Wi-Fi for the Internet of Things
In the future, we believe that a mix of technologies will be used for connecting all devices together. Wi-Fi is one candidate technology for capillary radio – connecting tiny devices into the Internet of Things – but some adaptations will be needed.
Let’s take the example of your own home, with a TV, printer, weather station, stereos, lights, sauna, etc. The devices that mainly remain inside your home can be connected with some short range technology such as Wi-Fi or Bluetooth. You may also want to connect to some of the devices in your home when you are away - at work or on vacation - and then you need to also connect the devices to the Internet. This in turn means you need interoperability throughout the layers. Our solution for this is capillary networks. In a previous blog post, Bluetooth Smart Mesh — why does it make sense for IoT?, we discussed how Bluethooth could be used, and Wi-Fi is another candidate technology for capillary radio.
Wi-Fi is a popular short range radio technology widely used today. Although mainly used for Internet access in the home and office, Wi-Fi is increasingly getting deployed for other use cases as well, spanning from industrial automation to the sensors and actuators at home. However, due to specific technical challenges with the underlying IEEE802.11 technology, such as high power consumption, scalability, and range, major adaptation of Wi-Fi for IoT is still required. To address these challenges, the IEEE 802.11ah amendment has been created.
A short summary of new features in extended range 802.11ah
By introducing the IEEE 802.11ah amendment, the Wi-Fi community is addressing the new technological challenges associated with IoT. The technology resembles Wi-Fi in the 2.4 and 5 GHz, and is intended to provide IP connectivity to more things and devices. Key targets introduced in IEEE 802.11ah are: longer transmission range, several new power-saving features, and an increased number of stations (STAs) associated with one access point (AP). To achieve these targets, important physical (PHY) layer modifications have been made. For example, the use of sub-1GHz communication bands and narrow bandwidths are new things in Wi-Fi and both of these features work to extend the communication coverage. Also, several medium access control (MAC) layer modifications are introduced with the aim to improve the power-saving mechanisms and enlarge the number of associated STAs.
How does ah work?
The PHY layer design is based on the IEEE 802.11ac standard just moved to the new spectrum. To accommodate narrower channel bandwidths, the IEEE 802.11ah physical layer is obtained by down-clocking 10 times the IEEE 802.11ac physical layer. This means that the sub-carrier spacing and the channel bandwidths are shrunk by a factor of 10, while the duration of the symbols is lengthened by a factor of 10, when compared to the corresponding parameters in IEEE 802.11ac. The IEEE 802.11ah PHY supports 1 MHz, 2 MHz, 4 MH, 8MHz and 16 MHz channel bandwidths. The narrower bandwidths, 1 MHz and 2 MHz, are intended primarily for use cases requiring lower data rates than conventional voice and video applications, such as sensor or metering scenarios. Moreover, a new modulation and coding scheme MCS10 has been introduced for the 1 MHz channel. It is more robust than the other modulation and coding schemes, thereby providing range extension up to 1 km.
The MAC layer bases on top of the IEEE802.11ac as well. The Traffic Indication Map (TIM) has been extended to allow for more STAs and a mechanism called page slicing is used to enable better power savings. New shorter beacons are introduced to reduce the signaling overhead from page slicing. The use of Null data packet (NDP) formats has been applied to more control messaging compared to ac, also lowering signaling overhead. Very long sleep intervals up to five years are supported with the unified scaling factor. To ensure power savings on sensors, new sensor STA type is introduced allowing faster medium access to minimize awake time for battery operated devices. Target Wake Time (TWT) may be used to reach the STA in the DL and Restricted Access Windows (RAW) may be used to reduce contention on the channel and thus save energy.
Introducing Wi-Fi to the sub 1 GHz spectrum has its challenges. There is no worldwide ISM band available but different bands need to be used depending on location and all bandwidth options are not available everywhere. Furthermore, duty cycle restrictions may limit the number of transmissions a STA is allowed to perform in specific regions. Enhancing power savings by allowing more sleep, on the other hand, introduces problems such as reachability to the sleeping STAs and synchronization when waking up.
IEEE is currently finalizing the IEEE 802.11ah standard, with the expected final approval of the standard in March 2016. Also, members of Wi-Fi Alliance have formed a task group to investigate requirements for an interoperability certification program based on this upcoming standard.
Wi-Fi is already used for many IoT use cases today, however, the use cases requiring low power, scalability and longer range have not been sufficiently addressed in the legacy versions. The upcoming IEEE802.11ah amendment brings the support for these requirements to Wi-Fi thus making it an interesting candidate to be used as a capillary radio.