We consider the error floor of binary frequency-shift keying (FSK) due to intersymbol interference in time-dispersive mobile radio channels, with a limiter-discriminator-integrator detector. The errors are caused by bursts in the instantaneous frequency. We propose and verify that a nonlinear frequency discriminator can achieve zero error floor for pure FSK by clipping off these bursts. For filtered FSK, the error floor is not completely removed, but strongly reduced - typically by one or two orders of magnitude. The tighter the filtering, the less effective this nonlinearity is. The nonlinear discriminator can also be used in conjunction with adaptive sampling.

We analyze how multipath effects namely errors due to channel-induced intersymbol interference, can be reduced by frequency-discriminator detection for FSK. We first introduce a new mathematical technique that can be considered as generalization of FM noise-click theory, and thus allows insights into the error mechanisms. Building on those insights, we propose and verify that a nonlinear frequency discriminator can achieve zero error floor for pure binary FSK and drastically reduced the error floor for filtered FSK by clipping off the ISI-induced FM clicks. The stronger the filtering (before or after the frequency discriminator), the more the beneficial effect of the nonlinearity is reduced. Finally, we show that increasing the modulation index decreases the error floor, especially in the presence of frequency shifts. Our results suggest the design of improved receivers that are less sensitive to ISI.

The bandwidth requirements of binary coded baseband signals are usually determined from spectral considerations. It is shown, by bit error rate calculations, that this approach can be misleading. The influence of pulse shapes on the performance of Miller, Manchester, and CRL codes is discussed.

This paper investigates the effects of a multilevel signal applied in an optical communication system with a step-index fiber. The model for description of the fiber impulse response used here is more accurate than previously used models, which influenced conclusions significantly. It is found that multilevel signals can improve performances of a digital optical system when intersymbol interference is significant.