RFIEEE 802.11ac WLAN Basics
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This tutorial covers the basics of WLAN-11ac, including:
- Main page
- Frame
- PHY layer
- MAC layer
- Data rates
- Spectral mask
- Beamforming
- Radio planning
This page provides an overview of the IEEE 802.11ac technology, its features, references, and solution providers/vendors.
IEEE WLAN 802.11-ac is a member of the 802.11 family of standards for WLANs (Wireless Local Area Networks) and is currently under development to provide high throughput below 6 GHz.
Compared to its predecessor, 802.11n, it incorporates the following additional features:
- RF Bandwidth of 80 and 160 MHz.
- MIMO spatial streams increased up to 8.
- High-density modulation schemes up to 256QAM.
- Multi-user MIMO (MU-MIMO), where multiple Stations (STAs) with multiple antennas use the same radio channel to transmit/receive separate data streams simultaneously. This technique typically uses more than one antenna.
Similar to its predecessors 802.11a and 802.11n, it also uses OFDM as a modulation technique to map symbols onto multiple subcarriers. The frequency spacing between subcarriers is 312.5 KHz, resulting in a symbol duration of approximately 3.2 microseconds, similar to the 802.11a symbol period (Ts). It supports 20 MHz and 40 MHz bandwidths and MCS 0 to MCS 7 for a single stream.
To maintain a constant spacing of 312.5 KHz between subcarriers, the number of subcarriers across the channel bandwidth is kept proportional. Hence, 64 subcarriers are used for 20 MHz, 128 for 40 MHz, 256 for 80 MHz, and 512 for 160 MHz. Accordingly, IFFT and FFT are used. In 802.11ac, the multi-user transmit path supports up to 4 users, with each user supporting up to 4 STS (space-time streams). The number of STS should not exceed 8.
PPDU Transmission
For transmission, the PSDU from the MAC layer is appended with a PLCP preamble to form a PPDU. The PPDU, or Physical Layer Convergence Procedure Protocol Data Unit, transmitted by a WLAN 11ac compliant Station (STA) follows various frame structures, such as Non-HT mode, HT mixed mode, HT greenfield mode, and VHT mode.
Figure 1 depicts the VHT (Very High Throughput) frame format.
For more details on the VHT mode frame format, read the published standard or draft standard by IEEE on the website mentioned below. In the 11ac Physical layer, an LDPC tone mapper is used in addition to the constellation mapper.
