On the availability of four-level CPFSK systems with modulation pulse shaping during multipath propagation

This paper may be included in the field of research which has been developing in the last few years to analyse the availability of digital modulation systems during multipath propagation. The continuous-phase FSK systems (CPFSK) with rectangular modulation pulses shaped by a duty-cycle, τ/T, admitting of any value between 0 and 1 is considered. This system has already been studied in normal propagation conditions where a significant improvement in performance has been enphasized with respect to conventional CPFSK systems (τ/T=1 ). Now its behaviour during multipath fading is tested.We show that, with suitable exploitation of the degrees of freedom available (as an example duty-cycle of modulation pulses and maximum frequency deviation), it is always possible to obtain, as far as outage probability conditioned to multipath fading is concerned, decreasing values for increasing transmitted power, even in absence of diversity techniques.The numerical results reported with reference to a four-level CPFSK system show that, by means of suitable choice of system parameters, an improvement of more than one decade can be obtained in the outage probability, with respect to the 4-PSK system, in the range of transmitting power of practical interest. Consequently, the problem of using diversity techniques in the design of CPFSK radio links is considerably simplified.