# Quadrature Phase Shift Keying

A Modulation Scheme where the Phase of the modulated signal is shifted depending on the incoming bitstream by an angle of -135°, -45°, +45° or +135°. Thus 4 states (a two-digit binary symbol) can get expressed in every transition:

The resulting waveform looks like if it would be assembled of several sine wave segments from 4 phase-shifted base sine signals.

*here a picture of a sine sigal with shifted phase would be nice*

In order to minimize the used Bandwidth allocated on the Transponder the outgoing signal is usually filtered using digital low-pass FIR Filters with smooth roll-off so that the high-order noise of the sudden state changes gets removed.

*here we may show the smoothed version of the sine sigal with shifted phase. Maybe also the spectrum diagrams?*

*Has anybody the standard number of the paper that specifies usage of QPSK in DVB-S handy? Any more useful information in there?*

*a few words about allocated bandwidth and noise sensitivity of QPSK modulated signals would be great.*