When introduced to the radio frequency created by the reader/controller – the tag will charge its internal capacitor wirelessly and communicate directly with the reader using the RF backscatter principle. Unlike inductive field modulation in HF frequencies (13.56 MHz ISO 15693) – the tag rebroadcasts its communication in the ~900-920 MHz frequency band (FCC) which allows for significant increases in read/write distances compared to HF.
EPC (Electronic Product Code) Generation 2 standards allow for the communication between a number of tags (up to thousands) and a single antenna. Due to the re-transmission of the signal, many tags can be read at once at a distance up to 10 m. The tag is first identified with its EPC, but many databases can be accessed.
EPC Global -- EPC Class 1 Generation 2 standards are the most common in industrial production.
ISO standards -- ISO and WTO accepted the EPC Global standards for Class 1 Gen 2 tags and this became the ISO 1800-6C air interface protocol.
A UHF tag contains four main memory banks for manipulation and storage of information (under the UHF Gen2 standard) and they are summarized below.
|EPC (Electronic Product Code)||TID (Tag Identifier)||User Memory||RFU (Reserve for Future Use)|
|Identifying information for the tag to initiate communication||Locked unique identifier for chipset||Full read / write capability||Full read / write capability|
|Full read / write capability with redundancy check for data integrity||Non-repeatable, cannot be rewritten||512 bits to 8 kbits typical||Storage for passwords, kill, lock and unlock passwords|
|96 or 128 bits typical, expandable to 496 bits||96 bits typical||Used for additional information outside of hte EPC memory||May not be included in all chipsets|
|Present on all chipsets||Present on all chipsets||May not be included in all chipsets|
Typical memory structure of UHF RFID tags
Since UHF antennas read many tags at once, it is possible for two tags to broadcast their EPC memory bank at the same time. Gen 2 standards provide an algorithm tags report EPC information in sequence so no tags are missed.
Tag reading and writing
UHF RFID communications relies on the tag's ability to effectively receive and retransmit radio frequency signals produced.
Linear polarization is achieved by broadcasting electromagnetic waves on a single plane. By focusing communication on a single plane, more power can be effectively used to achieve distance and still adhere by the FCC standards for the UHF spectrum (increase in about 3dB on a single plane). While this does not seem like much, it can allow for significantly longer read distances, with some major drawbacks:
While there are some specific applications where this antenna polarization is beneficial, when investigating an asset tracking program, circular polarization should be used.
Circular polarization utilizes RF transmission in two planes, which results in a rotating electromagnetic wave, completing a whole revolution throughout a single wavelength emitted. Simply, this results in a corkscrew-like emission of the RF signal. By broadcasting on two planes, the effective emission loss is about 3dB, resulting in a shortened read range than its linear counterpart. However, when discussing asset tracking with UHF, the benefits of the circular polarization far outweigh the loss in read range.
Because of the corkscrew-like nature of the field – the tags orientation can be effectively omni-directional, and the tags no longer need to be on the same plane as the reader. This is significant in discussing the tracking of assets when location, orientation/plane, and distance are never guaranteed. Circular antenna polarization is what allows for tracking portals and UHF mobile automation tracking for intralogistics applications.
The two main factors influencing the speed of data transfer are local storage / memory and the amount and type of data read / written.
Local storage / memory
Amount and type of data