23.3 A Passive Crystal-Less Wi-Fi-to-BLE Tag Demonstrating Battery-Free FDD Communication with Smartphones.

As the Internet of Things (IoT) applications become increasingly widespread, wireless connectivity presents stringent requirements on low power, low cost, and compatibility with widely deployed commodity hardware such as BLE/Wi-Fi-embedded devices. Supporting established communication standards is necessary for seamless integration with current infrastructure. Conventional BLE/Wi-Fi-compatible IoT devices require a mW-level power supply, which limits the lifetime and increases the hardware cost of battery or wall power [1]. In recent years, passive IoT tags based on backscatter communication have become a good candidate for low-power low-cost IoT applications due to battery elimination [2–5]. For example, a continuous-waveform (CW) signal source serves as the power transmitter, and the tag can backscatter a BLE/Wi-Fi signal based on the incident power (Fig. 23.3.1, top, first) [2, 3]. However, the CW incident signal source should be dedicated instead of a commodity tablet/smartphone, so its deployment takes additional infrastructure cost. The tag in [4] is powered by LTE energy, which utilizes an incident reversely-whitened CW-like BLE tone to backscatter a Wi-Fi signal (Fig. 23.3.1, top, second). With an incident reversely-whitened BLE tone, both the tag-to-smartphone uplink and the smartphone-to-tag downlink can be conducted in a half-duplex way [5] (Fig. 23.3.1, top, third). However, there are several shortcomings in the existing passive IoT tags: 1) Due to the complexity of Wi-Fi protocol, none of the prior works have realized backscatter on incident Wi-Fi to provide a standard-compatible signal for commodity devices, despite that nowadays Wi-Fi is widespread throughout our homes, offices, and other environments. 2) The clocks of all the existing tags rely on crystals, which increases the cost of deployment. 3) The communication can only be simplex or half-duplex, while simultaneous uplink and downlink can increase the throughput of a wireless network.