White noise generation for different bandwidth signals in MATLAB
One technicality to be aware of is that white Gaussian noise has infinite bandwidth. What you are asking for is filtered white noise, which has a flat PSD in a given bandwidth, and zero power outside of it. It is common to call this filtered noise &quot;white noise&quot;, but you should be aware of the difference.
With that out of the way, consider the channel between the ground station and the satellite. The noise in this channel is modeled as white Gaussian, with $\text{PSD} = N/2$. Assuming a filter with bandwidth $B$, the power of the filtered noise is, as you say, $NB$ (because the power is the integral of the PSD from minus infinity to infinity, which includes negative frequencies).
Now you need to define the receiver filter. It is very unusual, in my experience, to transmit a signal with two different bandwidths. You have two choices for the receiver design: a single filter with bandwidth 180 kHz, or two filters, one of 180 kHz and another of 15 kHz.
The single-filter design is obviously cheaper and much simpler to implement. The drawback is that it is quite suboptimal, as the data signal will be subject to much more noise than it would with a 15 kHz filter.
The two-filter design will be much more complex (I have never seen a real system implemented like this). The benefit is that it will be optimal.
In Matlab, you can generate filtered white noise of variance 1 with randn. After that, it's just a matter of scaling the noise appropriately to get the variance you want.