A Forward Link That Closes on Paper

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

A Forward Link That Closes on Paper

A UHF RFID link budget at 915 MHz puts a reader configured at 30 dBm and a 6 dBi antenna against 3 m of free-space path loss and a tag whose sensitivity is -18 dBm, and the chapter’s arithmetic lands on a forward-link estimate of -11.2 dBm. The same portal gives a pallet an 8-round inventory window — a 2.0 s dwell at 250 ms per round — before it leaves the read zone. This audit asks the question the chapter’s own caution invites: does a forward link that closes on paper with 6.8 dB of margin prove the tag will be heard, or only that it can wake?

Companion to the chapter RFID Design and Deployment — every number here comes from that chapter.

Ada: The chapter puts numbers beside the physical choices, which is exactly right — so let me verify the forward-link screen and its margin, then name what the screen still cannot promise.

• Wavelength at 915 MHz: 3e8 / 915e6 = 0.328 m.
• Free-space path loss at 3 m: 20 x log10(4 x pi x 3 / 0.328) = 20 x log10(114.9) = 41.2 dB.
• Forward link at the tag: 30 + 6 - 2 - 41.2 - 4 = -11.2 dBm.
• Margin against a -18 dBm tag sensitivity: -11.2 - (-18) = 6.8 dB of forward-link headroom.
• Dwell rounds in the same portal: a 2.0 s window at 250 ms per round gives 2.0 / 0.25 = 8 inventory rounds.

The audit conclusion is bounded on purpose: 6.8 dB and 8 rounds both reproduce, but the forward-link budget only proves the tag can wake — it says nothing about whether the reader can hear the far weaker backscatter on the return path, and 8 rounds only helps if tag orientation and anti-collision cooperate. A design that closes on paper still has to survive the return link and the real tag population before it earns a pass.

Every number above is taken from the chapter’s own material and re-derived step by step.