Page "Multiple frequency-shift keying" Paragraph 20

The most challenging case is when the delay and Doppler spreads are both large, i. e., the coherence bandwidth and coherence time are both small.

This is more common on auroral and EME channels than on HF, but it can occur.

A short coherence time limits the symbol time, or more precisely, the maximum coherent detection interval at the receiver.

If the symbol energy is too small for an adequate per-symbol detection SNR, then one alternative is transmit a symbol longer than the coherence time but to detect it with a filter much