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RADIO FREQUENCY SYSTEMS
R F S C o n n e c t w i t h t h e b e s t ®
COVERAGEEVALUATIONS
IN TUNNELSAPPLYING RADIATINGCABLES
Reprint from the proceedings of the
ITC Conference Amsterdam, March 1997
RFS kabelmetal
H.-D. Hettstedt,
M. Davies,
B. Herbig,
R. Nagel
A4 RZ COVERAGE orange 3.FE 08.07.1999 9:55 Uhr Seite 2
R F S C o n n e c t w i t h t h e b e s t ®
Innens. A4 -Orange 1-3 08.07.1999 9:49 Uhr Seite 1
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 coverage
are 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 for
the 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 the
order 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
Circular
ContourFigure 2.1:
Sketch of a Free Space Scenario
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 1
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
rated into 4 different segments representing very diffe-
rent conditions for radio wave propagation. This extre-
me inhomogenity is to demonstrate the complexity of
exact 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
Radiating
CableDirect
Path
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).
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 2
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
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 a
12 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].
e
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
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 3
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
4
Figure 3.3:
Measurement Results of Coupled Mode Cables in Free Space and Tunnel
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 4
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
5
Figure 3.4:
Measurement Results of Radiating Mode Cables in Free Space and Tunnel
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 5
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
So radiating cables show fading characteristics even
without tunnel environments. They can be classified into
two general types, operating either in the coupled
mode 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 the
cross-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
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 6
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 Rice
or 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.
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
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
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 7
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
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 where
two 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
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 8
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
9
Figure 4.2:
Theoretical System Loss Results Extrapolated from Measurements on a Cable of 150m
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 9
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
10
Figure 5.1: System Loss Results Measured in a Concrete Tunnel and in a Tunnel
of a Combination of a Concrete Section with a Steel Tube
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 10
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
11
Figure 5.2: Comparison of Coupling Loss Results Gained in Sections
of a Concrete Tunnel and of a Steel Tube
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 11
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
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 the
UESTRA 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
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 12
COVERAGE EVALUATIONS IN TUNNELS
APPLYING RADIATING CABLES
13
Text COVERAGE EVALUA….FE 08.07.1999 9:54 Uhr Seite 13
R F S C o n n
R F S G R O U P M E M B E R SR F S G R O U P M E M B E R S
S A L E S O F F I C E S
2299.050.PM 171 001 56 - 01
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