  WiFi signal has become accessible everywhere, providing high-speed data
transmission experience. Besides the communication service, channel state
information (CSI) of the WiFi signals is widely employed for numerous Internet
of Things (IoT) applications. Recently, most of these applications are based on
analysis of the microwave reflections caused by physical movement of the
objective. In this paper, a novel contactless wireless sensing technique named
WiEps is developed to measure the dielectric properties of the material,
exploiting the transmission characteristics of the WiFi signals. In WiEps, the
material under test is placed between the transmitter antenna and receiver
antenna. A theoretical model is proposed to quantitatively describe the
relationship between CSI data and dielectric properties of the material. During
the experiment, the phase and amplitude of the transmitted WiFi signals are
extracted from the measured CSI data. The parameters of the theoretical model
are calculated using measured data from the known materials. Then, WiEps is
utilized to estimate the dielectric properties of unknown materials. The
proposed technique is first applied to the ethanol/water mixtures. Then,
additional liquids are measured for further verification. The estimated
permittivities and conductivities show good agreement with the actual values,
with the average error of 4.0% and 8.9%, respectively, indicating the efficacy
of WiEps. By measuring the dielectric property, this technique is promising to
be applied to new IoT applications using ubiquitous WiFi signals, such as food
engineering, material manufacturing process monitoring, and security check.
