I. Introduction
With Radio Frequency Identification (RFID) technology development, many commercial devices are applied in diverse scenarios [1]–[3] such as health-caring monitoring [4], goods tracking [5], [6], classification, localization and stuff management [7]. Especially in health-caring monitoring scenarios, the wireless RFID devices are expected to play a role as respiration sensors [8], [9], skin temperature sensors [5], [14], [15] and strain sensors [16], [17]. Generally, the operation frequency selection and structure designs of RFID sensors depend on the required sensing functions. With regard to the working frequency selection, Ultra High Frequency (UHF) RFID devices are getting more attention due to their lower cost, long-distance transmission and fast-easy identification. There are two ways to achieve sensing functions for wireless UHF-RFID devices. One way is that tags can be designed to connect sensors with different functions [18]. For these types of UHF-RFID sensors, chips need to have extendable ports for sensors connection (such as the ROCKY100) [18]. Another way is to use the antenna as a sensor.