Modulation

Now that we’ve seen a few of the multiplexing technologies which help divide wireless spectrum, let’s take a deeper look at how the spectrum is put to use via modulation techniques (overlaying voice for communication).

*Note: CDMA, TDMA, etc are typically looked upon as “channel access methods” but actually employ modulation techniques to transmit data (e.g. a CDMA signal being put onto a carrier)

Modulation: The process of varying some characteristic of the electrical carrier wave as the information to be transmitted on that carrier waves varies. In the wireless world, we first take a signal, such as a telephone conversation, and impress it upon a constant radio wave called a carrier. Once this is done, the voice signal varies (or modulates) this radio wave. The two (voice signal and carrier wave) go together over the air. A voice frequency in the audible range (which we can hear) thus modulates or varies a constant frequency in the radio range (which we cannot hear). Essentially, modulation makes voice band and radio band frequencies work together. Three types of modulation are commonly used for communications:

- Amplitude Modulation
- Frequency Modulation
- Phase Modulation
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And there are variations on these themes called Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). In the next few days, we will cover all of these types of modulation and their associated themes.