Generally, downlink freq. > uplink freq. in mobile communications. I was able to gather some answers as to why is so. But my seniors (in my company) seem not to be satisfied with the below answers.
High freq. means higher attenuation. So lets suppose that BTS (as in case of GSM) transmits using low frequency on the downlink. So, that means lower attenuation. Hence, data is received as it should be (without attenuation). So, MS(mobile) has to transmit on uplink using higher freq. in order to avoid interference. But high freq. means higher attenuation. So, in order to overcome the attenuation, MS has to transmit on high power. But due to the constraint on the amount of energy available on the MS battery, this approach is not feasible. Hence, downlink freq. should be greater than uplink freq.
This point is from user perspective. Generally, people download more than upload (however, its converging now). So, we require more bandwidth in DL than in UL and it can be attained only if DL freq. > UL freq. Hence DL freq. > UL freq.
I have searched almost everything (however, that's not possible). If you could help me regarding the same, it would be very kind.