fading rate. This is Doppler spreading, the frequency domain counterpart of coherence time. The shorter the coherence time, the greater the Doppler spread and vice versa. Designing MFSK for HF With appropriate parameter selection, MFSK can tolerate significant Doppler or delay spreads, especially when augmented with forward error correction. (Mitigating large amounts of Doppler and delay spread is significantly more challenging, but it is still possible). A long delay spread with little Doppler spreading can be mitigated with a relatively long MFSK symbol period to allow the channel to "settle down" quickly at the start of each new symbol. Because a long symbol contains more energy than a short one for a given transmitter power, the detector can more easily attain a sufficiently high signal-to-noise ratio (SNR). The resultant throughput reduction can be partly compensated with a large tone set so that each symbol represents several data bits; a long symbol interval allows these tones to be packed more closely in frequency while maintaining orthogonality. This is limited by the exponential growth of tone set size with the number of data bits/symbol. Conversely, if the Doppler spread is large while the delay spread is small, then a shorter symbol period may permit coherent tone detection and the tones must be spaced more widely to maintain orthogonality. 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 wider than one matched to the transmitted symbol. (The filter should instead be matched to the tone spectrum expected at the receiver). This will capture much of the symbol energy despite Doppler spreading, but it will necessarily do so inefficiently. A wider tone spacing, i.e., a wider channel, is also required. Forward error correction is especially helpful in this case. MFSK schemes for HF Because of the wide variety of conditions found on HF, a wide variety of MFSK schemes, some of them experimental, have been developed for HF. Some of them