Thursday, October 9, 2014

U-NII Unlicensed Spectrum Inventory in 5 GHz Bands

Given the recent FCC Report & Order on U-NII (Unlicensed National Information Infrastructure) rule changes in March/April of 2014, I thought it would be helpful to recap the new regulations in the United States regarding the 5 GHz unlicensed spectrum bands. I've put together the following table for quick reference:
U-NII Unlicensed Spectrum in 5 GHz
(Click to Download PDF)

Additionally, here is a graphic of the 5 GHz U-NII bands, both current and proposed, from the NTIA report made in January 2013 (note - this graphic does NOT reflect the change with regards to the extension of U-NII 3 up to 5.850 GHz).

NTIA Graphic of U-NII Unlicensed Spectrum in 5 GHz

Cheers,
Andrew von Nagy

Saturday, September 27, 2014

Wi-Fi SNR to MCS Data Rate Mapping Reference

I previously posted a picture of an SNR to MCS data rate mapping table that I have compiled based on various sources of credible research. Keith Parsons has kindly put this information into a printable format for reference. You can download them below.

It should be noted that individual devices perform differently. These tables are simply generic estimates that are a good approximation for many Wi-Fi devices. In other words, it's not perfect.

Click to Download Full Version (PDF)

This table maps client SNR values to MCS indexes for the purpose of determining the data rates that clients can achieve based on the signal quality of their connection to the AP.


SNR is also related to RSSI. Two RSSI values are of importance: the Minimum Receiver Sensitivity and the Expected Receiver Sensitivity. The 802.11 minimum receiver sensitivity tables often referenced in research and testing material are the required minimum RSSI values that a radio should be able to decode a given modulation type and encoding rate (MCS index) with a packet error rate (PER) less than 10%. Most 802.11 radios provide better receiver sensitivity than the minimum requirement. Therefore, the "Expected Receiver Sensitivity" reflects the typical receive sensitivity of clients with the ability to achieve any given MCS index at a lower RSSI than the minimum receiver sensitivity required to pass testing. For example, the minimum receiver sensitivity for an 802.11ac 20 MHz PPDU at MCS 9 is -57dBm, but most 802.11ac radios can decode this PPDU at a lower RSSI such as -62dBm. 

It should also be noted that a receiver's ability to perform Maximal Ratio Combining (MRC) across multiple receive antenna chains is not reflected in this SNR chart. MRC can allow a device to receive the incoming signal at a lower energy level at each of the individual antenna inputs to the RF front-end radio circuitry which are then combined using digital signal processing (DSP) to provide additive gain. This effectively increases the SNR the client experiences. MRC is based on each client device's receive antenna chain specifications and the number of spatial streams being used for the link between the client and AP, with extra receive radio chains being used for MRC. After MRC gain is added, you can use this table to lookup the MCS rate the client may be able to achieve given it's final resulting SNR . Also be aware that many manufacturer receive sensitivity specifications will list RSSI and SNR values 3-6 dB lower than what is specified here because they list the signal level at the antenna input prior to DSP and MRC gain.

Some of the references used to help compile this table (not an exhaustive list):

  1. IEEE and Realtek - Receiver Sensitivity Tables for MIMO-OFDM 802.11n (PPT) - See tables in appendix
  2. Heegard - Range versus Rate in IEEE 802.11g Wireless Local Area Networks (PDF)
  3. IEEE 802.11-2012 Standard - Sections 16.4.8.2 (802.11 DSSS), 17.4.8.2 (802.11b HR-DSSS), 18.3.10.2 (802.11a OFDM), 19.5.2 (802.11g ERP), 20.3.21.1 (802.11n HT)
  4. IEEE 802.11ac-2013 - Section 22.3.19.1 (802.11ac VHT)
  5. Aruba 802.11ac In-Depth (PDF) - See figure 19, page 25


Cheers,
Andrew von Nagy

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.











Cheers,
Andrew