base station transmit at down link ferquency band of930–960mhz because as u know higer the frequency lesser is the travelled distance.so in case of base station we can transmit at higer power and at higer frequency.
,while ms(mobile station )should be transmitting at lower frequency band ie—890–915Mhz(uplink) Mhz so it is more easy for mobile station to transmit to the base transiever station at lower power and that will consume less battery power .