“communication systems” or “communications systems”?

Question

I know, we add s to the scientific contents when want to mention them as a field of study. Compare

Mathematics, Thermodynamics, Physics, ....

My field of study revolves around transmitters, receivers, wireless channels, and others of that ilk. In English, this field is called communications^†. Now, I am curious to know which one of the below phrases is correct and what is the difference between them:

1. communication systems
2. communications systems

The Ngram results:

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Specifically, these two book titles are about the field, but one uses s and one does not:

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^† In some universities it is still called Telecommunications.

Answer 1

If you look at this definition of communication, you will see that the meaning the various methods of sending information between people and places, especially phones, computers, radio, etc. is only valid in the plural.

When we make plurals for compound nouns, we normally only make the last word plural. So, most people will follow this rule and make only the last word plural, saying "wireless communication systems" and "wireless communications". Others will insist that communications only carries the correct meaning in the plural, and will say "communications systems".