I don't understand the concept of dimension of a signal. I ran into it in an explanation of Shannon Capacity, and in a paper on spread spectrum. I was hoping somebody could explain with an example. Does it apply to analog as well as digital? For instance is AM two dimensional considering time and amplitude? Or is QPSK two dimensional because it is a combination of a sin and cosine term? Or does multilevel signaling like Pulse Amplitude Modulation have as many dimensions as discrete levels it can take, or would dimensionality refer to the number of possible symbols it can represent in a pulse?