ThingMagic Mercury API v1.23.0 - Manual (Page 123)

How UHF RFID Works (Gen2)
Performance Tuning 123
Transmit Power
/reader/radio/readPower
/reader/radio/writePower
Although not directly related to Gen2 settings, the role of transmit power is so central to
successful RFID applications, a few words must be said about it. In order to have
successful communications between a reader and a tag, the tag must be able to be
powered-up by the reader’s signal. No communication at all occurs if the tag remains
dormant and does not respond to the reader. To achieve maximum performance, always
configure the reader to transmit at the highest permitted power into an antenna of highest
permitted gain, allowed by local regulations. (In regions governed by the FCC regulations,
this would be a +30 dBm power level into an antenna with a maximum gain of 9 dBiC, if
circularly polarized, and 6dBiL if linearly polarized.)
Reader-to-Tag Settings
/reader/gen2/Tari
There are two primary Gen2 settings that control reader-to-tag communications, “Tari”
and “link rate”. In our flashlight and mirror analogy, if the communication “protocol” was
Morse code, the “Tari” would control the length of a dot (or dash) and the link rate would
control how quickly the dots and dashes are sent. Just as with Morse code, the maximum
speed at which words can be sent is limited by the length of the dots and dashes .
ThingMagic adjusts both of these Gen2 settings together, so as the user selects a smaller
Tari, the link rate is automatically increased. Tari values offered are 6.25 usec, 12.5 usec,
and
2 5 usec, which are automatically paired with link rates of 40 kbps, 80 kbps and 160
kbps by the ThingMagic reader.
Factors to consider when selecting the Tari value (and therefore the link rate) are:
1. Will I be writing data to the tag often? The Gen2 protocol is designed to minimize
communication from the reader to the tag (during an inventory round, the messages sent
are very short.) If obtaining the EPC of the tag is essentially all that is required, then the
Tari/link- rate can be optimized for other factors – they won’t affect performance that
much. If tags are being written to, then decreasing the Tari (to increase the link rate) could
have noticeable benefits.
2. Are there many other readers in the area? Without going into too much explanation,
faster communication on a channel can cause interference with adjacent channels if the
channels are closely spaced. If there are other readers in the area and performance of
the first reader seems to degrade as more readers are turned on, then the system might
be experiencing adjacent channel interference between readers. This is rarely seen when
readers are operated in the North American (FCC) region as it contains 50 well-spaced