Design Article
Narrowband PLC and the power line medium
Zeev Collin, vice president engineering, Semitech Semiconductor
2/13/2012 8:09 AM EST
PLC technologies
The most prevalent Smart Grid application today is connecting the consumer premises to utilities for Automatic Meter Reading (AMR), which requires a very limited amount of bandwidth. Emerging Smart Grid applications employ periodic readings and active control in an attempt to manage the load on the grid (also called Advanced Metering Infrastructure or AMI). Other rapidly emerging applications include Street Light Control (SLC), Vending Machines, Solar Panels, Electrical Vehicle Charging, Smart Appliances and in general any application that involves an electrically connected device requiring monitoring and control. The bandwidth demand of such applications is higher and typically requires between 15Kbps and 30Kbps of reliable data.
As applications develop, the communication techniques employed over the power line have evolved. Initially deployed schemes often included variations of basic single carrier Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) techniques. Such techniques provide limited bandwidth and are limited in their ability to cope with the harsh power line environment reliably.
Figure 3 illustrates FSK and Binary PSK modulations. FSK is highly affected by impulse noise that can spread over a number of bits. PSK is more resistant to noise, but is affected by phase distortion and impedance variation. Combining the two schemes concurrently provides additional level of robustness. Some devices (such as the SM6401 from Semitech Semiconductor) provide the flexibility of the combined approach.
Other techniques deployed in early systems to avoid impulse and tonal noise involve “spreading” of the communication signal over a wide band or Spread Spectrum. Spread Spectrum technology has its roots in the military. It intentionally uses broad, randomized (noise like) signals that are much wider band than the information they are carrying to make them more noise-like. Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate. These special "spreading" codes are called "pseudo random" or "pseudo noise" codes. Spread Spectrum reception is then performed by correlating the received spread spectrum signal with a replica of the expected waveform. Spread Spectrum communication techniques perform well in the presence of Gaussian noise. However, they tend to struggle with propagation delays and tonal interference that are common in power line environments.
Just like in other domains, the new wave of N-PLC implementations adopts advanced modulation approaches like Orthogonal Frequency Division Multiplexing (OFDM) to better address the increasing data bandwidth and reliability needs. Multiple emerging N-PLC standards, such as ITU G.hnem and IEEE 1901.2, are using OFDM as their underlying technique.
OFDM is a technique for transmitting large amounts of digital data over a noisy channel. OFDM gained considerable success in wireless and other noisy communication environments, as it combines many slow data rate carriers to form an overall higher data rate. The technology works by splitting the signal into multiple smaller sub-signals that are then transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to an individual data sub-carrier and modulated using PSK or QAM (Quadrature Amplitude Modulation). The primary advantages of OFDM over single carrier schemes are its ability to cope with severe channel conditions and higher data rates. If parts of the spectrum are blocked by noise, with error correction, the data can still be received without errors. Figure 4 illustrates the spectrum of a typical OFDM modem with a single and five sub-carriers. Using orthogonal sub-carriers assures that there is no crosstalk between the sub-carriers. Compared to single carrier modems, OFDM implementations take advantage of more advanced digital signal processing techniques, such as Fast Fourier Transform (FFT).
OFDM is a well-established and well researched technology that no doubt takes N-PLC to the next level of performance. However, as we have seen, the power line noise environment has unique enough characteristics that may make conventional OFDM insufficient. While OFDM is an inherently adaptive technique, it relies on successful communication over sufficient number of carriers and in particular successful transmission of the frame header and preamble (the exact number depends on the error correction techniques employed and the structure of the frame). The harsh noise conditions of the power line and the fact that many frequencies are temporarily or permanently blocked to communication still present a challenge in that regard. The emerging OFDM based IEEE 1901.2 standard recognized the issue of scattered usable frequencies and has implemented a sub-banding mechanism to filter out noisy portions of the available spectrum (Figure 5 illustrates the structure of an OFDM frame without (a) and with (b) sub-banding). This is a step in the right direction; however, it still does not resolve the vulnerability of the frame header. There is room for even more flexible schemes that can adjust to the power line noise that is time and frequency dependent by adapting modulations, frequencies and the power spread to achieve better communications performance. As an example, the SM2200 device from Semitech Semiconductor implements an OFDMA-like scheme that improves on OFDM by allowing complete independence between the communication channels and enables dynamic channel selection that adapts continuously to changes in the channel characteristics. It is being successfully deployed in China as part of one of its first Advanced Metering Infrastructure deployments.
Figure 5 - Single OFDM frame structure in time and frequency domains
Next: Conclusion
The most prevalent Smart Grid application today is connecting the consumer premises to utilities for Automatic Meter Reading (AMR), which requires a very limited amount of bandwidth. Emerging Smart Grid applications employ periodic readings and active control in an attempt to manage the load on the grid (also called Advanced Metering Infrastructure or AMI). Other rapidly emerging applications include Street Light Control (SLC), Vending Machines, Solar Panels, Electrical Vehicle Charging, Smart Appliances and in general any application that involves an electrically connected device requiring monitoring and control. The bandwidth demand of such applications is higher and typically requires between 15Kbps and 30Kbps of reliable data.
As applications develop, the communication techniques employed over the power line have evolved. Initially deployed schemes often included variations of basic single carrier Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) techniques. Such techniques provide limited bandwidth and are limited in their ability to cope with the harsh power line environment reliably.
Figure 3 illustrates FSK and Binary PSK modulations. FSK is highly affected by impulse noise that can spread over a number of bits. PSK is more resistant to noise, but is affected by phase distortion and impedance variation. Combining the two schemes concurrently provides additional level of robustness. Some devices (such as the SM6401 from Semitech Semiconductor) provide the flexibility of the combined approach.
Figure 3 - FSK and BPSK modulations
Other techniques deployed in early systems to avoid impulse and tonal noise involve “spreading” of the communication signal over a wide band or Spread Spectrum. Spread Spectrum technology has its roots in the military. It intentionally uses broad, randomized (noise like) signals that are much wider band than the information they are carrying to make them more noise-like. Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate. These special "spreading" codes are called "pseudo random" or "pseudo noise" codes. Spread Spectrum reception is then performed by correlating the received spread spectrum signal with a replica of the expected waveform. Spread Spectrum communication techniques perform well in the presence of Gaussian noise. However, they tend to struggle with propagation delays and tonal interference that are common in power line environments.
Just like in other domains, the new wave of N-PLC implementations adopts advanced modulation approaches like Orthogonal Frequency Division Multiplexing (OFDM) to better address the increasing data bandwidth and reliability needs. Multiple emerging N-PLC standards, such as ITU G.hnem and IEEE 1901.2, are using OFDM as their underlying technique.
OFDM is a technique for transmitting large amounts of digital data over a noisy channel. OFDM gained considerable success in wireless and other noisy communication environments, as it combines many slow data rate carriers to form an overall higher data rate. The technology works by splitting the signal into multiple smaller sub-signals that are then transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to an individual data sub-carrier and modulated using PSK or QAM (Quadrature Amplitude Modulation). The primary advantages of OFDM over single carrier schemes are its ability to cope with severe channel conditions and higher data rates. If parts of the spectrum are blocked by noise, with error correction, the data can still be received without errors. Figure 4 illustrates the spectrum of a typical OFDM modem with a single and five sub-carriers. Using orthogonal sub-carriers assures that there is no crosstalk between the sub-carriers. Compared to single carrier modems, OFDM implementations take advantage of more advanced digital signal processing techniques, such as Fast Fourier Transform (FFT).
Figure 4 - Examples of OFDM spectrum
OFDM is a well-established and well researched technology that no doubt takes N-PLC to the next level of performance. However, as we have seen, the power line noise environment has unique enough characteristics that may make conventional OFDM insufficient. While OFDM is an inherently adaptive technique, it relies on successful communication over sufficient number of carriers and in particular successful transmission of the frame header and preamble (the exact number depends on the error correction techniques employed and the structure of the frame). The harsh noise conditions of the power line and the fact that many frequencies are temporarily or permanently blocked to communication still present a challenge in that regard. The emerging OFDM based IEEE 1901.2 standard recognized the issue of scattered usable frequencies and has implemented a sub-banding mechanism to filter out noisy portions of the available spectrum (Figure 5 illustrates the structure of an OFDM frame without (a) and with (b) sub-banding). This is a step in the right direction; however, it still does not resolve the vulnerability of the frame header. There is room for even more flexible schemes that can adjust to the power line noise that is time and frequency dependent by adapting modulations, frequencies and the power spread to achieve better communications performance. As an example, the SM2200 device from Semitech Semiconductor implements an OFDMA-like scheme that improves on OFDM by allowing complete independence between the communication channels and enables dynamic channel selection that adapts continuously to changes in the channel characteristics. It is being successfully deployed in China as part of one of its first Advanced Metering Infrastructure deployments.
Figure 5 - Single OFDM frame structure in time and frequency domains
Next: Conclusion
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JimJarvis
2/15/2012 7:29 AM EST
The author sumarizes: "power line as communications medium presets unique challenges."
An understatement, to say the least.
The entire concept of PLC makes no sense, either from an engineering or business perspective. And I think you have to state your objective, in analyzing any proposal.
IF you want to enable remote or automatic meter reading, to save utilities labor expense-- forget about power comms--use available wideband--and either provide consumers a discount for automatic reading, or cut a deal with cable companies or cellphone carriers, for data backhaul from a smart meter.
IF you want to provide realtime command and control of the national grid, to provide smart power routing, and source switching, to match peak demand and alternative source variabiity, you need an agile wideband system which is separate from the network you want to control. And we've already got that over 95% of the US, with the internet.
From an investment standpoint, as well as a communications engineering perspective, PLC is a technology which offers nothing.
The only reason it's still around is the relative expense of wideband (way down from where it was 20 years ago, and falling) versus sunk cost in the existing utilities outside plant.
Overlay divested power generation and distribution companies on that backdrop, and it makes even less sense.
It's time for a national energy vision, and a policy for implementation. We're wasting time on PLC.
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docdivakar
2/15/2012 2:35 PM EST
I am not ready to say narrowband PLC has out-lived its usefulness. The early implementations were really for fault detection in power transmission (though there were challenges to 'jump' across transformers) and the repair. From the utility provider's point of view, this was and still is a necessary piece of the communication needed in a system, even if it ends up being a backup/redundant.
MP Divakar
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sfertig
2/15/2012 9:50 AM EST
PLC from a technology point of view isn't useless at all. It depends on the application but obviously there is not a single technology that fits all.
Even if get closer to standards on communications providing solutions for a wide range of scenarios, some special cases will always have their own rules.
Coming up with business models generating revenue with PLC isn't that easy but also a complete different story.
Research in PLC has brought up good results which diffused to other areas too. In some parts of the world PLC is used successfully. Beside that technology in general provides diversity in people's life and business. PLC is one of it.
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WKetel
2/15/2012 1:54 PM EST
Narrow band communication over power lines certainly would face a few challenges, but not the legal ones that broadband faced, caused by it's interference generation. The reason for seeking narrow band communication in some areas is because there is no internet there, so the assertion that internet is a better choice is sort of not the issue. It is possible that other methods might be better, but the questions about relative cost and reliability need to be answered in detail before that call can be made correctly. Glittering generalities have no place in the decision making process, only actual data.
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luchenbe
2/16/2012 5:48 AM EST
It certainly is true that implementing a successful PLC system is a challenge, but the same can be said for wireless: meters are often installed in unfavourable locations like basements and in most of the world outside the US, houses are built using solid materials like stone or concrete which are far from transparant for radio waves. Using PLC is very attractive for grid operators as this is the only solution offering plug&play: you connect the meter to the LV-network and at the same time it can contact its concentrator. Noise and variable impedances at PLC-frequencies is a real and tough problem however and one can say that in the future one can only expect the number of noise sources in appliances to further increase (electronic ballasts for energy efficient lighting, Photo-voltaic convertors, electric cars, ...). We at Eandis have developed a solution for these problems: our patented technology uses filters to isolate the customers' LV-loads from the outside grid and we decided to use multiple gateways on each network segment to further increase reliability of the powerline communication. It's implemented in a test area with about 3000 customers and we are able to consistently read out over 95 % of the meters every hour and over 99% daily. In fact we had more issues with the firmware and software than with the PLC. By using multiple gateways we get a bandwidth of up to 9.6 kbps per 10 connected meters average, which we consider sufficient to be able to support IP over PLC in the future.
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anne-francoise.pele
7/30/2012 5:46 PM EDT
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http://www.eetimes.com/design/smart-energy-design/4390953/Power-Line-Communication---Design-archive?Ecosystem=smart-energy-design
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