adding noise to qpsk signal in ADS

dear all, i want to measure the BER of qpsk in presence of noise, i have used system level blocks for qpsk modulator, amplifier and attenuator and simulated using circuit envelope. i have used the ber_qpsk() function to plot the BER vs Eb/No plot, which gives me an ideal curve, now i want to add some noise in between the system, so that i get a deviation in the ber curve. please help with a suitable block for noise, which should add noise to qpsk signal...

i want to add noise block to the ADS example project BER_Env_pj (examples/tutorial) so that i get simulated BER curve deviated from the ideal curve. please help me to find a suitable noise block. thanks