Nouns:
* Signalman: A person who operates signals, especially on a railway.
* Signalwoman: A female signalman.
* Signal light: A light used for signaling.
* Signal flare: A pyrotechnic device used for signaling.
* Signal tower: A tower containing signaling equipment.
* Signal flag: A flag used for signaling.
* Signal buoy: A buoy used for signaling.
* Signal processing: The manipulation of signals for various purposes.
Verbs:
* Signalize: To give a signal.
* Signalize: To mark with a signal.
Adjectives:
* Signal-strength: The intensity of a signal.
These are just a few examples, and there are many other potential compound words that could be created using "signal."