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ABUAD Journal of Engineering Research and Development (AJERD)
Volume 1, Issue 1, 95-99
http://ajerd.abuad.edu.ng/ 95
Theoretical Analysis of Transmission Parameters and
Interference Issues in Power Line Communication
Systems
Adedayo O. AJIBADE1,*, Ilesanmi B. OLUWAFEMI2, Adedayo O. OJO1, Kehinde A. ADENIJI1.
1Department of Electrical Electronic and Computer Engineering, Afe Babalola University, P.M.B. 5454, Ado-Ekiti, Ekiti,
Nigeria.
ajibadea@abuad.edu.ng/ojoao@abuad.edu.ng/adenijika@abuad.edu.ng
2Department of Electrical and Electronic Engineering, Ekiti State University, P.M.B. 5353, Ado-Ekiti, Ekiti, Nigeria.
ibto75@gmail.com
*Corresponding Author: ajibadea@abuad.edu.ng
Date of First Submission: 28/09/2017
Date Accepted: 01/11/2017
Abstract: Communication over the wireless network is becoming
increasingly saturated as a result of global proliferation of wireless
and mobile devices. Coupled with the requirement for in-home
devices and appliances to share information in real-time, powerline
communication (PLC) systems are gaining rapid popularity as a
cost-effective alternative to wireless and other wire communication
techniques. In this work, the salient properties of PLC systems
including mechanism of signal propagation, channel and noise
characteristics, as well as signal interference within the PLC
network are expounded. Furthermore, the effect of interference of
PLC signals with signals from other existing communication
channels are emphasized.
Keywords: Power line communication (PLC), signal propagation,
channel, noise, interference
1. INTRODUCTION
The concept of communication over the powerline is not a
new one; power companies have always used PLC technique
to send low-data control and monitoring signals over the
power network [1], [2]. Smart grid and smart metering are
evolving technologies that rely on PLC for their functioning.
While smart grid allows devices and appliance connected to
the electrical network to share information, and ensure
performance optimisation, smart metering helps to reduce
power wastage by constantly learning and monitoring power
behaviour and consumption pattern of appliances on a
network [3], [4]. With these technologies, remote monitoring
and fault detection on the network as well as automated billing
are achieved [4]. Given the successful deployment of PLC for
the above technologies, it has become a strong consideration
for providing reliable and affordable in-home (or in-building)
communication services. Powerline networks are ubiquitous in
nature i.e. they exist virtually everywhere across the globe;
this makes PLC a cheaper alternative to other existing
communication techniques [3]. The deployment of PLC does
not require installation of any major infrastructure of software;
PLC couplers or adapters and only a few other accessories are
needed [3], [4]. Also, they display strong resilience against
natural hazards, and are still capable of transmitting low-
voltage communication signals even with the occurrence
faults that render them incapable of transmitting high-voltage
electric power [1]. However, the use of powerline cables as a
channel/medium for information and data transfer is fraught
with some challenges [6]. First, powerline networks were
originally designed to carry high-voltage (relative to data
signal voltage) signal, which are typically at a low frequency
of 50/60Hz, while data signals are low-voltage signals at
frequencies in the MHz range [6], [13]. This frequency
mismatch makes the powerline network a harsh
communication channel for transmitting higher-frequency data
signals [9], [13]. Thus, the propagation mechanism, channel
and noise characteristics of PLC channels are slightly different
from those of wireless and other commonplace wire
communication channels like coaxial cables, optic-fibre cables
and twisted-pair cables [7]. Secondly, interference in PLC –
mainly due to leakage of radio frequency (RF) signals
propagating the powerline cables. This interference may occur
among devices that share data within the PLC network,
resulting in undesirable degradation of transmitted signals and
reduction in throughput [18] or leakages from several devices
may combine to form a strong signal that can totally distort
other signals transmitted on same range of frequencies [19].
Because of similarities in many aspects of wireless and PLC
signal propagation and channels, references to wireless
systems will be made when necessary within this write-up.
The subsequent sections of this paper give an overview of
signal propagation behaviour, channel and noise
characteristics, and interference in PLC systems.
ABUAD Journal of Engineering Research and Development (AJERD)
Volume 1, Issue 1, 95-99
http://ajerd.abuad.edu.ng/ 96
2. SIGNAL PROPAGATION MECHANISM IN PLC
NETWORKS
Like any other communication channel, it is important to
discuss extensively, the signal propagation mechanism of PLC
channels, as this will enhance better understanding of the
channel and noise behaviour of the channel. While reflection,
diffraction, and scattering are the main mechanisms of
propagation in wireless channels, propagation in PLC
channels is primarily based on reflection [11]. When a signal
propagates the PLC channel from the transmitter to the
receiver, the signal experiences reflections at impedance
discontinuities along the path. These discontinuities mainly
result from line branching and load terminations on the
network as shown in figure 1 [7], [8], [12].
Figure 1: Customer Premise PLC Network
As a result, the received signal comprises of several
attenuated, delayed, and phase-shifted copies of the
transmitted signal, leading to time dispersion [7], [25].
Generally, in communications parlance, a parameter known as
root-mean-squared (RMS) delay spread is measure of the
extent of time dispersion [7]. Several factors on the PLC
network determine the extent of the RMS delay spread [7],
[30]. These include the number and length of branching nodes
between the transmitter and receiver, the separating distance
between the transmitter and receiver, as well as the impedance
values of terminal loads. It has been observed that worst
values of RMS delay spread are obtained for low and high
terminal load impedances [16], [17]. Several studies and
measurements on RMS delay spread values in the frequency
range of 30 MHz for low-voltage PLC networks have shown
that it ranges between 2 and 6 microseconds [18], [19].
Asides time dispersion, wireless and PLC channels vary
with time i.e. are time-selective [9], [25]. Time-selectivity in
wireless channels is due to relative mobility between the
transmitter and receiver [9], [11]. In PLC channels, it is
primarily due to impedance variations at load terminations on
the network [7]. Time variation in PLC channels are induced
by changes in the reflection factors of the propagation paths,
and are categorised into long-term and short-term variation
[7], [17]. The former is caused by constantly-changing
impedance status of the PLC network termination points from
switching on/off of connected devices. This changing
impedance values at termination points induce a change in the
reflection and transmission coefficients of some propagation
paths, resulting in variation of the channel response. Long-
term impedance variations also depend on the state of the
connected electrical load – plugged and active, plugged but
inactive or unplugged [7], [26]. Short-term impedance
variation on the other hand stems from the cyclic nature of the
mains alternating current (AC) [24]. The separation distance
between transmitter and receiver determines the extent of
channel variation from impedance dependency of the
electrical loads on the mains cycle [31]. Short-term impedance
variation in low-voltage PLC channels has a coherence time of
at least 600 microseconds [17], [24]. Coherence time in
communication is the duration of time over which the
channels remains time-invariant [7].
3. PLC CHANNEL CHARACTERISTICS
3.1 Multipath Propagation
In wireless propagation environments, the presence of
natural and man-made objects like trees, buildings, hills,
atmospheric moisture e.t.c., along the signal path causes the
transmitted signals undergo the effects of reflection,
diffraction, and scattering. This results in multiple copies of
the transmitted signals being received at the receiver i.e.
multipath propagation [9], [11]. These signals comprise the
direct path signal and many other attenuated and delayed
copies of the signal. These signal copies will induce deep nulls
at certain frequencies as a result of destructive interference
between signal paths. This phenomenon is referred to as
frequency-selective fading [9], [11], [13]. Similar to wireless
channel environments, the powerline channel is an unstable
transmission environment due to impedance variations from
the characteristic impedance of the cable, the topology of the
network, and the nature of the connected electrical appliances
[7], [8]. This time-variation of the PLC channel causes signals
propagating it to experience multipath propagation effects.
The multipath in PLC depends on the physical topology as
well as the physical characteristics of the channel [4], [7].
Hence the PLC channel becomes frequency selective mainly
from reflections and transmissions caused by impedance
mismatches at branch discontinuities and network
terminations. Thus, the transmitted signals will arrive the
receiver with varying attenuation and delay [7], [10]. From
Fourier analysis, a delay in the time domain will result in a
periodic a phase-shift in the frequency domain. At the
receiver, the direct signal and the phase-shifted signals
combine to produce notches/nulls at certain frequencies,
resulting in frequency-selective fading [7].
According to Zimmermann and Dostert [10], the PLC
channel can be completely characterised by its channel
frequency response as:
()=
(,) (1)
ABUAD Journal of Engineering Research and Development (AJERD)
Volume 1, Issue 1, 95-99
http://ajerd.abuad.edu.ng/ 97
where Γ and are the reflection and transmission coefficients
along the propagation path respectively; and represent
the number of reflection and transmission coefficients
experienced by the propagating signal along a particular path
; (,) is the frequency- and distance-dependent
attenuation arising from the physical characteristics of the
cable; exp(2) represents the phase of the
component due to the time delay . The time delay, , is
related to the speed of propagation within the communication
channel (here, powerline cables) as:
=
(2)
where is the dielectric constant of the insulation material;
and is the speed of light in a vacuum.
3.2 Attenuation
In wireless channels, signal attenuation is regarded as path
loss, determined by transmission frequency and propagation
distance [9]. In a similar manner, the electrical characteristics
of the powerline cable and the transmission frequency band
determine the extent of signal attenuation in PLC channels [4].
The signal attenuation in PLC systems is a result of the loss of
power as the signal propagates the powerlines, transformers,
couplers, e.t.c. From transmission line theory, four parameters
that describes the electric characteristics of a cable segment
include resistance, capacitance, conductivity, and inductance
[13], [22]. Equation (3) shows that the resistance of a unit-
length cable increases proportionally with the square-root of
transmitting frequency, hence the attenuation in PLC channels
is more pronounced at higher frequencies [13].
=
(3)
where is the free-space permeability; and are the
conductivity and radius of powerline cable respectively.
Also, from extensive measurements carried out on low-
voltage PLC networks, it has been shown that the attenuation
varies directly with frequency and distance, and is given by
the equation:
(,)=exp (() (4)
where is the frequency of the signal; is the distance
covered by the signal; ,, and are cable-dependent
parameters that are derived from empirical measurements
[10].
From equations (3) and (4), it can be deduced that the
attenuation experienced by signals propagating a powerline
channel increases as the transmitting frequency and distance
increase [7], [ 10].
4. PLC NOISE CHARACTERISTICS
Unlike wireless communication systems and most other
communication systems, where noise is modelled as an
additive white Gaussian noise (AWGN), noise on PLC
systems are complex in nature, consisting of a slow-varying
coloured background noise, narrowband noise, and a fast-
changing impulsive noise [8], [13]. The coloured background
noise stems from the sum total of the harmonics of the mains
cycle as well as other low-power noise sources within the PLC
system. This noise has a very low power spectral density that
varies directly with frequency. Narrowband noise is in the
form of sinusoidal signals with modulated amplitudes; it is
mainly induced by interference from nearby short and medium
wave broadcast radio signals. Since their amplitudes vary very
slowly over time, both coloured background noise and
narrowband noise make up the background noise in PLC
systems [7], [13], [27]. Impulsive noise is regarded as the
most significant noise in PLC systems; it is the major cause of
errors in data transmission over PLC channels. It is mainly
generated by intermittent and random interference on the
supply signal [27], caused by electrical appliances connected
to the PLC network. These noises are of three different types
[7] [8], [18]:
1) Periodic impulsive noise: This class of noise is
synchronous to the mains frequency, and is induced by the
power supplies within the PLC network.
2) Periodic impulsive noise asynchronous to the mains
cycle: This noise is mainly generated from switching on/off
power supplies.
3) Aperiodic impulsive noise: This noise is induced by
switching transients within the PLC network, this type of
noise usually has a power spectral density significantly above
the background level.
5. INTERFERENCE IN PLC
5.1 Electromagnetic Compatibility of Wireless and PLC
Systems
Electrical signals travelling along a wire or cable causes
some radiation of the signals into the surrounding. These
radiated signals could interfere with nearby electronic
equipment, resulting in malfunctioning of such equipment
[15], [28]. This is referred to as Electromagnetic Interference
(EMI) [20], [28]. Other cable types - coaxial, optic-fibre,
twisted-pair cables – by design are meant to transmit data
signal while limiting potential antenna effects, powerline
cables inherently tend to double as unintentional antennas
[15], [20]. Hence, when the powerline network is used to
transmit radio frequency (RF) signals (in the range of 10 kHz
– 30 MHz), some of these signals may leak, interfering with
signals from other devices that are connected on the PLC
network [18], [28], [29]. This results in undesirable
degradation of transmitted signals and reduction in throughput
[20], [21]; Some of the connected appliances my also inject
noise into powerline cables, contributing further to
performance degradation of the PLC network [29]. Of greater
concern is the interference with signals propagating other
communication channels like short-wave, medium-wave, FM
and DAB radio [28], [29]. RF signals leaking from several
devices on the PLC network in nearby buildings may combine
to form a composite signal significant enough to interfere
with, and distort signals transmitted over these channels [28].
Unfortunately, most of the regulations on PLC are restricted to
a single device under monitoring and testing [18]. In a
ABUAD Journal of Engineering Research and Development (AJERD)
Volume 1, Issue 1, 95-99
http://ajerd.abuad.edu.ng/ 98
widespread PLC scenario, where numerous sources are
unintentionally radiating in parallel, the wire structures that
transmit the PLC signals form an antenna array thereby
contributing to the far field. Expectedly, certain portion of the
transmitted power are radiated through ground and sky wave,
affecting highly sensitive short-wave radio services like
amateur radios, wireless security services, and military
surveillance stations [28].
In a nutshell, when PLC signal overlay frequency ranges of
wireless services, occurrence of interference becomes
unavoidable; the extent of this interference depends largely on
the transmission power and distance, as well as on the
structural layout of wires [21[, [28]. The network and cable
symmetry determine the fraction of the injected signal power
that will be emitted by radiation [28]. Symmetry is defined in
terms of the impedance between a conductor and ground. For
a two-wire power line, if the impedances between each
conductor and ground are equal, the line is regarded as being
balanced or symmetrical. For symmetrical lines, signal
propagation in the desired differential mode is possible, while
non-symmetrical lines results in an undesired common-mode
propagation [28]. For a common-mode cable pair, current
signals flow in the same direction on both conductors, where
the return portions follow the ground path. Differential mode
on the other hand, ensures that current signals flow in opposite
directions on the cable pair. Thus, a highly symmetrical line
implies a large ratio of differential to common mode current
flow, and by extension, very weak radiation, while a non-
symmetrical line results in common-mode current flow,
inducing high radiation [28].
5.2 Mitigating Radiation Effects in PLC
As discussed in the preceding section, wireless signals
transmitting on short-wave, high-frequency spectrum are
susceptible to interference and in the worst case, blockage
from PLC signals. In light of this, it is highly expedient to
minimise radiation and unwanted radiation from PLC. The
following steps are recommended to achieve this:
1) Network conditioners need to be incorporated into
PLC adapters or networks. These conditioners will help
reduce
radiation significantly by maintaining the symmetrical balance
in the power line cables.
2) Installing high-frequency filters at line ends keeps the
PLC signals on the intended propagation paths and also
prevents them from entering attached devices or conductors
with high radiation efficiency. These filters are very effective
in limiting PLC radiation to acceptable levels, but are also
costly [28].
3) Using appropriate power supply cable configuration
that exploits the “natural” symmetry inherent in certain
configurations. For example, for a three-phase four-wire
configuration, the PLC signals may be injected between two
of the phases. This will generate a significantly higher
symmetry than injecting the signal between the phase and
neutral of a single-phase two-wire configuration [28]. This
technique, however, is better suited for outdoor access
network and not for indoor networks. Indoor networks cannot
exploit the natural symmetry since PLC signals are only
injected between phase and neutral lines [28].
4) Reducing the power spectral density of PLC signals:
Since PLC signal emissions are measured within a limited
bandwidth, shrinking the power spectral density immediately
lowers the radiation levels, while maintaining the same
transmitted power. This technique will facilitate the use of
broadband multicarrier modulation schemes (like Orthogonal
Frequency-Division Multiplexing or OFDM) that will spread
the transmitted power across large ranges of frequencies.
6. CONCLUSION
In this article, the salient physical attributes that
characterize power line networks have been outlined. First, the
signal propagation mechanism was shown to be primarily by
reflection, that induces a phenomenon termed RMS delay
spread. This RMS delay spread depends on the impedance
values of connected loads. Multipath propagation and
attenuation in PLC were also presented; different factors that
induce multipath propagation were identified while the
transmission distance and frequency were shown to determine
attenuation in PLC. The complex noise nature of PLC was
also highlighted. Interference issues in PLC, especially with
signals that share the same frequency was considered, and
techniques of mitigating against it were suggested.
Research efforts on the effects of interference of PLC
signals with other signals that share the frequency spectrum,
to the best of our knowledge, remains scanty in literature.
Therefore, concerted research efforts need to be directed
towards analysis of interference effects of PLC signal on other
signals propagating other communication channels. It is also
imperative that international regulators consider embedding
PLC into existing electromagnetic compatibility (EMC) rules.
This will go well to resolve any interference concerns with
other spectrum users, and ensure faster adoption of PLC
globally as a standard communication technology.
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