Digital Modulation

Learn how information is communicated in digital systems from point A to point B.

A digital source produces a sequence of bits in the form of 00s and 11s. The question is how to send these bits from point A to point B. For this purpose, an electromagnetic wave acts as a carrier of information.

Remember that the signal produced by an unmodulated electromagnetic wave is a sinusoid given as:

x(t)=Acos(2πft+θ)x(t) = A \cos \left(2\pi ft + \theta\right)

There are three tunable parameters, which are listed below:

  • Amplitude AA
  • Frequency ff
  • Phase θ\theta

Information can be conveyed by altering any of these parameters.

Let’s explore them below.

Amplitude shift keying

The process starts with converting the bits of 00s and 11s into voltage levels known as symbols, for example:

0112\begin{align*} 0 \quad &\longrightarrow \quad 1 \\ 1 \quad &\longrightarrow \quad 2 \end{align*}

This is called binary modulation. To keep the contents simple, we will focus on binary modulations only. Otherwise, multiple bits can also be assigned to a symbol. For instance, a quaternary modulation can represent the sets of 22 bits as:

00301110+111+3\begin{align*} 00 \quad &\longrightarrow \quad -3 \\ 01 \quad &\longrightarrow \quad -1 \\ 10 \quad &\longrightarrow \quad +1 \\ 11 \quad &\longrightarrow \quad +3 \end{align*}

The serial-to-parallel converter we saw in the OFDM block diagram represents this aggregation of bits (22 in this case) for subsequent mapping to a signal level.

Let’s investigate how a sinusoidal wave can be modulated in amplitude.

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