## 4 Hardware Design  
This section describes the hardware circuit board and the design principle for our proposed LRWuR. To wake up the desired radio, the transmitter sends a wake-up signal in a packet that encodes the 16-bit  
Figure 4: Overview of LDWuR for Wi-Fi power savings.  
Figure 5: Comparison of the proposed MAC protocol with a conventional MAC protocol. (**a**) Conventional MAC protocol; (**b**) proposed MAC protocol.  
wake-up identification (ID) value and 24-bit additional out-of-band data for a total of 40 bits. If the wake-up ID matches the node ID, the wake-up radio sends an interrupt signal to wake up the receiving node's MCU. Then, the transmitter will send a message to the data radio. If the wake-up ID does not match the node ID, the receiving radio ignores the wake-up signal and waits for the next wake-up signal. Thus, the latencies are the same whether a false or successful wake-up signal is received.  
Figure 6a shows the block diagram, while Figure 6b shows a photo of the prototype. The sender node consists of a microcontroller unit (MCU) and an IEEE 802.15.4 data radio. The MCU encodes the data with a spreading code and uses the data radio to send an OOK-modulated data sequence to the receiver node. As with previous wake-up radio proposals, we select OOK modulation rather than more complex schemes, since it requires simple hardware and has a low implementation cost. The spreading code scheme consists of 16 chips for each pattern (symbol \(A\) or symbol \(B\)). Symbols \(A\) and \(B\) represent binary one and zero, respectively. The receiver node employs the wake-up radio to decode the signal using the same spreading code to obtain the wake-up bit sequence.  
Figure 6: LDWuR prototype. (**a**) Block diagram of LDWuR and nodes; (**b**) circuit board.  
We use simple components to implement the proposed LRWuR with proper settings on capacitors and the crystal for the wake-up radio's bandwidth. Our wake-up radio prototype is separate from the sensor node with a built-in main data receiver. It includes a Fleck3b [25] circuit board and an off-the-shelf OOK receiver QwikRadio [26] circuit board. The OOK receiver includes the image-rejection filter, amplifier, AGC (Automatic Gain Control) and OOK demodulation. The OOK receiver circuit board includes impedance matching and a band pass filter for the OOK receiver. The demodulator's bandwidth configuration can be adjusted through jumpers and capacitors on the OOK receiver circuit board. The demodulation bandwidth is set at 6.85 kHz with 22-nF and 1-\(\mu\)F capacitors. The crystal, the reference clock at 14.29983 MHz for all of the OOK receiver's internal circuits, provides the carrier frequency at 916.5 MHz. To improve the sensitivity, our wake-up radio prototype uses a Fleck3b circuit board to process the block orthogonal codes' algorithm.  
Other existing wake-up radios [8, 27, 28, 29, 30] are also applicable if they are tuned to operate at a frequency of 916.5 MHz and a low data rate below 10 kbit/s. The designer can choose a proper OOK receiver to be an alternative solution. In terms of the transmitter, the designer can select any IEEE 802.15.4-compliant data radio to operate at two frequency channels of 916.5 MHz, \(916.5+/-0.2\) MHz and a data rate of about 20 kbit/s to be the other design option.