## Example of complex eye diagram |
Question 3.2

#### Shown here are the eye diagrams for a modulation scheme with four states and 16 states in the demodulated signal. This would be typical for one branch of a 16-QAM (Quadrature Amplitude Modulation) and 256-QAM modem. The eye diagram as a diagnosis tool comes into its own here, showing clearly the individual 'eyes' between the discrete states, and also illustrating how critical the sample timing must be to detect the symbol at the maximum eye opening. |

#### It also serves to show how much more susceptible higher-order modulation formats are to noise and distortion when compared with a binary system of equivalent energy per bit. The eye opening for the four-level system is so much narrower and the spacing between decision boundaries so much smaller than those in the previous two-state examples and this gets progressively worse as the number of symbol states rises. Great care should be exercised when using the eye diagram for diagnosis to ensure that the observation is made after all the filtering within the system.