This paper proposes a semi-analytical approach to characterise the performance of digital cellular systems when cochannel interference is determined by a small number of interferers. Linearly modulated signals (such as BPSK, QPSK, M-ASK, M-QAM, MSK, O-QPSK, etc.) with linear receivers are investigated by considering the probability density function of the interference at the input of the decision device. Non-linear (Continuous Phase) modulation signals (such as GMSK) and non coherent detection are also investigated by means of simulation. As a consequence, a general approach to outage probability definition considering the protection ratio as a function of the interferers number and useful signal-to-noise ratio is introduced
