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Wireless

Sharing Bandwidth: Cyclic Prefix Elimination

Tom Williams
Principal Architect, Network Technologies

Dec 20, 2017

Unfortunately, there is only so much over-the-air wireless bandwidth, and it must be shared between a lot of folks. And the situation is not getting any better.  While you can usually run another wire or fiber optic cable between two locations to get more bandwidth, if you have a wireless application you must share this scarce resource.

New applications, such as IoT (internet of things), 3-D Virtual Reality headsets, and new cell phone applications are demanding more and more bandwidth. With cable subscribers watching video on portable devices, such as tablets and phones, interference problems (such as frozen pictures and tiling) are becoming more frequent problems. More than half of customer complaints are caused by wireless problems, and the most common problem is Wi-Fi interference, frequently from a neighbor’s service.

Solutions to the Problem

  • One solution to the problem of more bandwidth is to use cellular technology and make the cell size smaller. Have you ever observed that out in the country cell towers are tall for a long reach? But in crowded cities, they are much closer to the ground, and the antennas are pointed downward. This is to reduce cell diameter in highly populated areas, allowing bandwidth reuse in non-overlapping cells. Transmitted power is also reduced for small cells to limit signal reach, thus reducing interference.  However, large numbers of cell sites are expensive to deploy and maintain - and the bandwidth itself can be expensive. In the latest FCC bandwidth auction, the 600MHz band in the United States was sold for almost $20 billion!
  • Other techniques to increase bandwidth include steerable beams and a technique called MIMO (multiple input, multiple output). This is a system for reusing the spectrum with more unique signals in the same air, by transmitting 2 or more signals on different antennas which are physically separated. At a receive site, sophisticated signal processing, using 2 or more antennas, separates the two signals.

CableLabs Innovation: Cyclic Prefix Elimination

CableLabs researchers are constantly looking for efficiency improvements, and they have found one way to improve wireless signals to make them use less bandwidth. This method, called “OFDM CP Elimination” (the full mouthful is Orthogonal Frequency Division Multiplex Cyclic Prefix Elimination!), allows the data to be sent in less time, increasing the resolution of pictures, and reducing the time for screen updates. Their method eliminates an overhead called a “Cyclic Prefix”, thereby improving efficiency by up to 25%.  A side benefit of finishing transmissions earlier is increasing battery life for handheld devices.

Interested in a deep dive into cyclic prefix elimination? Check out my video on the subject, my blog post "Getting Rid of a Big Communications Tax on OFDM Transmissions" and my technical paper in the December issue of the SCTE ISBE Journal titled "OFDM Cyclic Prefix Elimination."

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