This paper is concerned with estimating the performance of a multi-user frequency hopping (FH) spread spectrum mobile radio system with binary frequency-shift keying (BFSK) modulation over Rician and Rayleigh frequency-selective fading channels and, more specifically, with estimating the effect of time delay spread on that performance. Previous analyses of FH-BFSK communications over frequency-selective fading channels have typically assumed that the time delay spread models are symmetrical. This paper proposes the use of non-symmetrical time delay spread models, and introduces a new method to estimate average bit error probability, thus opening the way to the use of site-specific time-delay spread models in performance estimation and in mobile and personal communication system design.>