# 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
The first type of system uses standard IEEE 802.11 frame transmissions and, therefore, are compatible with off-the-shelf Wi-Fi devices with only a software update. On the other hand, the WuC bit rate offered by these systems is low, limited by the achievable frame rate (note that these systems can encode, at most, one WuC symbol per IEEE 802.11 frame). Besides, WuCs comprising long sequences of frames can be broken by interfering transmissions due to the random access nature of the IEEE 802.11 MAC. In [12], for example, a simple, non-addressable WuC is encoded by the transmission of any IEEE 802.11 frame; the WuRx will wake up the device upon detecting any energy above a given threshold in a Wi-Fi channel. The solution is easy to implement but is prone to frequent false detections in the presence of other transmissions in the same band (e.g., Wi-Fi, Bluetooth, etc.). The same authors extend this idea to send a sequence of frames, combined with periods of silence to encode binary data (e.g., an address) [13]. In [14], WuC symbols are encoded using the length of IEEE 802.11 data frames, but the WuRx design consumes power in the order of the mW. Authors in [15] presented a \(\mu\)W order WuR system wherein an On-Off Keying (OOK) signal is emulated by combining silent periods, representing OFF symbols, and ON symbols generated by the transmission of successive IEEE 802.11 frames of minimum length. Due to the large WuC symbol duration, the effective bit rate of this solution is lower than 1 kbps. Reference [10] employs the WuR system of [15] and presents a proof-of-concept smart plug system implementation for a use case consisting of a green Wi-Fi application. WuR solutions of the second type (i.e., IEEE 802.11's OFDM symbol level), on the other hand, allow a higher WuC bit rate, leveraging 250 kBd of the IEEE 802.11's OFDM PHY using 4 \(\upmu\)s symbols. Authors in [16] manipulate OFDM symbols, forcing some subcarrier sets to zero, thus encoding the WuC's binary data in the frequency domain. Similarly, authors in [17] modulate the WuC using OOK by activating/deactivating the central subcarriers of an IEEE 802.11g/n OFDM signal. The system in [17] was integrated using 14-nm FinFET CMOS technology and consumed less than 100 \(\upmu\)W. A system using the symbols of an IEEE 802.11 ah 256-QAM constellation is presented in [18], the implementation of which was tested on a software-defined radio platform. Those approaches present a common drawback in their implementation complexity since they require low-level access to PHY functions (e.g., direct access to I/Q samples), which is not supported by legacy Wi-Fi interfaces, thus preventing its use from billions of already existing devices [2]. This drawback, however, is partially overcome by the solution proposed in [11]. In [11], authors reverse-engineer the full IEEE 802.11's OFDM PHY to generate specific data bit sequences at the output of the MAC layer, which will translate into the wanted OFDM symbols at the output of the PHY. In this case, a simple software application can be used to inject those bit sequences through common raw socket interfaces. This approach, however, requires prior knowledge of the random seed used by the scrambler block at the input of the PHY, something manufacturers are reluctant to share. In light of the growing interest in this topic, the IEEE P802.11 WG started to work on standardizing a WuR solution and created the TGba task group. TGba's IEEE 802.11 ba specification was released in October 2021 [19]. In fact, different receiver designs, compatible with the current specification, have already been published [20; 21]. The IEEE 802.11 ba also operates at the OFDM symbol level and uses OOK to modulate WuC data [10; 22]. Following the same principles defined in [17], IEEE 802.11 ba WuC follows a 20 MHz non-High Throughput (non-HT) preamble, used to allow legacy STAs to detect the WuC and, thus, prevent collisions.