For Natin: Electrical tilt of the antenna usually CANNOT be modified! There are antennas with two or more connections for one feeder(each connection for a different tilt value) but those are pretty expensive
For Van 14 _9_2002:
1 -diversity antenna (in mobile communication) is an antenna used to improve the uplink path with some dB(usually 3 dB). Briefly one can use the 2nd antenna in order to pick the higher signal among the 2 received(for doing that BTS i.e. RBS must pick the highest received signal or sum the signals or make the difference of the received signals, it depends..). It can be either space diversity or polarity diversity in GSM
2- about antenna (type, config, characteristics …)GSM antenna are omni antenna(with a gain of 11 dB) or sectorized antenna( Typical with an HPBV of 60 degrees and a gain of 17dB)
3 -what is cross antenna ? cross antenna means you have a cross-polar antenna. You income 2 feeders to it: one with a +45 degrees and the other with -45 degrees related to verticality. 4- How to install antenna (for Base Transceiver Station ).U use a pole mounting and you ask the radio network planner the following things: antenna height, azimuth to be pointed to, electrical tilt and mechanical tilt, and to what sector(cell) will be mapped( in order to connect it to its designated feeder).Mail me if there are more to be discussed
For rajib : One reason why antennas are not mounted horizontally: because vertical polarization waves are less attenuated than the horizontal ones. If anybody else knows a different reason I look forward for it!