π/4-QPSK modems for satellite sound/data broadcast systems

The use of π/4 quadrature phase-shift keying(QPSK) modems for satellite sound broadcast systems (SSBS) broadcasting to mobile or portable receivers is proposed. Three different differential detectors (including the FM-discriminator followed by integrate-sample-and-dump filter) and a novel coherent detector are discussed. The degradation caused by the frequency offset between the local oscillator and the unmodulated carrier in the baseband differential detector is studied. The performance of both coherently and differentially detected π/4-QPSK in a Gaussian channel is also studied. It is shown that with a frequency offset of more than 3% of the symbol rate, the performance degradation is more than 1 dB at 10^-4. The out-of-band power of the nonlinearly amplified bandlimited π/4-QPSK signals is reduced from -13 dB to -37 dB if a 2-dB output back-off amplifier is used instead of a hardlimiter. The performance of the π/4-QPSK is equivalent to that of QPSK, although the π/4-QPSK has the advantage of less spectrum restoration after nonlinear amplification. The coherent demodulator and differential decoder avoid the three-level detection and achieve the same bit-error-rate (BER) performance as DEQPSK with a simple circuit