Zimu Zhou et al.: SensorlessSensingwithWiFi 5
applications might have already gone beyond the capability of CSI. Apart from further exploring and exploiting the frequency diversity and the phase information of CSI, researchers also began to identify its limitations in practice, as well as seek other techniques to extend CSI-based sensing to general WiFi-based sensorless sensing or WiFi radar. In Ref. [21], researchers pointed out via ambiguity function analysis that the range resolution of WiFi-compatible passive bistatic radars can only reach meters, which is fundamentally constrained by the bandwidth of WiFi signals. To overcome this intrinsic constraint, researchersalternativelyincorporateMulti-Input-MultiOutput (MIMO) technology. Researchers in Ref. [22] exploited antenna cancellation techniques to eliminate the impact of static clutters to enable through-wall sensing of human movements. In Ref. [23], the authors achieved computational imaging using WiFi, and built a MIMO-based prototype on software defined radio platforms. They experimentally demonstrated that the size, material, and orientation of the target objects can significantly affect the performance of WiFi imaging, and a one-fit-all solution is still to be explored. 4 Conclusions Wireless and sensorless sensing seeks breakthroughs in the contradiction between the limitation of WiFi and the growing demand for environment perception in daily life, seeks a balance between low cost and high accuracy, explores solutions via frequency diversity and spatial diversity, and creates applications that are previously infeasible in wireless communications and mobilecomputing.Weenvisiontechnologicaladvances would boost the capability of wireless sensing to finer granularity and higher sensitivity, which will in turn fostervariousnewapplications.Thisarticleonlyserves as an introduction on the concept of wireless and sensorless sensing, and we refer interested readers to the corresponding references for in-depth information. IfweconsiderWiFiasasidesensor,thenWiFi-based sensorlesssensingcanberegardedasoneoftheworld’s largest wireless sensor networks, spreading over office buildings, shopping malls, other public places and homes, and silently watching the activities of humans therein. Living inside such a network, every individual in the physical world has been bestowed with unique being in the digital world. So the next time you want a secret meeting, after shutting the doors, pulling down thecurtains,andevencheckingforwiretapsbeneaththe table, do not forget to turn off the WiFi!