Antenna gain of cell tower
Start by looking at the geometry.  Power doesn't come from nowhere.  You have to take it from other areas.  So, we have a tower with a cell antenna on it.  If you read up on cell towers, they usually have three antennas, each operating a 120 degree section.  So, we can take that power and direct it elsewhere for some gain.  There's also no sense transmitting into space or the ground, so that'll give us some more gain to play with.  I'm not going to answer this for you, but I'm sure that you can come up with a decent approximation if you search out the radiation pattern of one antenna of a cell tower.  It's a decent starting location at least.  If all else fails, assume a gain of 0 dB.  You're guaranteed not to underestimate in that case.