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Dynamic Performance of Passive-Tag Inductive RFID Systems
Michael L. Beigel
President, Beigel Technology Corporation, 1982 Sage Ave., Corona, CA 91720
Email: firstname.lastname@example.org Web: www.beitec.com
REVISION: February 21, 1999
COPYRIGHT Michael Beigel 1999
ALL RIGHTS RESERVED
The operating principles of passive-tag inductive RF-ID tag and reader systems are summarized, and system design parameters are applied to performance requirements. Dynamic interactions between tag and reader are summarized with respect to the probability of completed data transactions when reader and tag are in motion with respect to each other.
TABLE OF CONTENTS
1.1 Operation of RF-ID SYSTEMS
Passive tag inductive RF-ID systems function as follows: A READER supplies power and possibly a timing signal to the passive tag. A coil in the READER radiates an alternating magnetic field into space at a constant frequency. A coil in the ID TAG receives energy from the magnetic field generated by the reader, and supplies POWER and optionally a TIMING SIGNAL from the field to the tag electronics. The activated TAG sequences through its internal data and sequentially LOADS its coil according to the DATA information, thereby modulating the amount of power drawn from the reader field. The READER senses the variations in field power consumption congruent to the DATA INFORMATION in the tag, and decodes the variations to re-construct 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.
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.
1.2 Definition of Performance
The function of the RF-ID system is to provide an exchange of useful information between readers and tags connected with a population of objects. Radio-frequency identification 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 formats vary in specific embodiments according to the requirements and constraints of the target application and environment. Reading range (distance for a reliable data transaction) and the ability to communicate with tags in motion with respect to a defined "reading volume" are aspects of RF-ID system performance.
1.3 General Design Objectives for RF-ID Systems
Activate the tag as far as possible from the reader coil.
Activate the tag at any orientation to the reader field.
Communicate with the tag at the tag activation distance.
Communicate with the tag within a single message period (shortest time).
Communicate with the tag without errors.
1.4 Measuring RFID system performance
The extent to which a product design approaches the theoretical optimum performance for a given type of system can be measured by comparing measured performance with theoretical performance in those areas in which the comparison is meaningful. By identifying the performance aspects of the ID system for which theoretical benchmarks can be derived, one can measure the relative performance of a given product implementation, and predict the extent of improvement in system performance achievable with product upgrades.
2 SYSTEM SPECIFICATION
2.1 Size of Data Space
The required amount of data space (For example, the size of a population of objects to be tagged) determines the number of unique codes needed during the use of the ID system. Since the code space (number of unique codes possible for a system) determines both the ID tag memory length and the speed of transmitting the data transaction, the code space should be as small as possible while sufficiently serving the needs of the system over the expected product life.
2.2 Reading Volume Geometry
The "reading volume" is the 3-dimensional space in which the reader can read (communicate with) a tag. Defining the reading volume dictates the specific design of the reading system. The requirements of the reading volume define the design parameters for the whole system. If the reader can be moved to find a relatively stationary tag, the requirements for the size, shape and intensity of the field are different than the case in which the reader is stationary and the tags move through the reading volume.
2.3 Tag Size and Geometry
RF-ID tags can be designed in a wide variety of sizes and shapes, corresponding to the needs of specific applications. For a given geometry of tag, a larger tag will give a greater reading distance. Therefore for maximum signal transmission the tag should be as large as possible given the other product design constraints, and the tag antenna should have a shape which minimizes its directionality of response in the reading volume.
2.4 Tag Velocity
The highest velocity at which tags can move through any path in the reading volume determines the amount of time available for a completed data transaction. A tag and reader must complete at least one cycle of the shortest valid data transaction, without transmission errors, between the time the tag is activated and the time it is no longer activated in the reading volume. This is one factor which determines the design of the data structure and signal transmission protocol .
2.5 Reliability of Data Transmission
Reliability 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. To prevent erroneous readings, a number of extra bits of information are programmed in the tag to check or correct the accuracy if the main "message" bits, according to algorithms well known in data transmission theory. Increasing he number of error checking/correcting bits defines the reliability of the system; however, the number of extra bits in the tag also complicates the tag design, increases the tag chip size and increases the data transaction time. Therefore, the reliability algorithm should be chosen to utilize the minimum "reliability" bits consistent with the needs of the system.
2.6 Multiple Tag Protocols
Tags can be made with differing signal transmission systems and encoding formats. In many situations, multiple tag types must be read simultaneously by a single reader system.
2.7 Anti-Collision Strategy
For systems in which multiple tags within the reading volume must all be recognized and read, an "anti-collision" method must be employed. The most common anti-collision methods use a method to cause multiple tags active in the 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 least the transaction time of a single tag times the number of tags in the reading volume.
2.8 Expandability of product and system designs
We can assume that new types of tags will develop over the installed life of the ID system. Therefore 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.
3 TRANSMISSION PROTOCOL
3.1 Transmission Layer
3.1.1 Excitation Field Generation Pattern
For this paper, we assume the type of system (full-duplex) in which the reader emits a continuous RF field at a constant frequency and the tag produces a modulation signal while energized and clocked by the reader field. Other systems exist (half-duplex) which employ a pulsed field and a transponder that emits an ID code in the "quiet" time intervals between the field pulses. Because of the requirement for continuous reading at high speed, the continuous field approach is described for the reasons that it allows the tags to be activated at any time they come into the reader field and thereby be sensed and decoded in the minimum possible time.
3.1.2 Excitation Frequency
The excitation frequency for the system is the most basic of all the system specifications, since the reader-tag system is based on a transfer of energy between resonant systems in the reader and the tag. The frequency and power of RF emissions is subject to worldwide regulation. In the "low-frequency RF" domain utilized by present transponder systems, frequencies between 100 KHz and 135 KHz are chosen for worldwide regulatory acceptance. Since power transfer between the tag and the reader is more efficient at higher frequencies, the likely frequency of choice for these systems will be as close as possible to 135 KHz.
3.1.3 Tag Modulation Method
Modulation method is the pattern with which the tag absorbs power from the reading system in order to transmit information back to the reader.
3.2 Signal Transmission Protocol
A few basic types are in use presently, all of which are based on superimposing a secondary absorption pattern on the intrinsic mechanism of variably loading the tag coil by a switching element controlled by the tag circuitry. The modulation patterns presently in use are: Amplitude Shift Keying (ASK): The varying absorption of power (loading) at a sub-modulation frequency cnstitutes logical "1", the non-absorption of power constitutes a logical "0". Frequency Shift Keying (FSK): The loading varies at two different sub-modulation frequencies, corresponding to logical "0" and logical"1". Phase Shift Keying (PSK): The loading varies at a single sub-modulation frequency, but provides phase changes at specific time
intervals to denote logical "0" and "1". Both FSK and PSK are secondary variants on ASK, using the fundamental principle of variable loading and superimposing extra frequencies or phase shifts by varying the pattern of the loading sequence. Each type of modulation has advantages and disadvantages in terms of signal transmission rate, noise immunity and system complexity.
3.1.4 Bit Period
All "full duplex" systems currently in use derive the tag timing from the frequency of the excitation field of the reader. By counting cycles of the excitation field, the modulation periods are obtained, as well as the time length for a transmitted "bit" of information.
The fewer cycles per bit (i.e. shorter time length), the faster the message transmission will be. The more cycles per bit, the more reliable the message transmission will be.
3.2.1 Code Structure
The code structure for an ID message is the system of organization to transmit a coherent, reliable and decode-able information sequence to a reader. RF-ID tags generally transmit a message consisting of:
"PREAMBLE" or starting bits to indicate the beginning of the message,
"DATA" bits to transmit the ID information,
"CHECKSUM" bits to insure the reliability of the transmitted data.
The PREAMBLE field may also be used to define a particular type of tag
and to allow reader timing synchronization. PREAMBLE field is often
called the SYNC field.
The DATA field may contain other information besides an ID code, for instance a country code or a manufacturer code. The CHECKSUM field may be a separate field at the end of the data transmission, or it may be distributed within the sequence of bits in the message.
3.2.2 Message Length (Bits)
The total of the tag information bits is the message length. The message length times the TIME PER BIT equals the message transmission time. The TIME PER BIT equals the TIME PER CLOCK CYCLE (inverse of the reader field frequency) times the number of cycles per bit. Generally an ID tag will transmit the message in a complete and repetitive sequence, repeating the sequence as long as it is energized by the reader.
3.2.3 Error Checking
The CHECKSUM is calculated from the other data in the tag and essentially "summarizes" the contents of the data. When the reader receives a tag code, it re-calculates the checksum and compares it with the data sequence as received. If the data transmission is correct, the calculated checksum will equal the received checksum. Depending on the degree of reliability needed for the data transmission, the checksum will vary in length and complexity of calculation.
4.1 Coil Size
For a tag of a given volume (diameter times length), the amount of space occupied by the energy transforming coil structure is a primary determinant of the tags ability to receive operating energy and modulate it with coded information. The coil size should generally be maximized within the tag volume. Coil size is not the only factor in energy transfer to the tag. Increased resonance also leads to higher energy transfer. A combination of a coil with a capacitor will generally form a more highly resonant circuit than a coil alone.
4.2 Operating Power Level
The power level at which the IC in the tag begins to function reliably is another determining parameter of tag performance. An IC which operates at a lower power level will begin to function farther away from the source of the reader field, giving potentially greater reading distance.
4.3 Modulation Strength
The intensity with which the tag varies the loading of its resonant circuit while maintaining reliable operation determines its "signal strength" to the reader. Higher signal strength makes it easier for the reader to detect and decode the tag signal.
5.1 Field Generation
The first function of the reader system is to activate the tags in its reading volume. To do this optimally, the reader must create an energizing magnetic field appropriate to the geometry of the reading volume and the most probable orientation of tags passing through the volume. For large reading volumes such as are presently used and proposed for fisheries applications, designing a field generation system with sufficient strength, size and consistency is a defining problem for "state of the art" R&D.
5.1.2 Power output
Power output of a reader's magnetic field generator may vary by orders of magnitude from the smallest hand held systems to large fixed-point installations. The requirements for constructing large and powerful magnetic field generators for proposed fisheries projects demand very efficient, low-distortion electronics and resonant electromagnetic networks.
The power output of field generators sufficient to meet reading requirements for the largest systems may exceed regulatory agency specifications for RF emissions. In this case, electromagnetic shielding may be necessary to reduce RF emissions outside the reading volume to acceptable levels.
5.2 Tag Data Acquisition
5.2.1 Analog Signal Processing
The analog signal processing section of the reader performs detection of a very weak perturbation signal from a tag in the presence of a strong energizing field signal. Then it transforms the signal by filtering and amplification to a level appropriate to digitization and further processing in the digital domain.
5.2.2 Decoding and Event Transmission
The amplified signal from the tag modulation of the reader field is digitized and the resulting digital signal is analyzed to detect modulation patterns indicating a valid tag signal. Further analysis verifies that the signal received came from a valid tag with a specific ID number. This processing should occur in "real time", that is, almost simultaneously with the tag passing through thefield. The decoded ID tag events must be stored, transmitted to a central location and recorded or displayed for analysis purposes.
6 PROBABILITY OF READING IN DYNAMIC TAG-READER INTERACTION
6.1 Reader Field Pattern
The electromagnetic field in the reading volume, defined by the reader coil geomerty, the environment near the reader coil(s) and Maxwell’s equations, will generally not be consistent in intensity or orientation.. Therefore a deterministic function of tag activation is associated with the variation of magnetic field strength and orientation in the reading volume, for any given position and orientation of a tag stationary in the reading volume.
6.2 Tag Orientation, Speed and Trajectory
A tag will have the greatest reading distance at optimum orientation, and lesser reading distance as a function of sub-optimal orientation. The average reading distance of a specific stationary tag in the reading volume can be calculated by [integrating the reading distance for all orientations by the probability of orientation in the given direction]. The reading distance for a stationary tag in the reader field is a function of the field strength and the tag orientation in the field. The probability of reading therefore varies proportional to the field strength and is an inverse function of the distance between the tag and the reader. The amount of time the tag is activated by the reader field also affects probability of reading. The theoretical optimum is that the reader can read the tag if it is active for one message period. A tag can move through the reading volume at a variety of speeds. For a given trajectory through reading volume, there is a maximum speed at which a tag can move through the volume and remain active for a sufficient length of time to transmit a complete code message. An "ideal" reader could receive and decode the message. Above this speed, the probability for obtaining a reading is zero. For all speeds below the maximum speed, the probability of reading increases according to a function dependent on tag orientation, reader signal-to-noise ratio and other factors. A tag can also move through the reading volume with varying orientation, thereby varying its relative signal strength or even going through periods of de-activation on its way. Another probability function is therefore the probability that a tag will be readable on account of its trajectory.
6.3 Multiple Tags
If more than one tag is activated within the reading volume at a given time, the tag signals will interfere with each other, giving an ambiguous message to the reader. Depending on the modulation method used in the tags, this mutual interference has a variable effect on whether a valid reading of any tag in the field will take place. Even in systems which utilize "anti-collision" methods, multiple tags in the field will increase the amount of time necessary for completed data transactions of all the tags. Therefore another probability function is whether multiple tags will be in the reading volume simultaneously.
6.4 Noise Sources
Electromagnetic noise sources in the vicinity of the reader sensing apparatus will decrease the probability of a successful reading operation. If the tag outputs a perfectly good signal in the presence of noise, the probability of the reader receiving erroneous information along with the correct tag signal increases according to a complex function of the noise intensity and frequency spectrum as related to the signal processing characteristics of the reader.
 "Fisheries Paper"
 Temic RFID Handbook
 Patent 4,333,072
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