On the irreducible error floor in fast fading channels

In fast fading channels without pilot information, existing receiver structures exhibit an “irreducible error floor,” where their bit error rate (BER) levels out at high signal-to-noise ratio (SNR). This is even observed in practical maximum likelihood sequence detectors (i.e., MLSDs with certain approximations, in order to achieve finite per-symbol complexity). The conditions for optimality and for an error floor are examined in detail. We find that the standard receiver design choices induce an error floor unless the channel has very specific properties