Enhancing reliability in medium-voltage power line communications

Using OFDM and OFDMA to improve smart-grid communications
Orthogonal frequency division multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a noisy channel, such as the power grid. 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 some sort of PSK (phase shift keying) or QAM (quadrature amplitude modulation) i.e. BPSK, QPSK.

In addition to its high spectral efficiency, an OFDM system reduces the amount of crosstalk in signal transmissions, and can efficiently overcome interference and frequency-selective fading caused by multipath.

While OFDM addresses communications in noisy smart grid environments, it is still insufficient to achieve reliable communications in very harsh conditions. To further improve reliability, the OFDM method can be combined with a multiple access scheme. The approach is called orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the OFDM scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual data streams. This allows simultaneous transmission of several individual data streams.

OFDMA further improves OFDM robustness to fading and interference, but more importantly, the individual data streams can be used either to communicate with multiple nodes (power meters) simultaneously or for redundancy, thus greatly improving the reliability of the system.

Figure 3 presents the result of a communication trial using OFDMA with a redundancy scheme. This approach results in performance with signals reduced by 18 dB more than existing approaches. The result is that although the channel capacity was reduced to less than half its previous value, OFDMA was still able to reliably communicate with 100% of the packets successfully transmitted across a small (and variable) number of the sub-channels. It can be seen that OFDMA significantly enhances both throughput of data, as well as the reliability when part of the carrier frequency spectrum is blocked by either noise or attenuation.


Conclusion
Reliable communications increase throughput by reducing data packet retries. This is especially
important for smart-grid implementations, as it allows concentrators to communicate with a larger number of meters. This throughput enables multiple daily readings, enabling better control over the grid. Typical power line communications techniques like BPSK and OFDM are insufficient in these noisy environments. Adding frequency flexibility by adding multiple access channels by assigning subsets of subcarriers to individual data streams results in improved communications reliability.

About the Authors:
Dr. Song Cui is director of product development at Semitech. She holds a Ph.D in electronic engineering from University of Adelaide in Australia and has been working in the semiconductor industry for more than two decades. Prior, Song worked as chief technology officer at Semitech Innovations in charge of all company’s projects and ASIC product development.

During more than a decade working in the semiconductor industry in Australia, she maintained close collaborations with local universities in an effort to bring the latest technologies into new products.


Lizhou Dai is a lecturer at the Harbin Institute of Technology, Harbin, China.