Complete Link Budgets for Backscatter-Radio and RFID Systems

Backscatter radio - wireless communication by modulating signals scattered from a transponder (RF tag) - is fundamentally different from conventional radio because it involves two distinct links: the power-up link for powering passive RF tags, and the backscatter link for describing backscatter communication. Because of severe power constraints on the RF tag, a thorough knowledge of the backscatter channel is necessary to maximize backscatter-radio and radio-frequency identification (RFID) system performance. This article presents four link budgets that account for the major propagation mechanisms of the backscatter channel, along with a detailed discussion of each. Use of the link budgets is demonstrated by a practical UHF RFID portal example. The benefits of future 5.8 GHz multi-antenna backscatter-radio systems are shown. An intuitive analogy for understanding the antenna polarization of RF tag systems is presented.

1. Introduction

In recent years, reductions in the manufacturing costs of high-frequency integrated circuits have sparked explosive growth and research interest in backscatter-radio systems, of which the most well-known application is radio-frequency identification (RFID) for industrial supply chains. Of course, supply-chain management is only one of the many uses of backscatter radio-frequency (RF) tags. Other uses include medical telemetry, low-cost sensors, remote switches, position location and tracking devices, and passive data exchange, to name just a few. It is easy to get swept away dreaming up all of the possible uses of low-cost, low- or no-power radio tags: contact-less memory sticks, subcutaneous glucose sensors for diabetics, environmental tracking of food items, the famous “instant wireless checkout” of tagged items at the grocery store, and so on. However, whether or not any of these applications are even possible depends on the multi-faceted physics of the RF tag's radio link budget, which is the subject of this article.