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The emerging Internet of Things (IoT) and machine-to-machine (M2M) communication markets demand wireless networking standards that operate in the sub-1 GHz spectrum, providing long-range and low-power operation. There also is a need to offload the data demands of smartphones and portable electronics from the cellular network.

The IEEE 802.11af and 802.11ah standards aim to solve these challenges by offering a Wi-Fi-like experience with reasonable data rates up to and beyond a kilometer. To do so, they occupy different parts of the 1-GHz spectrum and exhibit numerous other differences  (Fig. 1 and Table 1).

Fig. 1

What is IEEE 802.11ah?

Making use of the 900-MHz licensed exempt bands could enable long-range and low-power wireless sensor networks (WSNs) and other massive, multiple-node wireless networks based on stations and relays. With IEEE 802.11ah, the concept of a Wi-Fi-like wireless station can be realized. It promises range to 1 km at 1-, 2-, 4-, 8-, and 16-MHz channels with a minimum of 100-kbps throughput (Fig. 2). Maximum throughput for IEEE 802.11ah may reach as high as 40 Mbps.

This low-power and low-throughput mode enables short bursty data packets, which enable a very short on-time for remote or battery-powered sensors. The medium-access-control (MAC) protocols of the upcoming standard also enable smaller-frame formats, beaconless paging modes, and sensor traffic priority for lower-power applications.

It is likely that IEEE 802.11ah will use a downsampled version of the IEEE 802.11a/g specifications to service the 26 channels around 900 MHz (Fig. 3). Set to be finalized in early 2016, IEEE 802.11ah makes use of relay access points (RAPs) and network stations (STAs) in order to communicate frames from device to device. This relay function enables intelligent and low-power networking schemes that limit power use through an expansive network.

In addition, the modulation and coding scheme (MCS) level can be adjusted based on the quantity of data that needs to be transmitted. To limit energy consumption due to the overhead of relays and hopping, bi-directional hopping will be limited to two exchanges.

Fig. 2

To increase energy efficiency and power savings, the target-wake-time (TWT) function in IEEE 802.11ah permits a routine and scheduled sleep time for each access point and station. The access points are grouped within a basic service set (BSS) along with restricted channel access to a designated group. The goal of this type of partitioning is to prevent multiple transmissions from networks that are unable to see each other. Sectorization can be implemented with electronically controlled antenna beams or a diversified set of antennas.

Fig. 3

In the United States, up to 26 MHz of spectrum is available at 900 MHz, enabling up to 16 MHz of bandwidth for the standard. This increased bandwidth enables even higher-data-rate applications if necessary. Additionally, the MAC protocol is designed to account for a massive amount of nodes in an environment with the timing, paging, and sectorization protocols.

Given these factors, IEEE 802.11ah can provide an IP-based Wi-Fi-like system for M2M applications with much longer range and better material penetrating frequencies over earlier versions of Wi-Fi. IEEE 802.11ah is now included as part of an amendment to the 802.11REVmc standard with working group approval planned for January 2016.

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