Reduction of multipath effects for FSK with frequency-discriminator detection

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.