AWGN Channel - Simulink

Hi guys, I need to use AWGN channel block in simulink but I don't understand why, when I insert SNR mode, it asks me to insert also the Input Signal Power.... What is it? Because when I rice the Input Signal Power (with SNR kept constant), the BER became worse!!! So The signal who I specify the Power seems to be another noise source....

Can you explain me better?

P.S. I've a Bernoulli source, with GMSK modulation and after I've the channel.

Thank's everybody. Andrea