IEEE WLAN 802.11af Tutorial-What is White-Fi
This WLAN IEEE 802.11af tutorial covers 802.11af basic features,802.11af physical layer and provide link to cognitive radio technique used for 11af to operate.
The frequency spectrum has been allocated for two types of services viz. licensed and unlicensed. Day by day use of wireless spectrum is becoming more and more due to advancement of wireless technologies and standards. IEEE 802.11af standard is developed for spectrum sharing. This standard helps unlicensed White space devices and other licensed services to share the spectrum. The standard also covers rules to fulfill international regulatory domains.
Unused frequency spectrum is referred as white space and devices designed to use this through spectrum sharing is referred as White Space Devices(WSDs). TV white space exists in the broadcast VHF/UHF television bands. As it operates below 1GHz this signal frequency will go through less material obstruction compare to other bands and hence it is ideal for non line of sight communication/coverage. Due to lower frequency, TVWS frequency band used in IEEE 802.11af will have less path loss compare to other 2.4 GHz and 5.7 GHz bands. This technology is also referred as White-Fi.
The technology White-Fi uses the unused frequencies in TV signal spectrum. Microsoft has pioneered the project to provide last mile connectivity in the countries which include India, Kenya, singapore, US and UK.
As we know 200-600 MHz frequency is used for TV channels to carry the data. In India, 93% of this spectrum is not utilised. White-Fi will make use of this vacant spectrum to provide the connectivity.
The unused spectrum spaces are commonly referred as White space and hence the name White-Fi. Many technology companies are looking at using this unused portion to provide the last mile internet access to the users.
Following are the silent features of IEEE 802.11af White-Fi:
• Operates in TV bands 470 to 790 MHz (Europe) and 54 to 698 MHz (USA)
• Single Channel Bandwidth(W) supported are 6,7,8 MHz, Other possible supported combinations are 2W,W+W,4W,2W+2W.
• Single spatial stream is mandatory in WSDs.
• MIMO with 4x STBC and 4x Multiuser diversity
802.11af Physical Layer
IEEE 802.11af defines physical layer for WSD devices to use and operate in TVWS frequency bands.
This physical layer replaces HT and VHT specifications developed previously for
WLAN 802.11a OFDM and 802.11ac variants.
As mentioned PHY supports channel bandwidth combinations as outlined above.
It supports 144 OFDM subcarriers for 6 and 8 MHz bandwidth channels.
It supports 168 subcarriers for 7 MHz bandwidth channels.
In as single channel bandwidth subcarriers are arranged as follows:
• Data Subcarriers: -58 to -2, 2 to 58
• DC carrier: Index 0
• Pilot carriers: Total 6 at indices -53,-25,-11,+11,+25,+53