System DescriptionWe analyze the occurs error in binary FSK due to inter symbol interference (ISI) in mobile radioWe use here an equivalent baseband representation, so channels, with a limiter-filtering-integrator detector.the carrier frequency f, is no further importance. At the The errors are caused by bursts in the instantaneous transmitter, a binary data stream frequency modulates frequency. We propose and verify that a nonlinear the carrier, the maximum frequency deviation is ±fd frequency filtering can achieve zero error floors for and the modulation index hmod is defined as 2fdT, pure FSK by clipping offthese bursts. Forfiltered FSK,where T is the bit duration [1 ]. (Fig. 1) the error floor is not completely removed, but strongly reducedtypically by one or two orders ofmagnitude