What is 802.11af?

What is 802.11af? What are its features and when is it used?

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- everything RF

Sep 11, 2021

IEEE 802.11af is a wireless networking standard that is designed for low-power, long-range wireless local area network (WLAN) operation for smart homes and IoT devices. It operates in the TV white space (TVWS) frequency range, employing unused TV spectrum at frequencies between 54 to 790 MHz for short periods of time. 802.11af utilizes cognitive radio techniques to provide an indoor range of a few hundred metres and an outdoor range up to and beyond one kilometre. It is also called “Super Wi-Fi” or “White-Fi” due to its capability to utilize unused radio spectrum or white spaces within the television bands. 

Why use the White spaces?

In the presence of two or more high-powered wireless transmissions, there must be a particular physical distance between them for proper function i.e., to avoid interference. It is demonstrated in the case of mobile base stations where geographical locations are divided into cells to avoid inter-cell interference. However, there are many small pockets of space left inside the coverage area of the main transmitter for low-powered access points to operate.

This led to the introduction of 802.11af White-Fi concept. In this long-range network standard, which resembles traditional Wi-Fi, the access point transmits low-power signal inside the area covered by the main broadcast transmitters. These access points are designed to provide wireless LAN without hampering already existing services. 

This approach towards the effective use of radio spectrum is being increasingly implemented as the emerging wireless communications market demands networking standards that operate in the sub-1 GHz spectrum. It ensures that spectrum is being utilized to the maximum potential with frequency becoming a scarce resource as new technologies are being continuously introduced with ever-increasing bandwidth requirements.

802.11af White-Fi Technologies

As 802.11af access point are deployed inside the coverage area of the main broadcast transmitter, it is very important to ensure that the system does not create interference with existing television transmissions. Technologies adapted to achieve this are:

  • Cognitive Radio Technique: This technique makes it possible for IEEE 802.11af system to detect transmissions and move to alternative channels.
  • Geographic Sensing: This technique helps in creating a geographic database of what channels are available for use in which region.

Benefits of IEEE 802.11af White-Fi

  • The 802.11af standard delivers long-range Wi-Fi-Like experience with decent data rates (Up to 35.6 Mbps – can go higher with the use of channel bonding).
  • 802.11af system utilize unused frequencies in the TV white space (TVWS) spectrum, resulting in an effective use of available spectrum.

Data Rates of 802.11af Standard

802.11af makes it possible to achieve a maximum data rate of 35.6 Mbps per spatial stream at an 8 MHz channel bandwidth. The 6- and 7-MHz channel bandwidths can achieve data rates of up to 26.7 Mbps per spatial stream. By implementation channel bonding, the maximum data rate rises to 426.7 Mbps in 6 and 7 MHz channels. Under the same conditions, the 8-MHz mode reaches 568.9 Mbps.

Physical Layer of 802.11af

The physical (PHY) layer in 802.11af is based on orthogonal frequency division multiplexing (OFDM). 802.11af enhances the 802.11 WLAN PHY to support operation in the TV white space (TVWS) spectrum in the VHF and UHF bands. It introduces a new Television Very High Throughput (TVHT) PHY to accommodate the narrow TV channels that are available in the TVWS spectrum.

In addition to OFDM, 802.11af uses multiple operational enhancement techniques adopted by recent IEEE 802.11 standards, such as multiple-input multiple-output (MIMO), and channel bonding. Specifically, IEEE 802.11af has the provision to bond up to four of the 6 to 8 MHz-wide channels, depending on the regulatory domain. MIMO streams can be implemented (up to four) and can be either in mu-MIMO or space-time-block-code (STBC) operation.

Spectrum Regulation of IEEE 802.11af

In the US, only 6-MHz channel bandwidths are permitted in TV channels 2, 5, 6, 14-35, and 38-51 for a maximum of 48 hours of continuous use. The US also use the Internet to query a geolocation database (GDB) that typically updates the system on any regional regulations that are in effect. In the European Union, they permit TV white space operation in 8 MHz channels between 490 and 790 MHz, with the GDB granting use for up to 2 hours.