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The lowdown on power line communication

Posted: 02 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Power line communication? cables? Narrowband? Broadband?

One communication technology that allows sending of data over existing power cables is power line communication (PLC). Given that, with just power cables running to an electronic device (for example) one can both power it up and at the same time control/retrieve data from it in a half-duplex manner.

For the purpose of understanding, PLC can be broadly viewed as:
1. Narrowband PLC
2. Broadband PLC

Narrowband PLC works at lower frequencies (3-500kHz), lower data rates (up to 100s of kbps), and has longer range (up to several kilometers), which can be extended using repeaters. Broadband PLC works at higher frequencies (1.8-250MHz), high data rates (up to 100s ofMbit/s) and is used in shorter-range applications.

Recently, narrowband power line communication has been receiving widespread attention due to its applications in the Smart Grid. Another application that narrowband PLC has been used in is smart energy generation, particularly in micro-inverters for solar panels.

Broadband PLC, in contrast, has mainly found acceptance as a last-mile solution for Internet distribution and home networking. With its high data rates and no additional wiring, broadband PLC is seen as an exciting and effective technology for multimedia distribution within homes. This optimism in the market is reflected by the recent acquisitions of Intellon by Atheros, Coppergate by Sigma, DS2 by Marvell, and Gigle by Broadcom, all in the Home Area Networking (HAN) segment.

There is another way to classify power line communication and that is:
1. PLC over AC lines
2. PLC over DC lines

While most companies are currently geared toward 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 and Microchip.

Companies in the broadband PLC segment include Atheros, Sigma, Marvell, Broadcom, Lantiq, Maxim and Plugtek.

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 C 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.

Modulation schemes
A variety of modulation schemes can be used in PLC. Some of these are Orthogonal Frequency Division Multiplexing (OFDM), Binary Phase Shift Keying (BPSK), Frequency Shift Keying (FSK), Spread-FSK (S-FSK) and proprietary schemes too (for example Differential Code Shift Keying (DCSK) from Yitran). In the table below, BPSK, FSK, SFSK and OFDM are compared on the basis of two important criteria C bandwidth efficiency and complexity (cost).

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