel （TPCCH）. These channels are mapped to the PDSCH. To implement the trunking service management， two logical function entities， i.e. TCF and TMF， are included in EPC， and the non?access stratum （NAS） signaling messages related with Trunking Service Management （TSM） are designed. The B?TrunC system architecture is shown in Fig. 1.
The 3GPP designs the GCSE architecture to realize group communications through the PMCH in MCPTT system based on the eMBMS architecture. The 3GPP MCPTT system architecture is shown in Fig. 2. The PMCH allocation is fixed in different uplink?downlink configurations and the trunking applications are provided by the MCPTT server in the application layer.
In summary， B?TrunC redesigns the LTE bearer layer and service layer of Uu and NAS interfaces in LTE system to support a variety of trunking services. 3GPP MCPTT enhances the bearing capability for trunking services based on the mature commercial LTE product; all the special applications are realized in the application layer， which is independent with the system architecture. Therefore， the 3GPP MCPTT architecture is similar to the decoupled design version of LTE?R.
Because the 3GPP scheme can support the trunking services on the application layer of LTE system， the implementation of railway functional is flexible. GCSE realizes the group communication via fix allocated PMCH， while the PDSCH is used to implement the group communication， so the resource utilization of B?TrunC system is better.
5.2 System Functionality and Performance
Besides the narrow band trunking communication services borne in the TETRA system， B?TrunC is designed to bear more multimedia trunking services， such as video group call， video individual call， video forwarding， and delivery to group.
3GPP MCPTT system collects the trunking service requirements of PPDR in MCPTT over LTE stage 1， which includes group call， broadband， late call entry， dynamic group call， call prioritization， etc.
For railway trunking communications， functional addressing and location dependent addressing are two typical services. The functionality and technical realization of the two services in the B?TrunC and 3GPP are being standardized. Due to the decoupling design of MCPTT system， more extra undefined new service requirements can be developed in the application layer of communication system according 3GPP specifications， the new service requirements of FRMCS can simply be supported by the 3GPP solution. The system performance is also an important aspect to evaluate the system suitability. The key performance metrics for trunking communication of B?TrunC and MCPTT system are listed in Tables 1 and 2. From the tables we can see that the QoS requirements of B?TrunC are stricter than those of MCPTT system. This is because that B?TurnC has simplified system architecture and technology realization.
5.3 Standardization
The standardization process of B?TrunC system is shown in Fig. 3. B?TrunC Release 1 was finished in 2014. At this stage， the broadband trunking functionality including group multimedia services is enhanced in a local network system， and the radio interface and interface between core network and dispatcher， i.e. Uu and D， are standardized respectively. The network interoperability and roaming architecture of B?TrunC is designed in Release 2， which was finished in the beginning of 2017. Besides that， more technical realizations for rail transportation communication service requirements would be considered at this stage. In Release 3， the additional functionalities including device direct communication and cognitive radio had been considered by the end of 2017.
The standardization process of 3GPP MCPTT system is also shown in Fig. 3. 3GPP Release 13 was frozen in 2016; the functional architecture and signaling flows to support MCPTT services were finished and eMBMS functionality were enhanced. In Release 14， more mission critical video and data service requirements has been considered. The roaming， preemption， service continuity of MCPTT services were enhanced in this version.
In summary， in terms of multimedia trunking communication functionality， B?TrunC has realized the multimedia group communication at stage 1， so B?TrunC is developed ahead of 3GPP MCPTT. However， by the end of 2017， the two systems had similar functionalities.
5.4 Interoperability
In most of countries and regions， such as Europe and China， different rail lines are operated by different railway operators. For example， two railway operators， i.e. Guangzhou Railway and Hong Kong Railway Corporation Limited （MTR）， manage the rails from Guangzhou to Shenzhen and to Hong Kong. The radio access subsystem in the Hong Kong section has to interconnect with the core network layer in the Guangzhou?Shenzhen section. Because the signaling of the two sections is provided by different vendors， the device interoperability and interface openness are mandatory requirements for the railway mobile communications. Therefore， GSM?R requires the interoperability as a mandatory feature， which should be inherited by LTE?R. For 3GPP R13 specifications， besides the standardized interfaces， the interfaces of group communication entities are also standardized， such as the MB2 interface between the MCPTT server and Broadcast Multicast?Service Centre， M1 interface between the Evolved UMTS Terrestrial Radio Access Network （E?UTRAN） and multimedia broadcast/multicast service gateway （MBMS?GW）， and GC1 interface between MCPTT client and MCPTT server.
For the B?TrunC system， the Uu?T interface between the Trunking UE and eNode?B and the D interface between Trunking core network and dispatcher are defined in Release 1. The enhanced S1 interface and the interfaces of Trunking core network are standardized in Release 2.
In summary， as the 3GPP is a global mobile telecommunications standardization organization， MCPTT specifications have better interoperability and interface openness.
5.5 High Mobility Support Capability
High speed trains will be operated up to 500 km/h in Europe and China in the near future. The severe Doppler shift of uplink and downlink signals and frequently handover have great impact on the reliability of services transmission， so LTE?R requires the mandatory high mobility support.
For LTE system， in terms of mobility， 3GPP Release 9 is designed aiming primarily at low mobility applications in the range of 0 to 15 km/h to show the highest system performance. The system is capable of working well at higher speeds from 15 km/h to 120 km/h and providing the basic functional support from 120 km/h to 350 km/h. In Release 14， the high speed railway is considered as a typical scenario， so the high mobility support is required to achieve up to 500 km/h. In Release 14， the frequency offset correction algorithm and multi?RRU co?cell sharing technology are adopted to improve the system performance with high speed railway scenarios.
To evaluate the effects of high mobility on the B?TrunC system and MCPTT system， we set up the hard?in?the?loop simulation system in the lab. Because there is no MCPTT products currently， the MCPTT is designed based on the Frequency Division Duplex （FDD） system. Therefore， the FDD LTE system is used to illustrate the performance evaluation results of MCPTT and the simulation results are shown in Table 3. The simulation results show that the throughput and block error rate of FDD LTE are a little better than B?TrunC， and the two systems have similar performance in terms of ping delay. Therefore， the MCPTT system has better performance in a high mobility environment. 5.6 Backward Compatibility with GSM?R
LTE?R requires backward compatibility with GSM?R. On one hand， it means that the borne services can be automatic transferred between GSM?R network and LTE?R. On the other hand， it should be enabled to reuse the existing equipment of GSM?R which has not reached the end of its lifecycle， such as the towers， hardware of base stations， and on?board devices.
The two feasible solutions are designed based on the commercial LTE system， so the backward compatibility with GSM system can be guaranteed. GSM?R was designed by the ETSI but involved based on 3GPP specifications， so the backward compatibility of MCPTT with GSM?R is considered in 3GPP Release 14 and 15 specifications. For the B?TrunC system， there is no plan to investigate the backward compatibility with GSM?R. Besides， the most GSM?R macro base station products support a seamless upgrade to LTE eNode B that satisfies 3GPP specifications.
6 Conclusions
The development of LTE technology offers an excellent opportunity to improve both the performance and capabilities of railway mobile communication systems. In this work， we analyzed the suitability of the B?TrunC and 3GPP MCPTT on LTE?R. In terms of system functionality， performance， and standardization process， the B?TrunC system keeps ahead of MCPTT system. In terms of system architecture， interoperability， high mobility support capability， and backward compatibility with GSM?R， the MCPTT system performs better than B?TrunC. Besides， 3GPP MCPTT is an international standard， so LTE?R is designed based on the 3GPP MCPTT which is benefit for globalization of railways.
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Biographies
SUN Bin received the B.S. degree in electronic engineering and the M.S. degree in electronic engineering from Beijing Jiaotong University， China in 2004 and 2007， respectively. From 2007 to 2015， he was a R&D manager with BeiJing LiuJie Technology Co.， Ltd. He is currently an assistant researcher with National Research Center of Railway Safety Assessment， Beijing Jiaotong University. His main research interest is the interconnection and interworking of core network for dedicated railway mobile communication system.
DING Jianwen （[email protected]） received his B.S. and M.S. degrees from Beijing Jiaotong University， China in 2002 and 2005， respectively. He is currently an associate researcher with National Research Center of Railway Safety Assessment， Beijing Jiaotong University. He received the second prize of progress in science and technology of the Chinese Railway Society. His research interests are broadband mobile communication and personal communication， dedicated mobile communication system for railway， and safety communication technology for train control system. LIN Siyu received the B.S. degree in electronic engineering and Ph.D. degree in electronic engineering from Beijing Jiaotong University， China in 2007 and 2013， respectively. From 2009 to 2010， he was an exchange student with the Universidad Politécnica de Madrid， Spain. From 2011 to 2012， he was a visiting student with the University of Victoria， Canada. He is currently an associate professor with Beijing Engineering Research Center of High?speed Railway Broadband Mobile Communications， Beijing Jiaotong University. His main research interest is performance analysis and channel modeling for wireless communication networks， dedicated railway mobile communication system.
WANG Wei is the LTE?R technical director and a railway wireless communication system expert of ZTE Corporation， with rich experience of the GSM?R system design. He has a deep understanding of GSM?R and LTE?R and has undertaken several major railway?related projects on wireless communication systems.
CHEN Qiang is a LTE?R product manager and railway wireless communication system expert of ZTE Corporation， with rich experience of railway communications. He is familiar with international and domestic railway standards， service applications， and service processes.
ZHONG Zhangdui is a professor and supervisor of Ph.D. candidates at Beijing Jiaotong University， China. He is now a Chief Scientist of the State Key Laboratory of Rail Traffic Control and Safety， Beijing Jiaotong University. He is also a director of the Innovative Research Team of Ministry of Education of China and a Chief Scientist of Ministry of Railways of China. He is an executive council member of the Radio Association of China and a deputy director of Radio Association of Beijing， China. His research interests are wireless communications for railways， control theory and techniques for railways， and GSM?R system. He has authored/co?authored seven books， five invention patents， and over 200 scientific research papers in his research areas. He received the MaoYiSheng Scientific Award of China， ZhanTianYou Railway Honorary Award of China， and Top 10 Science/Technology Achievements Award of Chinese Universities.