coverage evaluations in tunnels

8/18/2019 Coverage evaluations in Tunnels

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RADIOFREQUENCYSYSTEMS

R F S C o n n e c t w i t h t h e b e s t ®

COVERAGEEVALUATIONS

IN TUNNELS

APPLYING RADIATING

CABLES

Reprint from the proceedings of theITC Conference Amsterdam, March 1997

RFS kabelmetal

H.-D. Hettstedt,

M. Davies,

B. Herbig,

R. Nagel


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R F S C o n n e c t w i t h t h e b e s t ®


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COVERAGE EVALUATIONS IN TUNNELS

APPLYING RADIATING CABLES

ABSTRACT

This paper analyses coverage predictions in tunnels

using radiating cables. It is demonstrated that cable

data gained from free space measurements results are

applicable to tunnel environments. Fading models used

for radio communication in free space applied to

tunnels explain effects of electrical behaviour of radia-

ting cables in tunnels. A coverage prediction is perfor-

med by system loss calculations based on cable data.

The coverage model for tunnels is similar to that of free

space. Measurement results are presented from a typi-

cally equipped metro tunnel considering a rectangular

concrete section and another consisting of a combina-

tion of a concrete section with a steel tube. The results

show concurrence between theory and practice.

1. INTRODUCTION

For radio coverage in free space, two types of coverageare important for the system design, the area coverage

and the contour coverage. Assuming a specific fading

characteristic, e.g. Rayleigh fading or Ricean fading, the

area coverage can be computed from the coverage

measured on any contour surrounding this area.

In a tunnel along a radiating cable the coverage can be

computed directly from measurement results of the

system loss. A normal system design approach is to

predict the reception probability at the end of the cable

section using known cable data: cable loss and coupling

loss. The reception probability thus gained is compa-

rable with the contour coverage in free space.

Of special interest are the influences of the tunnel

surroundings on the coverage characteristics and com-

parison between predicted and measured coverage

values. This papers helps to clarify this complex. A fur-

ther interesting point relevant to system design is that

of matching the coverage in free space with that in

tunnels. Fading models in tunnels are investigated forthe case of radiating cables.

2. COVERAGE MODELS

In order to meet specific requirements of system reliabi-

lity, radio coverage normally has to be confirmed theo-

retically and by measurements. In free space applicati-

ons, the situation and design procedure is well known.

In tunnels the situation is very different, though the

measurement conditions are actually simpler. Normally

a tunnel radio system is a portion of a larger radio

system supplying both free space and tunnels. So the

coverage in both areas is of special interest as well as

the interfaces between them. Therefore both situations

are considered in this paper.

2.1 Free Space

In free space, coverage requirements are defined by a

specific minimum signal reception level within a defined

percentage of an area. For public safety e.g. an area

coverage of 98 % is needed. FIG. 2.1 shows an area in a

simplified form within a circular contour enclosing a

Base Station antenna. As the dimensions can be of theorder of several kilometres, the difficulty of confirmati-

on by direct measurements is obvious. So the procedure

of measuring only the contour coverage is a useful

simplification, but still involves considerable effort. A

mathematical relationship between area and contour

coverage is given in [1] assuming Rayleigh fading under

these specific conditions. The area in FIG. 2.1 is sepa-

1

Hilly Area

Antenna Urban Area

Wood Area

Plane Area

CircularContour

Figure 2.1:

Sketch of a Free Space Scenario


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rated into 4 different segments representing very diffe-

rent conditions for radio wave propagation. This extre-

me inhomogenity is to demonstrate the complexity ofexact coverage evaluation in large areas.

2.2 Tunnels

Confirmation of radio coverage in tunnels is easier in

practice because the space is clearly limited to a narrow

area which is normally directly available for measure-

ments. So the results of field strength measurements

along the radiating cable can be used to evaluate recep-

tion probability and coverage evaluation.

FIG. 2.2 shows the situation inside a tunnel cross-

section giving an image that signal reflections from the

tunnel wall are an important factor. To gain theoretical

coverage predictions as part of the system design as a

whole, the conditions for electromagnetic propagation

in the tunnel must be analysed carefully.

• Measurement results for specific antenna heights and

distances from the cable should be valid within the

whole cross-section.

• Cable parameters gained from free space measure-

ments must be transferable to tunnel applications.

• Tunnel influences must be clear and predictable.

• The characteristics of the signal strength variations

along the cable must be analysed mathematically.

2

Mobile

Tunnel Cross-Section

RadiatingCable

DirectPath

Multipath

Figure 2.2:

Sketch of a Tunnel Scenario

3. FADING MODELS

Signal transmissions generally show field strength va-

riations depending on local characteristics. These phe-

nomena, known as fading, must all be considered in

evaluating radio communication parameters. In order to

obtain comparable procedures, conditions in free space

and in tunnels must be analysed.

3.1 Fading in Free Space

In free space, the fading effects can be separated into

two parts, a long and a short-term fading, see e.g. [1].

The long-term fading represents signal strength variati-

ons due to specific local attenuation and blocking

effects additionally to the normal attenuation of e.m.

propagation over distance. The name is due to its nearly

constant characteristic on time. The statistical distribu-

tion derives from a lognormal function with a standard

deviation of typically 5.5 dB for frequencies up to

1 GHz.

Superposed on this effect there is a short-term fading

resulting from multipath propagation with a high den-

sity of signal variations. The statistical distribution is

related to Rice or Rayleigh functions, see e.g. [1]. Rice

fading is typical when more than 50% of the signals are

propagated on the direct path. The Rayleigh function

must be applied when multipath propagation predo-

minates.

FIG. 3.1 shows these relationships resulting in a free

space attenuation at a specific distance to a BTS anten-

na. Moving along the circular contour in FIG. 2.1, we

obtain both lognormal fading over a constant attenu-

ation D due to the area specific characteristics as well as

multipath effected fading. In case of Rayleigh fading the

mean value will have a lognormal distribution. In this

case the mean value differs from the median (i.e. 50%

reception probability).


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3

Each of these effects must be considered in system

calculations, see FIG. 3.2. In order to achieve a required

coverage, a fading margin is needed which represents

both the long-term and short-term fading. In [1] the

mathematical relationship between contour and area

coverage is given. A typical calculation resulting from

Rayleigh fading is: A 95% contour coverage requires a12 dB short-term margin and results in a 97% area

coverage.

3.2 Fading in Tunnels

In tunnels a line of sight to the radiating cable can nor-

mally be assumed. The conditions for Rice fading are

evidently given. However, the situation is complicated

by the cable itself. There is no discrete radiating source,

instead the cable has a function of a distributed anten-na. This leads to an electrical field along the cable with

typical interference from interactions of different types

of e.m. waves generated from the cable. For a detailed

description of the function of radiating cables see [2].

Multipath DominatingDirect Path Dominating

Free Space Attenuation

Distance

(Log. Scale)

Short-term Fading

Attenuation

Lognormal Fading

(ca. 12dB for 98% Coverage)

(ca. 5dB)

(e.g. 12dB SINAD, 50% Coverage)

(C/N = 1)

Total Margin

Noise Figure

Fading Margin

Margin for MultipathMading

Static C/N

Inferred Noise Level

Thermal Noise Level

e.g. Frequency

Signal Levels

Margin for LognormalFading

Figure 3.1:

Fading Effects in

Free Space

Figure 3.2:

Fading Margins


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4

Figure 3.3:

Measurement Results of Coupled Mode Cables in Free Space and Tunnel


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5

Figure 3.4:

Measurement Results of Radiating Mode Cables in Free Space and Tunnel


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So radiating cables show fading characteristics even

without tunnel environments. They can be classified into

two general types, operating either in the coupledmode or the radiating mode. Both types show different

field characteristics. FIG. 3.3 shows measurement results

of the coupling loss of an RLF cable operating in the

coupled mode at 960 MHz. The upper diagram shows

results gained from free space measurements under

standard conditions. The lower diagram shows results

from tunnel measurements under equivalent conditions.

In FIG. 3.4 similar results are shown for a RAY type cable

operating in the radiating mode.

6

Both types of cables show similar statistical distributions

of the coupling loss for free space and tunnel environ-

ments. This means that multipath in tunnels within thecross-section is of secondary order only and that the

cable characteristics are clearly dominated by their

functions. These are results of a more exhaustive investi-

gation of different types of cables, see [3]. [3] also

shows that the same cables give results in free space

and tunnels which can differ up to ± 5 dB, although, at

discrete frequencies, the 5%, 50% and 95% reception

probability values relative to each other, remain the

same.

Upper Part: Coupled Mode Cable,

Lower Part: Radiating Mode Cable

Figure 3.5:Probability Density Functions for Cables

in Free Space and Tunnel


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The statistical distributions of field strength in free

space and tunnels are thus apparently equivalent, but

the tunnel environments lead to an offset of max. ± 5 dB.These results would suggest that the tunnel environ-

ments result in a type of fading comparable with the

lognormal fading in free space. A thorough analysis of

the distribution of this effect would require a very great

number of tunnel measurements under otherwise

unchanged conditions.

The probability density function of the cable itself,

whether in free space or in tunnels, does not fulfil the

parameters of the Rayleigh or the Ricean characteristic.

The mean value of a Rayleigh distribution is 1.25 times

the standard deviation and that of a Ricean distribution

is even higher (see [1]), whilst graphical and numerical

analysis of radiating cable coupling loss shows the mean

value to be actually less than that of the standard devi-

ation (typically 90%). This effect is only possible when a

high degree of asymmetry is present in the distribution

(remembering that negative values for coupling loss in

non-logarithmic terms are not possible). The asymmetry

apparent in the PDF’s shown in FIG. 3.5 is characterised

by a very steep incline towards zero (compared to Riceor Rayleigh distributions) and a very gradual decline

towards higher signal strengths. This tendency is typical

of logarithmic scaling, so that evidently a lognormal

element predominates to an extent that other super-

imposed characteristics are no longer recognisable.

We can thus suppose that the radiating cable pattern

contains both a Rayleigh and a lognormal component

which would mean that the resulting distribution is

then a Suzuki distribution (see [1]). Further research is

necessary to verify this, deriving the appropriate para-

meters for a hypothesis and subjecting it to a chi squa-

red test, and is the subject of present research.



7

4. COVERAGE PREDICTIONS IN TUNNELS

For the coverage prediction in tunnels an approach is

useful which accords with that for free space. The

system loss for the end section of a long radiating cable

can be computed using cable data gained from free

space measurements. The considerations of the previous

chapters show this procedure to be correct. This compu-

tation leads to a coverage comparable with the contour

coverage in free space and is the basis for the overall

coverage along the whole cable.

Fig. 4.1 shows the relationship between cable loss,

coupling loss, system loss, margin for „lognormal“ fa-

ding and the min. reception level. Using the cable data

from standard tests the length of the end-section is of

course the same as of the test length, normally 100 m.

This procedure of system loss computations leads to

situations where the min. reception level at the end of

the cable always meets the coverage requirement. This

is a desirable result for system design.

The difference between the coverage along the whole

cable and that of the end-section depends on the

amount of total cable loss. Fig. 4.2 shows the system

loss diagrams of an RLF type cable for a 500 m and a

1000 m section, theoretically evaluated. A comparison

of the reception probability curves show that the distri-

bution is stretched by increasing the cable losses due to

the double cable length. The differences between the

50%- and 95%-values are remarkable. The divergence of

end-section coverage and area coverage is shown by a

brief analysis:

Cable type: RLF 13/33-1800

Frequency: 900 MHz

Cable loss: 3.4 dB/100 m

Coupling loss: 71.3 dB/50%, 82.5 dB/95%

End-Section Coverage Whole-Section Coverage

Length [m] Sys. loss (50%) Sys. loss (95%) Sys. loss (50%) Sys. loss (95%) Sys. loss (98%)

100 74.7 dB 85.9 dB - -

500 88.3 dB 99.5 dB 80.3 dB 93.4 dB 97.3 dB

1000 105.3 dB 116.5 dB 89.1 dB 106.9 dB 111.1 dB


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8

This analysis shows that the procedure of computing

the system loss for a 95% coverage of the end-section

leads to an area coverage of more than 98%. The diffe-rences depend on cable types, lengths and frequencies.

The coverage prediction has to be performed analyti-

cally case by case.

5. MEASUREMENT RESULTS

In a longer section of the UESTRA metro tunnels in

Hanover, different types of radiating cables were tested.

The cables are installed on side walls at train window

level in a single bore in one direction. The tunnels are

mostly of concrete material and of rectangular size in-

cluding niches, side changes of the cabling and diverse

typical discontinuities. These environments offer the

opportunity for tests under typical installation conditi-

ons.

Two representative test results were chosen. FIG. 5.1

shows in the upper part a diagram of system loss mea-

sured in a rectangular tunnel section between two met-

ro stations. The diagram below shows the system loss

resulting from measurements in a tunnel section wheretwo different types are combined: a concrete rectangu-

lar tunnel with a metal tube. Both diagrams are related

to cable lengths of appr. 500 m. For both tests the same

cable type RLF 17/44 was used for the tests at 960 MHz.

The test antenna was fixed outside the train in the cen-

tre of the front window at vertical orientation. So the

test conditions were equal.

In the upper diagram for the system loss in the rectan-

gular tunnel it can be seen that the slope shows small

variations which can be explained by discontinuities in

the tunnel and local additional cable losses from con-

necting jumper cables. The analysis of the extracted

coupling loss shows that there is a nearly constant off-

set between the free space and tunnel values which are

within the ± 5 dB variation due to the expected „lognor-

mal“ fading effect.

Comparing the measured system loss with the calcu-lated one it can be seen that the 50% values fit very

well. The predicted 95% value is again equivalent with

the measured 98% value. The difference between the

5% values are comparable with the difference between

Cable Loss

LognormalMargin

CouplingLoss

Min. Reception Level (C/N= 12dB)

Lognormal Fading 95%

100m

(Linear Scale)

Distance

System Loss

Figure 4.1:

Diagram of System Loss

in Tunnels


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9

Figure 4.2:

Theoretical System Loss Results Extrapolated from Measurements on a Cable of 150m


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11

Figure 5.2: Comparison of Coupling Loss Results Gained in Sections

of a Concrete Tunnel and of a Steel Tube


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12

the 95% values, see the table below. In conclusion, this

example of practical application shows that the pre-

dicted coverage of 95% at the far end of the cable leads

to a total coverage of 98%.

Further interesting results were gained from the

measurements in the section where two very different

types of tunnels are connected directly. As can be seen

from the lower diagram in FIG. 5.1 there is a step of

appr. 10 dB in system loss at the junction from the

concrete tunnel section to the steel tube. This means

there is a remarkable difference between the coupling

losses in the different sections.

In FIG. 5.2 the coupling loss characteristics for both

tunnel sections are shown separately. The upper

diagram shows the coupling loss for the concrete

section, the lower one that for the first 130 m of the

steel tube. Direct comparison demonstrates an effective

lower coupling loss in the tube at a difference of appr.

10 dB and a more regular fading characteristic. The

reason for this effect is of course higher reflections

resulting in lower losses. Another interesting result is

that there is no change in the distribution function for

the tube. It can be assumed that even under highly

reflective environments the cable characteristics do not

change and that a constant offset based on typical

influences of a tunnel can again be observed, in this

case with positive results.

6. CONCLUSIONS

It was demonstrated that distribution characteristics of

radiating cables in tunnels are equivalent to those in

free space. Differences of electrical behaviour can be

explained by fading models from free space. A coverage

Coupling Loss System Loss

Reception Free Space Data Tunnel Data Offset Predicted from Measured Data

Probability Tunnel Data

5% 65.7 dB 69.3 dB + 3.4 dB 73.4 dB 77.5 dB

50% 72.9 dB 76.7 dB + 3.8 dB 88.2 dB 88.4 dB

95% 84.6 dB 88.0 dB + 3.4 dB 106.9 dB 102.0 dB

prediction for tunnel sections supplied by radiating cab-

les can be performed from system loss calculations ba-

sed on cable data. Measurement results of a metro tun-

nel in typical concrete environments and in a highly re-

flective steel tube confirm theoretical calculations made

in free space, demonstrating the independence of the

cable’s behaviour to the environment.

7. ACKNOWLEDGEMENTS

The authors would like to extend their thanks to theUESTRA AG, Hanover, for their kind permission to use

the tunnels and to Mr. Witte and Mr. Reuter for their

good collaboration in forms of advice and supply.

Furthermore, they wish to thank Mr. Mahlandt from the

RFS Cable Development Department for his helpful ad-

vice and for the supply of software.

8. REFERENCES

[1] M. D. Yacoub: Foundations of Mobile Radio

Engineering, CRC Press, 1993

[2] H.-D. Hettstedt: Development and Applications of

Leaky Feeders, International Seminar on

Communications Systems For Tunnels, London, 1993

[3] H.-D. Hettstedt, B. Herbig, G. Klauke, R. Nagel:

Comparison of Performances of different

Leaky Feeders in a Metro Tunnel, Tunnel Control

& Communication, Basel, 1994


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13


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