Digital Communication Techniques

1Modulation

1.1. Modulation?

1.1.1. Main reasons for modulation

• – In the modulation process, one characteristic of a high-frequency (passband) carrier signal is modified according to the instantaneous amplitude of the signal to be processed (in its baseband).
• – Why does modulation make sense for signal transmission (distance, etc.)?
• – Several signals are transmitted on the same channel.
• – Capacitative or inductive devices need high frequency (carrier) to operate (to ensure Stability and a good Noise Rejection).

1.1.2. Main modulation schemas

• – Examples of application: transmitting audio and video signals.
• – Mobile radio communications, such as cell phones.

  Basic modulation types are as follows:

• – amplitude modulation (AM): modifies the amplitude;
• – frequency modulation (FM): changes the frequency;
• – phase modulation (PM): changes the phase.

1.1.3. Criteria for modulation via electronics

Requirements for signal processing are as follows:

• – speed;
• – reliability and precision;
• – low energy consumption.

Electronics serve to transport information at least energy cost, as quick as possible, in the greatest quantity possible, with the highest quality possible and with maximum reliability.

NOTE.– In electrical engineering, the electrical signal serves to transport the energy and not the information.

1.1.4. Digital modulation: why do it?

Digital modulation provides more information capacity, compatibility with digital data services, better security levels, better communication ...