Saturday, June 1, 2013

Powerline Communications

Powerline communications is one of the less known digital communications technologies. The work on the powerline communications started a long time ago and there has been some products. But the speed and quality of those products were sub-par. The real effort to use powerline for high-speed data communications started about 15 years ago. The idea behind the power line communications is this: as in almost every home there are already powerlines installed, why not use them as a communication medium?


There are many advantages in using powerlines for communications:
  • They are already installed, so the medium comes for free and there is no hassle to install new wires
  • If the place can be used for data communications, with a very high probability it already has powerlines installed
  • The gateway to the powerline is the wall outlets. There are many of them in any home, so access to the powerline is virtually possible from any place in the home
  • Unlike wireless signal that has difficulty penetrating the walls, powerline cables have already installed all over a building and pass the walls  


However there are a number of issues and challenges in using powerline for data communications:
  • The channel is noise as there is already high voltage signal on the line and also, a lot of appliances connected to the powerline. These appliances can inject wideband noise over the power lines. 
  • Also since powerline are very line cables, they can act as an antenna and radiate signals and also receive signals. So designers should be careful to comply with EMI limits, and also RFI signal can effect the operation of powerline systems.
  • Since there are a lots of cables connected to each other, in a branch type shape, the channel has a lot of frequency dependent notches. So equalization is not too easy.
  • The channel is time varying because of so many devices that are connected to the channel get turn on and off continually.

Physical layer technology

Since the channel is heavily frequency selective and time varying, OFDM base modulation is used. This is for ease of equalization as well as for ease of transmit power spectral shaping to comply with FCC and other regulations. Also since the channel is time varying periodically some know data is being transmitted. This data is used for channel estimation and also for timing recovery. Since in OFDM there is a large peak-to-average ratio (PAR) sometimes techniques to reduce the PAR is used. Also because of heavy noise on the channel, both channel coding and also interleaving is performed on block of data.

Available products

Currently there are a number of high quality powerline modems are available in the marketplace. The chips for these modems are build by companies like Broadcom and Atheros (bough by Qualcom). For example Linksys has a 200 Mbps powerline adapter, or Netgear also has a 200 Mbps adapter.

What do you think about Powerline communications? If you have any question or suggestion we would love to hear from you so leave a comment for us.

If you are interested in short technical blog posts take a look at our friends blog post Minutify.

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