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Overview of IEEE 802.11ah low-frequency WiFi standards

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  • Time of issue:2018-02-01 00:00
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Overview of IEEE 802.11ah low-frequency WiFi standards

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How to improve the diffraction (ie diffraction) capability of WiFi signals? Conventional WiFi devices work in the 2.4GHz and 5GHz frequency bands. The electromagnetic wave wavelengths corresponding to these two frequency bands are usually smaller than the size of the obstacle. No matter how advanced algorithms or circuit design can change the characteristics of the electromagnetic waves themselves, the WiFi signal Diffraction becomes impossible.

But the good news is that the IEEE 802.11ah standard, which is expected to be released in 2016, will solve this problem because its specified operating frequency is below 1GHz.

Recently, when evaluating the requirements of a new software radio project, I briefly learned about IEEE 802.11ah. The IEEE 802.11ah physical layer is a down-frequency version of IEEE 802.11ac. As an upgraded version of 802.11n, 802.11ac supports 80MHz, 160MHz, 80+80MHz channel bandwidth, and also supports 20/40MHz channel bandwidth of 802.11n. 802.11ah defines 2MHz, 4MHz, 8MHz, and 16MHz channel bandwidth, which is exactly 1/10 of the previous standard. In addition, 802.11ah also defines a 1MHz channel bandwidth for longer-distance wireless transmission.

Its target application scenarios include sensors and smart meter reading, sensors and smart meter reading backhaul links, and WiFi coverage extension. At present, the IEEE 802.11ah draft can be downloaded, but it will cost $302.00, as shown in the figure below.

IEEE 802.11ah channel

Similar to the 5GHz frequency band, the available frequency bands in the frequency spectrum below 1GHz vary from country to country. Therefore, 802.11ah defines different frequency ranges according to the actual conditions of different countries, including the United States, South Korea, China, Europe, Japan and Singapore. Taking the United States as an example, the frequency range is 902-928MHz, with a total of 28MHz spectrum resources, and there are 26 available channels in the case of a 1MHz channel bandwidth. In order to achieve higher throughput, 802.11ah retains the channel bonding method in 802.11n, 802.11ac, that is, multiple adjacent channels form a wider channel bandwidth. Therefore, the 2MHz channel bandwidth is composed of two adjacent 1MHz channels, 4MHz is composed of four adjacent channels, and so on. For 802.11ah, the United States can support a maximum of 16MHz channel bandwidth. The following figure shows the composition of the 802.11ah channel in the United States.

The available 802.11ah spectrum in different countries is shown in the figure below. It can be seen that the available frequency in South Korea is 917.5-923.5MHz, and the 0.5MHz frequency offset is to reduce possible interference; the available frequency range in Europe is 863-868MHz; the available frequency range in China is 755-787MHz (to be verified); The usable frequency range in Japan is 916.5-927.5MHz; the usable frequency in Singapore is composed of two discontinuous spectrums. From this picture, it is enough to see how precious the spectrum resources are.

IEEE 802.11ah transmission mode

In the 802.11ah standard, 1MHz and 2MHz are used as common channel bandwidths, and all 802.11ah devices must also support these two channel bandwidths. The physical layer design of 802.11ah can be divided into two categories, the first category is a channel bandwidth higher than (including) 2MHz, and the second category is a 1MHz channel bandwidth.

For modes higher than (including) 2MHz, such as 4MHz, 8MHz, 16MHz, the 802.11ah physical layer is precisely designed to be 1/10 of the 802.11ac physical layer clock, and has the following characteristics:

Support OFDM

Support MIMO

Support MU-MIMO

Same number of subcarriers as 802.11ac

OFDM symbol period is exactly 10 times that of 802.11ac

Support MCS0-9

The 802.11ah rate set under the 2MHz channel bandwidth is shown in the following table. It is obvious that the 802.11ah rate is exactly 1/10 of that of 802.11ac. In order to reduce power consumption, 802.11ah supports up to 4 spatial streams, while 802.11ac can support up to 8 spatial streams.

MCS Inde

Modulation

Code Rate

NSD

NDBPS

Data Rate
(Normal GI)

Data Rate
(Short GI)

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

For the 1MHz transmission mode, the 802.11ah physical layer has the following characteristics:

FFT points: 32

Number of subcarriers: 24

MCS type: MCS10

Sub-carrier spacing: 31.25KHz

IEEE 802.11ah MAC layer

In the 802.11ah MAC layer design process, some enhanced features and new frame structures are introduced, with the following features:

Support a large number of terminal access

Enhance power saving performance

Efficient media access mechanism

Enhanced throughput

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