# Bandwidth-Based Wake-Up Radio Solution through IEEE 802.11 Technology
## 2 State of the Art on IEEE 802.11-Based Wake-Up Radio Solutions  
In general, any WuR system can be depicted by the scheme in Figure 1a, where a power-limited device is required to save energy by disabling one or more power-hungry subsystems. Said device is equipped with a WuRx, a simple radio receiver circuit (i.e., no transmission capability) designed to have a very low energy consumption. When those inactive subsystems are required to resume their normal operation, a particular radio signal, called WuC, is transmitted from the WuTx to the WuRx, which will wake up those subsystems from their power-saving state. The WuRx can be as simple as a capacitor and a rectifying diode [6]. More sophisticated WuRx designs include active elements, offering additional features, such as the possibility to send WuC to a specific WuRx or group of receivers (i.e., addressable WuC) or the capacity to decode and process binary data embedded in the WuC. Still, even with the presence of active elements, a WuRx requires power in the scale of \(\upmu\)W, or even nW [7]. In the case of IEEE 802.11-based WuR, the Wi-Fi radio is considered as the primary radio used for the regular exchange of data frames, which benefits from the security features, and high transmission rates supported by the IEEE 802.11 specifications. However, although highly efficient in terms of energy per bit [8], Wi-Fi radios show high power consumption figures, even in an idle state (i.e., neither transmitting nor receiving). In an idle state, STAs must always be ready to detect known preamb  
Figure 1: (**a**) Generic Wake-up Radio (WuR) system, where a Wake-up Transmitter (WuTx) sends a Wake-up Call (WuC) to a Wake-up Receiver (WuRx); (**b**) Wi-Fi-based WuR system where a Wi-Fi radio is used as a WuTx; (**c**) Wi-Fi-based WuR where primary radios are used for a regular data frame exchange. of a frame), and this implies a power consumption in the scale of hundreds of mW. It is, therefore, a good idea to put that primary radio to sleep while a very low-power radio (the WuRx or secondary radio) remains active, ready to wake up the primary radio upon request. When a transmitter needs to deliver IEEE 802.11 frames to the sleeping STA, it will buffer those frames and send a WuC. Solutions like the one in [9] use out-of-band signaling for the WuC, which requires a secondary radio also at the transmitter side. This implies that (i) primary and secondary radios may operate at different distances (e.g., the WuC reaches the WuRx, but then, communications using the primary radio are not possible at that distance), and (ii) the need for a dedicated secondary radio to work as WuTx limits the applicability of such solutions. For those reasons, in our approach, we build the WuC from an IEEE 802.11-compliant interface, which is used both as WuTx and as the primary radio for high-rate data transfers, as shown in Figure 1b,c. In Figure 1b, the concept of WuR presented in Figure 1a is applied for the usage of the IEEE 802.11-compliant interface as WuTx and, thus, the WuC is built from legacy IEEE 802.11 transmissions. Figure 1c shows an example in which the primary interface in the receiver side is awake after the WuRx has received the IEEE 802.11-based WuC, and IEEE 802.11 primary interfaces at transmitter and receiver can start exchanging regular data frames. As discussed in [10], an IEEE 802.11-based WuTx enables the implementation of multiple applications using the concept of WuR shown in Figure 1a, given that Wi-Fi hardware is easily available at low costs. Different WuR systems have been studied in the literature, where researchers present different solutions that can use a Wi-Fi device (with modifications or completely off-the-shelf) as WuTx [10]. According to how IEEE 802.11 signals are used to generate the WuC, we classify these WuR systems into two groups [11]: (i) systems that use sequences of standard IEEE 802.11 frames to encode the WuC; and (ii) systems that encode the WuC within IEEE 802.11 Orthogonal Frequency-Division Multiplexing (OFDM) signals at the symbol level.