RF fundamentals: dBm, SNR, MCS and why signal strength lies

You can go a long way in Wi-Fi with four numbers and one rule of thumb. This page is those four numbers.

The decibel, quickly

dB is a ratio. dBm is a power referenced to one milliwatt. The only arithmetic you need:

+3 dB = double the power. -3 dB = half.
+10 dB = ten times the power. -10 dB = one tenth.
0 dBm = 1 mW. 20 dBm = 100 mW. 30 dBm = 1 W.

So an AP at 17 dBm is transmitting at half the power of one at 20 dBm, and a client reporting -70 dBm is receiving one tenth the power of one at -60 dBm.

Signal, noise and the number that matters

RSSI is how loud the signal is at the receiver. Noise floor is everything else — thermal noise plus any energy the receiver cannot decode. Typical noise floors run around -95 to -92 dBm in clean spectrum, worse in congested environments and wider channels.

SNR = RSSI - noise floor. This is the number that determines your data rate. A client at -65 dBm with a -95 dBm noise floor has 30 dB SNR and will do very well. The same -65 dBm with a -75 dBm noise floor has 10 dB SNR and will crawl.

This is why signal strength alone lies. A survey heat map showing -60 dBm everywhere tells you almost nothing if you did not also measure noise.

Channel width raises the noise floor

Widening the channel gathers more thermal noise:

Channel width	Noise floor penalty
20 MHz	baseline
40 MHz	+3 dB
80 MHz	+6 dB
160 MHz	+9 dB
320 MHz	+12 dB

A 160 MHz channel needs 9 dB more signal than a 20 MHz channel to achieve the same SNR. This is the real reason wide channels shrink coverage — and the reason "just use 160 MHz" is usually bad advice in an office.

The MCS ladder

Modulation and Coding Scheme indexes step from robust-and-slow to fast-and-fragile. Roughly, for 802.11ax:

MCS	Modulation	Approx. SNR needed	Character
0	BPSK	~5 dB	Survives almost anything
4	16-QAM	~15 dB	Comfortable
7	64-QAM	~22 dB	Good connection
9	256-QAM	~28 dB	Very good
11	1024-QAM	~35 dB	Close to the AP, clean air
13 (be)	4096-QAM	~40+ dB	Marketing conditions

Note what this means: 4096-QAM needs roughly 40 dB SNR, which means around -55 dBm in a clean environment. It exists a few metres from the AP and nowhere else. Data-sheet peak rates are quoted at this rung of the ladder.

Design targets worth using

-67 dBm — the standard primary-coverage target for voice and video, at the band you are designing for.
-70 dBm — acceptable for data-only coverage.
-75 dBm and below — treat as a hole, whatever the client says about "still connected".
SNR 25 dB or better in primary coverage; below 20 dB users will complain even with a strong signal.
Cell overlap of 15 to 20 percent at the design threshold, so roaming has somewhere to go.

The rule of thumb

Free-space loss is roughly 6 dB per doubling of distance. Every 6 dB you lose halves your range. That is why a 3 dB antenna gain change or a 3 dB power change matters far more than it sounds like it should, and why an AP behind a metal-framed glass partition can be effectively invisible three metres away.

Transmit power and the asymmetry trap

Turning AP power up does not fix a coverage hole. The AP can shout further; the client — a phone with a small antenna and a battery to protect — still cannot shout back. You create a region where clients associate at good signal, then fail on the uplink.

Match AP transmit power to the weakest client class you must support, typically 11 to 17 dBm indoors, not the 23 dBm maximum. If coverage is inadequate at that power, you need another AP, not more power.