Power line communication impedance profiling and matching for broadband applications.
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Date
2018
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Abstract
Power line communication(PLC) is a wired communication technology that has recently re-
ceived a lot of attention due to its attractive prospects towards home and /or neighborhood network
applications as well as smart grid technologies. It allows establishing digital com- munications
without any additional wiring requirements. Effectively, one’s home and/or neighborhood wiring
contributes into a smart grid to deploy various data services. It is well known that the
power grid is one of the most pervasive infrastructure built to provide electricity to customers,
therefore, utilizing this infrastructure for digital communications will only result in an
ubiquitous telecommunications network. It is common practice to use wires to establish a physical
connection in many telecommunications channels, but most electronic devices already have a
pair of wires connected to the power lines. Therefore, these wires can be used to
simultaneously establish digital communications. Thus, power line communications can be used as an
alternative solution to more established technologies such as wireless, coaxial and optical
communications. As a promising technology, PLC has attracted a lot of research and has become an
active area of research which continues to evolve over time. Notwithstanding its advantages, PLC
has issues, namely, severe noise at low frequencies and varying characteristic impedance. This is
primarily because the power line channel was not originally designed to be used for communications,
thus, it remains a harsh channel. Other challenges arise from the fact that there are
different wiring practices around the world, unpredictable loading characteristics as well
as differential- and common-mode characteristic impedance. As a result, there is a considerable
amount of noise signal attenuation during data transmission. Loss of signal can be
addressed by increasing the power at the transmitter, noise reduction and/or reducing
channel attenuation to improve the signal-to-noise ratio. However, PLC modems are
subject to legislation that impose a limit with regards to the signal levels in the
lines. Power lines are good radiators at high frequencies which makes them behave like large
antennas with the ability to intercept other radiations in the same frequency range. The radiated
signal is proportional to the currents in the line, thus, increasing line currents will
not solve the problem but would rather lead to violation of electromagnetic
compatibility (EMC)
regulations.
In this work, an alternative solution is provided which seeks to address the issue of signal
attenuation caused by the changing input impedance of a typical power line channel. The
deleterious effects of noise are not considered since this work focuses on broadband
PLC in the 1–30 MHz frequency range. The objective of this work was to design and
build an impedance adaptive coupler to mitigate effects of channel attenuation caused by varying
impedance. In this way, the propagating signal will “see” a uniform impedance and as a
result the data output will be improved. The work was facilitated by measuring several impedance
profiles of PLC channels in the band of interest. Typically, the network topology of PLC
networks is not known and the building architectural blueprints are
not always readily available. To overcome this issue,this work was performed on power
line test-beds designed to mimic varied typical PLC network topologies. Moreover, there is an
additional benefit in that it is possible to relate the output impedance profile to
the network topology. The channel input impedance characteristics were determined in a
deterministic manner by considering a power line network as a cascade of parallel
resonant circuits and applying transmission line theory to develop the model. The
model was validated by measurements with good agreement over the frequency range was considered.
Several measurements were then used to determine the minimum, average and maximum input
impedance that a signal will experience as it traverses the channel. It was found that,
regardless of the network size (in terms of number of branches), the average input
impedance is 354 ± 1.1 % Ω in the 1-30 MHz frequency band. Due to the
unpredictable nature of the input impedance of the power line network, an impedance adaptive
bidirectional coupler for broadband power line communications was designed. The impedance
matching is achieved by using typical L-section matching networks in the 1–30 MHz band.
The matching section of the coupler has the characteristics of a lowpass filter while
the coupling section is a highpass filter, effectively forming a bandpass network. The simulated
transfer characteristics of the designed coupler performs very well for impedances starting around
150 Ω and the performance improves a great deal as the impedance increases. The coupler can still
be improved to accommodate much lower input impedances (as low as 50 Ω). However, based on the
measured results of input impedance, it was observed that the power line channel impedance is
statistically higher than 200 Ω most of the time which makes the presented design acceptable.
Description
Masters Degree. University of KwaZulu-Natal, Durban.