Cellular IoT -- short-range radio solutions: Wi-Fi
Editor's Note: Growing requirements for increased availability of IoT devices coincide with the emergence of cellular technologies well suited for the IoT. For developers, the need has never been more acute for more detailed information about cellular technologies and their application to the IoT. Excerpted from the book, Cellular Internet of Things, this series introduces key concepts and technologies in this arena.
In an earlier series, the authors described the evolving landscape for cellular, its role in the IoT, and technologies for massive machine-type communications (mMTC) and ultra reliable low latency communications (URLLC).
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Adapted from Cellular Internet of Things, by Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs.
Chapter 9. The competitive Internet of Things technology landscape (Cont.)
By Olof Liberg, Marten Sundberg, Eric Wang, Johan Bergman, Joachim Sachs
Wi-Fi based on IEEE 802.11 is one of the most used unlicensed radio access technologies and its focus is on providing high data rate services to a range of mainly consumer electronics devices. Very long battery life has not been in focus. Also the scalability of IEEE 802.11 has mainly addressed being able to provide a high total throughput for a number of connected devices in an area; from the early Wi-Fi version IEEE 802.11b to the version IEEE 802.11ac the theoretically achievable physical layer peak data rates have increased from 11 Mbps to 6.9 Gbps . The typical operation of IEEE 802.11 is in the 2.4 and 5 GHz unlicensed spectrum.
IEEE 802.11ah is an amendment to the IEEE 802.11 standard that is focused on IoT applications. The Wi-Fi Alliance has chosen Wi-Fi HaLow as the marketing term to be used for the IEEE 802.11ah amendment. IEEE 802.11ah has some design targets that significantly differ from the high-data rate focused IEEE 802.11 variants. First, IEEE 802.11ah addresses the unlicensed spectrum below 1 GHz, which is in the range 902e928 MHz in the United States and 863e868 MHz in Europe; other regions also have unlicensed spectrum regions somewhere in the range 750e928 MHz . Differences of the sub-1-GHz spectrum versus higher spectrum bands are as follows:
- The propagation conditions sub-1-GHz facilitate longer range. For a wide-area usage and spread of IoT devices, transmission range is a key property to provide sufficient coverage to IoT services with limited amount of access points. At the same time, the IoT devices are expected to transmit only limited amounts of data. For mobile broadband Wi-Fi usage, where devices are expected to transmit a lot of data, the extended range would mean that the channel is blocked for a longer time and the channel access time per device would reduce.
- There is less unlicensed spectrum available than at higher spectrum bands. This also means that the total capacity of data that can be provided within an area is lower than at higher spectrum bands. For mobile broadband focused Wi-Fi usage, this is a disadvantage because one focus is to provide high capacity in combination with high per user data rates. For IoT-focused Wi-Fi this is less of a problem, as the total amount of data transmitted even by a very large group of IoT devices is expected to remain modest.
The IEEE 802.11ah physical layer design is derived from the IEEE 802.11ac . To address the lower spectrum bands, with less available bandwidth, and to enable robust long range transmission, the bandwidth of the IEEE 802.11ah has been scaled down by a factor of 10 compared to 802.11ac. That means that IEEE 802.11ah supports different carrier bandwidths of 2e16 MHz in comparison with the 20e160 MHz carriers of 802.11ac. In addition, an extra robust carrier configuration with 1 MHz bandwidth has been defined. Reference  describes a 24.5 dB link budget gain of IEEE 802.11ah at 900 MHz compared with 802.11n at 2.4 GHz. The gains stem from reduced path loss at low frequency (8.5 dB), reduced noise bandwidth due to narrower carriers (10 dB), further reduced noise bandwidth and repetition coding gains of the new robust 1 MHz carrier configuration (6 dB). The achievable data rates with IEEE 802.11ah are between 150 kbps and 347 Mbps. Several MAC features have been introduced to reduce power consumption for a client device and support more devices being connected to the same access point. IEEE 802.11 applies LBT in form of CSMA/CA. A larger number of connected devices lead to increased collision probabilities, which can be accentuated with the increased effect of hidden nodes with outdoor deployments . To reduce the collision probability, the restricted access window (RAW) has been introduced. It divides the contention period into up to 64 RAW slots. Devices are allocated to particular RAW slots; and the number of devices, which are contending simultaneously for channel access, can be reduced to those devices being allocated to the same RAW slot. Device battery consumption can be significantly reduced, by enabling communication in uplink and downlink direction in new bidirectional transmission opportunities, where reverse link traffic can follow closely on forward link traffic. This enables long sleep cycles for devices. In addition, with a new target wake time the device and an access point can agree on certain fixed time periods, when data that the access point receives for a device shall be forwarded to the device. This reduces the amount of activity of a device to be able to receive data. Furthermore, the maximum idle period for a device has been extended in IEEE 802.11ah so that devices can be configured with sleep periods of up to around five years, and such devices only need to connect once every maximum idle period to the access point to avoid being automatically disassociated from the access point. IEEE 802.11ah also introduces new frame formats, which reduce the overhead of control information added in messages. This is significant for IoT traffic because the data payloads are often very small (e.g., a few bytes for a meter reading) and control info can quickly introduce significant overhead. For data transmission a short MAC frame format is added, and for control messages a null data packet has been introduced.
A more extensive description and evaluation of Wi-Fi IEEE 802.11ah can be found in References [39-43].