Monday, August 4, 2014

802.11ac Adjacent Channel Interference (ACI)

I was reading this article on development of 5G cellular technologies when this bit on OFDM deficiencies and the need for new waveforms to support higher capacities and user densities caught my attention (emphasis added by me):
4G and 4G+ networks employ a type of waveform called orthogonal frequency division multiplexing (OFDM) as the fundamental element in the physical layer (PHY).  In fact, almost all modern communication networks are built on OFDM because OFDM improved data rates and network reliability significantly by taking advantage of multi-path a common artifact of wireless transmissions.  However as time and demands progress, OFDM technology suffers from out-of-band spectrum regrowth resulting in high side lobes that limit spectral efficiency.  In other words, network operators cannot efficiently use their available spectrum because two users on adjacent channels would interfere with one another.  OFDM also suffers from high peak-to-average ratio of the power amplifier, resulting in lower battery life of the mobile device.  To address OFDM deficiencies, researchers are investigating alternative methods including generalized frequency division multiplexing, filter bank multi-carrier, and universal filter multi-carrier.  Researchers speculate that using one of these approaches over OFDM may improve network capacity by 30 percent or more while improving the battery life for all mobile devices."

This aligns with most Wi-Fi professionals' recommendations to deploy 5 GHz radios on non-adjacent channels to avoid that dreaded adjacent channel interference (ACI). 

And if you look at an OFDM Wi-Fi transmit spectral mask, either the limits defined in the standard or using a spectrum analyzer, you will see rather significant side lobes that can impact adjacent channels (and channels even further away, depending on proximity and power levels). I have even considered including discussion of OFDM spectral masks within my 802.11ac presentations and writing due to the fact that as channel widths get wider, so to do their side lobes because the frequency distance from the main carrier signal at which the relative power level must be reduced to be in compliance increases as well. 

Here is an illustration that I put together over a year ago but never published and kept in the appendix of my 11ac presentation. It illustrates how ACI can increase due to the spectral mask differences as channel widths get larger. I have inlaid two 20 MHz spectral masks inside the 40 MHz mask, and two 40 MHz masks inside the 80 MHz mask. Essentially, the side lobe power level reduction requirements are based on the size of the main signal lobe; as the main signal lobe gets larger, so too does the allowed power in side band lobes.

Spectral Mask Comparison of 20, 40, and 80 MHz Wi-Fi Channels
And below is a capture from a spectrum analyzer approximately 10 feet away from an 802.11ac AP operating in 80 MHz mode with a large amount of traffic. Notice the high signal level in adjacent channels (52-64, and likely would impact the as-of-yet unapproved U-NII 2B band). 

Spectrum Analysis Capture of an 802.11ac 80 MHz Waveform

This is why you need a minimum of 10 feet of separation between radios operating in the same frequency band (unless other shielding mechanisms are used, which increase cost), as well as the recommendation to have adjacent 5 GHz radios operating on non-adjacent channels. This will start to become a bigger issue as we deploy more 5 GHz radios to handle capacity and user density demands. More manufacturers are considering developing software-defined radios (SDR) as well as multi-radio APs that have more than one radio operating in the 5 GHz band. You should carefully research and verify (through real-world testing) these solutions to ensure that interference within the AP is not an issue.

As always, the better you understand what's going on at the physical layer, the better wireless engineer and architect you will be. 

Happy signal hunting,


  1. Hi Andrew,
    Thanks for the great article! I noticed in the spectrum screenshot that the adjacent channel power looked to be a difference of about 10dbm.

    I'm curious if this gap gets any wider over distance, say 20 or 30 feet. Would it still be 10dbm between the intended signal and the side lobes?

    I'd love the opportunity to experiment with this. I'd like to see the performance impact of running two adjacent channel APs at different distances from each other.

    Anyways, thanks again for the great articles. They're always thought provoking and a pleasure to read.

    Best Regards,

    1. Hi Greg,
      It's actually about a -20 dB drop from the main signal lobe to the first sideband lobe. The relative level of signal drop is due to the quality of the filters in the transmitting device, and should remain the same at further distances. For instance, if you my spectrum analysis adapter were further away and recorded the main signal lobe at -65 dBm then I would expect the signal level in the immediately adjacent channels to be around -85 dBm (from this specific AP transmitting a signal). The increased signal level in adjacent channels will most often not be recognized and decoded as Wi-Fi frame by stations on the adjacent channel, but will increase the noise floor and degrade the signal quality (SNR) for stations on the adjacent channel thus impacting performance (CRC errors, retries, and dynamic rate switching to lower data rates).


  2. Great article Andrew, thanks. I fear the day that the standard consumer Wi-Fi kit is all set to use 80MHz channels by default. Sure, it enable the advertisement of those high headline speed figures... but... you could just imagine an episode of Friends set in 2015 where they're waving fists at the people in the neighbouring apartments because their Wi-Fi is interfering :)

  3. Hey Andrew,

    Great post. Any idea what the cause is of the blue vertical lines in the spectrum analyzer?
    I also see these a lot assesing the 5GHz spectrum bands. Also using Chanalyzer.

    1. Hi Koen,
      I'm not sure what that is. I've never paid much attention to it but now I'm curious. Perhaps the MetaGeek guys know?