Performance Analysis of Quaternary CP-FSK Systems with Modulation Pulse Shaping and Limiter-Discriminator Detection

The multilevel CP-FSK transmission systems with limiter-discriminator detection in which the modulation pulses are rectangular with a length τ admitting of any value not higher than symbol time T, are here considered. First, a procedure is presented in order to determine the optimum values of the modulation levels and of the decision thresholds, which are strictly correlated with τ. Next, the optimization of four-level CP-FSK systems with reference to different types of transmission channels is carried out by determining for each the value of τ, the modulation and threshold levels and the equivalent noise bandwidth, which guarantee the error probability Pe = 10-6 with the minimum transmitting power. For comparison, the same evaluation is made for the conventional system, for which τ = T and the distribution of the modulation and threshold levels is uniform. The results obtained, experimentally verified, show that: -the optimum choice of the modulation and thresholds levels is important even if τ = T;-the optimum system performance takes place, in all the considered cases, for non-critical value τ = 0.2 T, for which the improvement compared to the conventional case is always very great.The experiments, carried out at the Telettra laboratories, have also shown that the considered modulation method allows the use of highly non linear power amplifiers in the transmitter, without any practical effect on system performance.