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PLC Technology

Digital communications over power lines is an old idea that dates back to the early 1920s, when the first patents were filed in this area. Since then, power utility companies have used power line communications (PLCs) for a couple of decades for narrowband applications such as metering and control. In the past decade, however, there has been a renewed interest in the possibility of exploiting power line cables as a broadband communications medium. Moreover, as opposed to the past where the focus was on low rate utility applications and broadband Internet access, today’s interests spans several important applications: indoor wired local area network (LAN) for residential and business premises, in-vehicle data communications, smart grid applications (advanced metering and control, peak shaving, mains monitoring, distribution automation), and other municipal applications, such as traffic lights and lighting control, security, etc. For some of these applications, products are already available on the market, allowing bit rates in the order of several hundreds of megabits per second (Mb/s). Such products are specified by IEEE and ITU-T standards, of by Industry Alliances.
The topic of PLCs is difficult as it lies at the intersection of several fields: circuit analysis, transmission line theory, electromagnetic theory, signal processing, and communications and information theory. It is certainly true that these considerations also apply to other (and more conventional) communications channels such as the wireless or the telephone channel, however, today, communications engineers have the availability of abstracted and simplified models for the wireless and telephone channels because the initial efforts devoted to the modeling of these classical channels date back many decades. Therefore, a shift from the electromagnetic and circuit analysis to the communication domain has naturally occurred with time. This is not yet true for the power line channel, whose modeling is still tied to approaches and tools of other-than-communications disciplines, so that adequate channel models have not yet been standardized, and there is no widely accepted channel model similar to those derived for mobile radio or telephone channels. The consequence of this is that a solid communications and information theoretic approach to PLCs is still lacking, and general results on the ultimate performance achievable over the power line channel are scarce.
Most of the published papers on PLC have appeared in the IEEE Transactions on Consumer Electronics, the IEEE Transactions on Power Delivery, the IEEE Transactions on Industry Applications, and the IEEE Transactions on Industrial Electronics, whereas very few papers on PLCs have appeared in publications traditionally dealing with communications problems. This complicates bibliographic research so we believe that this Best Readings on PLC can become a valuable bibliographical resource to those starting to work on PLCs and who find themselves with the objective difficulty of dealing with a bibliography composed of many technical papers scattered across a very large number of diverse journals and conferences.
There is another way to classify Power Line Communication and that is:

  • PLC over AC lines
  • PLC over DC lines

 While most companies are currently geared towards providing AC-PLC solutions, PLC in DC lines also has applications. Two such applications are PLC over the DC-bus in distributed energy generation, and PLC in transportation (electronic controls in airplanes, automobiles and trains). This use reduces wiring complexity, weight, and ultimately cost of communications inside vehicles. However, in this article, we will be dealing mostly with narrowband PLC over AC lines.
The narrowband PLC market is seeing healthy competition, with a large number of PLC suppliers joining the fray, including:

  • Cypress Semiconductor
  • Echelon
  • ST Microelectronics
  • Yitran
  • Texas Instruments
  •  Maxim
  • Semitech Semiconductor
  •  Ariane Controls
  •  ADD Semiconductor
  •  Microchip

Companies in the broadband PLC segment include:

  • Atheros
  • Sigma
  • Marvell
  • Broadcom
  • Lantiq
  • Maxim
  • Plugtek

PLC Technology: How does it work?
PLC is like any other communication technology whereby a sender modulates the data to be sent, injects it onto medium, and the receiver de-modulates the data to read it. The major difference is that PLC does not need extra cabling, it re-uses existing wiring. Considering the pervasiveness of power lines, this means with PLC, virtually all line- powered devices can be controlled or monitored!
When discussing communication technology, it is often useful to refer to the 7-layer OSI model. Some PLC chips can implement only the Physical Layer of the OSI model, while others integrate all seven layers. One could use a Digital Signal Processor (DSP) with a pure software realization of the MAC and an external PHY circuit, or an optimized System-on-Chip (SoC) solution, which includes the complete PLC – MAC and PHY. The Cypress CY8CPLCXX series is an example of the latter, with a ready-to-use Physical and Network layer, and a user-programmable application layer. Before moving on to the applications of PLC, let’s first understand the various aspects of the Physical layer by viewing it as three segments on the basis of data rate.

Earlier, we saw that PLC is widely used in the Smart Grid and in micro-inverters. As the market gets familiar with this technology, PLC should see wider adoption in other applications like lighting (e.g. traffic light control, LED dimming), industrial (e.g. UPS communicating to a network device, irrigation control), machine-to-machine (e.g. vending machines, a hotel’s reception-to-room communication), telemetry (e.g. offshore oil rigs), transport (e.g. Electronics in cars, trains and airplanes) and indeed, applications of PLC are only limited by one’s creativity. In this article, we will find out a little more about PLC in energy generation and conservation markets.

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