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A look at IEEE 802.11ah: Wi-Fi below 1 GHz

A look at IEEE 802.11ah: Wi-Fi below 1 GHz

Technology News |
By eeNews Europe



The IEEE 802.11 working group is defining a new standard called 802.11ah. It operates in sub 1 GHz license-exempt bands, providing a much improved transmission range and can also be used for large scale sensor networks with low power consumption targeting for billions of IoT (Internet of Things) or M2M (Machine-to-Machine) device connections. 802.11ah is based on down-clocking of the 802.11ac standard and adds some enhancements in PHY and MAC layers such as power saving, large number of station support, better coverage, and mobile reception. This standard is still in the draft status, with a final version expected in 2016. The Wi-Fi Alliance has also begun to define certification programs based on 802.11ah.

IEEE 802.11 Wireless Local Area Network (WLAN) is mainly operated in the 2.4 GHz and 5 GHz frequency bands. However, these high-frequency bands limit the transmission range of 802.11n and 802.11ac for outdoor environments. Learn the basics of 802.11ah and about some of the test challenges.

Use Cases

In general, there are three use case categories for 802.11ah: sensor networks, backhaul networks for sensors and meter data, and Wi-Fi extended range networks. Large coverage, low power consumption, native IP support and large numbers of device support are the main advantages for 802.11ah. It’s characteristics include:

  1. 802.11ah can extend the range with 1 MHz and 2 MHz mandatory modes;
  2. There are some enhancements in 802.11ah PHY and MAC layers designed to achieve ultra-low power consumption and multi-year battery life for large scale sensor networks, optimized for small packet size and long sleep time;
  3. 802.11ah sensor is native IP support;
  4. Up to 8,191 devices associated with an AP (access point) through a hierarchical identifier structure.

Figure 1 shows an example of sensor networks in a future smart home. In this application, an AP (access point) with 802.11ah technology is placed indoors. A large number of devices such as temperature sensors, light sensors, and smart meters are deployed throughout the home, enabling home devices and appliance to be "smart."

Figure 1: 802.11ah will cover a greater distance than 801.11ac in smart home environment.


The second use case is in backhaul networks for sensors and meter data. The backhaul networks provide the connection between sensors and data collectors. IEEE 802.15.4g provides a link for the lower sensor and 802.11ah provides a wireless backhaul link to forward the aggregated data generated by the sensors to the data center. Figure 2 illustrates a wireless backhaul network in which the 802.11ah AP and gateway collect and forward the data from sensor devices to the data center.

Figure 2: 802.11ah can provides access to backhaul networks.

As an extended range Wi-Fi, 802.11ah can be used in homes, campuses, stadiums, shopping malls, and other locations. It provides a wider coverage range to reach garages, backyards, and basements than legacy WLAN technologies that operate at 2.4 GHz and 5 GHz. For example, a campus WLAN solution utilizes tri-band APs (2.4 GHz/5 GHz/ 900 MHz), so wireless access is provided everywhere on campus. Offices and classrooms are covered by 802.11ac and outdoor areas have 802.11ah APs to provide extended range coverage for space between buildings, parking lots, and sports fields. This helps cellular offloading with 802.11ah extended coverage. 802.11ah APs support wider coverage area and a larger number of students.

Channelization

Figure 3 shows the global channelization for 802.11ah. You can see that many countries have identified the spectrum for 802.11ah and have also specified the maximum bandwidth that they will support for 802.11ah. The maximum channel bandwidths obtained by the channel bonding are different due to the specific country regulations.

Figure 3: IEEE 802.11ah global channelization.


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.


Measurement Challenges

As shown in Table 1, the 802.11ah standard covers a wide range of data rates, modulation complexities, and multi-stream configurations, although the most common use cases are for single channel and low data rates. The test challenges for higher modulation schemes, higher MIMO order (up to 4×4), beamforming and multi-user MIMO are also optional to 802.11ah. For these applications that take advantage of more complex modulation schemes, test equipment with the necessary performance is required. For example, considering modulation accuracy, transmitter testing with a low enough EVM floor will be required for measuring 256QAM modulations. Knowing the limitation of the test equipment performance, including factors such as phase noise, is necessary and important. Also, the test instrument needs to support generating and analyzing much narrower bandwidth signals for 802.11ah RF test, as well as meet the standard requirements for narrow bandwidth uses.

Transmitter and receiver test items are defined in the 802.11ah draft standard (Table 2). These items are similar to the other WLAN standards, except scaled down according to the bandwidth and timing per packet. For example, receiver minimum input level sensitivity test needs to be 10 dB lower because 1/10 bandwidth is required compared with 802.11ac standards.

Table 2: IEEE 802.11ah transmitter and receiver test items.

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