September 22, 2005
UHF RFID deployments
To ensure interoperability between businesses that have deployed RFID systems purchased from different vendors, standards have to be in place. UHF RFID systems too need uniform standards. Along with interoperability, a uniform standard must also facilitate independent working of the different systems that exist in an environment.
In the case of UHF systems, there is a chance of RFID reader interference that could affect the widespread acceptance of UHF deployments. For small and medium scale installations, it is possible to use screens that shield readers from one another. Also, it is possible to switch one reader on and simultaneously switch-off all the other readers. These solutions however do not present a cost-effective alternative for large scale installations.
The RF energy emitted by a reader is inversely proportional to the square of the distance that it travels. The strength of a signal reflected by a passive UHF RFID tag is inversely proportional to the fourth power of the distance that it has to travel. This means that readers in an area can interfere with reader-tag communication occurring in the vicinity. In order to overcome this problem, the Auto-ID Center has proposed that the bandwidth be partitioned for the readers located in different areas in a warehouse.
This proposal will work only if there is enough bandwidth to apportion. In Europe, this idea may not work as the tags are required to operate in a range 865.6 MHz and 867.6 MHz. In America, the tags can work in the range 902 MHz to 928 MHz. Even though in the US the tags have sufficient range to work with, it will be difficult to accommodate readers that are operating using the same frequency far enough to prevent interference. Another alternative could be to have the readers oscillate between different frequencies in the UHF spectrum. However, this will not help to further the cause of a global standard as such a solution will not be workable in Europe.
Time division multiplexing enables readers to transmit in a synchronized manner. However, it is not an easy solution for deployment on a very large scale. In order to ensure that the tags are read by the synchronized readers, either the tags or the readers will have to move slowly enough so that no tags are missed. This will affect the reader throughput rate. It is possible to avoid collision between tag and reader waves if they are emitted at different frequencies. However, this solution needs to be implemented in a standardized manner to ensure that there is no jamming caused by incompatible systems and that tag data spectrums of one system are not affected by frequency- hopping readers from another system.
A possible solution is to treat read-only and read-write systems differently. In read-only systems, instead of the readers constantly asking the tags to identify themselves, the tags can dispatch their ID’s randomly. This will reduce the amount of interference that would otherwise happen due to the high energy emissions by the readers. This would also lead to the readers using less of the spectrum and facilitate the accommodation of more readers and tags. Read-write RFID systems can be placed in relative isolation so that there is minimum interference with the portal readers that only read the tags.
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