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Wireless

  Getting Rid of a Big Communications Tax on OFDM Transmissions

Tom Williams
Principal Architect, Network Technologies

Dec 20, 2017

You can find the background information for this article in the post "Sharing Bandwidth: Cyclic Prefix Elimination." 

Most wireless transmissions use a modulation technology called OFDM (orthogonal frequency-division multiplexing). This method was invented by Saltzburg and Chen at Bell Labs in the 1960s, but was not widely commercialized until the 1990s when faster signal processing chips became available.  This modulation method has now been adopted into DOCSIS 3.1 technology.

In essence, data symbols are formed into blocks comprised of a large number of cosine waves of differing magnitude and phase values. Because the waves all have an integer number of cycles in the block, they do not interfere with each other. That is, they have a mathematical property of orthogonality.

This modulation technique excels when there are a lot of reflections, a.k.a. multipath, echoes, or dispersion.  For this modulation to be successful, a portion of the transmitted signal must be copied from the back and pasted onto the front of the transmitted block.  This is illustrated in Figure 1. The copied and pasted signal is called a cyclic prefix (CP), or a cyclic extension, or sometimes a guard interval.  The function of the CP is to allow any echoes to die out before the remainder of the block is analyzed.

Figure 1: A cyclic prefix (CP) is made by copying a part of the end of a block and pasting it onto the front, providing immunity from echoes

Figure 1: A cyclic prefix (CP) is made by copying a part of the end of a block and pasting it onto the front, providing immunity from echoes

While this modulation technique works well, the CP is pure overhead, a waste of expensive scarce bandwidth, and depletes battery power on wireless transmitters. A CP is a communications tax on both bandwidth and battery power and CP overhead generally ranges between 5 and 25%.

CableLabs Invention

CableLabs has invented a method to get rid of the CP using a math trick called an “overlapped circular convolution” to remove the effect of echoes without the CP. Parts of neighboring preceding and subsequent blocks are used as “pseudo-prefixes.” After equalization, the neighboring blocks pieces are discarded, leaving a de-ghosted block.  Essentially the pseudo-prefix is applied at the receiver, and the transmitter doesn’t need to send any CP. That also means that the duration of the pseudo-prefix can be arbitrarily increased at the receiver for severe echo environments.

Figure 2 is a block diagram illustrating the CableLabs method, where a pseudo-prefix is created at the receiver using neighboring blocks. For wide-bandwidth applications, the overlapped circular convolution can be replaced with an overlapped Fourier transform with frequency domain equalization. This is more computationally efficient. For a user, the implementation of this technology means the cell phone data rates go up when receiving, and battery life is longer when transmitting.

Figure 2:  A “pseudo-prefix” for the X blocks is made with part of W block and part of the Y block.  After equalization using an overlapped circular convolution, the partial blocks are discarded, leaving block X fully equalized.  Next, the process is repeated to equalize the Y block.

Figure 2:  A “pseudo-prefix” for the X blocks is made with part of W block and part of the Y block. After equalization using an overlapped circular convolution, the partial blocks are discarded, leaving block X fully equalized. Next, the process is repeated to equalize the Y block.

Watch the video below to learn more about cyclic prefix elimination:

Video en Español

A technical paper describing the technique in detail is available in the December issue of the SCTE ISBE Journal of Network Operations on page 42 titled "OFDM Cyclic Prefix Elimination." You can download a copy for free once you register on the site. Subscribe to our blog or contact Principal Architect Tom Williams for more information

 

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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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CableLabs innovates to help our member companies provide better services to their customer including higher data rates, higher reliability and lower latency. Subscribe to our blog to find out more.

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