Introduction

Passive ultra high frequency (UHF) radio frequency identification (RFID) is an electronic tagging technology commercialized between 860 and 960 MHz that allows an object or person to be automatically identified at a distance of up to 10 m without a direct line-of-sight path using a radar-type radio wave exchange (Figure I.1). UHF is the dominant technology for supply chain management applications such as case and pallet tracking and returnable container identification. It is also widely used for real-time inventory, industrial automation, work-in-process tracking, asset management, forklift monitoring, personal identification (ID), vehicle access control, document security and authentication. There are numerous UHF standards, most notably ISO 18000-6 and EPCglobal Gen 2, which are the most widely supported RFID standards nowadays.

Figure I.1. Functional principle of a UHF RFID communication. Inlay attached to the item: substrate film onto which the antenna and the chip containing item data are combined. Radio frequency emitted by the reader installed on a gate, a cashier counter, etc. The tag sends the data in response to the radio frequency (backscattering modulation). The reader antenna transmits the modulated data to the reader. The reader decodes the data and sends it to the host computer

Today, many bar code processes involve bringing the bar-coded object to the reader and orienting the bar code for proper presentation to the reader. As a result, conveyor systems must run considerably slower than their top speeds so that bar code readers can identify passing objects. Unlike a bar code, the RFID tag does not necessarily need to be within the line of sight of the reader and it may be embedded into the tracked object. Moreover, RFID has the ability to identify multiple objects simultaneously. These are the important advantages of RFID over bar code technology, since they eliminate much of the labor currently required and increase the reading speeds.

Apart from the frequency, the main difference between high frequency (HF) and UHF RFID technologies is their read/write distance. The maximum distance for HF technology at 13.56 MHz is approximately 1 m and the maximum distance for UHF technology is approximately 10 m. When long read distances are required, 13.56 MHz technology is not an option: HF antennas do not radiate as their length is very small compared to the wavelength with an almost zero radiation resistance. HF antennas must be seen as near H-field sensors. Most UHF systems communicate by radio waves that provide longer distance. However, UHF systems can also work at close distance and one can find commercialized near-field UHF systems optimized for short-range reading. The same tags that can be read from a 10 m distance can also be read from 10 cm. For instance, UHF technologies can simultaneously satisfy requirements for short-range reading at assembly stations and long-range storage, shipping and receiving processes. UHF systems are then optimized for various processes and reading distances through the placement and configuration of readers.

In early deployments, the UHF systems suffered from performance degradation when used around liquid and metal compared to HF solutions. Advances in antenna design, reader tuning and best practices have overcome this limitation. For example, some UHF tags have been designed specifically for use in close proximity to metal and to take advantage of the conductive properties of the metal to enhance the RF performance. The idea that HF technology is required for use around metal and liquid is nowadays more of a perception than a reality. In supply chain applications, UHF is frequently used to successfully identify cases and entire pallets of consumer goods with high liquid or metal content.

The purpose of this book is not to be exhaustive but to focus on specific aspects of passive UHF RFID technologies. Its first objective is to provide a reference document on the tag antenna design and chip technologies, either from up-to-date academic papers, industrial data or author experience. The second objective is to include perspectives on end users, market and production. Nevertheless, important UHF RFID topics such as the architecture of the readers, the ISO 18000-6 air interface protocol standard and the worldwide regulations are beyond the scope of this book.

The book is organized as follows:

Chapter 1 provides an up-to-date state of the art in technologies and performances of UHF RFID integrated circuits (ICs). This includes the direct current (DC) voltage supply generation circuit and its regulator, the demodulator part to recover the data, the on-board oscillator to control the digital part and the organization of this digital part and memory. The focus is on the EPC Gen2 protocol adopted in the ISO 18000 Part 6. This added to a full description of the IC functionalities, fabrication issues, matching requirements and measurement tests and benchmarks should help chip designers to identify the main constraints of the technology.

Chapter 2 highlights the design and manufacturing issues of RFID tags. The antenna miniaturization on inexpensive materials is only one of several problems that a designer needs to solve. Tracking fluxes of goods between different companies and across the world must be performed with good read performance despite a close environment characterized by disturbances (associated items, other tags, surrounding objects, etc.). This can only be done if the tag design follows several rules, one of them being the wideband impedance matching to the RFID IC. Another rule is to limit the tag sensitivity to the environment by including the dielectric, conductivity and shape features of the tagged item in the analysis in order to take advantage of it or to fight against it.

As most UHF RFID tags are dipole-based structures, Chapter 2 first describes the fundamental circuit parameters of the dipole antenna (input impedance, radiation resistance, efficiency, Q-factor and impedance). The miniaturization strategies based on fat dipoles, tip loading and meanders are then presented followed by a description of the influence of the dielectric and metallic environments on the tag performance. The fundamental problem of impedance matching between the RFID chip and the antenna is clearly stated with a careful explanation of the single- and double-tuned matching strategies. It is demonstrated that the wide bandwidth characteristic of double-tuned matching circuit is crucial in the presence of dielectrics. Inductively coupled fed tag antennas, as well as the associated commercial loop-based modules, are also extensively detailed. Examples of their use on either filled or empty recipients are given. To conclude this chapter, a state of the art for tags on metal is proposed. Thin and thick structures are examined in succession.

In Chapter 3, special emphasis is put on the design methodology as a function of the radar cross section (RCS) and DeltaRCS with consequences on the near- field/far-field communications.

Chapter 4 is an overview of UHF RFID challenges including the applications, markets, trades and basic technologies, more specifically in the supply chain management and the retail inventory. It is demonstrated that return-on investment (ROI) is key to the RFID buying decision process. RFID technology must generate cost reduction and sales increase to trigger the associated investment. Key topics of future RFID are also detailed: use of tags throughout the whole product life, smart embedded RFID solutions, seamless and ubiquitous infrastructures, and future softwares in massive networks of small intelligent devices.