Is it safe to define that Data Whitening is always X-ORing a payload with a pheudorandom sequence known to both receiver and transmitter?
No, but that's a very common way of doing it. However, you can scramble and whiten a sequence by pushing it through an appropriate Linear Feedback Shift Register, and I'd assume that this is even more commonly applied in ...
Imagine your data is not white.
That means that at certain periods, it contains more variance, and hence information, than on others.
That's bad, because now certain frequencies become more sensitive to noise than others.
Other than that:
Many things that a receiver needs to do (AGC, equalization, synchronization) depend on the temporal stochastic ...