Powerline Communications Systems

2.2.1 Historical Overview
PowerLine Communications is the usage of electrical power supply networks for communications
purposes. In this case, electrical distribution grids are additionally used as a
transmission medium for the transfer of various telecommunications services. The main
idea behind PLC is the reduction of cost and expenditure in the realization of new telecommunications
networks.
High- or middle-voltage power supply networks could be used to bridge a longer distance
to avoid building an extra communications network. Low-voltage supply networks
are available worldwide in a very large number of households and can be used for the
realization of PLC access networks to overcome the so-called telecommunications “last
mile”. Powerline communications can also be applied within buildings or houses, where
an internal electrical installation is used for the realization of in-home PLC networks.
The application of electrical supply networks in telecommunications has been known
since the beginning of the twentieth century. The first Carrier Frequency Systems (CFS)
had been operated in high-voltage electrical networks that were able to span distances over
500 km using 10-W signal transmission power [Dost97]. Such systems have been used
for internal communications of electrical utilities and realization of remote measuring
and control tasks. Also, the communications over medium- and low-voltage electrical
networks has been realized. Ripple Carrier Signaling (RCS) systems have been applied to
medium- and low-voltage networks for the realization of load management in electrical
supply systems.
Internal electrical networks have been mostly used for realization of various automation
services. Application of in-home PLC systems makes possible the management of numerous
electrical devices within a building or a private house from a central control position
without the installation of an extra communications network. Typical PLC-based building
automation systems are used for security observance, supervision of heating devices, light
control, and so on.
2.2.2 Power Supply Networks
The electrical supply systems consist of three network levels that can be used as a transmission
medium for the realization of PLC networks (Fig. 2.7):
• High-voltage (110–380 kV) networks connect the power stations with large supply
regions or big customers. They usually span very long distances, allowing power
exchange within a continent. High-voltage networks are usually realized with overhead
supply cables.
• Medium-voltage (MV) (10–30 kV) networks supply larger areas, cities and big industrial
or commercial customers. Spanned distances are significantly shorter than in the
high-voltage networks. The medium-voltage networks are realized as both overhead
and underground networks.
• Low-voltage (230/400 V, in the USA 110 V) networks supply the end users either as
individual customers or as single users of a bigger customer. Their length is usually
up to a few hundred meters. In urban areas, low-voltage networks are realized with
underground cables, whereas in rural areas they exist usually as overhead networks.
In-home electrical installations belong to the low-voltage network level. However, internal
installations are usually owned by the users. They are connected to the supply network
over a meter unit (M). On the other hand, the rest of the low-voltage network (outdoor)
belongs to the electrical supply utilities.
Low-voltage supply networks directly connect the end customers in a very large number
of households worldwide. Therefore, the application of PLC technology in low-voltage
networks seems to have a perspective regarding the number of connected customers. On
the other hand, low-voltage networks cover the last few hundreds of meters between the
customers and the transformer unit and offer an alternative solution using PLC technology
for the realization of the so-called “last mile” in the telecommunications access area.
2.2.3 Standards
The communications over the electrical power supply networks is specified in a European
standard CENELEC EN 50065, providing a frequency spectrum from 9 to 140 kHz
for powerline communications (Tab. 2.2). CENELEC norm significantly differs from
American and Japanese standards, which specify a frequency range up to 500 kHz for
the application of PLC services.
CENELEC norm makes possible data rates up to several thousand bits per second,
which are sufficient only for some metering functions (load management for an electrical
network, remote meter reading, etc.), data transmission with very low bit rates and the
realization of few numbers of transmission channels for voice connections. However, for
application in modern telecommunications networks, PLC systems have to provide much
higher data rates (beyond 2Mbps). Only in this case, PLC networks are able to compete
with other communications technologies, especially in the access area (Sec. 2.1).
For the realization of the higher data rates, PLC transmission systems have to operate
in a wider frequency spectrum (up to 30 MHz). However, there are no PLC standards that
specify the operation of PLC systems out of the frequency bands defined by the CENELEC
norm. Currently, there are several bodies that try to lead the way for standardization of
broadband PLC networks, such as the following:
• PLCforum [PLCforum] is an international organization with the aim to unify and represent
the interests of players engaged in PLC from all over the world. There are more
than 50 members in the PLCforum; manufacturer companies, electrical supply utilities,
network providers, research organizations, and so on. PLCforum is organized into four
working groups: Technology, Regulatory, Marketing and Inhouse working group.
• The HomePlug Powerline Alliance [HomePlug] is a not-for-profit corporation formed to
provide a forum for the creation of open specifications for high-speed home powerline
networking products and services. HomePlug is concentrated on in-home PLC solutions
and it works close to PLCforum as well.
Standardization activities for broadband PLC technology are also included in the work
of European Telecommunications Standards Institute (ETSI) and CENELEC.
2.2.4 Narrowband PLC
The narrowband PLC networks operate within the frequency range specified by the CENELEC
norm (Tab. 2.2). This frequency range is divided into three bands: A, to be used by
power supply utilities, and B and C, which are provided for private usage. The utilities use
narrowband PLC for the realization of the so-called energy-related services. Frequency
bands B and C are mainly used for the realization of building and home automation.
Nowadays, the narrowband PLC systems provide data rates up to a few thousand bits per
second (bps) [Dost01]. The maximum distance between two PLC modems can be up to
1 km. To overcome longer distances, it is necessary to apply a repeater technique.
The narrowband PLC systems apply both narrowband and broadband modulation
schemes. First narrowband PLC networks have been realized by the usage of Amplitude
Shift Keying (ASK) [Dost01]. The ASK is not robust against disturbances and, therefore,
is not suitable for application in PLC networks. On the other hand, Binary Phase Shift
Keying (BPSK) is a robust scheme and, therefore, is more suitable for application in PLC.
However, phase detection, which is necessary for the realization of BPSK, seems to be
complex and BPSK-based systems are not commonly used. Most recent narrowband PLC
systems apply Frequency Shift Keying (FSK), and it is expected that BPSK will be used
in future communications systems [Dost01].
Broadband modulation schemes are also used in narrowband PLC systems. The advantages
of broadband modulation, such as various variants of spread spectrum, are its
robustness against narrowband noise and the selective attenuation effect that exists in
the PLC networks [Dost01]. A further transmission scheme also used in narrowband PLC
system is Orthogonal Frequency Division Multiplexing (OFDM) [Bumi03].
A comprehensive description of various narrowband PLC systems, including their realization
and development, can be found in [Dost01]. The aim of this book is a presentation
of broadband PLC systems, and, therefore, the narrowband systems are not discussed in
detail. However, to sketch the possibilities of the narrowband PLC, we present several
examples for application of this technology in the description below.
A very important area for the application of narrowband PLC is building/home automation.
PLC-based automation systems are realized without the installation of additional
communications networks (Fig. 2.8). Thus, the high costs that are necessary for the installation
of new networks within existing buildings can be significantly decreased by the
usage of PLC technology. Automation systems realized by PLC can be applied to different
tasks to be carried out within buildings:
• Control of various devices that are connected to the internal electro installation, such
as illumination, heating, air-conditioning, elevators, and so on.
• Centralized control of various building systems, such as window technique (darkening)
and door control.
• Security tasks; observance, sensor interconnection, and so on.
PLC-based automation systems are not only used in large buildings but they are also
very often present in private households for the realization of similar automation tasks
(home automation). In this case, several authors talk about so-called smart homes.
A PLC variant of the EIB (European Installation BUS) standard is named Powernet-
EIB. PLC modems designed according to the Powernet-EIB can be easily connected to
any wall socket or integrated in any device connected to the electrical installation. This
ensures communications between all parts of an internal electrical network. Nowadays,
the PLC modems using FSK achieve data rates up to 1200 bps [Dost01].
As it is specified in CENELEC standard, power supply utilities can use band A for
the realization of so-called energy-related services. In this way, a power utility can use
PLC to realize internal communications between its control center and different devices,
ensuring remote control functions, without building extra telecommunications network or
buying network resources at a network provider (Fig. 2.9). Simultaneously, PLC can be
used for remote reading of a customer’s meter units, which additionally saves cost on the
personnel needed for manual meter reading. Finally, PLC can also be used by the utilities
for dynamic pricing (e.g. depending on the day time, total energy offer, etc.), as well
as for observation and control of energy consumption and production. In the last case,
especially, the utilities have been trying to integrate an increasing number of small power
plants; for example, small hydroelectric power stations, wind plants, and so on. However,
the small power plants are not completely reliable and their power production varies
depending on the current weather conditions. Therefore, the regions that are supplied by
the small plants should also be supplied from other sources if necessary. For this purpose,
the utilities need a permanent communication between their system entities, which can
be at least partly realized by PLC as well.
The building automation is a typical indoor application of the narrowband PLC systems,
whereas the energy-related services are mainly (not only) indoor applications. In [BumiPi03],
we find a very interesting example of an application of a PLC-based automation system in the
outdoor area. In this case, a PLC-based airfield ground–lighting automation system is used
for individual switching and monitoring of airfield lighting. The length of the airfields and
accordingly the necessary communications networks in a large airport is very long (several
kilometers). So, the narrowband PLC can be applied to save costs on building a separate
communications network. This is also an example of PLC usage for the realization of socalled
critical automation services with very high security requirements, such as the light
control of ground aircraft movement in the airports.
2.2.5 Broadband PLC
Broadband PLC systems provide significantly higher data rates (more than 2 Mbps) than
narrowband PLC systems. Where the narrowband networks can realize only a small number
of voice channels and data transmission with very low bit rates, broadband PLC
networks offer the realization of more sophisticated telecommunication services; multiple
voice connections, high-speed data transmission, transfer of video signals, and narrowband
services as well. Therefore, PLC broadband systems are also considered a capable
telecommunications technology.
The realization of broadband communications services over powerline grids offers a
great opportunity for cost-effective telecommunications networks without the laying of
new cables. However, electrical supply networks are not designed for information transfer
and there are some limiting factors in the application of broadband PLC technology.
Therefore, the distances that can be covered, as well as the data rates that can be realized
by PLC systems, are limited. A further very important aspect for application of broadband
PLC is its Electromagnetic Compatibility (EMC). For the realization of broadband PLC, a
significantly wider frequency spectrum is needed (up to 30MHz) than is provided within
CENELEC bands. On the other hand, a PLC network acts as an antenna becoming a
noise source for other communication systems working in the same frequency range (e.g.
various radio services). Because of this, broadband PLC systems have to operate with a
limited signal power, which decreases their performance (data rates, distances).
Current broadband PLC systems provide data rates beyond 2Mbps in the outdoor arena,
which includes medium- and low-voltage supply networks (Fig. 2.7), and up to 12 Mbps
in the in-home area. Some manufacturers have already developed product prototypes
providing much higher data rates (about 40 Mbps). Medium-voltage PLC technology is
usually used for the realization of point-to-point connections bridging distances up to several
hundred meters. Typical application areas of such systems is the connection of local
area networks (LAN) networks between buildings or within a campus and the connection
of antennas and base stations of cellular communication systems to their backbone
networks. Low-voltage PLC technology is used for the realization of the so-called “last
mile” of telecommunication access networks. Because of the importance of telecommunication
access, current development of broadband PLC technology is mostly directed
toward applications in access networks including the in-home area. In contrast to narrowband
PLC systems, there are no specified standards that apply to broadband PLC networks