This answer claims that
Mostly, on HF, the FCC has chosen to limit modes of transmission and symbol rates rather than bandwidth
Why does the FCC limit symbol rates?
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1$\begingroup$ I was tempted to close this with the reason "primarily opinion-based", because the only people who know WHY are the FCC themselves. $\endgroup$ – Scott Earle♦ Aug 16 at 2:21
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That's a good question---and one that is heavily under debate currently. The FCC originally limited symbol rates as a way of limiting bandwidth for data modes indirectly (it made sense at the time).
But now that there are more advanced modulations (like the various forms of phase shift keying) that can exceed the symbol rate limitations in less bandwidth than an older modulation type like say, an FSK packet transmission, many people have been trying to get the FCC to change to a bandwidth limitation for data signals, rather than symbol rate, so that higher data throughputs can be obtained. It's a very controversial subject right now...
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1$\begingroup$ wait, you increase the symbol rate without increasing the channel bandwidth?! Either our understandings of symbol rate are different, or I'm missing something. $\endgroup$ – Marcus Müller Jan 12 '17 at 17:41
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2$\begingroup$ Within a given modulation type, yes, increasing the symbol rate always increases bandwidth, but some modulation schemes, such as the various forms of PSK (phase shift keying) are more efficient with bandwidth than others, so they can offer higher symbol rates at equal or lower bandwidths than older more conventional modulations such as those that are FSK (frequency shift keying) based. $\endgroup$ – ruhnet Jan 12 '17 at 17:51
