In powerline communications (PLC), OFDM makes better use of bandwidth than FSK.
Fast, reliable, and secure communications are critical for enabling the "energy Internet," an interconnected network of interactive smart devices. Therefore, selecting the right network infrastructure and communications technologies is crucial to achieving any Smart Grid vision.
One of the biggest challenges facing implementers is meeting both current and future Smart Grid requirements, while ensuring interoperability and open-endedness among grid elements. When evaluating communications platforms, it is important to look for a solution that:
- Provides a cost-effective system architecture
- Includes security mechanisms to protect grid assets
- Is standards-based to ensure interoperability and open-endedness
- Can support new distribution switches, sensors, and home-area network (HAN) applications
- Coexists with older technologies such as spaced frequency shift keying (SFSK)
- Is field proven with strong industry support
- Provides a solution with a path for migration to meet the changing needs of future requirements
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique that utilizes the frequency band efficiently, allowing the use of advanced channel-coding techniques. This powerline communications (PLC) technology enables highly robust communication in the presence of narrowband interference, impulsive noise, and frequency-selective attenuation.
OFDM also is superior to single-carrier techniques for data communications (see the figure). In this example, eight tones between 10 and 95 kHz are used, providing a usable channel bandwidth of 85 kHz. In contrast, a single-carrier solution uses only two tones to transmit data in that bandwidth.
In both cases, four data bits and four error correction bits are sent. In the top half of the figure, OFDM transmits all eight bits with a single symbol. In the bottom half, FSK needs four symbols to transmit the same payload. Because OFDM uses the spectrum more efficiently, it opens the channel for more data and, thereby, a higher data rate.
The higher number of tones available from OFDM enables advanced encoding techniques, which cannot be achieved with FSK approaches due to their bandwidth inefficiency. As an example, a typical FSK/SFSK modem in the CENELEC A band (10 to 95 kHz) can only transmit 2 kbits/s at 12-dB signal-to-noise ratio (SNR) with a bit-error rate (BER) of 10-4.
This means that 1 bit in 10,000 transmitted is lost. In contrast, an OFDM system can transmit up to 32 kbits/s at around 4-dB SNR in the same band. Therefore, OFDM modulation with error-correction techniques enables an 8-dB improvement in performance at 16x faster data rates. Data rates up to 300 kbits/s can be achieved in the Federal Communications Commission (FCC, 10 to 490 kHz) and Association of Radio Industries (ARIB, 10 to 450 kHz) bands due to their wider bandwidths.
G3-PLC Emerges
An open standard using OFDM-based PLC is available to all Smart Grid architects. In partnership with Électricité Réseau Distribution France (ERDF), Maxim Integrated Products developed the G3-PLC specification to promote interoperability and open-endedness among Smart Grid implementations.
G3-PLC is configurable for higher data rates and can support higher frequency bands to meet global requirements. It includes an OFDM-based physical-layer (PHY) to ensure robust operation in severe environments; an IEEE 802.15.4-based media access controller (MAC) layer that's well suited to low data rates; and a 6LoWPAN adaptation layer to allow transmission of IPv6 packets over bandwidth-limited powerline channels. The specification also includes:
- MAC-level security using an AES-128 cryptographic engine
- A mesh routing protocol to determine the best path between remote network nodes
- Adaptive tone mapping for optimal bandwidth utilization
- A robust mode of operation and two layers of forward error correction (FEC) to improve communication under noisy channel conditions
- Channel estimation to select the optimal modulation scheme between neighboring nodes
- Coexistence with existing SFSK systems
The robustness of G3-PLC technology allows long-distance (over 10 km) data transmission over medium-voltage lines, enabling the use of fewer repeaters. Moreover, G3-PLC achieves reliable communication across medium- to low-voltage transformers, reducing the number of data concentrators required by a factor of 3:1. This capability can substantially cut costs in rural areas by enabling fewer data concentrators to support isolated meters.
Additionally, G3-PLC delivers a faster application data rate than single-carrier schemes: 4 s to read 3300 bytes compared to 28 s for SFSK. This accelerated load-profile reading offers significant advantages to applications that demand real-time visibility into grid conditions and energy usage.
PLC technology provides high performance and cost efficiency for medium- and low-voltage power grids. By communicating on the very lines that it measures and controls, it minimizes infrastructure, installation, and maintenance costs for utilities.
Traditionally, it has been difficult to achieve fast, reliable communications in the severe conditions that characterize powerlines. The 10- to 500-kHz frequency region allocated worldwide (e.g., FCC, ARIB, CENELEC A) for powerline signaling is particularly susceptible to interference, background noise, impulsive noise, and group delays.
An additional difficulty for PLC has been in crossing transformers. This capability is needed in certain topologies, especially in the U.S., to minimize the amount of concentrators and repeaters required to move data.
Maxim's PLC technology employs innovative communications and networking techniques to solve the problems inherent to PLC. With the approval of G3-PLC as an international standard and with available solutions from multiple sources, utility companies can now plan their deployments utilizing G3-PLC to achieve cost-effective Smart Grid systems with confidence.