Parameters of AWGN Channel Simulink block.

I have some questions regarding the "AWGN Channel" block of simulink.

1) For the mode "Signal to noise ratio" how is the "Input to signal power, referenced to 1 ohm (watts)" is defined? Should i change its value from the default 1?

2) I need to add WGN of lets say SNR 5 dB noise (with noise_std about 0.01) to a constant value, e.g. 1. How is this possible?

Thanks