Michael Beigel is cited in many patents relating to injectable magnetic coupled transponders. This paper relates to the performance achievable from such devices.


OBJECTIVE MEASUREMENTS FOR RF-ID SYSTEM PERFORMANCE:
SYSTEM DESIGN AND EVALUATION CRITERIA

ABSTRACT

Objective measurements based on an understanding of the operating principles of RF-ID systems are necessary to evaluate the performance of these systems for fisheries projects.

The operating principles of RF-ID tag and reader systems are summarized, and design parameters of the systems are applied to performance requirements of installations used in fisheries work.

Performance considerations for un-attended underwater monitoring installations will be emphasized.

Michael L. Beigel
January 16, 1993
Email: Mike Beigel

President, Beigel Technology Corporation
1982 Sage Ave.
Corona, CA 91720
PHONE: 909-371-6116

c 1993, Michael Beigel. All rights reserved.


TABLE OF CONTENTS

1 INTRODUCTION
1.1 Operation of RF-ID SYSTEMS
1.2 Definition of Performance
1.3 General Design Objectives for RF-ID Systems
1.4 Product Designs
2 SYSTEM SPECIFICATION
2.1 Population Size
2.2 Reading Volume Geometry
2.3 Tag Size
2.4 Tag Velocity
2.5 Reliability of Reading
2.6 Multiple Tag Types
2.7 Expandability
3 TRANSMISSION PROTOCOL
3.1 Transmission Layer
3.1.1 Continuous Field
3.1.2 Excitation Frequency
3.1.3 Modulation Format
3.1.4 Bit Period
3.2 Code Protocol
3.2.1 Code Structure
3.2.2 Message Length (Bits)
3.2.3 Error Checking
4 TAG
4.1 Coil Size
4.2 Operating Power Level
4.3 Modulation Strength
5 READER
5.1 Field Generation
5.1.1 Geometry
5.1.2 Power output
5.1.3 Shielding
5.2 Tag Data Acquisition
5.2.1 Analog Signal Processing
5.2.2 Decoding and Event Transmission
6 PROBABILITY OF READING
6.1 Reader Field Pattern
6.2 Tag Orientation, Speed and Trajectory
6.3 Multiple Tags
6.4 Noise Sources
7 MEASUREMENT EQUIPMENT AND TECHNIQUES
7.1 RF Gaussmeter
7.2 Tag operation sensor
7.3 Test signal tags
7.4 Tag Speed Fixture
7.5 RF signal measurement equipment

1 INTRODUCTION

1.1 Operation of RF-ID SYSTEMS

The RF-ID "PIT Tag" systems presently used in fisheries applications function in the following manner:
A magnetic coil in the READER radiates a magnetic field into space at a constant frequency,
A coil in the ID TAG picks up energy from the magnetic field generated by the reader, and derives all of its POWER and a TIMING SIGNAL from the field,
The ID TAG sequences through a MEMORY which contains the ID number, at a rate determined by the TIMING SIGNAL,
The ID tag VARIABLY LOADS its coil according to the MEMORY information, drawing a variable amount of power from the reader field,
The READER senses the variations in field power consumption congruent to the MEMORY INFORMATION in the tag and decodes the variations to re-construct the ID NUMBER in the tag.

See FIGURE 1.

1.2 Definition of Performance

The ultimate purpose of the RF-ID system is to provide useful information connected with the identification of a population of animals or objects.
Performance is defined and evaluated by determining the extent to which a system meets the needs of the application.
Radio-frequency identification systems are highly application dependent. ID tags, readers and coding formats vary in specific embodiments according to the special needs and constraints of the application.
In fisheries applications, certain aspects of the technology are emphasized in terms of fulfilling the requirements. Reading range and the ability to read fast moving subjects traveling through a stationary "reading space" are the principle demanding aspects of RF-ID systems in fishery applications.

1.3 General Design Objectives for RF-ID Systems
A completely optimized RF-ID would exhibit the following properties:
Activate the tag as far as possible from the reader coil.
Activate the tag at any orientation to the reader field.
Read the tag at the tag activation distance.
Read the tag within a single message period (shortest time).
Read the tag without errors.

1.4 Product Designs

Every product design falls short of theoretical performance possibilities.
The extent to which a product approaches the theoretical performance for a given type of system can be measured to a limited extent by comparing measured performance with theoretical performance in those areas in which the comparison is possible.
By identifying the aspects of the ID system for which theoretical benchmarks can be derived, one can predict the extent improvement in system performance achievable with product upgrades.

2 SYSTEM SPECIFICATION

2.1 Population Size
The size of the population of animals 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 and ID code, the code space should be as small as possible while sufficiently serving the needs of the population over the product life.
2.2 Reading Volume Geometry
The "reading volume" is the 3-dimensional space in which the reader can read a tag. Defining the space in which reading must take place 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 of a reader which is stationary and the tags move through the reading volume.
In the case of fisheries applications, the most challenging application is that of large volumes through fish can move at high velocity. Adult interrogation fish ladders are perhaps the best example of the requirement for a physically large and deep space responsive to ID tags moving at high velocity.
2.3 Tag Size
RF-ID tags can be designed in a wide variety of sizes and shapes, each corresponding to the needs of a specific application.
The syringe-implantable ID transponder now used in fisheries applications could be produced either larger or smaller than the present 12 mm length by 2.1 mm width cylindrical package, while keeping the same form factor.
For a given technology implementation of tag, a larger tag will give a better reading distance. Therefore for maximum signal transmission the tag should be as large as possible consistent with the size of the animal using it. However, for minimum invasiveness to the host animal the tag should be as small as possible. Therefore tag optimization in terms of size vs. performance is a principle issue in RF-ID system design.
2.4 Tag Velocity
Assuming a fixed location for the reader defining a volume in which the tags enter and exit, the highest speed at which tags can move through any path in the reader volume determines the minimum operating parameters for the system.
2.5 Reliability of Reading
Reliability of reading an ID tag, i.e. obtaining a correct reading at the reader corresponding to the ID code programmed in the tag, 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" the accuracy if the main "message" bits. By checking these bits against the rest of the received code, the reader validates the information in the tag. If the messages do not "check out", the reader can issue an error message, or ignore the reading.
The number of error checking bits defines the reliability of the system. However the number of extra bits in the tag complicates the tag design, increases the ID chip size and slows down the transmission time for the ID message. Therefore, the reliability requirement should be chosen to require the minimum extra bits consistent with the needs of the system.
2.6 Multiple Tag Types
Tags can be made with differing signal transmission systems and encoding formats. in most cases, multiple tag types can be read simultaneously by a single reader system.
2.7 Expandability
We can assume that new types of tags will develop over the life of the ID system. Therefore reader systems must be expandable in the aspects which are easiest to change (signal and code processing, field activation, and very durable in the aspects (field activation and sensing) which must remain in place for a long time.

3 TRANSMISSION PROTOCOL

3.1 Transmission Layer
3.1.1 Continuous Field
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 Modulation Format
Modulation Format is the pattern with which the tag absorbs power from the reading system in order to transmit information back to the reader.
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 constitutes 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 "rhythm" 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 Code Protocol
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 TAG

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 READER

5.1 Field Generation
5.1.1 Geometry
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.
5.1.3 Shielding
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 the field.
The decoded ID tag events must be stored, transmitted to a central location and recorded or displayed for analysis purposes.

6 PROBABILITY OF READING

6.1 Reader Field Pattern
The electromagnetic field in the reading volume will generally not be consistent in intensity or orientation, due to the laws of electromagnetism and the geometric constraints of the reading volume. Therefore a probability function of tag activation is associated with the variation of magnetic field strength and orientation 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 the tag 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 inversely proportional to the distance from the reader.
The time the tag is in 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 a section of the reading volume, there is a maximum speed at which a tag can move through the volume and remain active in the volume 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 approaches (one) 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 and 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. Therefore another probability function is whether multiple tags will be in the reading volume simultaneously.
Note that the "multiple tag" probability is decreased by having a shorter linear distance of tag trajectory through the reading volume, while the probability of reading tags at high velocity is increased by having a longer distance.
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.

7 MEASUREMENT EQUIPMENT AND TECHNIQUES

7.1 RF Gaussmeter
To measure the reader energizing field strength and orientation, an RF Gaussmeter is the appropriate instrument. The device should have a magnetic field sensor of small area and unambiguous orientation sensitivity. Either a small circular coil or a Hall-effect element may be used. The gaussmeter should have a flat frequency response in the range of interest for the ID system, and be calibrated to a known electromagnetic field standard.
7.2 Tag operation sensor
A special fixture associated with a test tag, which senses the correct operation of the tag in the reader field. Since the reader itself may not be able to read the signal from the tag even if the signal is correct, because of signal processing limitations, the measurement of the correct activation of the tag defines the theoretical maximum reading distance of a completely optimized reader.
7.3 Test signal tags
The test signal tag is a standard ID tag construction, except with the tag memory encoded to produce a simple periodic test pattern that can be traced as a signal through the reader signal processing sections.
Sending a "test signal" tag through the reader at a given velocity enables determination of the effective transmission time length for the tag.
7.4 Tag Speed Fixture
A device which runs tags through a space with controllable speed and orientation, to test the effectiveness of a reading volume. The device may place tags at varying orientations and place tags in close proximity to each other to test aspects of the reading system.
7.5 RF signal measurement equipment
For measuring emitted radiation of the RF-ID equipment to ascertain compliance with government regulations. Standard equipment is available for this purpose, including calibrated antennas, amplifiers, spectrum analyzers and graphical recording equipment.
The same equipment may also be used to measure ambient electromagnetic interference signals in the reading volume, to identify and solve potential reading problems.

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