# A Low-Power WLAN Communication Scheme for IoT WLAN Devices Using Wake-Up Receivers
## 2 Background
### IEEE 802.11 ba Standard  
To achieve low data transmission delay and power consumption, the communication method using wake-up operation is considered by the IEEE 802.11 group. The IEEE 802.11 ba, which targets low-power IoT devices, is under standardization using wake-up radio with a simple OOK signal [19].  
In this standard, each STA consists of a conventional IEEE 802.11 transceiver and a low-power operating wake-up radio, as shown in Figure 1. The wake-up radio includes the wake-up receiver and only decodes the wake-up frame and wakes up the IEEE 802.11 transceiver. The wake-up frame only includes the essential information to wake up the STA. The wake-up radio is added to the conventional IEEE 802.11 transceiver. IEEE 802.11 transceivers can be any kind of conventional IEEE 802.11 standard device.  
The wake-up frame consists of the legacy IEEE 802.11 preamble, one symbol of binary phase-shift keying (BPSK) signal, and the wake-up payload, as depicted in Figure 2. The legacy IEEE 802.11 preamble and the one symbol of BPSK signal serves to prevent other IEEE 802.11 devices from transmitting packets and causing collisions for the duration of the wake-up frame. The wake-up payload is modulated by OOK, and includes synchronization information, the receiver's address, and other information. In the standard, it is proposed that the wake-up payload uses the narrower bandwidth channel (about 4 MHz) and has the same transmission range as other IEEE 802.11 devices.  
Using the structure of the receiver STA, the transmission procedure in wake-up radio starts with the wake-up frame transmitted by the AP, as in Figure 3. When the AP transmits frames to the STA in the wake-up radio state, the AP sends the wake-up frame before the data packets are  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & **Frequency** & \multirow{2}{*}{**Modulation**} & **Sensitivity** & **Data Rate** & **Power** \\  & **(GHz)** & & **(dBm)** & **(Kbps)** & **Consumption (\(\mu\)W)** \\ \hline
[11] & 2 & OOK & \(-72\) & \(100\) & \(52\) \\
[12] & 868 & OOK & \(-71\) & \(20\)–\(200\) & \(2.4\) \\
[13] & 2.4 & OOK & \(-97/-92\) & \(10/50\) & \(99\) \\
[14] & 2.4 & PWM & \(-50\) & \(50\) & \(19\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of developed wake-up receivers. OOK = On-Off Keying. PWM = Pulse Width Modulation.  
transmitted. After the indicated wake-up delay, the STA that receives the wake-up frame may transmit the acknowledgment of the wake-up frame, which is the poll frame, unscheduled automatic power save delivery (U-APSD) trigger frame, etc. After receiving the poll frame from the STA, the AP may start the data transmission. In the case where the response frame of the wake-up frame is not needed, the AP may start transmission of the data frame after the wake-up delay.  
In the case of transmission failure, because the AP cannot detect whether the wake-up frame is transmitted successfully, it should send the wake-up frame and again wait for the wake-up delay. However, the long retransmission delay because of the wake-up delay results in poor delay performance of the data transmission, especially in the dense communication environment. In this case, the wake-up delay may be long, as in [20], compared with other transmission procedures.