Actually the earth is consider to be positive so we apply negative supply to our BTS so that when lightening occurs the charge travels to the positive terminal like earth not to the BTS. if we apply positive to our equipment then both the equipment and the earth will be connected and our instrument can be damaged during lightening
So negative polarity seperates our BTS from earth.
One more thing which i had heard is that all the digital equipments are configured to work on negative polarity
CH2MHill Communication group ltd.