Narrowband PLC and the power line medium
Introduction
The introduction of the Smart Grid on a mass scale requires robust and reliable communication services. In general, a combination of two communication systems is taking on the challenge: short range or mesh wireless and narrowband power line communication (N-PLC). Both systems face challenges when millions of devices send and receive information and each of them has its place in various deployment environments and regional regulatory climates.
The idea of using the AC mains for communications is not new. The concept of sending communication signals on the same pair of wires that are used for power distribution dates to patents from 1924 implementing "Carrier Transmission Over Power Circuits”. The simple carrier signaling evolved to using various modulation schemes that comprise N-PLC. Only in the last two decades, however, advancements in communications technology made N-PLC a commercially viable solution for large scale deployments.
Narrowband power line Communication (N-PLC) is generally defined as communication over power line that is typically operating in transmission frequencies of up to 500 kHz (as opposed to Broadband PLC that targets much higher bandwidth at shorter distances and operates over a much higher frequency band). Specifically, frequencies of 148.5 kHz and less have been recognized by Europe’s CENELEC standards body for use in N-PLC systems on a public utility’s power wires. Within this frequency range the resulting data rates are modest, ranging from 1Kbps to less than 100Kbps. These rates are appropriate for telemetry and control applications. In North America, Japan and China, the frequency range of up to 500 kHz are viable under local regulations for N-PLC and offers a reasonably wide communications bandwidth (up to above 300Kbps) and a broader range of applications can be considered.
In a power transmission and distribution system the conduit available to all nodes by definition is the power line. An N-PLC system that can provide reliable and cost effective data communication capabilities is an ideal and natural solution to grid communication needs. However, due to the characteristics of the power line noise environment, its changing conditions and variations in equipment and standards, communications over the power grid are difficult. To both reliably operate in this challenging environment and to successfully co-exist with previously installed equipment requires new approaches. This article focuses on the characteristics of PLC within this frequency range and presents the common communication techniques currently used within this band.
Power line channel characteristics
Assessment of commercial viability of a communication technology (or any technology for that matter) is only relevant in the context of its operating environment rather than the theory of communications in general. While the evolution of wireless communication for decades yielded significant characterization of the wireless medium resulting in huge advancements in wireless communications and communication technology in general, there was, in comparison, only small amount of characterization performed on the power line as a ubiquitous communication medium, and its specific challenges only now are becoming better understood.
The typical noise on the power line network is both time and frequency dependent. Some of the key characteristics of the power line environment, especially in the lower frequency region are:
• Impulse and tonal noise
• Significant and variable attenuation and propagation loss
• Often severe interference with time varying noise sources
• Dynamically changing channel due to load and noise variation
As one would expect, there are many sources of noise on the power line network. Some are due to the devices connecting to it and others due to the network itself, which in many cases is old and was never provisioned with communication in mind. Below are a few typical examples.
Activation of many kinds of devices can be a source of impulse noise. However, the most common impulse noise sources are light dimmers. These devices introduce high impulse noise, as they connect the lamp to the AC line part way through each half AC cycle. When the bulb is set to medium brightness impulses of several tens of volts are imposed onto the power lines at twice the AC line frequency.
Another form of noise is “tones”. The most common sources of tonal noise are switching power supplies, which are common in many electronic devices such as personal computers and electronic fluorescent ballasts. Many devices, such as televisions and computer monitors contain other high speed switching systems. The fundamental frequency of these systems is anywhere from 15 kHz to 1 MHz or more. The noise that these devices inject onto the power mains is typically rich in harmonics of the switching frequency. Figure 1 presents the spectrum of a typical DC charger injecting into the power main switching frequency harmonics at 70 kHz (main frequency), 140 kHz, 210 kHz, 280 kHz, etc.
Figure 1 – Typical DC charger spectrum
Another challenge presented by the high variety of devices connected to the power line and the variation of load is the variance in line attenuation and its frequency dependence. Loads that present low network impedance at communication frequencies compared to the characteristic impedance of the wiring (e.g. heating elements), cause the wiring inductance, rather than its capacitance, to dominate the propagation effects of the communication signal.
Figure 2 presents an example of a real life power line channel that combines many of the noise sources discussed above.
Figure 2 – Unpredictable noise signature in narrowband channel (10-500KHz)
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
Conclusion
In summary, the power line as communication medium presents unique challenges. Much technological advancement in communications and the increasing need for more intelligent resource management drive N-PLC to its next generation. Significant progress has been and is being made to make the communication adequate to meet the needs of Smart Grid applications. The next few years and the strong support of the industry will be pivotal in making N-PLC a reliable mainstream technology of the Smart Grid.
About the author:
Zeev Collin brings over 20 years of management experience in software/semiconductors and a broad understanding of communications and multimedia products. Zeev is a founder of Montage Systems, a wireless M2M company and technology incubator. Prior to founding Montage, Zeev carried several VP level product development and business management roles at Conexant Systems, where he managed diverse international organizations of over 200 professionals spanning US, Israel, Europe, India and China, and led the development of over 20 successful products generating billions in sales. In 1996 Zeev pioneered the soft modem technology and built it into products delivering $600M after acquired by Conexant.
Thanks to Matt Rhodes for his comments and to Dr. Amir Kamalizad for his art.
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