Electronic Communication Systems 2nd Edition by Roy Blake

Exercise 4

Telephone signals can be carried by radio using either analog or digital modulation schemes. Compare the bandwidth required to carry each of the following signals:
(a) an analog voice signal with a baseband bandwidth of 4 kHz using SSBSC AM (often used for terrestrial microwave links where many voice signals are transmitted together)
(b) the same analog voice signal using FM with a frequency deviation of 12 kHz (used for cellular phones)
(c) a standard digital voice signal using QPSK and assuming a channel with a signal-to-noise ratio of 20 dB. (Your answer to this question may suggest why data compression and vocoders are often used when digital signals are to be transmitted by radio.)