OFDM avoids AC power-line noise

With the convergence of broadcast entertainment and broadband access, the demand to send digital voice, video and Internet data within the home will increase. The cost of installing in-home wires to support this is expensive, disruptive and time consuming.

With multiple outlets in every room, residential power lines are already the most pervasive network in the world. However, communication on the AC power line is extremely difficult because of its unfavorable communication characteristics - specifically, line noise.

Rather than fighting through the noise, some companies are working with a technology that lets signals travel around the noise. Orthogonal Frequency Division Multiplexing (OFDM), which is the foundation of the HomePlug Powerline Alliance industry specification, is one such approach.

OFDM is a discrete multitone technology in which numerous signals of different frequencies, called carriers, are combined to form a single signal for transmission. Prior to combining, each carrier is first "phase shifted," or modulated, for the purpose of representing data bits. By modulating data bits on individual signals prior to combining them, many data bits can be transmitted over a small amount of time.

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Maximizing bits while minimizing the time to transport those bits increases throughput. After each carrier is modulated, the signals are sent through the OFDM engine, which combines the carriers into one signal that represents all the bits to be transmitted. As an example, HomePlug technology modulates data bits on 84 individual carrier frequencies ranging from 4 MHz to 21 MHz.

HomePlug technology enhances OFDM's basic functionality by comparing each individual modulated carrier with the characteristics of the power line medium. It then determines which specific carriers will experience high attenuation or noise impulses that will affect their abilities to transport data successfully. HomePlug technology automatically adapts to characteristics of the medium by determining a threshold in which successful communication can occur. If the attenuation or noise is too great for successful communication at a specific frequency, HomePlug technology will not use carriers in that frequency.

HomePlug technology constantly monitors the power-line medium for sudden changes in communication characteristics. As the communication path's ability to carry data changes with respect to time, the technology dynamically adapts and turns on and off carriers to ensure successful communication continues. By constantly adapting to the power-line medium, high-speed communication can be guaranteed at every outlet pair.

In addition to the changing power-line characteristics, noise spikes, which can occur at any time and last just a microsecond, can also disrupt a datastream. Given their random nature, simple adaptation is not dynamic enough to mitigate the problem, and forward error correction is employed. By incorporating error correction methods, the technology can reconstruct any bits that have been damaged during transmission. These error correction methods surround the information bits with data "protection" bits that contain the necessary information to reconstruct the information if the data arrives damaged.

HomePlug leverages the performance and pervasiveness of the Ethernet protocol while making enhancements to address today's real-time applications, such as priority bit fields. This ensures timing-critical applications, such as voice over IP, get priority.

HomePlug technology incorporates sophisticated encryption techniques built into the hardware so all packets automatically are encrypted before transmission over the power-line medium. This results in a clean transmission from one room of your house to the next while appearing as noise to your neighbor.

Markwalter is the director of program management at Intellon. He can be reached at brian.markwalter@intellon.com.