Transmitter power VS BER is the different plot,which represents the variation of BER with the input power.Usually for lower values of input power BER decrease with the increase of input power unless non linear effects comes into play.But the question is of plotting Rx power with BER which will come like waterfall as written above.Aluisthu i think your system plot defines the effects of non linearities due to the non linear variation of in the value of ber with input power.