1) First of all,to find out the power in dBm :
P[dBm]=10*log( P[mW] )
20W => 10*log(20000 mW)=43dBm
50’11873W => 10*log(50118’73 mW)=47dBm
2) The reason for using higher frequencies in the downlink and lower frequencies in the uplink is power saving in the batteries of the mobile station.
Transmitting in high frequencies involve more power consumption than transmitting in low frequencies.As the battery in MS is limited due to its mobility, power availability in BTS is greater, and low frequencies are used by the MS to transmit.
In wireless Satellite systems, satellites use low frequencies to transmit in order to save battery.