Why UL speed is always lesser than DL speed?

Question

In any technology (GSM/CDMA/UMTS/LTE) UL speed is always lesser then DL speed.
why?

Answer 1

Because UL data is from UE and power is main challenge at UE side, So UE is using only 1 transmitter. that's why UL data speed is lessor than DL data speed

Answer 2

In addition to deepak answer:
In LTE Uplink is SCFDMA while downlink is OFDMA which is also a factor (only for LTE and WiMax)

Answer 3

Also most of the commercial UEs don't support MIMO in the uplink and higher order modulation in the uplink (64 QAM etc.) due to UE complexity and cost. Moreover, I don't know of any commercial handsets supporting DC-HSUPA yet and UL carrier aggregation for LTE. This results in lower speeds in the uplink compared to downlink.