REPOSITORY > RESULTS

Abstract

In this doctoral thesis, radio signal propagation in special environments is discussed. The
primary focus is on the development of a path loss model for radio signal propagation in
long road tunnels and underground passageways, which allows a rather precise
calculation of radio signal path loss and, consequently, a quick assessment of the
communication system range. The developed path loss channel model is particularly
useful for terrestrial telecommunication systems used in public protection and disaster
relief (PPDR).
The proposed channel model consists of four segments with diverse slopes of path loss
and break points between those segments. The model was designed based on the received
signal strength measurements at various carrier frequencies of radio signal along two
tunnels with different shape, size and purpose of use. By analysing the measurements, we
established the general path of radio signal attenuation in long tunnels. These findings
were used for analytical determination of characteristic zones of radio signal expansion in
tunnels with four slopes which, together with break points, present a new empirical model
of signal attenuation. Furthermore, we established that radio signal attenuation along a
tunnel could be presented with a function comprising four segments, as specified by the
distance between transmitter and receiver: (i) area of close proximity, (ii) nearby area,
(iii) far area and (iv) very far area.
In the first segment, the area of close proximity, the signal exhibits free space loss. In
the second segment, the nearby area, signal attenuation gradient is significantly reduced
and represented by the empirically estimated attenuation factor a. In the third segment,
the far area, the signal level decreases like in the waveguide, while the attenuation
gradient in the fourth segment, the very far area, corresponds to free space loss again.
Break points, separating the individual propagation areas, were defined empirically and
with calculation. The position of the first break point corresponds to the maximum
distance at which the first Fresnel zone is still free of obstacles. The second break point
occurs due to losses in the walls of the tunnel.
If the walls of the tunnel were an ideal conductor, the transmitter would be perceived
by the receiver as an infinite field of transmitter images and, consequently, radio waves
would propagate as in a waveguide. However, since the tunnel walls are not made of the
perfect conductor, the receiver sees the transmitter antenna as a limited field of
transmitter images, which results in greater attenuation gradient of radio signal in the
nearby field. The last break point, at which the waveguide effect disappears, was
determined empirically from measurement results in the tunnel Karavanke.
Model parameters were determined for four telecommunication systems, which are
expected to be used for terrestrial communications in emergencies and special
environments in the near future – namely the TETRA and WiMAX systems, and
telecommunication systems for wireless local networks (WLAN) based on IEEE 802.11
standards and telecommunication systems for data transmission in wireless sensor
networks based on IEEE 802.15.4.
The parameters of the proposed path loss model were estimated from measurements in
the Strunjan-Portorož tunnel and the Karavanke tunnel. Due to the length of the Strunjan-
Portorož arcade tunnel, path loss is estimated only for three segments. At the carrier
frequency 400 MHz, signal attenuation drops sharply until the first break point, which is
at a distance of about 10 m. Thereafter, the downward slope decreases to approximate
0.25 dB/m. In the last part of the curve, the attenuation gradient is further reduced to
0.1 dB/m. The attenuation gradient of the third (waveguide) section of the curve for
higher frequency signals is significantly lower, as expected. At 868 MHz frequency,
signal attenuation gradient is 0.042 dB/m, but only 0.032 dB/m at 3.5 GHz. In the straight
section of the Karavanke long road tunnel, the path loss function has four distinctive
sections. In the nearby area, the signal attenuation gradient is 0.13 dB/m. In the central
part, where the phenomenon of waveguides effect occurs, the attenuation gradient of
radio signal frequency 400 MHz is 0.025 dB/m. The measurement results also show that
the attenuation gradient in the central part is affected by the position of the transmitter and
the receiver. The transmitter placed in the tunnel lateral niche caused duplicate signal
attenuation gradient in the waveguide part. Results of measurements taken in the curved
part of the tunnel showed that path loss is decreasing with the factor of 0.075 dB/m.
Greater attenuation gradient is due to the lack of direct visibility between the transmitter
and the receiver which results in a significantly shorter communication range.
In addition, the validity of the proposed model was evaluated through simulations,
which were based on ray–tracing method. The influences of individual tunnel parameters
(shape, transverse dimensions, electromagnetic properties of walls and floor, the signal
carrier frequencies and different transmitters and receivers) on signal attenuation gradient
were examined.
It was established that the radio signal propagation in a tunnel is strongly influenced by
the size and shape of the tunnel, as well as the carrier frequency of radio signal. On the
one hand, the position of the receiver and transmitter in the tunnel showed a negligible
impact on the attenuation gradient, while on the other hand the material properties from
which the walls, the ceiling and the floor of the tunnel were built showed a noticeable
impact on the attenuation gradient.
The simulation results showed that the proposed empirical model adequately takes into
consideration all the essential parameters that affect the propagation of radio signals in
special environments. In conclusion, the proposed empirical radio signal propagation
model with four slopes, which is the essential original contribution of the thesis, enables a
simple and sufficiently accurate calculation of the radio signal propagation of frequencies
up to 1 GHz in straight tunnels and underground passageways of different sizes and
shapes.