Tuesday, August 19, 2014

Visualizing How Wi-Fi SNR Helps Determine the Achievable MCS Data Rate

If a Wi-Fi station has a better signal, you get more throughput. Everyone knows that. Here is a handy chart to help visualize it.

This table shows the "typical" data rates that Wi-Fi stations can achieve based on their SNR (signal to noise ratio). I say "typical" because it actually varies based on the radio chipset receiver sensitivity, but these values are a good starting point for most devices.

The achievable data rate (MCS rate) varies based on a number of variables:
  1. The 802.11 protocol - really a function of the increasing maturity of chipsets over time to handle more complex modulation types even when SNR is a bit lower.
  2. The channel width - typically doubling the channel width increases the noise floor by 3 dB, which decreases SNR. So to get the same MCS rate on wider channels you need higher SNR.
  3. The complexity of the modulation - notice as you get into more complex modulations like 64-QAM and 256-QAM that it doesn't take much more SNR to move from the lower encoding rate to the higher encoding rate, and vice versa in the opposite direction.
Typical Wi-Fi SNR to MCS Data Rate Mappings
(Download for full resolution image)

The table is color-coded based on modulation type:
- BPSK = Red
- QPSK = Orange
- 16-QAM = Yellow
- 64-QAM = Blue
- 256-QAM = Green

Update: Keith Parsons was kind enough to put this chart into a printable format in PDF. Download the printable version here (not color coded).


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