Question E8C13:
What is described by the constellation diagram of a QAM or QPSK signal?  

Choices:  
A: How many carriers may be present at the same time  
B: The possible phase and amplitude states for each symbol  
C: Frequency response of the signal stream  
D: The number of bits used for error correction in the protocol    

Correct Answer is  B: The possible phase and amplitude states for each symbol  

To interpret a constellation diagram:

Correct answer (B): The possible phase and amplitude states for each symbol
- A constellation diagram plots the complex baseband symbol values on I-Q axes, where each point is a distinct symbol state.
- For QPSK, there are 4 points at equal radius (constant amplitude) with 90° phase separation.
- For QAM, points vary in both amplitude and phase; e.g., 16-QAM shows 16 points arranged in a grid.

Key relationships:
- Complex symbol: $s = I + jQ$
- Amplitude and phase of a point:
  $$A = \sqrt{I^2 + Q^2}, \quad \phi = \operatorname{atan2}(Q, I)$$
- Number of bits per symbol:
  $$k = \log_2 M$$
  where $M$ is the number of constellation points (e.g., QPSK: $M=4$, $k=2$; 16-QAM: $M=16$, $k=4$).
- Rule of thumb: More points (higher $M$) increases data rate but requires higher SNR because points are closer together.

Why the other choices are incorrect:
- A: How many carriers may be present at the same time  
  Constellation diagrams describe symbol states on one carrier’s I/Q plane, not the count of carriers (that’s a multiplexing/spectral question).
- C: Frequency response of the signal stream  
  Frequency response is shown by a spectrum or transfer function, not a scatter plot of I/Q symbol points.
- D: The number of bits used for error correction in the protocol  
  FEC overhead is a coding parameter at the bitstream/protocol level; it does not appear directly in the constellation plot (though coding affects error performance, not the diagram’s point locations).

In summary: The constellation diagram shows where valid symbol states lie in I-Q space, i.e., their phase and amplitude.