*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 modulation technique used in digital communication. 

### Correct Answer Explanation:
- **Correct Answer (D)**: QPSK modulation transmits digital data by using 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.

### Incorrect Answer Explanations:
- **Choice A**: This describes a method of reducing bandwidth but does not accurately define QPSK. QPSK does not involve quasi-parallel to serial conversion; it focuses on phase shifts to encode data.
  
- **Choice B**: This option mentions "quadra-pole sideband keying" and spread spectrum signals, which are not relevant to QPSK. QPSK is not specifically designed for spread spectrum applications, and the terminology used here is incorrect.

- **Choice C**: This choice refers to using Fast Fourier Transforms (FFT) and harmonics, which is unrelated to QPSK. QPSK does not involve generating frequencies at harmonics; it relies solely on phase shifts to convey information.

### Summary:
- QPSK uses four phase shifts to represent pairs of bits, making it efficient for digital data transmission.
- The other choices either misrepresent QPSK or describe unrelated concepts.

### Useful Rule of Thumb:
- Remember that QPSK effectively doubles the data rate of BPSK by encoding two bits per symbol, which is a key advantage in digital communication systems.