Improved performance QPSK demodulator for digital satellite radio

Practical realizations of the optimum closed loop structure for coherent demodulation of QPSK signals demands the approximation of the hyperbolic tangent nonlinearity (Tanh) with simpler and more easily implementable functions. This paper compares the commonly considered approximations to this nonlinearity and the practical structures which result, with an alternative in which the hyperbolic tangent nonlinearity is approximated by a saturated amplifier characteristic. It is shown that the performance of this new loop is for all practical purposes identical to the optimum theoretical loop. A practical implementation of this loop is proposed for the digital satellite radio system, and its practical performance is evaluated. The results validate the theoretical study, as well as the practical solutions adopted, concluding for the feasibility of a QPSK demodulator with a performance very close to the theoretical limit