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In this paper, a wireless energy?harvested ultra?high frequency (UHF) wireless sensor network (WSN) tag is designed and imple?mented for cellular IoT applications. The WSN tag is made up of a wireless energy harvesting circuit, a temperature sensing cir?cuit, and a radio frequency identification (RFID) tag. The developed WSN tag is compatible with the ISO/IEC18000?6C protocol. The WSN tag can receive the GSM RF energy operating in China GSM900 and GSM1800 bands in the surrounding environment and the solar energy, then converts the RF energy to direct current (DC) by schottky diode?based rectifying circuit, and finally stores the DC energy in a supercapacitor through a DC?DC booster circuit. The DC?DC booster circuit drives the front?end circuit, TI MSP430 microcontroller, temperature sensing circuit, and other active circuits in the tag. The MSP430 works in low?power mode when it is powered up, and it can also reduce power consumption more by reducing main clock (MCLK) frequency accord?ing to different forward link rates. The implemented WSN tag demonstrated that the RF?to?DC conversion efficiency is higher than 39％ when the receiving 900 MHz RF signal power is from ?14 dBm to 0 dBm and could make the tag work normally. The signal receiving sensitivity of the WSN tag is up to ?32 dBm at the rate of 40 kbit/s from the Reader to the WSN tag. The WSN tag sup?ports Miller coding and extended Miller coding. This wireless energy harvested UHF WSN tag, compared with conventional UHF passive tags and battery?powered active UHF RFID Tags, has many advantages, such as far communication distance, long service life, and sensing functionality. It will have wide applications in the Intet of Things (IoT).