When performing 5GHz channel planning the first step is creating your channel list (i.e. what channels you will use.) This decision is based on a few primary factors:
Client support of DFS Channels
What channels do the clients in your network support? Many older clients may not support UNII-2 or UNII-2-Extended channels. If the clients do not support a channel it should either not be used at all or it should only be used in an area where it is providing redundant coverage.
Proximity to RADAR
Is your location close to an area that may contain a weather RADAR? If so, you may want to consider not using some DFS channels (116-132.) When RADAR is detected in the area, APs will be forced to move to another channel. This channel may already be in use by a neighboring access point, causing co-channel interference.
Channel Width (20MHz, 40MHz, 80MHz, or 160 MHz)
The wider the channel, the higher the data rate and bandwidth that access point will be able support. However, this comes at a cost. The wider the channel, the fewer channels that are available for channel planning. In dense environments this will lead to increased co-channel interference. Only 2 160 MHz channels are available, making 160 MHz channels only an option in very small or very well designed environments with little to no neighboring interference. If DFS channels can be used, 80 MHz offers 6 non-overlapping channels and 40 MHz 12 non-overlapping channels respectively. 20 MHz channels are abundant with many possible combinations.
In general, very high density deployments or non-DFS deployments should use 20 MHz channels. In lower density environments or environments where DFS channels can be used 40 MHz may be preferred. In well designed DFS-friendly environments 80 MHz channels could be an option. 160 MHz channels are really not a viable option outside of SOHO deployments.
Obviously, all APs must support the selected channel width. The majority of clients should also support the channel width, otherwise the clients will not be able to take advantage of the additional bandwidth.
A helpful graphic that illustrates the different channel widths that are available in the 5GHz spectrum is provided below:
When implementing voice over WiFi it may be best to limit the number of channels used. In order to roam effectively phones must be able to quickly find their next AP. Phones can sometimes be programmed to only scan certain channels to optimize roaming performance. Using a large number of channels can increase the amount of time required for a phone to scan for a new AP.
Some channels are allowed to transmit at higher powers. These higher powers may be required in certain areas to provide the necessary coverage (especially in outdoor scenarios.)
Translating the Channel List to the Design
Once you have your channel list, you can then begin planning each APs channel.
Each AP should use a channel that is not used by any of its neighbor APs. What is a neighbor AP? As a general rule, any AP whose coverage would overlap with this AP's coverage at a signal level of -85 dBm or higher.
Additionally, placing an AP near a neighboring AP that is on a directly adjacent channel should be avoided. In other words, a 36+ (40 MHz) channel should not be placed beside a 44+ channel. This is because the sidebands of adjacent channels at high transmit powers (or when placed very closely to each other) can cause interference. This can not always be avoided and will not always result in a performance impact, but is a good general practice to follow.
The channels would be selected similarly to the following diagram:
Obviously, APs do not propagate a signal in perfect hexagons, the actual coverage of each AP would need to be determined either via a predictive or passive RF survey.