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IEEE 802.11ah: WLAN Technology Basics

ieee 802.11ah wlan iot phy layer mac layer

This page describes the basics of the IEEE WLAN 802.11ah PHY and MAC layers and mentions the 802.11ah technology features used for IoT (Internet of Things). IEEE 802.11ah is a WLAN standard operating under the sub-1GHz frequency range (about 900MHz).

It’s designed to extend the range of existing 11ac WiFi networks operating in the 2.4/5GHz bands, as well as to leverage the growing opportunities in the IoT (Internet of Things) space.

It will also find applications in home automation, smart grids, low-power sensors, and wearable consumer electronic devices. It supports various bandwidths: 1MHz, 2MHz, 4MHz, 8MHz, and 16MHz. Hence, it’s often considered a 10-times downclocked version of the IEEE 802.11ac standard.

Like other WiFi networks, an 802.11ah network consists of an 11ah AP (Access Point) and 11ah-compliant devices such as water meters, power meters, gas meters, and systems compliant with the IEEE 802.11ah protocol stack.

IEEE 802.11ah supports a completely new PHY (Physical) layer and MAC (Medium Access Control) layer. These are described below.

The following table-1 mentions the 802.11ah frequencies supported in different parts of the world. The table covers 11ah frequency allocations in the 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
Singapore	866-869MHz, 920-925MHz

WLAN 11ah Physical Layer (PHY)

There are two physical layer variants based on the supported channel bandwidth: one designed for 1MHz BW and the other designed for 2MHz and above.

The WLAN-11ah 2MHz and above Physical layer supports 2MHz, 4MHz, 8MHz, and 16MHz channel BWs. The 1MHz version is new and designed to extend the coverage for low data rate applications in WiFi-based networks.

Here are the features of the 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 compared to 11ac
MCS and number of data subcarriers are the 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	N_SD	N_DBPS	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
MCS-3	16-QAM	1/2	52	104	2.6 Mbps	2.89 Mbps
MCS-4	16-QAM	3/4	52	156	3.9 Mbps	4.33 Mbps
MCS-5	64-QAM	2/3	52	208	5.2 Mbps	5.78 Mbps
MCS-6	64-QAM	3/4	52	234	5.85 Mbps	6.5 Mbps
MCS-7	64-QAM	5/6	52	260	6.5 Mbps	7.22 Mbps
MCS-8	256-QAM	3/4	52	312	7.8 Mbps	8.67 Mbps
MCS-9	256-QAM	5/6	-	-	-	-

The 1MHz version of the 11ah physical layer has the 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 the 802.11ah MAC layer design and development, some features are enhanced compared to the 802.11 MAC layer, and a new frame structure has been introduced.

The improvements in the 11ah MAC layer are targeted at the following features: