Quadrature Phase Shift Keying

by craig miles Wireless Communications Blog

Quadrature Phase Shift Keying (QPSK) is a digital modulation scheme that represents two bits of data per symbol. In QPSK, four different phase shifts of the carrier signal are used to encode the binary data. Each phase shift corresponds to a specific combination of two bits (00, 01, 10, and 11).

The term “quadrature” refers to the use of two carriers that are 90 degrees out of phase with each other. The QPSK modulation constellation diagram typically illustrates the four different phase positions.

Here’s a basic breakdown of how QPSK works:

  1. Mapping Bits to Phase Shifts:
    • 00 is represented by a 0-degree phase shift.
    • 01 is represented by a 90-degree phase shift.
    • 10 is represented by a 180-degree phase shift.
    • 11 is represented by a 270-degree phase shift.
  2. Transmission:
    • Each symbol represents two bits of information.
    • The carrier signal is modulated with the appropriate phase shift based on the two-bit data.

QPSK is used in various communication systems, including satellite communication, digital television, and some wireless communication standards. While Quadrature Phase Shift Keying provides a higher data rate compared to Binary Phase Shift Keying (BPSK), it is more susceptible to noise and interference than higher-order modulations like 16-QAM or 64-QAM.