# Bandwidth-Based Wake-Up Radio Solution through IEEE 802.11 Technology
## 3 Bandwidth-Based Wake-Up Radio Solution through IEEE 802.11
### Wake-Up Radio Receiver  
As stated, the proposed Wake-up Radio system works with a simple WuRx design, provided that it is capable of distinguishing transmissions using different bandwidths in Wi-Fi frequencies (i.e., 2.4, 5 GHz bands). The WuRx presented in this paper is composed of three components: High-Pass Filter (HPF), envelope detector, and comparator (Figure 5). The HPF is the key component of this WuRx, as it is responsible for filtering the incoming signal in order to distinguish among the different signal bandwidths. OFDM signals with different bandwidth values contain a different number of subcarriers. In the case of IEEE 802.11n and 11 ac, for example, 20 MHz signals are built of 64 subcarriers, 40 MHz signals of 128 subcarriers, 80 MHz signals of 256 subcarriers, and 160 MHz signals of 512 subcarriers, containing the signals with larger bandwidth also the subcarriers of the signals with shorter bandwidth (i.e., the 512 subcarriers of the 160 MHz signal include the 64 subcarriers of the 20 MHz, the 128 subcarriers of the 80 MHz signal, and the 256 subcarriers of the 80 MHz signal). Given the aforementioned characteristics of OFDM signals, the usage of an HPF with the appropriate cut-off frequency will allow the differentiation among signals with different bandwidths, i.e., a different number of subcarriers. In this way, for instance, a 20 MHz signal can be filtered using a cut-off frequency above the frequency components of its 64 subcarriers, whereas signals with larger bandwidth values will still maintain high signal levels at this filter output. After filtering, the signal is handled by the envelope detector block in order to convert the signal to a DC voltage. Finally, the output of the envelope detector is introduced in the comparator block and compared with a pre-defined threshold value. Based on this comparison, the signal bandwidth of the incoming signal at the receiver input is decided, thus determining the symbol and corresponding bit values composing the WuC. For instance, after filtering using a cut-off frequency above the frequency components of 64 subcarriers, if the signal at the input of the comparator block is below the threshold, the decision is that a 20 MHz signal is received; otherwise, it is decided that a signal above 20 MHz is received. Note that an initial pre-defined preamble will be included at the beginning of the WuC, thus indicating the start of the WuC frame, followed by the sequence of coded bits (i.e., address or other binary data). The aforementioned process and WuRx illustrated in Figure 5 are feasible for distinguishing between two signal bandwidths, and thus, for the scenario employing two different symbols and one bit per symbol. Besides the elements shown in Figure 5, additional hardware will be necessary for the identification of the address or of the binary data received (e.g., data slicer and address correlator); however, this part is out of the scope of the present research, and we leave it for future work. For the scenario with four different symbols and two bits per symbol, the WuRx of Figure 5 is insufficient to distinguish among signals with four different bandwidth values. Thus, the WuRx of Figure 6 is proposed, which includes three receiver chains as the one shown in Figure 5 and a logical decision block. The incoming signal is handled by each receiver chain, being the differences among chains, the cut-off frequency of the filters, and the threshold value of the comparator blocks. The objective of the first filter and chain consists in distinguishing between a 20 MHz signal and signals with larger bandwidth values. The second filter and chain aim at differentiating between 40 MHz signals and  
Figure 5: Main composition of WuRx. signals above 40 MHz of bandwidth. The third filter and chain target is to discriminate between an 80 MHz signal and a 160 MHz signal.