Wi-Fi Airtime, SNR, and Modulation Margin
Ada re-checks the Wi-Fi burst airtime, signal-to-noise headroom, and modulation margin from the chapter’s own link budget
ADA · CALCULATION AUDIT
Wi-Fi Airtime, SNR, and Modulation Margin
The chapter already gives enough numbers to check the design without inventing measurements. The useful question is not whether Wi-Fi can move the payload in theory; it is whether the selected modulation still has margin when interference appears — the chapter’s link budget lands a worst-case 29 dB SNR against a 25 dB requirement for 64-QAM, leaving a 4 dB cushion. This audit re-checks the airtime, SNR, and modulation margin from those figures.
Companion to the chapter Wireless Access: Wi-Fi — every number here comes from that chapter.
The airtime and margin checks
| Check | Arithmetic | Result |
| Burst payload | 10 KB x 1024 x 8 |
81,920 bits |
| Average report rate | 81,920 bits / 30 s |
2,730.7 bps = 2.73 kbps |
| Free-space path loss (25 m, 2.4 GHz) | 32.45 + 20log10(25) + 20log10(2.4) |
68.0 dB |
| Received power | 20 dBm - 68 dB - 8 dB |
-56 dBm |
| Clean SNR | -56 dBm - (-95 dBm) |
39 dB |
| Worst-case SNR | 39 dB - 10 dB |
29 dB |
| 64-QAM margin | 29 dB - 25 dB |
4 dB |
| 1 Mbps airtime | 81,920 / 1,000,000 |
81.92 ms |
| 54 Mbps airtime | 81,920 / 54,000,000 |
1.52 ms |
| 72.2 Mbps airtime | 81,920 / 72,200,000 |
1.13 ms |
Audit conclusion: selecting 64-QAM 3/4 is the conservative answer because the worst-case SNR still keeps a 4 dB cushion above the 25 dB requirement. The faster 256-QAM option would need the whole 32 dB worst-case budget and falls 3 dB short of it, guaranteeing link failures for people, doors, multipath, or another busy neighbor, not just risking them.
CORRECTED: this audit previously computed the free-space path loss at 25 m as 60 dB, which silently drops the chapter’s own 20log10(f_GHz) frequency term (about 8 dB at 2.4 GHz) from its stated FSPL formula. Using that formula in full gives 68 dB, cascading to -56 dBm received power, 39 dB clean SNR, and (with the same 10 dB interference budget the chapter uses elsewhere) a 29 dB worst-case SNR and 4 dB 64-QAM margin, not 7 dB. The table and conclusion above now match the chapter’s own corrected derivation.
Every number above is taken from the chapter’s own Wi-Fi link-budget example and re-derived step by step.