For a simple project like one or two APs, simply assign the least used frequencies from the site survey. For more complex projects involving three or more APs, pick a frequency reuse pattern for the frequencies that are used for the project; start with the most complicated part of your site survey and start assigning frequencies. Plan the location of APs initially for coverage, not capacity. Avoid overlapping channels if possible. However, if an area has to be overlapped, plan it such that it is naturally an area where the most capacity would be required, such as in a library, conference room, or lecture hall (see Figure rf-6).
Overlap in a central library
For multiple-floor installations, if more than 30 dB of isolation is used between floors (such as concrete and rebar floors), try not to use the same frequency directly above or below a cell that has already been allocated. Where less than 30 dB of isolation is used between floors (such as a two-by-four framed apartment building), the plan needs to take the three-dimensionality of the cell into account (see Figure rf-7).
Cells are 3-D in buildings
Some of the most complex problems are areas where not enough channels are available to plan out the space. In a two-dimensional space, this can happen in areas where a central room has to be covered with surrounding classrooms or offices, such as a library or lab. In a three-dimensional space, this can happen in a tri-level portion of a building (see Figure rf-8).
A case where RF planning is difficult
The signals from difference floors can overlap and intrude on another. In some cases when you have no way around two cells that use the same frequency being bordered against each other, plan the seam to be in areas where no coverage is necessary, such as equipment rooms, restrooms, wiring closets, stairwells, and janitorial supply rooms.