Adjacent Channel Interference (ACI) solution
- Increase physical distance
- Change channel (Non-overlap)
- Decrease transmission power of interference source
- Change channel (overlap)
Once again, the reason why ACI exist is because channels have overlap component, especially in 2.4 GHz band. So immediately adjacent channel may not be enough separation of two channels to elimnate ACI.
Definitive solution to avoid ACI is to use orthogonal/non-overlapping channel for any APs that are in the close enough physical proximity where they can hear each other. In simple term, separate far enough channel distance (frequency separation).
The non-overlapping channels depends on the channel width and frequency band of the Wi-Fi. Below is the table of non-overlapping channels for most commonly used channel width for each band. Basically, one can think as 5 GHz has no channel overlap.
Bold are DFS Channels
*Non-orthogonal i.e. overlapping channels may be utilized but this is not most common practice and academic articles tries to look into this. It gets very complex when trying to incorporate non-orthogonal channel for potential channel selection. One study tried to compare use of orthogonal channel only and non-orthogonal channel for automated optimal channel assignment logic creation and concluded it is better to just stick with orthogonal channels (ref).