Thursday, August 21, 2014

Quick Take: Wider Channel Widths Are Flashy but Not Efficient

I've been thinking of writing a well-articulated blog post on why the preference for high-density Wi-Fi networks is smaller channel width over larger channel width. This post is NOT that.

Instead, I was on Twitter articulating some of the logical points why smaller channel widths provide better aggregate capacity than larger channel widths (assuming you deploy enough radios and take advantage of all the spectrum at your disposal). Here is a quick recap of those points.

You might want to reference my SNR to MCS Index Mapping Table, which shows why larger channels result in a reduction in modulation rate that can often offset the gain from using the wider bandwidth in the first place. And my 802.11ac Receiver Sensitivity charts show that you have to have a really great signal strength for wider channels to even be considered, but watch out in your design because overcompensating to achieve higher signal strength will increase co-channel interference (CCI) which travels a LONG ways! Finally, my post on 802.11ac Adjacent Channel Interference (ACI) shows that wider channels create more ACI than smaller channels, and ACI is even more detrimental and unfriendly than CCI. Therefore, radio receivers require greater adjacent channel rejection (up to 8dB more), and with fewer channels for frequency re-use ACI is more likely.


Wednesday, August 20, 2014

802.11ac Receiver Sensitivity

Following my previous post regarding typical SNR to MCS rate mappings for Wi-Fi clients, an interesting discussion was held on Twitter regarding the effects of increased channel width on the ability of a client to decode frames at any given SNR. Long story short, wider channels increase the noise power captured by the receiving radio which reduces its SNR. For every doubling of channel width, you require 3dB better signal to achieve the same MCS rate.

George Ou created a chart showing the relative range of each MCS rate based at various channel widths:

Following up on his work, I thought it would be useful to provide some context around these coverage ranges by referencing it against a typical noise floor of -93 dBm found in many environments. Using this noise floor and the SNR to MCS rate mapping table, combined with the relative coverage ranges (based on RF signal propagation using the inverse square law) we can visualize what data rates a typical 802.11ac radio will experience at various RSSI and SNR signal levels for each channel width.

Note - these receiver sensitivities are not absolute. Wi-Fi radios vary. But this chart is a good approximation for many radios and provides a generic reference for you to visualize and understand this effect.

802.11ac Receiver Sensitivity (Down to -91 dBm)

Update: as requested by various folks, here is a zoomed-in version of the same chart so that the higher data rates are easier to distinguish.

802.11ac Receiver Sensitivity (Down to -82 dBm)

Thanks to George Ou for his work on the initial chart!


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).