Post History

<blockquote>with just one mode</blockquote>

Others have already explained that "discrete" means a finite set of values, not just two.

This is to point out that having just one "mode" (it seems you mean symbol) doesn't make any sense.  A signal that can only ever be in one value is no signal at all since it can't carry any information.  At a minimum, any "signal" needs to have at least two distinguishable states.

If there is no choice in what a signal can tell you, then any receiver can just substitute the signal value with a constant locally.