IEEE 802.11af White-Fi Technology

- an overview of the IEEE 802.11af or White-Fi proposal for Wi-Fi using the TV White spaces using cognitive radio technology.

White-fi is a term being used to describe the use of a Wi-Fi technology within the TV unused spectrum, or TV white space. The IEEE 802.11af working group has been set up to define a standard to implement this.

With a number of administrations around the globe taking a more flexible approach to spectrum allocations, the idea of low power systems that are able to work within portions of spectrum that may need to be kept clear of high power transmitters to ensure coverage areas do not overlap is being seriously investigated.

When using systems like White-Fi technology, IEEE 802.11af that use TV white space, the overall system must not cause interference to the primary users. With processing technology developing further, this is now becoming more of a possibility.

White-Fi concept

The basic concept behind White-Fi technology, IEEE 802.11af is that broadcast television coverage has to be organised so that space is left between the coverage area of different transmitters using the same channels so that interference does not occur.

Sufficient space has to be left so that even when tropospheric propagation conditions increase the distances over which signals can be received, interference does not normally occur.

This means that there are significant areas where these channels are unused and this leads to very poor spectrum use efficiency.

Diagram showing the concept behind IEEE802.11af White-Fi and how the small White-Fi cells can use spectrum between the coverage areas of adjacent TV transmitters
IEEE802.11af White-Fi Concept

With Wi-Fi applications only requiring low power, it is possible to use this unused spectrum between the coverage areas without the fear of interference being caused within the television transmitter coverage area.

Benefits of IEEE 802.11af, White-Fi

There are many benefits for a system such as IEEE 802.11af from using TV white space. While the exact nature of the IEEE 802.11af system has not been fully defined, it is still possible to see many of the benefits that can be gained from White-Fi technology:

  • Propagation characteristics:   In view of the fact that the 802.11af white-fi system operating the TV white spaces would use frequencies below 1 GHz, this would allow for greater distances to be achieved. Current Wi-Fi systems use frequencies in the ISM bands - the lowest band is 2.4 GHz and here signals are easily absorbed.
  • Additional bandwidth:   One of the advantages of using TV white space is that additional otherwise unused frequencies can be accessed. However, it will be necessary to aggregate several TV channels to provide the bandwidths that Wi-Fi uses on 2.4 and 5.6 GHz, to achieve the required data throughput rates. It is possible that vacant channels in any given area will vary widely in frequency and this presents some challenges in managing the data sharing across the different channels, although this has been successfully achieved in technologies such as LTE.

Looking at these benefits, it is believed that the White-Fi system offers sufficient advantages to enable development to be undertaken.

IEEE 802.1af white-fi technologies

In order for white-fi 802.11af to be able to operate, it is necessary to ensure that the system does not create any undue interference with existing television transmissions. To achieve this there are a number of technologies and rules that may be utilised.

  • Cognitive radio:   One way in which a white-fi system would be able to operate is to use cognitive radio technology;

    Note on Cognitive Radio:

    With pressure on radio spectrum increasing all the time, it is necessary to utilise the available spectrum as efficently as possible. One method of helping to achieve this is utlise radio technology that is able to sense the environment and configure itself accordingly - Cognitive Radio. The technology is heavily dependent upon Software Defined Radio technology as the radio needs to be configurable according to the previaling radio environment.

    Click on the link for further information about Cognitive Radio technology


    Using this technology, it will be possible for the white-fi, IEEE 802.11af system to detect transmissions and move to alternative channels.
  • Geographic sensing:   Another method that is favoured by many is geographic sensing. Although details are not fully defined, having a geographic database and a knowledge of what channels are available there is another way of allowing the system to avoid used channels.

Salient features

The table below gives a summary of the salient features of 802.11af technology.


802.11af
Characteristic
Description
Operating frequency range 470 - 710MHz
Channel bandwidth 6MHz
Transmission power 20dBm
Modulation format BPSK
Antenna gain 0dBi

The proposal for the implementation of White-Fi is sill in its draft or development stages. However it provides an effective way of accessing more radio spectrum in an area where available bandwidth is at a premium, and utilising the resource more effectively.

By Ian Poole


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