Digital Communications 1

7Digital Transmissions with Carrier Modulation

7.1. Introduction and schematic diagram of a digital radio transmission

In Chapter 6, we studied the methods used for digital baseband transmissions where the transmission medium is likened to a low-pass filter. However, most of the transmission media are of the bandpass type. This is the case for:

• – the analog telephone channel for which the authorized frequency band resulting from all the filters involved is 300 Hz-3,400 Hz;
• – the radio channel (radio communications), the case of the propagation of terrestrial electromagnetic waves (EMW), for which the attenuation decreases when the frequency increases (law in 1/f²). The presence of the ionospheric layer causes refractions leading to non-linear propagation and the generation of multiple propagation paths. In addition, the absorption of waves due to water vapor in the troposphere causes significant attenuations at very high frequencies (several tens of GHz) but selectively (in certain frequency bands);
• – the earth-satellite channel, the case of the propagation of EMWs in space, comparable to terrestrial hertzian transmissions but with the distinction that ionospheric refraction is much less important because of much more favorable angles of incidence.

In all these situations, the technique used consists of transmitting a signal whose frequencies are compatible with the transmission channel and obtained by modulating a single-frequency signal by the digital baseband signal. ...