Overview of IEEE 802.11af
The 802.11af amendment defines enhancements to the 802.11 WLAN physical layer (PHY) and medium access control (MAC) specifications to support operation in the TV white space (TVWS) spectrum in the VHF and UHF bands between 54 and 790 MHz. 802.11af introduces a new Television Very High Throughput (TVHT) PHY to accommodate the narrow TV channels that are available in the TVWS spectrum. The TVHT PHY is based on the Very High Throughput (VHT) PHY defined in the IEEE 802.11ac-2013 amendment, but it is modified to support basic channel units (BCU) of 6, 7, or 8 MHz. 802.11af also supports aggregation of these BCUs, including the following configurations:
- TVHT_2W: 2 contiguous BCUs (12, 14, or 16 MHz)
- TVHT_W+W: 2 non-contiguous BCUs (6+6, 7+7, or 8+8 MHz)
- TVHT_4W: 4 contiguous BCUs (24, 28, or 32 MHz)
- TVHT_2W+2W: 2 non-contiguous segments composed of 2 BCUs (12+12 MHz, 14+14 MHz, or 16+16 MHz)
The transmission in each BCU uses the VHT PHY parameters for a 40 MHz channel with a sampling clock change to fit into each of the BCU bandwidths. The number of encoders NES is always set to one for each user. Single-user MIMO and multi-user MIMO with up to 4 spatial streams are supported. For one spatial stream and one channel, the maximum data rate is 35.6 Mbps.
To avoid interference with protected services that use the TVWS spectrum, 802.11af devices determine their position using a global navigation satellite system such as GPS and query a geolocation database via the Internet to determine what channels are available for use. The mechanism for this process is determined by local regulatory requirements.
The 802.11af amendment is going through the final stages of IEEE approval and is scheduled to be approved in March 2014.
Keysight does not have specific signal generation and analysis solutions for 802.11af yet, but the solutions for 802.11ac can be configured to perform some limited testing for 802.11af devices.
In N7617B Signal Studio for WLAN, the 802.11ac options provide a parameter called “Downclocking Ratio” that can be used to adjust the sampling rate and bandwidth of the signal. For example, the subcarrier spacing for a 6 MHz 802.11af signal is (6 MHz/144) = 41.6667 kHz. Users can select the 40 MHz bandwidth for 802.11ac, and use a downclocking ratio of (normal WLAN subcarrier spacing 312.5 kHz/41.6667 kHz) = 7.5 to generate the 6 MHz bandwidth signal. This signal can be used for testing components such as power amplifiers. However, the MAC layer information will not be correct for 802.11af, so this signal may not work for receiver testing applications.
For modulation analysis, the 802.11ac demodulation options in the 89600 VSA software or N9077A WLAN measurement application can be used to demodulate an 802.11af signal by changing the subcarrier spacing to match the downclocked value. In the VSA software, the subcarrier spacing parameter can be found in the “Demodulation Properties” window’s “Advanced” tab for 802.11n/ac demodulation. In the N9077A WLAN application for the X-Series signal analyzers, users can go to the Mode Setup menu, select the Radio Std softkey, choose 802.11ac, select 802.11ac (40 MHz BW), select Subcarrier Spacing, then enter the appropriate subcarrier spacing.