*Question G8A12:
What is QPSK modulation?*  

*Correct Answer is  D: Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits*  

### AI Response: (may contain errors)  

QPSK, or Quadrature Phase Shift Keying, is a type of digital modulation technique. Here's why the correct answer is correct and why the other choices are incorrect:

1. **Correct Answer: D**  
   QPSK modulation transmits digital data by changing the phase of the carrier signal. It uses four distinct phase shifts: 0 degrees, 90 degrees, 180 degrees, and 270 degrees. Each phase shift represents a unique pair of bits (00, 01, 10, 11). This allows QPSK to transmit two bits per symbol, effectively doubling the data rate compared to Binary Phase Shift Keying (BPSK), which only uses two phases.

2. **Why the Other Choices are Incorrect:**
   - **Choice A:** Quasi-parallel to serial conversion is not a modulation technique. It refers to a method of data handling, not modulation.
   - **Choice B:** Quadra-pole sideband keying is not a recognized modulation method. Spread spectrum signals are typically generated using techniques like Frequency Hopping or Direct Sequence Spread Spectrum, not QPSK.
   - **Choice C:** Fast Fourier Transforms (FFT) are used in signal processing to analyze frequency components, not directly in modulation. The description of generating harmonics is unrelated to QPSK, which focuses on phase shifts rather than frequency harmonics.

3. **Useful Concepts:**
   - **Symbol Rate:** In QPSK, each symbol represents two bits. If the bit rate is $R_b$, the symbol rate $R_s$ is given by:
     $$ R_s = \frac{R_b}{2} $$
   - **Phase Shifts:** The four phase shifts in QPSK are used to encode the following bit pairs:
     - 0 degrees: 00
     - 90 degrees: 01
     - 180 degrees: 10
     - 270 degrees: 11

Understanding QPSK is crucial for recognizing how digital data can be efficiently transmitted using phase modulation techniques.