Leong et al. (2005, 2006b) have
presented detailed discussions and analysis on this matter and Table 7.2, as extracted from
Leong et al. (2006b), summarizes the minimum distance (calculated using a piecewise path
loss model with variable environmental factor) between two antennas connected to readers
before one antenna operating at a certain channel will prevent the other antenna from using
that channel. It should be noted that these results are obtained using a 0 dB isotropic
receiving antenna, and do not represent any real life situation, as a typical RFID antenna
will be a directional antenna. Nonetheless, the data presented in the table gives suf?¬?cient
128 RFID Handbook: Applications, Technology, Security, and Privacy
evidence that a low threshold value for the LBT as speci?¬?ed in ETSI 302 208 is severe enough
to impede the reader deployment in a dense RFID reader system.
7.3 Reader Synchronization
Under the concept of reader synchronization, all the RFID readers in a certain area, for
example all the readers in a warehouse, are networked together through a central control
unit. The connection method can be the common Ethernet connection, or equivalent, and
will be discussed in the next section.
Since all the readers are linked together, physically or wirelessly, they can be directed to
carry commands at the same time. In addition, they can be assigned channels dynamically
so that the spectrum management is optimized while the reader collision is minimized.
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