# Bandwidth-Based Wake-Up Radio Solution through IEEE 802.11 Technology
## 1 Introduction  
More than two decades after the first IEEE 802.11 specification saw the light of day, the devices using that technology are counted in billions, and it continues to gain momentum [1]. IEEE 802.11 (or Wi-Fi) devices are nowadays present in almost all types of applications and appliances where network connectivity is required. However, many of the scenarios where we find Wi-Fi in use today are far out of the scope of the original specification, focused on the provision of broadband internet access to computers in a Wireless Local Area Network (WLAN). In this regard, the IEEE P802.11 Working Group has continued to improve this technology in an attempt to keep meeting the requirements of always-evolving user demands and to enable new scenarios and use cases. We can consider Wireless Sensor Networks (WSN) and the Internet of Things (IoT) as two of those new scenarios where Wi-Fi is trying to find its niche. Those scenarios have their particular requirements; for example, WSN should cope with the failure of nodes, support mobility, and enable device-to-device communications through a multi-hop path [2], while IoT often requires the operation over long-distance point-to-multi-point wireless links. Nevertheless, both the IoT and WSN pose similar challenges to communication because (i) they have to provide support to a potentially large number of connected devices; (ii) most of the connected devices are resource-constrained (i.e., limited processing power, storage capacity); and (iii) devices have limited access to a power source. Different amendments produced by IEEE P802.11 task groups offer partial support for those requirements. IEEE 802.11s, for example, supports reliable multi-hop transmissions at layer 2, enabling mesh-like topologies [3] to meet some of the requirements of WSNs. IEEE 802.11 ah (certified under the name of Wi-Fi HaLow), on the other hand, focuses more on the IoT case. Regarding the support of a large number of devices, IEEE 802.11 ah allows more than 8000 connected stations (STAs); note that legacy Wi-Fi has a (theoretical) upper limit of 2007 STAs per access point (AP), which is impractical due to the high collision probability. Recall that legacy IEEE 802.11 defined a Carrier Sense Multiple Access scheme with Collision Avoidance (CSMA/CA); that is, an STA willing to transmit first senses the medium, if it is free, the STA starts transmitting one or more frames within a Transmission Opportunity (TXOP) of limited duration. If the channel is sensed to be busy, and before a new attempt to gain access to the medium, the STA sets a backoff timer, choosing a random number of slots between 0 and a given Contention Window value (CW). The backoff timer is paused while the medium stays busy and resumes when the medium is free. If the transmission fails, CW is doubled. Despite these measures, the performance of legacy Wi-Fi networks degrades drastically in the presence of hundreds, even tens of simultaneous transmitting STAs. To minimize the impact of collisions in denser scenarios, IEEE 802.11 ah also introduces a new channel access mechanism called Restricted Access Window (RAW). RAW divides STAs into different groups and restricts channel access only to a group during a particular time period, reducing the number of simultaneous contenders and, thus, reducing collisions [4]. This amendment also extends coverage by allowing more robust Modulation and Coding Schemes (MCS) and using a lower frequency band below 1 GHz, while most Wi-Fi networks use either 2.4 GHz (case of IEEE 802.11b/g/n/ax) or 5 GHz (case of 11a/n/ac/ax) Industrial, Scientific and Medical (ISM) bands. IEEE 802.11 ah also extends range by allowing signals of different bandwidths (i.e., concentrating energy over a narrower band, increases Signal to Interference and Noise Ratio (SINR)) from 1 to 16 MHz, while legacy Wi-Fi uses 20 MHz of bandwidth. IEEE 802.11n, on the other hand, allows transmissions of 20 or 40 MHz, and IEEE 802.11 ac and 11 ax add the options of 80 and 160 MHz, which can be set on a per-packet basis.