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#1 2012-07-08 20:56:06

atrox
Contributor
Registered: 2010-01-08
Posts: 35

Collission detection

I did some research into implementing collision detection into the iso15693 code but have a fundamental problem.

The demodulator currently used is a correlation type demodulator. This means, over the incoming samples a correlation value for each symbol is calculated. The symbol with the highest value "wins" - i.e. is returned as the decoded symbol.

The three symbols the code looks for is "logic 1", "logic 0" and "End of Frame".

This type of demodulator is usually quite robust, also in low SNR conditions - since it decides for the symbol with likelihood.

Here lays the problem for collision detection: In an multi-card environment one card will produce a slightly stronger signal than the other(s), e.g. because its slightly nearer than the other one(s). The correlation demodulator will always decodes the strongest signal, even if it is only slightly stronger.

The only solution I could think of at the moment is to introduce a fourth symbol, which is a mixture of logic-0 and logic-1. Is this feasible? Won't it hurt the detection performance in bad SNR conditions, as it is similar to the other both bit symbols?

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