Hello,Fauza khair
Alessandro said correct that PRBS is used to transmit random sequence of 0 & 1 and there is no power associated with PRBS. Now according to your second question about using 4QAM and 16QAM , that power change associated in OFDM system. According to my knowledge , 4QAM and 16QAM referred to 2 Bits/symbol and 4 Bits/ symbol respectively. 16QAM is different to 4QAM in the way that it is more bandwidth efficient and capable to transmit more data. However, using 16QAM increases noise level in OFDM system. I don’t think by using 4QAM or 16QAM there will be any change in power associate to it.