UHF RFID is probably the most noticed RFID at this time. While there have been more HF tags issued, the high profile nature of UHF has meant that many more people are aware of UHF RFID.
In the world of RFID, UHF is the newcomer to the fold. HF and LF tags were is use in many places when UHF started to become available. The industry needed a longer range passive tag and UHF is the answer.
The technology normally uses propogative technology like 2.45 GHz, and so like 2.45 GHz it has problems around liquids and materials that absorb the frequency. However it is possible to use the technology in a near field (or inductive) mode similar to LF and HF and this gets around the liquid problems.
The technology in Propogative mode can work at distances of 6 – 8 metres which is a big step up from the range of the other technologies. In Near Field mode this is much less (only 10s of centimeters) but it is possible to build tags with antennas that have the capabilities of both modes.
The frequency range 860 – 960 MHz is a very large range, but it is necessary to have a range this large to encompass all of the regulatory areas in the world. In the USA, the frequency is 902 – 928 MHz, but in Europe it is 865 – 868 MHz, and in Japan the frequency available is 952 – 957.6 Mhz. So if an UHF tag is going to work all over the world it has to be able to work from 860 – 960 MHz. If you are interested in the various UHF frequencies available around the world you can see them here.
UHF has gained a lot of publicity and use with the release of the EPCglobal UHF Gen 2 standard. This standards improved the performance of the original ISO/IEC 18000-6 Type A and Type B air interface protocols. ISO took the work of EPCglobal, and suggested some changes which were then incorporated back into the EPCglobal standard. ISO released this work as Type C in ISO/IEC 18000-6.
Recently the ISO workgroup has just finished work on changes to ISO/IEC 18000-6 and a new version was published on December 1, 2010. This new version includes several new features:
The release of the EPCglobal standard lead to the adoption of UHF by several groups. First of these was Wal-Mart. Their mandates to force suppliers into tagging cases and pallets were the first time that RFID was seriously used in the retail market place. The US Department of Defense followed shortly after with a mandate to tag items over a specified value. Other organizations have also taken the use of UHF RFID to heart and required the tagging of items to the standards for this frequency.
The latest use of UHF is the tagging of apparel in a test of item level tagging. Wal-Mart has lead the charge with the decision to tag specialty jeans and underwear. This was quickly adopted by other retailers and is rapidly becoming a major test for RFID.
This part of ISO/IEC 18000 defines the air interface for radio frequency identification (RFID) devices operating in the 860 MHz to 960 MHz Industrial, Scientific, and Medical (ISM) band used in item management applications. It provides a common technical specification for RFID devices that can be used by ISO committees developing RFID application standards. This part of ISO/IEC 18000 is intended to allow for compatibility and to encourage inter-operability of products for the growing RFID market in the international marketplace. It defines the forward and return link parameters for technical attributes including, but not limited to, operating frequency, operating channel accuracy, occupied channel bandwidth, maximum effective isotropic radiated power (EIRP), spurious emissions, modulation, duty cycle, data coding, bit rate, bit rate accuracy, bit transmission order, and, where appropriate, operating channels, frequency hop rate, hop sequence, spreading sequence, and chip rate. It further defines the communications protocol used in the air interface.
This part of ISO/IEC 18000 specifies the physical and logical requirements for a passive-backscatter, Interrogator-Talks-First (ITF) or tag-talks-only-after-listening (TOTAL) RFID system. The system comprises Interrogators, and tags, also known as labels. An Interrogator receives information from a tag by transmitting a continuous-wave (CW) RF signal to the tag; the tag responds by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the Interrogator. The system is ITF, meaning that a tag modulates its antenna reflection coefficient with an information signal only after being directed to do so by an Interrogator, or TOTAL, meaning that a tag modulates its antenna reflection coefficient with an information signal upon entering an Interrogator's field after first listening for Interrogator modulation in order to determine if the system is ITF or not.
In detail, this part of ISO/IEC 18000 contains one mode with four types. The detailed technical differences between the four types are shown in the associated parameter tables.
Types A, B and C are ITF. Type A uses Pulse-Interval Encoding (PIE) in the forward link and an adaptive ALOHA collision-arbitration algorithm. Type B uses Manchester in the forward link and an adaptive binary-tree collision-arbitration algorithm. Type C uses PIE in the forward link and a random slotted collision-arbitration algorithm.
Type D is TOTAL based on Pulse Position Encoding or Miller M=2 encoded subcarrier.
So called microwave RFID operates in the 2.45 GHz band and is available all over the world. The technology uses radio propagation techniques which mean that range is typically limited by the amount of power transmitted by the reader. (NOTE: According to RF literature, 2.45 GHz falls in the UHF band, however, this frequency RFID tag is normally referred to as a microwave tag). Many companies offering 2.45 GHz also offer active tag systems which increase the range considerably.
The big problem with propogative RFID systems is that they are very vulnerable to the effects of moisture and metal. An exposed tag (paper label) can be almost useless if it gets wet.
Unlike HF and LF RFID systems that use inductive coupling, propogative systems use backscatter of the reader’s carrier wave to piggy-back their reply on. This means that the reader talks to the tag, then sends plain CW (carrier wave). The tag modulates this carrier wave with the response and the reader is able to detect the modulation of its own carrier. The signal from the tag is very small and so the reader has to be very sensitive to detect and demodulate the signal.
Systems that use 2.45 GHz can be either active or passive systems. The range of a passive system is limited to a few metres at best, and more typically around one metre. Active systems can give read ranges approaching several tens of metres.
In ISO/IEC JTC 1/SC 31 the standard is ISO/IEC 18000-4: Information technology — Radio-frequency identification for item management — Part 4: Parameters for air interface communications at 2,45 GHz
This part of ISO/IEC 18000 defines the air interface for radio frequency identification (RFID) devices operating in the 2,45 GHz Industrial, Scientific, and Medical (ISM) band used in item management applications. The purpose of this part of ISO/IEC 18000 is to provide a common technical specification for RFID devices that may be used by ISO committees developing RFID application standards. This part of ISO/IEC 18000 is intended to allow for compatibility and to encourage inter-operability of products for the growing RFID market in the international marketplace. This part of ISO/IEC 18000 defines the forward and return link parameters for technical attributes including, but not limited to, operating frequency, operating channel accuracy, occupied channel bandwidth, maximum EIRP, spurious emissions, modulation, duty cycle, data coding, bit rate, bit rate accuracy, bit transmission order, and where appropriate operating channels, frequency hop rate, hop sequence, spreading sequence, and chip rate. This part of ISO/IEC 18000 further defines the communications protocol used in the air interface.
This part of ISO/IEC 18000contains two modes. The first is a passive tag operating as an interrogator talks first while the second in a battery assisted tag operating as a tag talks first. The detailed technical differences between the modes are shown in the parameter tables.
The FHSS backscatter option or the narrow band operation RFID system shall include an interrogator that runs the FHSS backscatter option 1 RFID protocol or in narrow band operation, as well as one or more tags within the interrogation zone.
This clause describes a RFID system, offering a gross data rate up to 384 kbit/s at the air interface in case of Read/Write (R/W) tag. In case of Read Only (R/O) tag the data rate is 76,8 kbit/s. By using of battery powered tags such a system is well designed for long-range RFID applications. This air interface description does not explicit claim for battery assistance in the tag.
There are other technologies that use the 2.45 GHz frequency bands which could be considered similar to RFID. These will be discussed in another post.