The best replacement for modulation is frequency modulation.

The best alternatives to modulation are rated as following:
★★★★★  frequency modulation
★★★★  amplitude modulation
★★★★  phase modulation
★★★★  multiplexing
★★★  digital modulation
★★★  covalent modulation
★★  key change
★★  pcm
★★  analog modulation
★★  delta modulation
★  regulation
★  tonicization
★  fm modulation
  pulse code modulation
  am
  frequency
  external modulation

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Speaking about modulation, one probably shouldn't forget about square wave signal.

Other search terms related to modulation and their ratings are:
★★★★★ square wave signal
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modulation
frequency modulation
amplitude modulation