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A primer on narrowband PLC and power line medium

Posted: 05 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:narrowband power line communication? PLC? N-PLC?

The widespread deployment of the Smart Grid 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 500kHz (as opposed to Broadband PLC that targets much higher bandwidth at shorter distances and operates over a much higher frequency band). Specifically, frequencies of 148.5kHz 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 500kHz 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.

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