Wi-Fi Airtime, SNR, and Modulation Margin

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's illustrative link budget lands a worst-case 29 dB SNR against a 25 dB assumed threshold for 64-QAM, leaving a 4 dB cushion. This audit re-checks airtime and margin; the design's preferred 5 dB margin requires a more robust starting rate, followed by site measurement.

Companion to the chapter Wireless Access: Wi-Fi — every number here comes from that chapter.

See the relationship before changing it

Set the input, apply the rule, then read the result with its unit.

Derive the baseline in four named moves

1. 1

   Name the input. The chapter baseline is 10 dB.

2. 2

   Name the relationship. margin = 39 dB clean SNR - interference - 25 dB requirement

3. 3

   Substitute with units. 39 - 10 - 25 = 4.00 dB

4. 4

   Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change interference allowance

Try Predict the direction of margin = 39 dB clean SNR - interference - 25 dB requirement. Test another interference allowance, then compare 64-qam margin.

Interference allowance

10 dB

Chapter baseline

64-QAM margin

Observe Interference spends modulation headroom even when the free-space link looks strong. Reset interference allowance to 10 and compare 64-qam margin.

Explain Interference consumes the clean signal-to-noise margin needed for the selected modulation, so each additional decibel removes one decibel of remaining margin.

Check yourself

What should you do before trusting a moved-control result?

Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.

What does this small model leave out?

Answer: Only interference allowance moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only interference allowance. It is not a field prediction.

TryTreat the chapter's burst payload and 25 m, 2.4 GHz link with its 64-QAM requirement as the fixed audit case; press Calculate.

ObserveWorst-case SNR lands at 29 dB and the modulation-margin result shows only 4 dB above the 25 dB requirement.

ExplainPath loss and interference subtract from received signal headroom; a modulation remains viable only when the resulting SNR exceeds its decoding threshold.

Ready: use the stated baseline inputs, then compare each displayed result.

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
802.11n MCS 6 at 58.5 Mbps	81,920 / 58,500,000	1.40 ms
Selected MCS 3 at 26 Mbps	81,920 / 26,000,000	3.15 ms

Audit conclusion: 64-QAM 3/4 is 802.11n MCS 6, not MCS 7. Its illustrative 25 dB threshold leaves 4 dB at the assumed worst-case 29 dB SNR, short of this design's 5 dB margin rule. Start with 16-QAM 1/2 (MCS 3) and measure the installed link. The higher-rate 256-QAM example's assumed 32 dB threshold is 3 dB above the budget; a single threshold does not predict a guaranteed failure rate.

CORRECTED: this audit previously computed free-space path loss at 25 m as 60 dB, omitting the chapter’s 20log10(f_GHz) term (about 8 dB at 2.4 GHz). The full formula gives 68 dB, then -56 dBm received power and 39 dB clean SNR. With an assumed 10 dB fade/interference allowance, worst-case SNR is 29 dB and 64-QAM margin is 4 dB, not 7 dB. The rate and recommendation above now follow the chapter's 5 dB design rule.

Every number above is taken from the chapter’s own Wi-Fi link-budget example and re-derived step by step.

Technical boundaries. CSMA contention, multipath evolution, rate adaptation, adjacent-channel traffic, packet retries, and antenna orientation are not simulated by this fixed Wi-Fi ledger.