Digital Communications 1

6Transmission of an M-ary Digital Signal on a Low-pass Channel

6.1. Introduction

To transmit a digital signal without distortion on a channel (only attenuation α and a given delay τ are accepted), the bandwidth of the transmission medium must be very large compared to the spectrum of the transmitted digital signal. The latter is, in any case, limited to a frequency band Δ/f by filtering the signal at the output of the on-line encoder.

In practice, the bandwidth of the channel is limited. This limitation can come from the transmission medium itself, which can be frequency selective (all the spectral components of the modulated signal do not propagate at the same phase velocity, because of variable attenuation as a function of frequency). This is precisely the case for two-wire and coaxial metal cables, which have a frequency dependent attenuation .

The determining parameters of electric cables with regard to signal transmission are, for a given reference length:

• – frequency transfer function (attenuation and phase shift);
• – characteristic impedance;
• – group propagation delay.

Before tackling the problem of digital transmission, in the next section we present digital systems and standardization for high data rate transmissions.

6.2. Digital systems and standardization for high data rate transmissions

In digital transmission systems, the transmission is actually simultaneous ...