Communicating Over Power Lines Requires Advanced Technology

By Mike Holt, Semitech

The global focus on smart grid has placed power line communication (PLC) in the spotlight. Utilities often favor PLC because it is a natural approach that allows them to move data over an infrastructure they control. Advanced metering infrastructure, automated meter reading, street lighting control, smart energy home area networking, home automation and building automation require communications over existing power lines.

This natural approach, however, is not necessarily the simplest. PLC requires overlaying communications capability on a medium and network not designed for communications. Harsh noise, equipment variations and differing standards have kept PLC solutions from tolerating well the noisy power grid environment.

Although PLC has been difficult, its reliable communication increases throughput by reducing data packet retries. This is especially important for smart grid implementations because PLC allows concentrators to communicate with many meters. This throughput allows multiple daily readings, enabling better control over the grid. Typical PLC techniques like binary phase shift keying (BPSK) and orthogonal frequency division multiplexing (OFDM) are insufficient in these noisy environments. Communication reliability can be improved by increasing frequency flexibility. This can be accomplished by assigning subcarrier subsets to individual data streams, which adds multiple access channels. This approach is called orthogonal frequency division multiple access (OFDMA) and can improve communications in noisy smart grid environments.

Communications reliability also can be increased by adding frequencies to improve noise immunity. OFDM is a technique that transmits a lot of digital data over a noisy channel, such as the power grid. The technology splits the signal into multiple smaller subsignals that are then transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to an individual data subcarrier and modulated using some phase shift keying (PSK) method or quadrature amplitude modulation (QAM). In addition to its high spectral efficiency, an OFDM system reduces the crosstalk in signal transmissions and can efficiently overcome interference and frequency-selective fading caused by multipath.

OFDM addresses communications in noisy smart grid environments, however, it still cannot achieve reliable communications in harsh conditions. To further improve reliability, OFDM can be combined with a multiple access scheme, which is the OFDMA approach.

OFDMA is a multi-user version of the OFDM scheme. As previously mentioned, OFDMA assigns subsets of subcarriers to individual data streams, allowing simultaneous transmission of several individual data streams. OFDMA improves OFDM’s robustness to fading and interference. In addition, the individual data streams can be used to either communicate with multiple nodes such as power meters simultaneously or for redundancy, improving system reliability.

During simulation and field trials in which part of the carrier frequency spectrum is blocked by either noise or attenuation, OFDMA significantly enhanced data throughput and communication reliability,

Mike Holt is vice president of marketing and sales for Semitech.

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