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Multiple object identification with RFID technology has been analysed in some publications (Lim &
Mok 1998), (Vogt 2002), but focusing only on Aloha protocols and more sophisticated algorithms,
such as ID arbitration and code division multiple access. As passive RFID systems are typically
designed for extremely low cost applications, sophisticated algorithms and more efficient
microprocessors requiring systems are not applicable. Therefore, this paper focuses on simple
protocols suitable for identifying low cost tags.

PASSIVE RFID SYSTEM
An RFID system consists of tags, readers, and an application host. The readers communicate
wirelessly with the tags to obtain the information stored on them. The data sent by the reader is
modulated and backscattered from a number of tags. The cheapest RFID tags with the largest
commercial potential are passive, harvesting energy from the reader's communication signal to power
up their operation and communication with the reader (Auto-ID Labs 2001), (Vogt 2002). RFID
communication consists of a number of communication cycles. Each cycle consists of three sections:
first, the reader sends an activation field to the tags. Then, the reader sends a command to the tags, and
finally it sends a CW field that the tags modulate and backscatter back to the reader. The reader's
command field defines the content of the tags' replies. Communication bit rates are 70.18 kb/s for
forward link and 140.35 kb/s for backward link (Auto-ID Labs 2001).

ANTICOLLISION ANALYSIS

This chapter analyses EPC tree algorithm and Aloha protocols. EPC tree algorithm (Auto-ID Labs
2001) is chosen according to its wide popularity. Aloha protocols are included in ISO 18000-6
standard and also used by some RFID manufacturers (Vogt 2002).

EPC tree algorithm

EPC tree algorithm defined by Auto-ID labs (Auto-ID Labs 2001) goes through all possible code
combinations as a binary tree. It optimises the number of required time slots by ignoring those leaves
that do not respond without any further requests. Moreover, any collisions between replying tags do
not interfere with the identification procedure as the reader does not need to know the contents of
tags' replies, only whether any replies occur or not. This is because of the well-synchronized reply
window: it has eight slots and each tag will modulate the requested 3-bit section of its identification
code to one slot. Chose of the slot is based on the content of the reply. The eight slots allow each
different set of the tree bits to occupy different slots (2 = 8). The actual duration of the total
identification procedure of a number of tags lies between the maximum and minimum curves,
depending on the alignments of the identification codes of the tags in the current binary tree. These
maximum and minimum curves are presented in Figure 1. 64 bit tag identifiers were used in
calculations. The derivation of these curves is presented in publication (Penttila et al. 2004).

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40
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20
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Number of tags
Number of tags Number of tags
Number of tags
Figure 1 Maximum (left) and minimum (right) identification duration with EPC tree algorithm
Aloha family protocols