QPSK: - In QPSK each pair at consecutive data bit is treated as a two bit code which is switch the phase of the carrier sine wave between one at four phase 90° apart. The four possible combinations at bib it code are 0°, 01, 10, and 11 each code represents either a phase of 45°, 185°, 225°, and 315° lagging, relative to the phase at the original un modulated carrier QPSK offers an advantage over PSK is a no carrier that how each phase represents a two bit code rather than a single bit. This means that either we can charge phase per sec. or the same amount of data can be transmitted with.