A Forward Link That Closes on Paper
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.
See the relationship before changing it
The figure reads from left to right. The blue card is inventory-round time. The middle card applies this page's rule. The green card is rounds in dwell window. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only inventory-round time, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 250 ms.
- 2
Name the relationship. rounds = 2,000 ms dwell / round time
- 3
Substitute with units. 2,000 / 250 = 8.0 rounds
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change inventory-round time
Try Predict the direction of rounds = 2,000 ms dwell / round time. Test another inventory-round time, then compare rounds in dwell window.
Observe Shorter inventory rounds fit more read attempts inside the same two-second dwell. Reset inventory-round time to 250 and compare rounds in dwell window.
Explain Shorter inventory rounds fit more read attempts inside the same two-second dwell.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
With frequency at 915 MHz and range at 3 m, calculate wavelength before pressing Check audit.
The forward-link ledger settles at -11.2 dBm and shows 6.8 dB wake-up margin; Reset audit returns the status to Ready.
The 6.8 dB reader-to-tag margin proves chip activation only; backscatter must traverse a separate, weaker tag-to-reader path.
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 dBmtag sensitivity:-11.2 - (-18) = 6.8 dBof forward-link headroom. - Dwell rounds in the same portal: a
2.0 swindow at250 msper round gives2.0 / 0.25 = 8inventory 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.
Technical boundaries: Free-space loss here omits the backscatter return budget, antenna polarisation, tag detuning, multipath, reader noise floor, and collisions among tags sharing 8 inventory rounds.