15-06-2013, 04:51 PM
Classification of Transient EM Noises Depending on their Effect on the Quality of GSM-R Reception
[attachment=55515]
Abstract
The Global System for Mobile communications—
Railways (GSM-R) is being deployed in different countries to
develop an efficient communication-based train control (CBTC)
system. GSM-R participates to achieve railways interoperability,
replacing noninteroperable CBTC on existing networks and, thus,
facilitating cross-border train circulations. GSM-R ensures voice
and data transmissions between trains and control centers and also
between trains. As any radio equipment, it is subject to electromagnetic
(EM) disturbances present in the railway environment.
Therefore, the quality of GSM-R transmissions can deteriorate. It
is then important to evaluate and predict the effect of these disturbances
in order to avoid any loss of train operational capacity.
After an overview of the methods used for the characterization of
the EM environment, we describe the GSM-R and the EM disturbances
that can affect its operation. The reasons why the existing
characterization methods are not fully adapted to the GSM-R are
highlighted. The general principle of classification is briefly recalled.
The rest of this paper develops the methodology proposed
to perform the classification of transient EM noises and the presentation
of a test bench and its associated experimental results.
Finally, an application to an add-on electromagnetic compatibility
supervising equipment installed on board the train is described.
INTRODUCTION
THE characterization of the electromagnetic (EM) environment
in electromagnetic compatibility (EMC) relies
on well-known and mastered methods described in many standards,
such as for example the frequency sweep. However, in
the particular context of the characterization of a mobile environment
equipped with a digital transmission system, these
“classical” methods reach their limits [1]. Indeed, because of
the train movement, the communication signal power evolves
continuously, depending on the distance between the train mobile
station (MS) and the base transceiver stations (BTS) it is
connected to.
LIMITATIONS OF THE CURRENT EMC STANDARDS
EM Environment Characterization
Several standards exist for the evaluation of the environmental
EM noise. They are generally based on frequency-domain
methods, using swept frequency equipment (spectrum analyzers
and receivers). Standard measurement methods assign a frequency
range, a resolution bandwidth and a detection method,
together with the holding time for each measurement at each
point on the frequency axis. Some of these standards are specific
to railway applications, such as EN 50121-2 that describes
the measurement method to verify the emissions of the whole
railway system (including the vehicles and the infrastructure) to
the outside world and sets the emission limits to apply [5]. However,
they are not adapted to control on-board emissions and not
suitable to digital transmission systems, such as the GSM-R.
Consequently, it is necessary to propose an updated methodology
of characterization of the railway EMenvironment that takes
into account the specificities of the railway operating conditions
and the system to protect.
Disturbance Signals Present in the Railway Environment
Because of its proximity to the catenary, the GSM-R antenna
also receives EMtransient interferences coming from the sliding
contact between the pantograph and the catenary. The sliding
contact is characterized by multiple plasma arcs carrying the
pantograph current and showing a chaotic and erratic behavior
on the cathode surface [11], [12] (for a general description of
both low-frequency and high-frequency behavior, please see
[13]). When a loss of mechanical contact occurs, a transient
current appears on the pantograph and on the catenary. This
causes in turn a radiated EM field both directly from the sliding
contact spot and from the connected conductors, up to 1 GHz
[14]. Then, the GSM-R antenna is also subject to the radiation
of the metallic elements that constitute the pantograph and the
catenary. These transient emissions cover large frequency bands
including the frequency channels of the GSM-R.
Definition of the Descriptor
Itwas shown in [8] on the basis of the results presented in [22],
that to be relevant, the descriptor has to reflect both the time
evolution of the recurrence of transients and the signal-to-noise
ratio (SNR) in order to be representative of the communication
quality. In our study, the SNR is defined as the ratio between
the power of the GSM-R signal and the power of the noise
level induced by transient signals on the GSM-R channel when
they occur. We also need a time resolution fine enough to be
comparable with the transmission duration of one GSM-R bit
(3.7 μs). Then, it becomes possible to evaluate the impact on
each GSM-R bit, at each occurrence of a transient.
Extraction of Parameters From the Descriptor’s Curve
Once the descriptor’s representation is obtained, different
“second-order” parameters can be extracted. The first parameter
is the number of minima. It is indicative of the recurrence
of the transients. The second one corresponds to the sum of the
values (in linear values) reached by D at these minima (i.e., the
sum of the minima). This second parameter is indicative of the
deterioration of the SNR when a transient occurs. Let us recall
that the recurrence of the transients and the deterioration of the
SNR at each appearance are the two parameters that affect the
quality of the GSM-R transmissions. The count of the number
of minima is performed by applying a threshold. All the values
below this threshold are considered as minima, as illustrated in
Fig. 4.
CONCLUSION AND PERSPECTIVES
A classification methodology of EM transient noises depending
on the effect they produce on the quality of digital transmissions
was presented. The objective is to propose a tool that
can be embedded on board trains to identify when the quality
of GSM-R transmissions could be insufficient to respect the
specifications due to the presence of EM transient interferences.
The methodology was detailed, from the research of a relevant
descriptor to the construction of the reference diagram used
for the classification of “unknown” signals (i.e., signals whose
class has not been determined during the record of the measurement
file). Several transient EM scenarios have been considered
and the associated scatter diagrams have been analyzed. These
analyses show that the descriptor and its two associated parameters
enable a good separation of classes. They are relevant for
the GSM-R system.
[attachment=55515]
Abstract
The Global System for Mobile communications—
Railways (GSM-R) is being deployed in different countries to
develop an efficient communication-based train control (CBTC)
system. GSM-R participates to achieve railways interoperability,
replacing noninteroperable CBTC on existing networks and, thus,
facilitating cross-border train circulations. GSM-R ensures voice
and data transmissions between trains and control centers and also
between trains. As any radio equipment, it is subject to electromagnetic
(EM) disturbances present in the railway environment.
Therefore, the quality of GSM-R transmissions can deteriorate. It
is then important to evaluate and predict the effect of these disturbances
in order to avoid any loss of train operational capacity.
After an overview of the methods used for the characterization of
the EM environment, we describe the GSM-R and the EM disturbances
that can affect its operation. The reasons why the existing
characterization methods are not fully adapted to the GSM-R are
highlighted. The general principle of classification is briefly recalled.
The rest of this paper develops the methodology proposed
to perform the classification of transient EM noises and the presentation
of a test bench and its associated experimental results.
Finally, an application to an add-on electromagnetic compatibility
supervising equipment installed on board the train is described.
INTRODUCTION
THE characterization of the electromagnetic (EM) environment
in electromagnetic compatibility (EMC) relies
on well-known and mastered methods described in many standards,
such as for example the frequency sweep. However, in
the particular context of the characterization of a mobile environment
equipped with a digital transmission system, these
“classical” methods reach their limits [1]. Indeed, because of
the train movement, the communication signal power evolves
continuously, depending on the distance between the train mobile
station (MS) and the base transceiver stations (BTS) it is
connected to.
LIMITATIONS OF THE CURRENT EMC STANDARDS
EM Environment Characterization
Several standards exist for the evaluation of the environmental
EM noise. They are generally based on frequency-domain
methods, using swept frequency equipment (spectrum analyzers
and receivers). Standard measurement methods assign a frequency
range, a resolution bandwidth and a detection method,
together with the holding time for each measurement at each
point on the frequency axis. Some of these standards are specific
to railway applications, such as EN 50121-2 that describes
the measurement method to verify the emissions of the whole
railway system (including the vehicles and the infrastructure) to
the outside world and sets the emission limits to apply [5]. However,
they are not adapted to control on-board emissions and not
suitable to digital transmission systems, such as the GSM-R.
Consequently, it is necessary to propose an updated methodology
of characterization of the railway EMenvironment that takes
into account the specificities of the railway operating conditions
and the system to protect.
Disturbance Signals Present in the Railway Environment
Because of its proximity to the catenary, the GSM-R antenna
also receives EMtransient interferences coming from the sliding
contact between the pantograph and the catenary. The sliding
contact is characterized by multiple plasma arcs carrying the
pantograph current and showing a chaotic and erratic behavior
on the cathode surface [11], [12] (for a general description of
both low-frequency and high-frequency behavior, please see
[13]). When a loss of mechanical contact occurs, a transient
current appears on the pantograph and on the catenary. This
causes in turn a radiated EM field both directly from the sliding
contact spot and from the connected conductors, up to 1 GHz
[14]. Then, the GSM-R antenna is also subject to the radiation
of the metallic elements that constitute the pantograph and the
catenary. These transient emissions cover large frequency bands
including the frequency channels of the GSM-R.
Definition of the Descriptor
Itwas shown in [8] on the basis of the results presented in [22],
that to be relevant, the descriptor has to reflect both the time
evolution of the recurrence of transients and the signal-to-noise
ratio (SNR) in order to be representative of the communication
quality. In our study, the SNR is defined as the ratio between
the power of the GSM-R signal and the power of the noise
level induced by transient signals on the GSM-R channel when
they occur. We also need a time resolution fine enough to be
comparable with the transmission duration of one GSM-R bit
(3.7 μs). Then, it becomes possible to evaluate the impact on
each GSM-R bit, at each occurrence of a transient.
Extraction of Parameters From the Descriptor’s Curve
Once the descriptor’s representation is obtained, different
“second-order” parameters can be extracted. The first parameter
is the number of minima. It is indicative of the recurrence
of the transients. The second one corresponds to the sum of the
values (in linear values) reached by D at these minima (i.e., the
sum of the minima). This second parameter is indicative of the
deterioration of the SNR when a transient occurs. Let us recall
that the recurrence of the transients and the deterioration of the
SNR at each appearance are the two parameters that affect the
quality of the GSM-R transmissions. The count of the number
of minima is performed by applying a threshold. All the values
below this threshold are considered as minima, as illustrated in
Fig. 4.
CONCLUSION AND PERSPECTIVES
A classification methodology of EM transient noises depending
on the effect they produce on the quality of digital transmissions
was presented. The objective is to propose a tool that
can be embedded on board trains to identify when the quality
of GSM-R transmissions could be insufficient to respect the
specifications due to the presence of EM transient interferences.
The methodology was detailed, from the research of a relevant
descriptor to the construction of the reference diagram used
for the classification of “unknown” signals (i.e., signals whose
class has not been determined during the record of the measurement
file). Several transient EM scenarios have been considered
and the associated scatter diagrams have been analyzed. These
analyses show that the descriptor and its two associated parameters
enable a good separation of classes. They are relevant for
the GSM-R system.