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First understand, lower the frequency more the distance it can travel with given power.
uplink--> when handset transmit-- Battery powered
Downlink--> when DCS transmit-- Mains powered
now , since lower frequency transmitted by the handset ,it can go to higher distance with low power. low power is important bcoz' it's battery power equipment.
And for downlink the frequency is higher hence need more power to travel larger distance but power here is not a problem bcoz' normelly DCS is mains powered.