Link Budget and Capacity Performance of Inductively Coupled Resonant Loops

A near field power transfer equation for an inductively coupled near field system, analogous to Friis transmission equation for far field communications, is derived based on the equivalent circuit model of the coupled resonant loops. Experimental results show the proposed near field coupling equation is trustworthy as it correctly predicts the transferred power versus distance relationship for different values of loaded quality factors at the transmitter and the receiver. Capacity performance of near field communication (NFC) links are analyzed based on information theory, respectively for noise limited and interference limited scenarios. The analytical results provide guidelines for power and capacity budget in inductively coupled antenna systems. Examples of inductively coupled VLF NFC links are evaluated for different operating scenarios, demonstrating the efficacy and importance of the proposed method for near field link budget.