A look at IEEE 802.11ah: Wi-Fi below 1 GHz: Page 3 of 4

August 26, 2015 //By Kevin Qian and Mingyan Wang
Kevin Qian and Mingyan Wang examine the key issues of the IEEE 802.11ah standard

The United States allocates 902 MHz to 928 MHz and the maximum bandwidth is 16 MHz; in China, 32 MHz bandwidth in total from 755 MHz to 787 MHz has been allocated for 802.11ah and the maximum bandwidth is 8 MHz; Korea has allocated 917.5 MHz to 923.5 with maximum bandwidth is 4 MHz; Japan has 11 1 MHZ channels from 916.5 MHz to 927.5 MHz; Singapore has two segments 866 MHz to 869 MHz and 920 to 925 MHzin total 8 MHz, and the maximum bandwidth is 4 MHz.

Physical layer

IEEE 802.11ah is mainly based on the 10-times down-clocked operation of IEEE 802.11ac's physical layer. 802.11ah defines the bandwidth as 2 MHz, 4 MHz, 8 MHz and 16 MHz. A 1 MHz channel is additionally defined for the purpose of further extended coverage. 1 MHz and 2 MHz support are mandatory. The physical layer can be divided into two categories, one category is the transmission mode of greater than or equal to 2 MHz bandwidth. The other category is the transmission of 1 MHz. For the first category, it can be considered as 10-times down-clocking of 802.11ac. Because the FFT size is also the same as 802.11ac, the subcarrier spacing is 31.25 kHz, which is only one-tenth of 312.25 kHz of the 802.11ac subcarrier spacing. 802.11ah orthogonal frequency-division multiplexing (OFDM) symbol duration is 10-times that of the 802.11ac, and the guard interval is also 10-times compared with 802.11ac, which can be 4 s, 8s or 16 s so that 802.11ah can meet the target of coverage range up to 1 km. For the 1 MHz transmission mode, it uses the same subcarrier spacing as 31.25 kHz, so the FFT size will be 32.

The goal of the 1 MHz channel is to further extend the transmission range. A new MCS (Modulation Coding Scheme) 10 is added for long range transmission. This MCS is same as MCS0 using code rate of one half, but it will repeat twice so the transmission range can be enlarged. The fixed pilot pattern has been defined in 802.11ac, but 802.11ah adds a new pilot pattern called travelling pilot. Travelling pilot can better mitigate the Doppler Effect to provide better support for mobile reception.

For the transmission mode, 802.11ah supports the normal S1G with short or long frame mode and the repetition of S1G_DUP_1MHz and S1G_DUP_2MHz modes. For S1G_DUP_1MHz mode, it will repeat the S1G 1MHz at all of the occupied bandwidths, so for the 4 MHz bandwidth, it will repeat S1G 1 MHz signal in all of the four 1 MHz channels. 802.11ah also supports multi-user MIMO but the MIMO stream is limited to four. It also supports beamforming of 802.11ac as an option.

The detailed peer-to-peer comparison between 802.11ac and 802.11ah is shown in Table 1.

Table 1: 802.11ah and 802.11ac physical layer comparison.

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