Narrowband Powerline Communication--Applications and Challenges—Part I

Transmitting data between remote devices has a long history and is becoming sine qua non in the modern world. Thanks to technological evolution and new emerging markets, powerline communication (PLC) is today a viable technology for both low speed and high speed networking. Broadband PLC represents an interesting alternative to cable, DSL or Wi-Fi as a last-mile solution for Internet distribution and home multimedia networking. For narrowband PLC, there is today a potentially vast market of command and control applications.

This article intends to give an overview of the state of the art and trends in narrowband PLC. The topics include basic modulation principles, characteristics of power lines, applicable regulations, typical applications and implementation challenges. Also, market perspectives and promising opportunities are discussed to assess the viability of the narrowband PLC in the Smart Energy world.

No new wires

The key advantage of PLC is the use of existing electrical lines as communication medium, which provides the major benefit of eliminating considerable costs of installing networking infrastructure, like dedicated cables or antennas.

Data is sent on the power lines by superposing a modulated high-frequency carrier signal on the line voltage, being high, medium or low, AC or DC. The carrier signal is then decoupled and demodulated at the receiving end to recover the information.

Once upon a time

Powerline communications have a long history; actually, it is almost as old as the power grid itself. The first research and developments date from end of the 19th century. In the beginning of the 20^th century, carrier frequency systems began to operate over high-voltage lines, then over medium and low voltage distribution systems for telemetry purposes.

The term Power Line Carriers emerged in the 1940s. In the 1970s and 1980s home-control PLC devices became commercially available, and at the same time the first standards were developed. The interest increased during the 1990s, with popular technologies like X-10, CEBus and LonWorks. The drive was to produce a reliable system which is cheap enough to be widely installed and able to compete cost effectively with wireless solutions. Over the last 20 years, research in PLC has intensified; new modulation and error control coding techniques were proposed, as well as new standards from industry alliances and professional associations. The emerging PLC technologies became promising for both consumers and energy providers.

Broad vs. Narrow

We distinguish today between two classes of PLC systems: narrowband and broadband. Usually, narrowband PLC (NB-PLC) refers to low bandwidth communication, utilizing the frequency band below 500kHz and providing data rates of tens of kpbs. Broadband PLC (BPL) utilizes a much wider frequency band, typically between 2MHz and 30MHz, and allows for data rates of hundreds of Mbps.

 

BPL technologies are used today for high-speed data transfer applications like Internet, HDTV, and audio. The optimism in this market is reflected by a number of solutions from companies like Atheros, Maxim, Sigma, all in the Home Area Networking. The NB-PLC finds many applications in the command, control and monitoring markets, like smart building automation, renewable energy generation, advanced metering, street lighting, plug-in electric vehicles, etc. Consequently, the narrowband PLC market is seeing healthy competition, with a large number of suppliers, including Echelon, Maxim, ST Microelectronics, Texas Instruments and Ariane Controls.

Modulation schemes

Different modulation schemes are used by the two categories of PLC technologies. Narrowband systems are mostly based on single-carrier modulation, while broadband technologies use multi-carrier techniques. Let’s look at the features of the main types of modulation used in narrowband PLC systems.

Single-carrier modulation

Single-carrier modulations, like Amplitude-Shift Keying (ASK), Frequency-Shift Keying (FSK) and Phase-Shift Keying (PSK), use a number of distinct signals to represent digital data. With these schemes, data is transmitted by changing, or modulating, a characteristic of a reference signal, for example the amplitude, the frequency or the phase. In the simplest form of these modulations, two discrete values of these parameters are used to represent binary data. For example, Binary Frequency-Shift Keying (BFSK) uses a pair of discrete frequencies to transmit the "1" (called the mark frequency) and the "0" (called the space frequency).

The performance of single-carrier PLC technologies has been proven in many implementations. X-10 is based on On-Off Keying modulation, the simplest form of ASK that represents data as the presence or absence of a carrier wave, synchronized with the zero-crossing of the power line voltage. Echelon’s transceivers use BPSK, ST’s ST7537 to ST7540 transceivers are based on BFSK, as well as Ariane’s PLM-1 modem. These solutions have the advantage of providing reliable communication at reduced power consumption and low cost. The low data rate, typically limited to 10kbps, makes these solutions appropriate for command and control, metering and monitoring applications.

The main drawback of single-carrier PLC systems is their sensitivity to narrowband noise and signal distortion. The robustness can be increased by error detection and correction mechanisms, which combined with message repetition can be very efficient.