IEEE 802.11ah WLAN technology basic features
This page describes IEEE WLAN 802.11ah PHY and MAC layer basics and mentions 802.11ah technology features used for IoT(Internet of Things).
IEEE 802.11ah is the WLAN standard operating under sub 1GHz frequency range (about 900MHz). It is designed to extend range of existing 11ac wifi network operating in 2.4/5GHz as well as to utilize the emerging opportunities in IoT(Internet of Things) space. It will also be used for home automation, smart grid, low power sensors and wearable consumer electronic devices. It supports various bandwidths viz. 1MHz, 2MHz, 4MHz, 8MHz and 16MHz. Hence it is considered to be 10 times downclocked version of IEEE 802.11ac standard. Like other wifi network, 802.11ah network also consists of 11ah AP(Access Point) and 11ah compliant devices such as water meter, power meter, Gas meter as well as systems compliant to IEEE 802.11ah protocol stack.
IEEE 802.11ah supports completely new PHY(Physical) layer and MAC(Medium Access Control) layer. These have been described below. Following table-1 mentions 802.11ah frequencies supported in different parts of the world. The table covers 11ah frequency allocations in US, Korea, Europe, China, Japan and singapore.
|Country||WLAN-11ah Frequency Range|
|US||902 to 928 MHz|
|Korea||917.5 to 923.5 MHz|
|Europe||863 to 868 MHz|
|China||755 to 787 MHz|
|Japan||916.5 to 927.5 MHz|
WLAN 11ah Physical Layer(PHY)
There are two physical layer varients based on channel bandwidth supported viz. one designed for 1MHz BW and the other designed for 2MHz and above. WLAN-11ah 2MHz and above Physical layer takes care of 2MHz, 4MHz, 8MHz and 16MHz channel BWs. The 1MHz version is new and designed to take care of extending the coverage for low data rate applications in wifi based networks.
Following are the features of WLAN-11ah Physical layer operating at 2MHz and above:
• Supports OFDM and MIMO
• Supports downlink Multi-user MIMO as introduced in legacy version 802.11ac
• 11ah Symbol duration is 10 times compare to 11ac
• MCS and number of data subcarriers are same as supported in 11ac from MCS0 to MCS9. MCS10 for 1MHz PHY is newly introduced.
• Supports 64 Point FFT with 52 data subcarriers.
The following table-2 defines modulation-code rates from MCS0 to MCS9 as supported in 802.11ah as well as 802.11ac.
|MCS Index||Modulation||Code Rate||NSD||NDBPS||Data Rate(Normal CP)||Data Rate(Short CP)|
|MCS-0||BPSK||1/2||52||26||0.65 Mbps||0.72 Mbps|
|MCS-1||QPSK||1/2||52||52||1.3 Mbps||1.44 Mbps|
|MCS-2||QPSK||3/4||52||78||1.95 Mbps||2.17 Mbps|
1MHz version of 11ah physical layer has following silent features:
Number of FFT points: 32
Number of data subcarriers: 24
MCS Type: MCS-10
Subcarrier spacing: 31.25KHz
WLAN IEEE802.11ah MAC Layer
In 802.11ah MAC layer design and development, some features are
enhanced compare to 802.11 MAC layer and new frame structure has been
The improvements in 11ah MAC layer are targetted for the following features.
➤Support for large number of stations
➤Enhancement in power savings
➤Efficient medium access mechanisms