Comment by sohkamyung

I'm not sure if this helps makes things clearer, but see this diagram for symbols in Quadrature Amplitude Modulation [1]. The valid symbols are mapped to certain points in the vector space. Now, imagine non-overlapping circles around each symbol. If a received signal falls within a circle, it would be mapped to that symbol in the centre of the circle.

This can be extended to 3-D or higher dimension spaces.

[1] https://en.wikipedia.org/wiki/Quadrature_amplitude_modulatio...