# 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 Transmitter and Wake-up Radio Call  
For the WuTx proposed in this research, first, a legacy IEEE 802.11n transmitter can be used, sending frames during a TXOP employing two different signal bandwidths, thus coding two different symbols, each of them composed by one bit (cf. Figure 3). On the other hand, for an IEEE 802.11 ac/ax transmitter, up to four different bandwidths for frame transmission during a TXOP can be used, thus allowing four different symbols, each of them carrying two bits (cf. Figure 4). Obviously, WuTx with the IEEE 802.11 ac/ax option also allows the one bit per symbol coding of Figure 3, but other combinations using two bandwidth values could be used to increase the distance between symbols (e.g., 20 and 80 MHz, or 20 and 160 MHz). Note that the number of frames involved in the TXOP of Figures 3 and 4 are only for the sake of example. IEEE 802.11 frames used in the proposed WuC are composed of PHY preamble and header and MAC header (i.e., empty data payload), thus consisting in IEEE 802.11 standard-compliant frames of minimum length available for transmission. The fastest MCS is chosen for the different signal bandwidths in order to minimize frame duration (i.e., number of OFDM symbols) employing one spatial stream. Thus, for 20 MHz of signal bandwidth, the minimum duration for an IEEE 802.11n transmission is 44 us (36 us for PHY preamble and header and 8 us for MAC header), whereas, for IEEE 802.11 ac, it is 48 us (40 us for PHY preamble and header and 8 us for MAC header). On the other hand, for higher values of signal bandwidth (i.e., 40, 80, and 160 MHz), MAC header duration can be reduced to the minimum value, i.e., one OFDM symbol of 4 us. This leads to a frame duration of 40 us for 40 MHz with IEEE 802.11n and of 44 us for 40, 80, and 160 MHz with IEEE 802.11 ac. With regard to IEEE 802.11 ax, the minimum frame duration employing signal bandwidths from 20 to 160 MHz is 57.6 us (44 us for PHY preamble and header, and 13.6 us for MAC header). Reference [24] shows detailed computation on frame transmission times for the different IEEE 802.11 amendments and configurations. Figure 2: Example of a standard-compliant transmission using a Transmission Opportunity (TXOP). Following IEEE 802.11 standard specification, consecutive frames within a TXOP are separated from each other by a SIFS interval, which is set to 16 \(\upmu\)s for IEEE 802.11 ac and IEEE 802.11n/ax amendments operating on the 5 GHz frequency band. For IEEE 802.11n/ax using 2.4 GHz spectrum, the SIFS value is 10 \(\upmu\)s; however, a signal extension of 6 \(\upmu\)s has to be added at the end of each frame, thus becoming 16 \(\upmu\)s the total amount of time between frames. Taking this into account, a symbol duration of 60 \(\upmu\)s is obtained when using IEEE 802.11n signals of 20 MHz and one bit per symbol (i.e., 44 \(\upmu\)s of frame duration plus 16 \(\upmu\)s of inter-frame interval), thus leading to a bit rate of 16.67 kbps. For IEEE 802.11n signals of 40 MHz and one bit per symbol, a bit period of 56 \(\upmu\)s and, consequently, a bit rate of 17.85 kbps is achieved. Thus, an average bit rate of 17.26 kbps is obtained when using IEEE 802.11n to encode a bandwidth-based WuC with two symbols. Bit rate is slightly reduced when using an IEEE 802.11 ac transmitter and one bit per symbol (15.63 kbps for signals of 20 MHz and 16.67 kbps for signals larger than 40 MHz, i.e., 16.15 kbps on average). For IEEE 802.11 ax, the achieved bit rate is 13.58 kbps. On the other hand, bit rates are obviously doubled for the scenario with two bits per symbol, leading for the case of IEEE 802.11 ac WuTx to 31.25 kbps for signals of 20 MHz and to 33.33 kbps for signals of 40, 80, or 160 MHz, i.e., 32.81 kbps on average. For the case of IEEE 802.11 ax, the bit rate is increased to 27.17 kbps for all the available signal bandwidths. The average symbol rate is expected to be used as the sample rate in the WuRx for symbol decoding, with synchronization guaranteed for typical WuC lengths of around 16 bits [9; 13; 14; 15; 25].