Taking Wi-Fi to Sub 1 GHz
December 22, 2014
The Internet of Things (IoT) is the catalyst for a number of new standards that will reshape wireless connectivity as we know it. Examples: Bluetooth LE, LTE-MTC, Zigbee, and LPWA standards. Wi-Fi (802.11) is also being reshaped to accommodate IoT applications. These applications have different requirements which make connectivity techniques for IoT fragmented. Competition is not limited to among these standards, but also extends to include proprietary protocols which makes developments in this space particularly interesting to watch in the next few years.
One emerging standard is IEEE 802.11ah which extends Wi-Fi to the sub 1 GHz bands. The limited availability of license-exempt spectrum in sub 1 GHz forces different considerations than extension of Wi-Fi to the 5 GHz band as happened with 802.11ac. For example, in the US only 26 MHz is available in the 900 MHz band (902 – 928 MHz). Other markets: Europe, Japan, China, South Korea and Singapore with similar of lower channel availability. Low channel bandwidth results in limited bandwidth but on the flip side, the low spectrum and channel size improves range: range is critical for IoT applications and techniques to extend range and support a large number of devices is at the heart of 802.11ah, which in brief has the following features:
Channelization: 1, 2, 4, 8, and 16 MHz. 1 and 2 MHz channels are common channel bandwidth required for all equipment to receive.
The physical layer: for bandwidth of 2 MHz and greater (Category 1), the PHY is a clocked-down version of 802.11ac supporting OFDM and MIMO including MU-MIMO on the downlink. Thus, the number of subcarriers in the 2, 4, 8 and 16 MHz is the same as in 20, 40, 80 and 160 MHz in 802.11ac, starting with 64 subcarriers for 2 MHz. The symbol duration is ten times that of 802.11ac. The modulation schemes are similar with support for 256 QAM 3/4. There are two guard interval options: 4 and 8 usec for a total of 36 and 40 usec symbol length, respectively. 802.11ah supports p to 4 spatial streams for MIMO. The data rates are also one tenth of those of 802.11ac.
|MCS Index||Modulation||Code Rate||Data Rate (Mbps)|
|Normal GI||Short GI|
The 1 MHz channel bandwidth is optimized for range. It supports 32 subcarriers. A new modulation index (MCS 10) is introduced with which is MCS 0 with 2x repetition.
The Medium Access Control Layer: There are enhancements to the number of supported devices, lower power consumption, channel access, and throughput improvements. 802.11ah access point assigns a hierarchical unique Association IDentifier (AID) to stations consisting of 13 bits. A new mechanism is introduced for power savings called ‘TIM and page segmentation’ as well as two classes of power savings stations (TIM and non-TIM) where it is possible to stay in doze mode over longer period of time. For each power saving class, channel access is improved. Channel access attempts for non-TIM stations are temporally spread out to support large number of devices. For TIM stations, 802.11ah defines a new type of contention-free access mechanisms. The MAC design includes compact frame format with reduced protocol overhead to increase throughput. Additionally, changes to feedback mechanism (ACK transmissions) are implemented improve throughput performance.
With these features, the range of 802.11ah in 1 MHz channel reaches 1 km for 200 mW transmitter in 900 MHz band, or about 5x the distance a 802.11n transmitter reaches in 2.4 GHz.
IEEE 802.11ah is still in process of standardization with expected ratification in 2015 and device availability in 2016.