Why LTE ( 3.9G) use OFDMA in DL, SC-FDMA in the UL ?

In simple terms OFDMA requires more power for transmission whereas SC-FDMA requires lesser power.. and since higher power can't be processed using UE (mostly mobile phones or CPEs) due to battery size limitations therefore SC-FDMA is used for uplink by UEs. Other reasons also include higher data rates achievable using OFDMA than the other one..

LTE use ofdma in DL for more data rate and more application .Base station can increase the power and have awideband power amplifier

.LTE in UL use S-CFDMA to overcome the high peak to average power ratio and be suitable for the lower reigon of power amplifier in the mobile and the low power in battery.     

4G technology uses OFDMA in DL because of the reason that data rate can be very high. OFDMA supports high data rate due to data transmission in parellel sub-carrier bands. However for UL SC-FDMA is used because of power limitation(For upload we have to use technique which consumes less power due to battery limitation on mobile phones). As SC-FDMA has less peak to power ratio than OFDMA. 

It's to do with Peak to Average Power Ratio (PAPR). OFDMA has higher PAPR than SC-FDMA therefore it is used in the downlink (from base station to mobile station). The base station can have a good linear amplifier for OFDMA causing little distortion. Mobile cannot have a very good linear amplifier so SC-FDMA is used on the uplink.

It is because of PAPR ,

because of PAPR and limitation of handset for providing power the SC-FDMA is more suitable for uplink.

BR//