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

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

### AI Response: (may contain errors)  

The correct answer to the question about the constellation diagram of a QAM (Quadrature Amplitude Modulation) or QPSK (Quadrature Phase Shift Keying) signal is:

**B: The possible phase and amplitude states for each symbol.**

### Explanation of the Correct Answer:
- A constellation diagram visually represents the different states (symbols) that a modulation scheme can take. 
- Each point on the diagram corresponds to a unique combination of amplitude and phase, which represents a specific symbol in the modulation scheme.
- For example, in QPSK, there are four points representing four different phase shifts, while in QAM, the points also vary in amplitude, allowing for more symbols to be represented.

### Why the Other Choices are Incorrect:
- **A: How many carriers may be present at the same time**
  - This choice refers to the number of distinct carrier frequencies used in a communication system, which is not depicted in a constellation diagram. The diagram focuses on the modulation states rather than the number of carriers.

- **C: Frequency response of the signal stream**
  - The frequency response describes how a system responds to different frequencies, which is not represented in a constellation diagram. The diagram is concerned with the modulation states rather than the frequency characteristics of the signal.

- **D: The number of bits used for error correction in the protocol**
  - Error correction relates to how data is protected against errors during transmission, which is a separate aspect of communication protocols. The constellation diagram does not provide information about error correction bits.

### Useful Concepts:
- In QPSK, each symbol represents 2 bits (since there are 4 states: $00$, $01$, $10$, $11$).
- In QAM, the number of bits per symbol can be calculated as:
  
$$
\text{Bits per symbol} = \log_2(M)
$$

where $M$ is the number of points in the constellation. For example, 16-QAM has 16 points, so:

$$
\text{Bits per symbol} = \log_2(16) = 4 \text{ bits}
$$

Understanding these concepts will help you grasp the significance of constellation diagrams in modulation schemes.