Passive sensors communicate by reflecting radio waves, like an echo. The energy savings are clear, but many reflections turn into a jumbled mess. It's like a choir where each singer repeats their own part without a conductor.
A new approach assigns each sensor a specific rhythm — a digital code. The receiver, like an experienced choirmaster, distinguishes the parts by this rhythm, even when signals arrive simultaneously.
The codes, together with the radio-frequency spectrum (set of frequencies), work like rules for merging onto a multi-lane highway: each sensor gets its own lane and timing. This reduces entropy (disorder) among the interference, and the receiver can pick out signals a hundred times weaker than the noise.
Some passive sensors can be powered directly by the energy of radio waves, so they don't need batteries at all.