IEEE 802.11af White-Fi Technology

IEEE 802.11af White-Fi system for Wi-Fi uses TV white spaces & cognitive radio for additional frequencies & more effective spectrum use.

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White-fi is a term that is used to describe the IEEE802.11af version of WiFi.

802.11af gains the name White-Fi from the fact that it is able to utilise spare radio spectrum or white spaces within the television bands.

To prevent interference between high powered there must be sufficient space between their coverage areas. In these areas, t is possible to utilise the spectrum because small low powered Wi-Fi access points will not radiate far and can be safely run in these areas without fear of causing interference.

This approach to the use of the radio spectrum is increasingly being investigated and regulatory authorities are keen to implement it as it provides more effective use of the spectrum.

White-Fi concept

As mentioned, the concept of 802.11af White-Fi is that the access point will transmit its low power Wi-Fi signal in an area that is between the coverage area of the main broadcast transmitters using that channel.

The broadcast system was set up so that a significant margin is placed between the coverage areas of broadcast TV transmitters to ensure that reception is not compromised even when tropospheric conditions mean that signals travel further.

This leads to very poor utilisation of the radio spectrum, and small low power transmitters can be used in the spaces between the coverage areas of different transmitters.

As the usage of different channels is different, the access point has to know which channel it can use. A variety of techniques can be used to achieve this, but whatever technique is adopted the White-Fi 802.11af access point must be able to automatically reconfigure itself.

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;

    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.

IEEE 802.11af White-Fi Features
Operating frequency range TV bands 470 to 790 MHz (Europe) and 54 to 698 MHz (USA)
Channel bandwidth 6MHz
Transmission power 20dBm
Modulation format BPSK
Antenna gain 0dBi
Signal format OFDM
OFDM carriers 144 OFDM subcarriers for 6 and 8 MHz bandwidth channels, 168 subcarriers for 7 MHz bandwidth channels.
Data subcarriers -58 to -2, 2 to 58
DC carrier Index 0
Pilot carriers Total of 6 at indices -53,-25,-11,+11,+25,+53

The use of white space technologies is on the increase. Although in some countries the introduction of digital television has reduced the amount of white space available, in others it is still huge - in India TV spectrum usage is only 7%, and this gives a huge opportunity for increased utilisation. Many other countries offer similar opportunities.

Although the many areas of this technology are still be developed and the overall landscape is changing, it nevertheless presents a very attractive option for many applications.

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