IEEE 802.11ah: sub GHz Wi-Fi

IEEE 802.11ah is a sub GHz Wi-Fi standard that utilises the licence exempt ISM bands below 1 GHz & provides longer range connectivity.

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IEEE 802.11ah is a Wi-Fi standard that has been designed to utilise the sub one GHz licence free ISM bands.

The radio propagation at these frequencies means that the signals are able to travel greater distances and this opens up opportunities for uses with the Internet of Things where sensors and control nodes may be located further apart.

The bands at these frequencies are much smaller than the 2,4GHz and 5GHz bands normally used for Wi-Fi and this limits the data rates that can be pushed over the links.

To accommodate the different aspects of sub GHz Wi-Fi a new physical layer and MAC has been developed to enable communications in these frequencies, albeit at a lower speed than that which is achievable for the main stream very high speed Wi-Fi variants.

IEEE 802.11ah sub GHz Wi-Fi basics

The IEEE 802.11ah standard is aimed at providing a global Wireless LAN, WLAN standard that operates within the unlicensed ISM, Industrial, Scientific, and Medical, bands that are available below 1 GHz. In this way IEEE 802.11ah will allow Wi-Fi-enabled devices to gain access for short-term transmissions in these frequency bands that are currently much less congested.

In addition to gaining access to additional spectrum, the use of 802.11ah will provide improved coverage range because of the propagating characteristics of these frequencies. This will open the applications of available to IEEE 802.11ah users to new opportunities including wide area based sensor networks, sensor backhaul systems and potential Wi-Fi off-loading.

ISM bands available

There are several ISM bands that are available for use by IEEE 802.11ah that exist below 1GHz. These are not globally available, but suitable bands do appear in most areas of the globe.

ISM Allocations applicable for IEEE 802.11ah
Country Band limits (MHz)
China 755 - 787
Europe 863 - 868
Japan 916.5 - 927.5
Korea 917.5 - 923.5
Singapore 866 - 869 &
920 - 925
USA 902 - 928

802.11ah channelization

IEEE 802.11ah sub GHz Wi-Fi defines the channels based upon the spectrum that is available in a given country.

The basic channel width is 1MHz, although it is possible to bond two adjacent channels together to form a 2 MHz channel to provide higher data throughput capability. Wider channels are available, the widest in the US being 16 MHz for the 902 - 928 MHz ISM band. Again this uses the same channel bonding method adopted for 802.11n and 11ac. Channel widths of 1, 2, 4, 8, and 16 MHz can be used.

Other countries have different spectrum allocations and accordingly the channels are on different frequencies, but the same basic methods are used, obviously with different limitations on the maximum number of channels that can be bonded together.

802.11ah PHY / radio interface

802.11ah uses orthogonal frequency division multiplexing, OFDM to provide the modulation scheme for the signal. However there are two categories into which the 802.11ah physical layer PHY can be split:

  • 1 MHz channel bandwidth: This mode of operation is aimed mainly at those applications requiring extended range. The narrower bandwidth and slower data rates enable signals at lower signal strengths to be accommodated. Typically these applications may be aimed to IoT or M2M applications where short bursts of data, normally at a low data rate may be required.

    In the 1 MHz bandwidth mode, 802.11ah uses the same subcarrier spacing as in the higher data rate mode, i.e. 31.25 kHz. The number of data subcarriers per OFDM symbol is 24. This is actually less than a half the number of data subcarriers in 2 MHz channel, because it utilises the bandwidth that would have been required for the guard-band between the two 1 MHz channels.

    As one of the main aims of the 1 MHz channel option is for extended range, a new Modulation and Coding Scheme, MCS index - MCS 10 - is included for long range transmission in addition to the 802.11ac's MCSs. This is effectively a mode of MCS 0 (see table below) but with a 2x repetition of the data to increase the resilience of the transmission.
  • Bandwidths of 2 MHz & more:   This mode uses bandwidths of 2, 4, 8, or 16 MHz. It again uses OFDM, and a design based on a tenth clocking rate of 802.11ac, i.e. symbol length of ten times that in 802.11ac. MIMO is also used within 802.11ah as well in this mode.

    In order to accommodate there is a variety of Modulation and Coding Scheme, MCS, options available. These are tabulated below:
    IEEE 802.11ah MCS for 2MHz Bandwidth Channels
      Data Rate (Mbps)
    MCS Index Modulation Code Rate Normal GI Short GI
    0 BPSK 1/2 0.65 0.72
    1 QPSK 1/2 1.3 1.44
    2 QPSK 3/4 1.95 2.17
    3 16-QAM 1/2 2.6 2.89
    4 16-QAM 3/4 3.9 4.33
    5 64-QAM 2/3 5.2 5.78
    6 64-QAM 3/4 5.85 6.5
    7 64-QAM 5/6 6.5 7.22
    8 256-QAM 3/4 7.8 8.67
    9 256-QAM 5/6 N/A N/A

    Within this table, it will be seen that MCS 9 does not have data rates entered. This is because MCS 9 is not valid for 802.11ah with a single spatial stream for a 2 MHz channel.

802.11ah MAC

The Media Access Control or MAC layer features a number of enhanced elements to provide support for large numbers of stations, power saving, and the like that.

  • Support for large number of stations:   802.11 access point allocate identifiers, called Association IDentifiers, AID, to stations that associate with the AP. For non-802.11ah systems, the maximum number of identifiers that can be allocated is 2007, but with 802.11ah being used for possibly IoT or M2M applications, this number could be exceeded. To overcome this issue, a hierarchical AID structure has been introduced. This hierarchical AID is 13 bits long, leading to a maximum number of associated stations of 8191 (2-1). This AID is split into four levels: page; block; sub-block; and station index within the sub-block.
  • Power saving:   Power saving is a growing issue, especially for IEEE 802.11ah that will be used for many IoT and M2M applications. Many of the remote nodes will need to run using batteries and these need to be able run for weeks, or even years without replacement.

    • TIM stations:   These stations remain awake all the time and continually monitor the beacon frames that are sent. It can receive data as soon as it is ready to send.
    • Non-TIM stations:   Non-Time 802.11ah stations have a doze state. When they are in this state they are unable to receive data and this is buffered ready for when they become active again.
  • Throughput enhancements:   In order to be able to make the best use of the available bandwidth, there have been a number of enhancements to ensure that the data is carried as efficiently as possible. To achieve this a number of new innovations have been introduced.
    • Compact MAC header format:   802.11ah has a proposed new MAC header format that is more compact than those used in legacy systems. This change shortens the legacy MAC header format and also moves some elements into other areas. The QoS and High Throughput, HT fields are moved into the Signal, SIG field in the PHY header and other unnecessary elements removed. Also there is no duration field in the short MAC header. These and a few other changes enable the MAC header to be shortened, freeing up valuable space and improving the efficiency of the system
    • MAC mechanism:   802.11ah defines a new medium access scheme whereby the channel access delay and ACK transmission overhead are eliminated.

Sub GHz Wi-Fi IEEE 802.11ah provides lower data rates but longer ranges than the Wi-Fi variants that operate in the bands above 1 GHz where absorption by buildings and other objects increases significantly. As such 802.11ah sub GHz Wi-Fi is ideal for use with the Internet of Things, IoT where longer distances may be required, and where the data throughput required is much lower.

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