S TUDY OF THE POSSIBILITY OF USING TRANSMISSION IN THE LTE SYSTEM ON A SELECTED RAILWAY LINE FOR THE PURPOSE OF RUNNING RAILWAY TRAFFIC

: The railway and the Rail Traffic Control Systems installed on it in Poland have recently experienced rapid technological development. This is undoubtedly due to the funds that Poland receives from the European Union for the modernization of railway infrastructure. The development of modern railway infrastructure means not only stations, modern rolling stock, but also safe and reliable train control systems based on the latest telecommunications and information technologies. For a longer time, radio communication based on the GSM-R (GSM for Railways) standard is being introduced on PKP. For this purpose, dedicated infrastructure is being built in order to use this technology for railway traffic. This is associated with huge investment costs. Since the beginning of its existence, research has been conducted worldwide on the use of LTE (Long Term Evolution) technology for conducting railway traffic. For economic and technological reasons, it is necessary to study other possibilities of using modern telecommunications infrastructure not yet dedicated to railway tasks in these open networks. The article will present research on the use of open radio communication network based on LTE standard for rail traffic and passenger comfort. It will discuss the research method and selected results of measurements made on railway line no. 4. The choice of this railway line was dictated by two factors. The first one resulted from the maximum train speed, the second one is the variety of rolling stock used on this line (compartmental and non-compartmental wagons). The part of research concerning collection of measurement material was performed within the framework of completed research work PBS3/A6/29/2015. As a result of conducted in-depth literature analysis as well as performed measurements and calculations, it allowed to develop a model and software for simulating system operation in real conditions. This system allows to send railway telegrams on appropriate safety level defined in standardization documents. The research proved the possibility of using an open system in the LTE standard for the transmission of signals for railway traffic control and passengers while maintaining an appropriate level of safety. The only limitations which were indicated by the tests are improper radio interface coverage of railway lines. Appropriate planning of radio coverage of railway areas by radio communication operators (so far ignored) with proper cooperation of infrastructure manager can lead to launching efficient communication system without necessity to build specialized infrastructure for railway.


Introduction
The introduction of microprocessor technology into the railway traffic control equipment took place at the turn of the century. However, the use of modern radio systems in rail transport is the moment when standard-based technology appeared in the world. GSM (Global System for Mobile Communications). For the needs of railways, and in particular aspects related to railway traffic control, this technology has been extended by giving it its name to the GSM-R standard. Unlike the GSM version, the frequency band in which the railway system operates has been extended and information transmission in two channels each 4 MHz wide has been introduced. FM (Frequency Modulation) systems currently used on the railways in Poland did not allow for data transmission due to their specific nature of modulation. However, some aspects of "data transmission" were used in radio-stop system. GSM-R transmission enables not only transmission in relation track -vehicle but also in relation track -track. Therefore, it has also become the basis for using it for remote wireless management of railway infrastructure. Due to the degradation of wired infrastructure connecting the element of railway control systems, and related primarily to their destruction, the use of radio transmission medium for independent management of railway traffic control equipment is increasingly considered. An unquestionable problem when implying such solutions is the safety of transmission in such systems. It should be emphasized that safety at a certain level of transmission is already offered by the radio transmission systems themselves, which was also used in the GSM-R standard. Creation of a separate system for the needs of railways is connected with huge expenditures on designing, testing, certification and finally building and implementing such technology. Therefore, it is possible to use public open radio networks for railways, which will reduce costs because such a system relies on existing infrastructure. A means of transmitting information that meets the requirements for safe transmission in the sense of railway traffic control systems should be developed. Therefore, a model for open radio transmission in public traffic control systems based on the public radio standard, LTE, will be presented later in this article.
The research methodology and assumptions will be presented. A measurement system will also be presented, which was used for actual measurements on the railway line based on the developed data transmission method. The obtained selected measurement results are original, as no research on the use of open data transmission for railway purposes using the LTE system has been conducted worldwide. Broadband signal propagation and related issues have been widely discussed, but no consideration has been given to rail issues under these headings. The methodology of research as well as results and analyses of computer simulations concerning mathematical modelling of the system and transmission channel, paying attention to ensuring adequate capacity and reliability of the system and taking into account the problem of train movement were described in (Chrzan, 2018;Chrzan, 2020, Chen et al., 2017Joshi et al., 2017, Kukulski et al., 2019). The LTE system architecture and issues related to broadband signal transmission have been standardized in (3GPP, 2020). The possibility of use in railway systems was not indicated there, however. Due to the specificity of access to the technology and frequency bands used, the solutions presented above are based on the analysis of electromagnetic compatibility of existing radio systems based on frequencies specific for given regions (800, 900, 1800, 2100, 2500 MHz) (Chrzan, 2018;Chrzan, 2020). In this work, however, the consideration will concern the bands available for Polish users in railway radio communication. Currently, there are no studies conducted in Poland aimed at using the LTE system on railways as a natural successor to the European system of railway radio communication based on the GSM-R standard, currently being introduced in our country. Within the framework of research works carried out at the WTEiI (Faculty of Transport, Electrical and Information Technology of the University of Technology and Humanities in Radom), an attempt was made to develop a method of safe data transmission between railway traffic control devices using fourth generation radio systems. These tests were limited to signal transmission in relation between the equipment installed on the track and LCS. However, there are no concepts and solutions for signal transmission in the track-vehicle relationship (Chrzan, 2018;Chrzan, 2020) 3. Analyses -case studies 3.1. Modelling the transmission system in a track-to-train relationship The following assumptions were made for modelling the system of open radio transmission of telegrams between the traffic control devices and the Local Control Centre at WTEiI: − the network load is a natural load, no influence on the network traffic generated by the measuring system -actual conditions in which the system is to operate, − for modelling purposes, we assume the GSM-R transmission parameters with the assumption that meeting these conditions meets the transmission security conditions (Siergiejczyk and Rosiński, 2019).

The transmission model and the measurement method
In order to make measurements in the physical environment, a transmission model was built for an open system based on LTE transmission. The system in which the transmission takes place in the first stage of tests between virtual software LCS (Local Control Centre) and virtual control device, located on the train path. Within the framework of measurements, a system which is not a real time system from the Microsoft Windows operating system family was used. As a field transmitting device, a computer with Microsoft Windows   proposed system belongs to SIL-3 layer. Specification for SIL-3 also enumerates transmission in GSM-R, GPRS and Wi-Fi (802.11) standards. LTE technology uses packet transmission and has built-in "responsibility" for transmission correction so that LTE can be regarded as single channel transmission for which THR varies in the rage of <10-9; 10-8). Therefore, in authors opinion klucza 1024 i 2048 bitów The designed measuring program includes two modules: Server (being an LCS simulator) -and Client (being a transmission receiver). The Server program allows to connect with the Client program after establishing common transmission parameters such as: IP address, TCP protocol port number, and determining the method of the signal encoding. In case of RSA transmission with the use of a key -the server program allows to generate the key on the basis of the entered password. The server program receives the transmitted telegram, decrypts it, checks its integrity and sends the receipt confirmation to the Client. The program which initiates the exchange of transmission with a specific data encryption is the Client's program. Additionally, it enables sending a fixed message (telegram) of a specified length cyclically, at a specified time, sending a variable message, generated separately to each connection with the server, of a fixed length, generated cyclically, at a specified time, sending a single message of a specified length. The telegram sent from the client to the server is constructed in such a way as to meet the safety requirements for open and closed transmission systems contained in the PN-EN50159:2011 standard, which is related to the used hardware and software working on it. Ensuring an appropriate safety level of telegram transmission should be carried out in a way that enables detection of signal transmission errors from the sender to the recipient, and in case of transmission interruption above a specified time, the system should cause a transition to a state ensuring safety. The results were saved to a file in excel spreadsheet format Fig. 3. Because of the high non-stationarity of the propagation medium, which is the LTE system, a single measurement result is unreliable or repeatable. In such a case, the measurements were classified in terms of the probability of exceeding a certain delay of the received signal. The value of this probability is in the range 1-99%, however, typical values of this parameter are: 1%, 10%, 50%, 90% and 99%. For the purpose of the study, which is the study of delays, medians were used, i.e. middle values of an ordered subset of measurement results, whose probability of exceeding by other results from this subset is 50% (Brazeetta et al., 2016). In practice it is connected with determining the median value of the measurement signal delay at the receiver's input for the i-th subset of measurement data, created from n measurement results, collected on the route with the length of 40 λ according to the relation below: is an orderly subset of n of signal delay measurement results at the receiver's input and n is the number of measurements taken on the i-th (i=1,2,3... ) section of the propagation path tested. The process of median determination was carried out in real time, during the measurement tests on the section of the railway line No 4, but according to the designed algorithm of the program described in Fig On the basis of the algorithm presented above and relation (1), all files with the results of field tests were integrated into one and data concerning the suspension height of transmitting and receiving antennas, frequency of the tested signal and observation data concerning propagation conditions prevailing on the Central Rail Line during particular measurement series were supplemented. Using the knowledge of characteristic points' coordinates (geographical coordinates of the position of traffic control devices) and the commonly known relation to the value of the angle between two straights on the plane, the angle of radio waves' arrival to the receiver in relation to the axis of the main communication line was calculated for each propagation case.

Fig. 3. Recording of telegram data communication parameters
In the next part it has been assumed that the same transmitting and receiving modems are used, the transmitting antennas are in a radio channel with the same parameters, so it can be assumed that the resulting antenna disappearances are correlated. For transmitting and receiving devices m correlated distribution of Rice'a in antenna n can be expressed as a relation: where Km parameter K Rice'a the physical channel from the base station to srk m devices, and the angle assumed in formula (2) where the distance d between two antenna elements is no more than the wavelength, Θm means the angle between the direct signal path and the position on the railway line of the srk equipment.
In equation (2) : where dm is the distance to the transmitting and receiving equipment. When exchanging information in the LTE system between the base station and the srk devices in the field in time slot t the same block of data symbols x(t) is multiplied by the composite number √ ( ) ( ( )) in the n-antenna and broadcast, as appropriate, from all the antennas, for n = 1,…, N tak, że ∑ ( ) =1 = 1 while maintaining total transmitting power. The received signal model from all Nt transmitting antennas to the transmitter-receiver m is described by the relation: ( ) is Gauss's additive white noise in ( )~(0, 2 ). Therefore, the overall channel gain for transmitterreceiver m is: For a correlated distribution of the Rice'a channel, dependency (6) can be transformed further as: where ~(0,2 ). For Rice's determined K-factor to maximize the total gain of the user channel m, equation (7)     The studies described in Tables 1-3 give rise to the thesis that appropriate spatial planning of LTE systems allows for the resignation from dedicated development of a rail-specific system such as GSM-R. It is possible to use an open LTE system for railway purposes, both for train control and signaling but also for passenger data transmission. The results of radio signal level tests generated from the terminals are shown in Figure 4. On some sections of the railway line the level of signal has been decreasing, due to the fact that the communication infrastructure makes it possible to cover urban areas and roads with signal. No rolling stock was included. In the case of the propagation measurements for the LTE system in rail transport, multiway signal and vehicle interference must be included, as well as the type of material used to build the wagons. In the case of network scaling and the signal coverage of the railway lines as well, the savings are enormous (

Summary
On the basis of studies carried out on the constructed measuring stand (Fig.1) it can be stated that using the records of PN-EN 50159:2011 standard it is possible to create a system that is able to satisfy not only the needs of data transmission for railway traffic control devices but also passengers. The use of protection methods described in PN-EN 50159:2011 standard and the use of VPN channel additionally for the safety of VPN channel gives a guarantee of safety at the level required by standardization documents. Therefore, it seems necessary to start talks between the railway infrastructure manager and mobile operators in order to develop a network creation model based on existing operators. Creation of appropriate radio coverage of railway communication lines creates great opportunities to use systems open to railway needs, without compromising the existing transmission safety. It is necessary to remember the next stage of mobile networks development in the 5G direction. As an example of the implementation of 5G technology on railroads, we can give Nokia, which is working with German Rail (DB) on the world's first standalone 5G system for automatic train running in Hamburg as part of the highly automated DB S-Bahn project. It will be verified if the 5G technology is sufficiently sophisticated to be used as a transmission layer for the future traffic control system.