Cisco Aironet 1850 Series Deployment Manual page 13

Note Installing APs above the ceiling tiles must be done only when mounting below the ceiling is not an option.
The tiles must not be conductive; such installations will degrade advanced RF features such as voice and
location, so verify coverage and performance. Always, try to mount the AP as close to the inside middle
of the tile as possible, and avoid areas with obstructions
Figure 11: Installing AP above ceiling tiles: Pick an area Clear of Obstructions and Avoid Ceiling Clutter
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