sorry to say like this i too donot know the exact answer,but i want to give u an idea so i am replying to u.i think the ans is to overcome the ring around condition which is same as multiple access techniques used in satellite communication.
In mobile system there are more user than BTS tower. So while finding the exact user transmiting antenna should have to choose the correct one from many user so it has to have high frequency than uplink. As we know that there are very less number of BTS as compared with user....
that higher frequencies hav higher free space loss.....now uplink is frequency from mob to bts.....now our mobile has lower power capacity as compared to bts....so we use lower frequency for it so that free spacce loss can be encountered....
that higher frequencies hav higher free space loss.....now uplink is frequency from mob to bts.....now our mobile has lower power capacity as compared to bts....so we use lower frequency for it so that free spacce loss can be encountered....
That makes sense. The mobile phone is powered by battery and we want our battery goes as long as possible. Less path-loss in the uplink (operating with smaller frequency) means that, to maintain the same receive power, the mobile phone needs to transmit less power. This translates to long battery life.