AWGN equation?

[david753]

I produce a signal equation of sinusoid with 8MHZ and 1Vp-p.
Then, take it to DFT.
I got the result, the peak is 4 dbm, noisefloor is about -170dbm in freqency domain.
If I want to add some noise to input signal, upgrade the noise floor to -30dbm.
What is the last equation of input signal?
What is the equation of noise in time domain?
Hope someone give me detalis explanation.
Appreciated.