TECHNICAL PAPER  


Addendum for Fisheries Applications

Mike Beigel

Presented at the PIT Tag Workshop 2000
January 12, 2000

Summary

The use of (PIT tag) RFID systems in fisheries real-time underwater monitoring is a very specialized application of RFID system and product design. The ability to read very small fast moving tags traveling through a large stationary "reading volume" under water is a main objective of fisheries RF-ID systems.

The tags are a specific size and shape (12mm length x 2.1mm diameter cylinder), with a fixed-length read-only code. Tag velocity through reading volume is up to approximately 5 meters per second. Readers are stationary and tags move through a fixed reading volume. Multiple tags may be present in the reading volume simultaneously.


Tag Design


Figure A.1: Implantable RFID transponder with ferrite coil and resonance capacitor
The implantable ID transponder now used in fisheries is a cylindrical package (12 mm length by 2.1 mm width). However, tags could be produced either larger or smaller the present size while keeping the same form factor. 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.


Tag optimization (size vs. performance) is a principle issue in RF-ID system design. For a given tag technology, a tag with a larger coil will activate in a lower field strength and give a better reading distance. For a given tag volume (diameter times length), the amount of the volume occupied by the coil should be maximized. Increased coil resonance also allows higher energy transfer, and a coil in combination with a parallel resonance capacitor will generally form a more highly resonant circuit than a coil alone.


Reader Design

Excitation Frequency
The frequency and power of RF emissions is subject to worldwide regulation. In the "low-frequency RF" domain utilized by present inductive transponder systems, frequencies between 100 KHz and 135 KHz are chosen for worldwide regulatory acceptance. In the US fisheries, systems using frequencies of 125 KHz, 134.2 KHz (ISO Standard frequency) and 400 KHz are presently in use.

Reading Volume Geometry
The most challenging application in fisheries applications is large reading volumes through which fish can move at high velocity. Adult interrogation fish ladders and channels are perhaps the best examples of the requirement for a physically large and deep reading volume responsive to ID tags moving at high velocity. 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 in the design of RFID systems. The requirements for constructing large and powerful magnetic field generators for proposed fisheries projects demand very efficient, low-distortion electronics and resonant electromagnetic networks.

Continuous Field FDX vs. HDX
Full-duplex systems employ a method in which the reader emits a continuous RF field at a constant and the tag produces a modulation signal while energized and clocked by the reader field. Half-duplex systems must employ a pulsed reader field and a transponder that emits an ID code in the "quiet" time intervals between the field pulses. The requirement for continuous reading at high speed favors the continuous field FDX method which allows the tags to be activated at any time they come into the reader field and be sensed and decoded in the minimum possible time.


Figure A.2: Reader system with large remote coil

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. Another benefit of electromagnetically shielded reading volume is the reduction of electromagnetic noise within the volume, thereby enhancing readability of the tag signal from a tag in the volume.

Multiple Tags in Reader Field
When a tag is in the reader field it will be activated, and multiple tags in the reader field will be simultaneously activated. Without an anti-collision protocol, it must be assumed that if multiple tags are activated their signals will interfere and no tag will be read. Present anti-collision protocols greatly increase the time necessary to read any one tag. Multiple tag activation 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. Thus, the "multiple tag" problem in the fisheries application is particularly difficult because of the simultaneous requirements for fast tag velocity and reliable reading of the tag code.


Code Protocol Design

The code protocol for fisheries should optimize the combination of fast reading and data reliability. Fast reading is obtained by a tag protocol with data bits that use fewer clock cycles per bit, and have the minimum number of bits necessary to uniquely identify the anticipated population of fish. Data reliability is optimized by having some redundancy within the data bits (example: FSK encoding) and by inserting sufficient CRC (cyclic redundancy check) bits to insure verification of the received tag code by the reader.


ISO Protocol

The use of the ISO protocol (ISO 11784 and ISO 11785) provides a standardized tag coding system and allows multiple providers for the tag products. The ID code specification is defined in ISO 11784 The code and transmission protocol are summarized as follows:

Reader field frequency: 134.2 KHz continuous field for FDX tags.
Tag modulation method: Modified differential bi-phase
Cycles per bit: 32
Bit time: 0.238 milliseconds
Bits per second: 4194
Bits per telegram: 128
Telegram time: 30.5 milliseconds for 128 bits at 32 cycles per bit
CRC Code: 16 bit, calculated only to 64 bit identification code


(click to enlarge)
(click to enlarge)
Figure A.3: Modulation diagrams from ISO 11785
International Standard.


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