Hi! Fady
As mohammad stated ver well and almost gave the answer to your question that how to plot ber vs rx power.i just add that you can find received power from
Project library>Ber analyzer>Results>Total power(dB)
Just drag this total power to x-axis of plot and log(ber) to y-axis side.In this way you will get tge plot in waterfall trend.
You only need to vary transmitted power for 5 or more iteration and plot is there
Back to back system is when you connect receiver rigth after the transmitter without transmitting signal over fiber or channel length distance.in this way you see tge back ti back signal without attenuation and dispersion effects.After this step you xan plot the the ber after some distance and see the change and panelty observed in power at FEC or 10^9 value of BER.
Hope you got the answer .
Cheers!!