Due to its ability to significantly enhance communication reliability, the fluid antenna system (FAS) has emerged as a promising technology for ultra-reliable communications (URC). However, existing expressions to evaluate the outage probability of FAS are either computationally complex or simplified at the cost of accuracy. To address this issue, we propose an accuracy-controllable approximation method under Rayleigh fading channels. Specifically, we first approximate the fully correlated Rayleigh fading model as a block-correlation Rayleigh model, where the channel gains of ports in different blocks are assumed to be mutually independent, while the channel gains of ports in the same block are considered equally correlated, to simplify the analysis while maintaining accuracy. Then, we propose a novel method to approximate the Bessel function I₀, introduced by the correlated Rayleigh fading, using an M -term series expansion, where the parameter M provides a tunable trade-off between approximation accuracy and computational complexity. Based on this approximation, a closed-form expression for the outage probability of FAS is derived. Simulation results demonstrate that our derived expression offers low computational complexity for evaluating the FAS outage probability while maintaining excellent approximation accuracy in the medium-to-high signal-to-noise ratio (SNR) range.

In this paper, a wireless energy-harvested ultra-high frequency (UHF) wireless sensor network (WSN) tag is designed and implemented for cellular IoT applications. The WSN tag is made up of a wireless energy harvesting circuit, a temperature sensing circuit, 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 according 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 supports 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 Internet of Things (IoT).