Prefiltered Quadrature Phase-Shift Keying: A Bandwidth- and Power-Efficient Modulation

Abstract

Prefiltered quadrature phase-shift keying (QPSK) is a QPSK-type modulation with sin (x)/x pulse shaping. The pulse amplitude shaping is applied strictly for the purpose of shaping the signal spectrum and is discarded at the quadrature demodulator. The data information is carried in the phase of the QPSK modulated signal. Performance of this modulation through linear and nonlinear channels is studied. An important feature of prefiltered QPSK is the rapid decay of its power spectral density outside the bandwidth of interest, thereby permitting closer spacing between multiple carriers in a single transponder. This modulation has a much narrower spectral main lobe than other QPSK-type modulations and has little or no side lobes. Prefiltered QPSK represents a departure from the use of constant envelope signals in nonlinear satellite communications channels. Performance results of prefiltered QPSK are compared with QPSK, staggered QPSK (SQPSK), and minimum shift keying (MSK). Under severe band-limited nonlinear conditions, prefiltered QPSK outperforms QPSK, SQPSK, and MSK. The effect of phase equalization of the channel filter is also investigated. Phase equalization produces drastic improvement over nonequalized filtering when the channel bandwidth is fully utilized.