Discussion
In digital terms, what is quadrature phase shift keying?
Answer: D
It is phase modulation restricted to four phases, carrying two bits per symbol, and it is the direct digital descendant of analogue PM. Four tones would be frequency shift keying. Four amplitude levels would be amplitude shift keying. And two separate carriers would need twice the spectrum, whereas quadrature keying achieves its two bits on one carrier by using phase alone. The lineage from analogue PM to modern digital radio runs directly through this scheme. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.
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