What are the problems associated with doing so, and how does one get around them?
There really are very few problems one faces when reading multiple tags. All RFID air-interface protocols—the rules governing how tags and readers communicate—include algorithms that prevent tags from interfering with one another.
One way to understand this is to think of a teacher trying to take attendance in a room containing a lot of students, but with no list of the children's names. The instructor might announce, "If your last name begins with the letter A, stand up." If five children stand up, he or she might say, "If your last name starts with AA, remain standing. If not, sit down." That might leave one child to communicate with. Then the teacher can say, "If your last name starts with AB, please stand up." And so on.
RFID systems have a variety of different means of doing this as rapidly as possible. But essentially, they all come down to the same processes of polling the tags, and narrowing down the requests until the reader can communicate with one tag at a time. It happens very quickly, so it appears you are reading 50 or so tags at the same time, when, in fact, the reader is communicating with only one tag in any given interval.
The only real problem inherent to reading multiple tags is having them block each other. If two tags are placed with one behind the other, the tag in front can block the signal of the one in back of it. If no energy reaches the tag behind, it can not be read. With high-frequency (HF) tags, a company called Magellan Technology has solved this issue using phased jitter modulation (PJM). I have seen the firm read 100 tags on documents stacked one on top of another. I have also witnessed NXP Semiconductors interrogate tags on 90 drug vials without a problem.
With ultrahigh-frequency (UHF) tags, this can sometimes be an issue, though UHF tags are improving all the time. The best way to avoid the problem is to make sure there is separation between tags, and to avoid placing one tag directly behind the other.
—Mark Roberti, Editor, RFID Journal
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