[IEEE 2012 IEEE 18th International Symposium for Design and Technology in Electronic Packaging (SIITME) - Alba Iulia, Romania (2012.10.25-2012.10.28)] 2012 IEEE 18th International

2012 IEEE 18th

International Symposium for Design and Technology in Electronic Packaging (SIITME)

978-1-4673-4760-0/12/$31.00 ©2012 IEEE 253 25-28 Oct 2012, Alba Iulia, Romania

Considerations regarding the radio design of GSM-R systems, inside railway tunnels

Lăcrămioara Mihaela Nemţoi, Ilona Mădălina Moise, Elena Alina Stanciu and Maria Claudia Surugiu

Faculty of Transports University “POLITEHNICA”

Bucharest, Romania [email protected], [email protected], [email protected], [email protected]

Abstract—The paper describes in detail an entirely original

radio design procedure, suitable to calculate and optimize the

radio coverage of GSM-R(ailway) systems inside arbitrarily

shaped railway tunnels. One of the major issues in modelling

radio waves propagation inside railway tunnels is the accurate

representation of tunnels’ geometry in the format of digital raster

maps, which are used to model indoor terrain features, by the

Geographical Information System (GIS) embedded in the radio

planning software ICS Telecom ® (produced by ATDI SA), used

throughout the radio design procedure presented herein. The

production of the tunnel digital raster maps relies on a very

simple and also original engineering method, which converts the

large scale situation plans of the railway tunnels, readily

available at the railway infrastructure operators in electronic

format (Autocad), to the desired raster map format, with a 0.1

meters resolution. The last section of the paper describes in detail

the selection and configuration of the indoor radio propagation

models and presents the results of the radio design procedure.

The whole approach of the paper is highly practical, being

presented as a radio design case study, for the “Dealul Negru”

railway tunnel, situated on the Braşov – Sibiu railway corridor.

Index Terms— GSM-R, railway tunnels, radio design, GIS

raster file format.

I. INTRODUCTION

The GSM-R(ailway) networks have an essential role to play in the functioning of modern automated train control systems, as they provide the bidirectional radio bearer for the train signaling systems (ERTMS – European Rail Traffic Management System and ETCS – European Train Control System). Since the railway safety relies on the continuous transmission link between train-borne and trackside ERTMS/ ETCS applications, the International Union of Railways (UIC) devised very strict technical requirements for the GSM-R networks, through the EIRENE (European Integrated Railway Radio Enhanced Network) system requirements specifications [ HYPERLINK \l "EIRENE_SRS" 1 ].

The simulation of wave propagation in arbitrarily shaped tunnels by applying the ray tracing techniques derived from the geometrical optics, a precise modeling of the railway tunnel itself is needed. Such modeling can be approached in an analytical way 2]}, by using mathematical equations that can describe both the tunnel course and the tunnel cross section, thus allowing the three-dimensional modeling of the tunnel boundaries. In simulation, the rays modeling the waves’

propagation are intersected with the analytically defined tunnel boundaries, using 3D intersection routines. However, it is obvious that the results of the mathematical equations that model the tunnel course must be further converted to one of the formats compatible with the Geographical Information Systems (GIS) embedded in the specialized software applications used in professional radio design to simulate radio waves propagation. The usual spatial data models used in GIS [ HYPERLINK \l "Dob10" 3 ] 4]} are the vector data format (characterized by points, lines and polygons) and the raster

data format (characterized by a grid-cell data structure, which involve the division of the geographical feature in identically sized cells, identified through row and column references).

A different and original approach to the railway tunnel modeling is presented in this paper, starting from the large scale situation plans of the railway tunnels, already available at the railway infrastructure operators, in electronic format (Autocad), and resulting directly in the railway tunnel maps in digital raster format on 16 bits. The digital raster format is used to model indoor terrain features (and hence the railway tunnel boundaries), by the Geographical Information System (GIS) embedded in the radio design software ICS Telecom ® (produced by ATDI SA), used throughout the radio design procedure presented herein.

We must underline from the beginning the limitations of ICS Telecom software, concerning the radio design in indoor spaces, as follows:

• Indoor spaces modeling does not take into account the height of the indoor space boundaries, thus for the particular case of the railway tunnels the characteristics of the tunnel’s transversal section cannot be entirely modeled, i.e. the tunnel ceiling and its influence on radio waves propagation (reflections) is neglected. The indoor module of ICS Telecom was mainly conceived to model office buildings with multiple levels, one of the tunable parameters being the floor factor, which is used to model the radio waves attenuation when crossing the floor/ceilings between different levels.

• The ray tracing module of ICS Telecom does only take into account one reflection for each ray, this limitation leading to a pessimistic prediction of the radio coverage inside the curved railway tunnels. As shown hereinafter in the present paper, this is a serious

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limitation for curved tunnels, where the multiple reflections of the radio waves from tunnel’s walls are very significant for the prediction of radio coverage.

ICS Telecom does not support the modeling of coaxial antennas (i.e. radiating coaxial cables or leaky feeders).

However, as shown further in this paper, ICS Telecom can be used with good results for the calculation and analysis of radio coverage inside straight railway tunnels, offering the following possibilities to model the geometry of the railway tunnels:

• The tunnel course and width can be modeled with a resolution that is entirely adequate for the prediction of radio coverage inside the tunnel, i.e. with a granularity of 10 centimeters for each pixel, or even better. Practically, the tunnel is represented in a top view, with the tunnel walls modeled as two parallel lines, spaced at a distance equal to the known width of the railway tunnel at the height of 4 meters height above the tunnel floor, equal to the nominal mobile antenna height specified in EIRENE Systems Requirements Specifications [ HYPERLINK \l "EIRENE_SRS" 1 ], Par. 3.2.1 “Coverage”.

• ICS Telecom provides a separate application for the advanced processing of digital maps, ICS Map Server, which allows a wide range of options for the import and post-processing of digital maps data and imagery, thus allowing the generation of digital raster maps to model indoor spaces, regardless of their complexity.

• Most of the propagation phenomena significant for the radio waves propagation in railway tunnels can be modelled by ICS Telecom: deterministic ITU-R P.525 model 5]} for free space propagation, adjusted for the radio waves attenuation calculation on partially or totally obstructed diffraction paths, additional attenuations due to absorption and multiple paths propagation, ray tracing module for 3D reflections [ HYPERLINK \l "ATD03" 6 ].

II. A PROCEDURE TO REPRESENT RAILWAY TUNNELS IN THE

GEOGRAPHICAL INFORMATION SYSTEM OF ICS TELECOM

As shown above, the large scale situation plans of railway tunnels are available at the railway infrastructure operators in electronic format, usually Autocad. In the following paragraphs, we will describe in detail a procedure to convert such situation plans in the digital raster format required by the GIS embedded in ICS Telecom, for the representation of indoor spaces, such as railway tunnels. The whole procedure is applied step-by-step for the “Dealul Negru” railway tunnel, situated on the Braşov-Sibiu railway corridor. The procedure consists of the following steps: 1) The situation plan of the Dealul Negru railway tunnel,

presented in Fig. 1, contains the top view of the tunnel and is available in Autocad format, the original scale of the model being 1:200. The dimensions of a rectangle that inscribes exactly the situation plan of the Dealul Negru tunnel, including the railway tracks adjacent to the

tunnel’s west and east entrances are of 629x291 meters, the length of the tunnel itself being of 443 meters.

Fig. 1. Situation plan of Dealul Negru tunnel (Autocad format, scale 1:200)

2) The following processing is performed in Autocad on the large scale situation plan of the tunnel:

• All the objects in the situation plan are eliminated, with the exception of the polygon line that represents the tunnel course and the railway tracks adjacent to the tunnel’s entrances.

• The polygonal line resulted is duplicated, then the new line resulted through duplication is placed in parallel with the original one, at the exact distance that equals the width of the tunnel at 4 meters above the tunnel floor, that in the particular case of Dealul Negru tunnel is 5.4 meters (the information is available in the transversal section plan. This is the nominal height at which the mobile GSM-R terminal antenna would be placed, and all the radio propagation predictions will take into account this height when calculating the received signal strength.

• Because the original situation plan also includes the railway tracks adjacent to the tunnel’s entrances, the positions of the tunnel’s portals are marked on the plan with two line segments, perpendicular on the polygonal lines representing the tunnel and railway course.

• The simplified situation plan, resulted from the processing presented above is exported in Microsoft Visio drawing format (*.vsd) for further processing. Such export preserves the scale and the aspect ratio of the original drawing. The resulted Visio drawing properties (available in the File/Page Setup menu) show that the drawing page dimensions are 629.54 x 291.22 millimeters and the drawing scale is 1:1.

Fig. 2. The simplified situation plan of the Dealul Negru tunnel (Microsoft

Visio format)

3) The tunnel’s simplified situation plan is exported from Microsoft Visio format in uncompressed Tag Image File format *.tif, using the command Save As.../Tag Image File

Format *.tif. The TIFF export is configured to preserve the

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dimensions and the aspect ratio of the original plan. This is

the stage of the procedure where the resolution of the

resulting tunnel map in digital raster format is decided. As shown in Fig. 3 below, the drawing dimensions are 62.918x29.078 centimeters, and if the export resolution is setup at 100 pixels/ centimeter, the resulting TIFF image will have 6292x2098 pixels. As we know that the real dimensions of the original situation plan (629x291 meters), it results that each pixel of the image represents 0.1 meters. If the export resolution is for example setup at 250 pixels/ centimeter, the resulting TIFF image will have 15730x7270 pixels, so that each pixel of the image represents 0.04 meters.

Fig. 3. Settings for the export of tunnel’s situation plan from Microsoft Visio

drawing to TIFF

4) The resulted TIFF file has to be geo-referenced, i.e. each pixel will be associated to a set of coordinates <x,y>. A new file is created with an ASCII text editor (e.g. Notepad), its filename must be identical with the filename of the TIFF file, and the extension must be *.tfw. The tfw

file is a text file (ASCII) which contains 6 numbers, each one on a different row, as follows:

• Line 1 – scale on X axis. Represents the horizontal distance in meters, represented by each image pixel. In our case, if the Visio export resolution was setup at 100 pixels/ centimeter, the scale on X axis is 0.1 meters/ pixel (i.e. for a Visio export resolution of 250 pixels/ centimeter, the scale on X axis is 0.04 meters/ pixel).

• Line 2 – rotation angle on X axis. In our case, the image is already north oriented, so this parameter is set to 0.

• Line 3 – rotation angle on Y axis. In our case, this parameter is set to 0.

• Line 4 – scale Y axis. Represents the vertical distance in meters, represented by each image pixel. The scale on Y axis is also 0.1 meters/ pixel (i.e. for a Visio export resolution of 250 pixels/ centimeter, the scale on Y axis is 0.04 meters/ pixel). However, this value is negative (i.e. -0.1), because the image origin is normally considered the upper-left corner, as opposed to a geographic file, where the origin is the lower-left corner.

• Line 5 – coordinates of the origin on X axis, which is the X coordinate of the upper-left corner of the image. In our case, this parameter is 0.

• Line 6 – coordinates of the origin on Y axis, which is the Y coordinate of the upper-left corner of the image. In our case, this parameter is 0.

5) The content of the resulted tfw file must be in our case: 0.1/0/0/-0.1/0/0 (the separator “/” is carriage return character, with each number on a separate line)

6) The digital raster file which is the result of this conversion procedure embeds in a single file, both the pixels of the image file (TIFF) and the geo-referencing information of the tfw file. This is accomplished with the ICS Map Server® application, as follows:

• The function used is Import IC1 from.../ Standard Images + TFW/TL2/GEO. The function requires as inputs the TIFF and tfw files obtained as specified above, and consistent with one another. The output of the function is an 8 bit digital raster file reproducing the railway tunnel’s large scale situation plan, with a resolution of 0.1 meters/ pixel, in accordance with the parameters in the two input files.

• As a result, two new files are automatically created in the same directory as the TIFF and tfw files: the 8 bits raster file <name>.ic1 and a palette file <name>.pal, the latter just controlling the correct display of the colors in the raster file, when loaded together in an ICS Telecom project.

Further processing of the resulted raster file is needed: creation of the indoor radio project 7]} in ICS Telecom (which puts together the digital elevation file – required to model indoor radio wave propagation – with an image file and a palette file – required to give the radio planner a meaningful visual representation of the tunnel), expanding the 8 bits raster file <*.ic1> to a 16 bits raster file <*.ic2>, removal from the 16 bits raster file of the railway tracks that are outside the tunnel itself, etc. However, we have already presented above the essential operations which actually define the conversion parameters and create the railway tunnel model in the raster file format, required by ICS Telecom to run the radio propagation prediction and analysis for indoor spaces like railway tunnels. The results of the conversion procedure described here can be summarized through the information provided by ICS Telecom in its map information function (see Fig. 4).

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Fig. 4. Digital cartography information provided by ICS Telecom – example

for the “Dealul Negru” tunnel

In the procedure described above, we cannot actually define a conversion error per se. The resulting raster file representing the tunnel course is as precise as the original Autocad file representing the situation plan of the railway tunnel, at a scale of 1:200. The “rasterization” procedure described above and the resulted raster file have a resolution that is established in the step c) of the procedure above (e.g. 0.1 meters/pixel, or 0.04 meters/pixel). The higher the resolution of the raster file, the smoother the resulted tunnel walls, especially when the tunnel walls are not just straight lines.

However, the setup of a TIFF export resolution of 250 pixels/ centimeter results in an image file of some 114 Megapixels which is difficult to handle in the radio design software ICS Telecom. In addition to that, a raster resolution of 0.1 meters/ pixel may be considered adequate for the radio waves propagation prediction in the GSM-R frequency band, where the wavelength is of about 0.3 meters, so that the size of the tunnel’s “building brick” (i.e. the resolution of the raster file) will not decisively alter the received signal level prediction performed by the radio design software ICS Telecom, except for the immediate proximity of the tunnel walls, where the resolution of the raster file can be considered equal to the roughness of the walls. With this interpretation (i.e. wall roughness = resolution of raster file), the 4 centimeters resolution may be considered insufficient, as the typical roughness of the tunnel walls is 2 centimeters [ HYPERLINK \l "Did00" 2 ] or even less. As shown in the predictions performed in the next chapter, the received signal level is severely attenuated in the proximity of tunnel walls, due to the higher roughness resulted from raster file resolution (10 centimeters). In reality, for GSM-R systems, the mobile terminal antenna is situated at 4 meters height above tracks, and in the middle of the tunnel course, so this issue may be easily neglected and the achievable resolution may be considered satisfactory.

III. CALCULATION OF THE RADIO COVERAGE INSIDE RAILWAY

TUNNELS, EQUIPPED WITH DISTRIBUTED ANTENNAS. CASE

STUDY FOR THE “DEALUL NEGRU” RAILWAY TUNNEL

A. Description and configuration of the propagation models

used by ICS Telecom for the calculation and analysis of

radio coverage inside railway tunnels

In indoor environments, such as the railway tunnels, the radio waves attenuation is caused by the following phenomena:

• Multiple reflections, diffractions and scattering on the tunnel’s inner walls, floor and ceiling;

• Absorption on all the tunnel’s elements mentioned above.

In order to account for all these phenomena, the use of the deterministic propagation model ITU-R P.5255]} is recommended, with ICS Telecom’s specific adaptation for the calculation of propagation losses on near-line of sight and diffraction paths. The indoor propagation model resulted takes into account not only the free space propagation losses but also additional radio signal degradation caused by absorption and multipath propagation [ HYPERLINK \l "ATD03" 6 ]. ICS Telecom ray tracing module can also be used, for the combined analysis of free space propagation and tunnel walls reflections.

All the methods described above can be utilized due to the highly accurate modeling of the tunnel course, obtained through the procedure described in detail in the previous chapter.

Following the above considerations, the propagation models configuration, recommended for the radio design of railway tunnels, is illustrated in Fig. 5 below:

• Propagation model: ITU-R P. 525 – calculation of free space attenuation.

• Diffraction geometry: Visibility/Indoor. The method selected to calculate propagation losses on diffraction paths, detects the obstacles and takes into account the signal absorption caused by these obstacles.

• Subpath attenuation: Standard subpath attenuation. The method calculates the attenuation caused by partially obstructed propagation paths (obstacles situated below the direct line of sight between transmitter and receiver, but above the 0.6 fraction of the first Fresnel ellipsoid). For the railway tunnel’s case, additional attenuation caused by multiple reflection paths along the tunnel floor must also be considered (modeled by the function Ground reflections minima/maxima, flat earth);

• Reflections on the tunnel walls: 3D coverage only function, with reflectance coefficient configured at 0.3 for concrete walls. This function calculates the received signal level in all the points of the modeled area, taking into account the contribution of reflected signals, according to the value of the reflectance

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coefficient. All the points situated in direct line of sight with the transmitter retransmit the power they receive, multiplied with the reflectance coefficient, over a solid angle of 360o, the gain of the reflexive surface being calculated according to the incidence angle averaged for each surface unit of one square meter.

Fig. 5. Propagation models configuration for radio design inside raiway tunnels

Apart from the deterministic propagation models configured in accordance with the principles specified above, the fine tuning of the models can be achieved by setting attenuation parameters for the tunnel boundaries, modeled as a 16 bits raster file, as detailed in the previous chapter.

The 16 bits raster file has a very simple structure: the raster grid cells which are situated on the tunnel course have the value “1” (symbolizing the tunnel walls), and any other grid cell of the raster file has the value “0”, symbolizing open space.

The configuration of the attenuation caused by the absorption on tunnel walls is configured through the Indoor

parameters… function. As shown in Fig. 6 below, the attenuation is only configured for the indoor material with code “1”, symbolizing the tunnel walls. We have already mentioned that one of the limitations of ICS Telecom in the radio design for railway tunnels is that the height of the tunnel walls and the tunnel ceiling are in fact neglected, as well as the shape of the transversal section of tunnel.

The significance of the “material attenuation” parameter for material code “1” in the figure below (i.e. 10 dB for concrete wall) is that any radio signal incident on the tunnel walls is attenuated with 10 dB. The significance of the “thickness factor” in the figure below is that any radio signal penetrating

the tunnel walls is attenuated with 10 dB*100 = 1000 dB, meaning in fact that the tunnel walls cannot be penetrated by radio waves, which for railway tunnels is actually the case. The tunnel walls attenuation parameter can be used for propagation model tuning, following site surveys and drive tests.

Fig. 6. Configuration of the attenuation parameters for railway tunnels

B. Results. “Dealul Negru” railway tunnel – GSM-R coverage

prediction for the straight tunnel section

The present case study calculates the radio coverage provided by a GSM-R base station (or repeater) panel antenna, placed at a 4 meters height at approximately ten meters from the West tunnel portal, oriented along the railway tunnel course8]}. See results in Fig. 7 below.

Fig. 7. GSM-R radio coverage of the “Dealul Negru” tunnel – straight tunnel section

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The results presented above demonstrate that a straight tunnel section of average length (around 430 meters in the case of “Dealul Negru” tunnel), can be covered successfully with a GSM-R base station or repeater antenna, placed in the proximity of the tunnel portal. The received signal level inside the tunnel does not range below 96 dBuV/m (-41 dBm). The effect of the reflections on tunnel walls is negligible in this hypothesis, as the direct line of sight radio radio coverage is much larger than the strength of reflected signals.

C. Results. “Dealul Negru” railway tunnel – GSM-R coverage

prediction for the curved tunnel section

The situation is however totally different for the case of a curved tunnel section. In order to simulate a curved tunnel section (in reality the “Dealul Negru” tunnel is almost straight), we modified the raster image file representing the railway tunnel course, by replacing the raster value “0” with “1” for the railway tracks adjacent to the West tunnel portal (curved), and vice versa for the straight tunnel section). The GSM-R transmitter panel antenna was placed inside the newly created curved tunnel section, in the proximity of the East portal, also oriented along the tunnel.

The GSM-R radio coverage prediction inside the curved tunnel section is presented in Fig. 8 below.

Fig. 8. GSM-R radio coverage of the “Dealul Negru” tunnel – curved tunnel section

In the case of the curved tunnel section, the reflections of the radio signals on tunnel walls have a very significant effect. The radio coverage is extended beyond the direct line of sight zone, along the tunnel. The coverage obtained exclusively through reflections is shown in the figure above, the reflected signal strength being 20-40 dB lower than the signal received on the tunnel section that is still in direct line of sight with the GSM-R transmitter. Although the ICS Telecom 3D ray tracing simulation engine is limited to only one reflection, we can infer that with one or two more reflections against the tunnel walls the radio signal strength would fade below the receiver sensitivity threshold.

IV. CONCLUSIONS

The conversion procedure from Autocad large scale situation plans of the railway tunnels, to ICS Telecom GIS raster format provides an entirely satisfactory resolution for the resulting railway tunnel model, which has been demonstrated herein to reach 4 centimeters/pixel. However, this resolution approaches the technical limits of raster files, as the embedded image files reach a resolution of 635 digits per inch, and cannot be increased, for example, to reach 2 centimeters/ pixel. Although the achieved resolution can be considered satisfactory from the GSM-R radio signal wavelength perspective (around 30 centimeters), the achieved tunnel walls roughness is higher than what is considered normal for concrete railway tunnels (2 centimeters). This limitation is resulting, when simulating radio waves propagation, in excessive scattering of radio waves on tunnel walls, and consequently the radio waves are excessively attenuated in the proximity of tunnel walls. However, from the operational point of view, this limitation is not very important, as the GSM-R mobile terminal antennas are placed centrally on the train, being situated on the longitudinal axis of the tunnel, at a large distance from tunnel walls.

The limitations of ICS Telecom in the radio design of GSM-R systems inside railway tunnels refer to the facts that the height of the tunnel and the tunnel ceiling are in fact neglected, as well as the shape of the transversal section of tunnel. However, ICS Telecom is a valuable tool for the accurate radio design of GSM-R systems in outdoor environments [ HYPERLINK \l "Nem11" 4 ] and straight railway tunnels.

REFERENCES

[1] UIC, International Union of Railways, "UIC Project EIRENE, System Requirements Specification," GSM-R Operators Group, PSA167D007, May 17, 2006. [Online]. www.uic.org/IMG/pdf/EIRENE_SRS_v15}

[2] Dirk Didascalou, "Ray-optical wave propagation modelling in arbitrarily shaped tunnels," Institut fur Hochstfrequenztechnik und Elektronik, Universitat Karlsruhe (TH), PhD Thesis ISSN 0942-2935, 2000.

[3] L. Dobrică, T. Ionescu, S.E. Colesca, "Spatial data acquisition, management and visualization in Geographic Information Systems," UPB Scientific Bulletin, Series C, vol. 72, no. 3, pp. 93-108, 2010.

[4] Lăcrămioara-Mihaela Nemţoi, "Study regarding the use of radio channels at the railway, on lines with specific works of art," University "POLITEHNICA", Bucharest, Ph.D. Thesis 2011.

[5] ITU Radiocommunication Assembly. (1994, August) ITU Web site. [Online]. www.itu.int/rec/R-REC-P.525-2-199408-I/en

[6] ATDI (Advanced Topographic Development & Images). (2003, January) ICS-telecom 6.++ Comprehensive Network Engineering on V/U/SHF Bands. Reference Manual.

[7] ATDI (Advanced Topographic Development & Images). (2005, January) ICS map server 7.x. Image Cartography System. Reference manual.

[8] C. Briso-Rodriguez, J.M. Cruz, J.I. Alonso, "Measurements and Modelling of Distributed Antenna Systems in Railway Tunnels," IEEE Transactions on Vehicular Technology, vol. 56, no. 5, pp. 2870-2879, September 2007.

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[IEEE 2012 IEEE 18th International Symposium for Design and Technology in Electronic Packaging (SIITME) - Alba Iulia, Romania (2012.10.25-2012.10.28)] 2012 IEEE 18th International