This is a general introduction and tutorial regarding inductively coupled RFID systems. It summarizes the operating principles and parameters of passive-tag inductive RFID system performance, focusing on dynamic interactions between tag and reader in relative motion and the probability of successfully completed data transactions. The full-duplex (FDX) operating model is assumed in most descriptions and examples.


Operation of passive tag RFID Systems: Inductively coupled RFID systems are best understood in context of the inter-relation between the systems, physics, communication, and component aspects.

An RFID READER supplies power and timing signals to the passive tag by radiating an alternating magnetic field coupled to an antenna coil into the surrounding space. An antenna coil in the ID TAG receives energy from the reader magnetic field, providing POWER and TIMING signals to the tag electronics.

The activated TAG accesses its internal DATA and sequentially varies the electrical loading of its coil according to the DATA information, modulating the amount of power drawn by the TAG from the reader field. The READER senses the variations in field power consumption corresponding to the DATA in the tag, decodes and outputs the DATA [1].

In Passive tag READ-WRITE systems, the reader can send DATA to the tag by sequentially modulating the energizing magnetic field. Additional circuitry in the tag senses and decodes the modulated reader field and puts the DATA into the tag memory or utilizes the DATA as operating commands (Figure 1).

A PROTOCOL between the reader and the tag allows for the systematic and reliable exchange of DATA in one or both directions. A DATA TRANSACTION is a completed exchange of data between reader and tag. The MESSAGE TIME is the time length for a single data transaction.

Modeling and Measuring RFID system performance: The function of the RFID system is to provide an exchange of data between readers and tags connected with a population of objects. RFID systems are highly application dependent. Performance is defined and evaluated by determining the extent to which a system meets the needs of the application. ID tags, readers and coding protocol formats vary in specific embodiments according to the requirements and constraints of the target application and environment.

Many aspects of RFID system performance can be mathematically modeled and simulated. Identifying the aspects of the system for which theoretical "ideal" performance benchmarks can be derived will enable the measurement of relative performance of a given product implementation. Comparison of the measured system performance with theoretical optimum performance allows prediction of the extent of improvement that can be achieved with subsequent product upgrades.

"Ideal" Design Objectives for RFID Systems: Some of the "ideal" performance benchmarks for RFID systems are listed below.
  1. Activate the tag as far as possible from the reader coil.
  2. Communicate with the tag at the tag activation distance.
  3. Communicate with the tag without errors within a single message period (shortest time).
  4. Activate the tag at any orientation to the reader field.

(click to enlarge)

Figure 1: System Diagram.
System Design

Size of Data Space: The required size of data space (for example, the size of a population of objects to be tagged) determines the number of unique codes needed during the useful lifetime of the ID system. Since the code space (number of unique codes possible for a system) determines both the ID tag memory size and the time length of the data transaction, the code space should be the minimum that sufficiently serves the needs of the system over the expected product life.

Reading Volume Geometry: The "reading volume" is the 3-dimensional space, referenced from the reader antenna, in which the reader can activate and communicate with a tag. Defining the required characteristics (the size, shape, orientation and intensity of field) of the reading volume dictates the specific design of the reading system. The requirements for the field geometry may differ according to whether the reader is stationary and the tags move through the reading volume, or if the reader can be moved to find a relatively stationary tag.

Tag Coil Size and Geometry: RFID tags may be designed in a variety of sizes and shapes corresponding to the needs of specific applications. For a given tag shape, a larger tag will give a greater reading distance. For maximum signal transmission the tag antenna should be as large as possible and have a shape which minimizes the directionality of response in the reading volume. Different tag coil shapes will give differing directional response to the reader field.

Tag Velocity: The highest velocity at which a tag moves through any path in the reading volume determines the minimum time length for a completed data transaction. For a successful event, the tag and reader must complete at least one sequence of a valid data transaction without transmission or reading errors during the minimum time length that the tag is activated in the reading volume.

Reliability of Data Transmission: Reliability of the data transaction, i.e. obtaining an error-free data exchange between tag and reader, can be designed into the ID system to the extent required for system performance. This is accomplished by utilizing error detecting and/or correcting code bits in the message. Increasing the number of error checking/correcting bits increases the reliability of the system performance; but also increases the tag chip size and the data transaction time. To optimize transmission efficiency versus data reliability, the reliability algorithm (checksum) should be chosen to utilize the minimum number of extra data bits adequate for the required data reliability.

Multiple Tag Protocols: Tags may have different signal transmission systems and encoding formats. In many situations, multiple tag types must be recognized and read simultaneously by a single reader system [2].

Anti-Collision: For systems in which multiple tags within the reading volume must all be recognized and read, an "anti-collision" protocol is used. The most common anti-collision protocols use methods to cause multiple tags active in the reader field to transmit their information in such a way that only one tag at a time is interacting with the reader. The transaction time for the group of tags in the reading volume must then be assumed to be at minimum the transaction time of a single tag multiplied by the number of tags in the reading volume.

Expandability of product and system designs: New types of tags will develop over the installed life of any RFID system. Reader systems must be expandable in the aspects which are easiest to change (signal and code processing), and very durable in the aspects (field activation and tag signal sensing) which must remain in place for a long time.

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