Also, bandwidth is a function of how much spectrum is available (for example between 1900MHz and 2000MHz there is a chunk of 100MHz) and the spectral efficiency (for example, 1 to 2 bits per Hz available).

For example:

If a carrier "owns" the block between 1900MHz and 1940MHz, and let's say LTE supports 2 bits/Hz using Frequency Division Duplexing (each direction, up and down, gets a chunk of spectrum), with 3 sectors per cellular tower, the carrier can support a total of 40x2x3 = 240Mbps per tower, or 120Mbps each way.

If the tower serves 1000 users (they serve far more in dense urban areas), each user will have 120kbps of capacity, and if they all use it at the same time, that's the speed they'll get. If there's a single user, the maximum down speed for that user would be 120/3 = 40Mbps.