Communications Toolbox

Digital Modulation Overview

Modulating a digital signal can be interpreted as a combination of two steps: mapping the digital signal to an analog signal and modulating the analog signal. These are depicted in the schematic below.

Figure 2-5: Two Steps of Digital Modulation

Except for FSK and MSK methods, when the receiver tries to recover a digital message from the analog signal that it receives, it performs two steps: demodulating the analog signal and demapping the demodulated analog signal to produce a digital message. These are depicted in the schematic below.

Figure 2-6: Two Steps of Digital Demodulation

For FSK and MSK methods, the demodulator uses correlation techniques instead of the two-stage process above.

The mapping process increases the sampling rate of the signal from Fd to Fs, whereas the demapping process decreases the sampling rate from Fs to Fd.

Functions in this toolbox can perform any of these steps, as summarized in the table below.

Table 2-7: Functions for the Steps of Digital Modulation and Demodulation 

Step	Function
Mapping and modulation	dmodce or dmod
Mapping only	modmap
Modulation without mapping	dmodce or dmod, with /nomap flag
Demodulation and demapping	ddemodce or ddemod
Demodulation without demapping (ASK, PSK, or QASK)	ddemodce or ddemod, with /nomap flag
Demapping only	demodmap

The functions are described in more detail in the sections that follow.

Filter Design Issues

Representing Digital Signals