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
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 .
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.
- Activate the tag as far as possible from the
- Communicate with the tag at the tag
- Communicate with the tag without errors
within a single message period (shortest time).
- Activate the tag at any orientation to the
| (click to enlarge)
Figure 1: System Diagram.
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
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
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
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 .
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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