QPSK, or Quadrature Phase Shift Keying, is a type of phase modulation technique used in digital communication. In QPSK, each symbol represents two bits of information. The phase of the carrier signal is shifted among four possible values: 0, 90, 180, and 270 degrees. This results in four distinct phase states that correspond to the four possible combinations of two-bit data. QPSK provides a balance between data rate and signal robustness, making it suitable for various communication systems.

### What is meant by QPSK?

QPSK, or Quadrature Phase Shift Keying, involves encoding data into the phase of a carrier signal. The term “quadrature” refers to the use of two orthogonal (90 degrees apart) signal components. In QPSK, these components allow for the transmission of two bits per symbol by altering the phase of the carrier signal. This technique improves spectral efficiency compared to binary phase shift keying (BPSK) by effectively doubling the data rate without requiring additional bandwidth.

The difference between PSK (Phase Shift Keying) and QPSK (Quadrature Phase Shift Keying) lies in the number of phase states used to represent data. PSK, typically referring to BPSK (Binary PSK), uses two phase states (0 and 180 degrees) to represent single bits. In contrast, QPSK uses four phase states (0, 90, 180, and 270 degrees), allowing it to represent two bits per symbol. This results in a higher data rate for QPSK compared to BPSK, without increasing the bandwidth.

8PSK, or Eight Phase Shift Keying, is a type of phase modulation that extends beyond QPSK by using eight distinct phase states. Each symbol in 8PSK represents three bits of information, with the phase of the carrier signal shifted among eight possible values. This allows for a higher data rate compared to QPSK while maintaining a similar bandwidth. However, the increased number of phase states also makes the signal more susceptible to noise and interference.

QPSK is indeed a constant envelope modulation scheme. In QPSK, the amplitude of the carrier signal remains constant while the phase is varied to encode the data. This constant envelope characteristic helps in maintaining signal integrity and reduces the effects of non-linear distortion in amplifiers, making it robust for transmission through various communication channels.