Why are most antennas in cellular networks +/- 45° polarized? I've just been asked a strange question that I cannot find an answer to (even on the internet it seems I can't find any explanation for this) and I ended up wondering why most of the antennas which work as "base stations" are nowadays +/- 45° polarized. While I do understand the meaning of polarization diversity, it is still not clear to me why antennas' producers chose those 2 polarizations instead of horizontal / vertical or other 2 orthogonal angles whatsoever. Moreover, I am sure that the radiators inside an antenna cannot perfectly have a +/- 45° polarization. What happens then if those are "rotated" a little bit (let's say +55° / - 35°)? Is the orthogonality the only important thing, or is a correct orientation (+/- 45°) important too and why is that?
Bonus question: I think that the polarization of mobile phones has nothing to do with the polarization of base station antennas (because of reflections and multi-paths their transmissions' polarizations could be received "rotated" in comparison to how it was sent), is that a correct assumption?
EDIT: I can't really remember the source, but I read somewhere that we don't use H-Pol, because the ground greatly attenuates the field in that case.
 A: A motive is: "Seen from the front or rear of a handset user, the polarization will be dominated by the vertical component (the most handsets have a linear polarization because they use patch or monopole antennas), but in the lateral direction a handset is typically held at a large angle to the vertical, between the mouth and the ear of the standing or seated user, typically at least 45°."
Antennas for base station in wireless communications, Zhi Ning Chen and Kwai-Man Luk. pp. 40
A: I'm going to clarify the whole questions one by one as they are needed to be explained as a chain.
First of all, the handset mobile antennas are linearly polarized because of the simplicity, compactness and not having 3(dB) gain drop due to the circular polarization. Given this and the fact that the base stations are supposed to have coverage for all mobile devices of a cellular network, they could have circular polarization or linear polarization; but circular polarization is always a costly solution. So, the vendors mostly go for linear polarization. Moreover, as two +45/-45 polarization are still orthogonal to each other, any dual polarized antenna could be easily used by a 45 degree physical rotation of the structure itself. The reason of this rotation is also coming from the nature of multi-path propagation model for intra-city radio coverage. A pure horizontal polarization is being blocked by the building and is not useful whatsoever. A pure vertical has the best availability for the network in long distances as the gap between the building and tower are vertical gaps; but the problem is with short range coverage and also the likely horizontal position of the handhelds which won't get any signal. Therefore, the slant polarization is the best way of retaining acceptable signal level for the whole devices of the network while having short-range and long-range coverage.
A: One aspect about the +-45 degree missed here is that it balances the average power required to be delivered to each antenna. With the V/H pair, the vertical would likely get more utilization (more power on average) than the horizontal (because of the user antenna orientations as mentioned in the other posts).
A: A simple answer is: with the exception of the occasional Frank Gehry building, urban areas consist of vertical and horizontal surfaces, so a diagonal linear polarization has the best chance of getting around.
