# Low-Power Wake-Up Receivers for Resilient Cellular Internet of Things
## 3 WUR Performance Metrics and Requirements  
While the use of a WUR has promising benefits of reducing system power consumption with low latency, there are several design challenges and tradeoffs that need to be kept in mind. An online survey [15] provides a database of the performance of ultra-low-power (ULP) radios published in top IEEE integrated circuit (IC) conferences and journals since 2005. The database is helpful in analyzing the trends of ULP WUR designs. Figure 3: Illustration of 4G and 5G device types (adapted from [14]). Power Consumption: While the exact power target depends on the specific scenario (MR power, average paging rate, WUR duty-cycling rate, etc. ), 3 GPP estimates a WUR active power of around 1 mW to achieve a power-saving gain greater than 90% [5; 12; 16];
2. Modulation Scheme and Standard Compatibility: A relatively stringent power budget limits the receiver design complexity, therefore limiting the choice of modulation schemes. Most ultra-low-power WURs in the existing literature only support simple non-coherent modulations, as seen in Figure 4, such as OOK (on-off keying) [17; 18; 19; 20; 21; 22; 23; 24; 25; 26], FSK (frequency shift keying) [27; 28; 29; 30; 31; 32; 33], or PSK (phase shift keying) [34; 35; 36; 37; 38]. However, these modulations have the drawbacks of low throughput and poor bandwidth efficiency. More importantly, they are not compliant with the existing cellular standards and cannot be adopted for wide use. On the other hand, OFDM QPSK/QAM used by 4G/5G are complex modulation schemes that support high data rates but demand more precise receiver designs, resulting in much higher power consumption. This motivates novel WUS designs like multi-carrier OOK [39; 40] that can be received and decoded with low-power receivers while being compatible with the existing base station hardware. 3. Sensitivity and Coverage: It is desirable to have the WUR achieve the same or better coverage as the MR. Receiver sensitivity, which is the minimum RF signal strength a receiver can reliably detect and decode, is often a direct proxy for communication range. It is expressed as \[\mathrm{P_{sen}}=-174\ \mathrm{dBm}/\mathrm{Hz}+10\mathrm{log}_{10}(\mathrm{BW})+ \mathrm{NF}+10\mathrm{log}10(\mathrm{SNR_{min}}),\] (1) where BW is the receiver bandwidth, NF is the receiver noise figure (in dB), and \(\mathrm{SNR_{min}}\) is the minimum signal-to-noise ratio (SNR) required to demodulate the signal with a specified bit error rate (BER). \(\mathrm{SNR_{min}}\) is determined by the modulation scheme, independent of receiver hardware design. Therefore, a receiver with low NF is required to achieve good sensitivity and wide coverage. However, a fundamental noise-power tradeoff exists, making it very difficult to achieve good NF with low power [15], as seen in Figure 4.
4. Latency: In most cases, latency can be defined as the interval between the time of data transmission from the base station and the time that the UE can monitor the paging occasion (PO) [5]. This then includes the WUS on-air time, the delay of the WUR (hardware processing delay and possible delay due to WUR duty cycling and synchronization), and the delay due to MR ramp-up and synchronization. Figure 4: Power vs. sensitivity [15]. Interference Resiliency: Cellular WURs would likely operate alongside congested wireless traffic, where wireless interferers can potentially cause undesirable false wake-ups of WUR nodes. The signal-to-interference ratio (SIR) is a performance metric for a receiver's interference resiliency (a more negative SIR is better). As shown in Figure 5, however, high interference rejection is a challenge for ULP receiver design [15]. 6. Data Rate: For a given amount of data, a larger data rate means less on-air time, which translates to lower delays and less energy consumption. On the other hand, some applications may require exchanges of large payloads, while others may not. In the former case, it is especially crucial to support a relatively high data rate. 7.