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Some notes from pointing on physical spacing between 2 antennas for LTE:
The reason we need a physical spacing between the antennas, is to obtain some spatial de-correlation between the antennas. The spacing required to get good de-correlation is a function of the environment in which the antennas are placed, hence it is not possible to give an exact measurement between the antennas as the environment is not known. In a very built up environment where the wave will experience many reflections good spatial de-correlation can be achieved with the antennas moderately close to each other. In a rural environment the physical spacing would have to be greater in order to achieve the same de-correlation.
Typical spacing between the antennas should be ½ wavelength and 2 wavelengths:

At 900 MHZ the wavelength is 30 cm
At 1800 MHz the wavelength is 16 cm
At 2700 MHz the wavelength is 11 cm.

A good de-correlation between antennas is required so that each antenna is able to receive a different signal at the same frequency. If this is achieved then you can effectively double the throughput of the system with 2 antennas and with 3 antennas you can triple the throughput.