Why do you want to do it in the time domain?  As opposed to simply integrating the noise as a function of frequency?

Few simulators support true time-domain noise.  Several have the built-in ability to integrate noise vs frequency.

I typically do what Geoffrey suggested, ie integrate the NF over the signal bandwidth. But there are a couple of points I would like to add:
1) make sure you integrate linearly in frequency, you might think it a nobrainer, but its all too simple in some tools to get it wrong be selecting a log freq NF plot and attempt to integrate.
2) Depending on your modulation scheme/burst lengths etc. noise at different parts of the signal bandwidth might be more or less important. An example would be how much flicker noise effects the SNR in a nominally 0Hz~3dBBW signal spectrum Direct sonversion receiver. The lower cutoff frequency in which to integrate the noise may be related to something like 1/burst time. For a modulation scheme like 2FSK with datarate=freq deviation, there is a null is signal at 0Hz, so even if you had loads of flicker noise it would really effect your likelihood for bit errors/impact SNR... Anaylsis of this kind of stuff is probably best done in matlab or similar.

ACWWong wrote on Jan 10^th, 2007, 2:43am:

Slight typo in my last post... This sentence should read:
For a modulation scheme like 2FSK with datarate=freq deviation, there is a null is signal at 0Hz, so even if you had loads of flicker noise it wouldn't really effect your likelihood for bit errors...

Hi aaron,
yes you consider all the noise within the filter bandwidth. My point is that the modulated signal spectrum (or noise or both) might NOT be uniform over the filter/signal bandwidth. For example in your 1MHz wide channel you would get a certain bit-error rate from your ADC if your noise was unifrom across the band and integrated to say a value x. A different noise profile (vs freq) across that 1MHz band could also integrate to x, but give rise to a different no. of bit errors (depending on signal psd, demodulation process etc.). This may or may not be relevant for you.
cheers
aw

yes, if there is no signal power between f1 & f2 then in some demodulation circumstances noise between f1 & f2 might not impact bit errors.
I make this point based from my own experience with simulation work in matlab. Given a particular signal modulation (and hence psd) & demodulator process, different noise profile within the signal band caused different error rates (although the SNR in the signal band is constant). This might not be important in your system (my system was at zeroIF so flicker noise profile), but a system simulation would tell you.