Copy / paste what one of my colleagues wrote:
I must admit I am not sure regarding the polarization used on our cell sites, an article stating that most operators use +/- 45° is probably correct. I’ll try to find out and let you know.
That said, knowing the base station antenna’s polarization is not as important as it might appear at first glance. In fact, it only matters in cases where a direct, uninterrupted, line-of-sight exists between the base station antenna and the UE antenna. In most practical cases, the signal the UE sees (and what the base station sees in the uplink case) is a mixture of signals that have been reflected off of and scattered by all kinds of objects (mountains, buildings, trees, vehicles etc.) in and around the line-of-sight. This is called “multipath”.
In a multipath environment, there is no way to predict the polarization of the signal at the receiving antenna, even if the polarization of the transmitting antenna is well known.
This is why the more complex variants of LTE (CAT.1 and above) require at least two antennas, which are typically set up to be sensitive to signals with different polarization.