  In this thesis, we will consider techniques to improve the spectral
efficiency of digital communication systems, operating on the whole transceiver
scheme. First, we will focus on receiver schemes having detection algorithms
with a complexity constraint. We will optimize the parameters of the reduced
detector with the aim of maximizing the achievable information rate. Namely, we
will adopt the channel shortening technique. Then, we will focus on a technique
that is getting very popular in the last years (although presented for the
first time in 1975): faster-than-Nyquist signaling, and its extension which is
time packing. Time packing is a very simple technique that consists in
introducing intersymbol interference on purpose with the aim of increasing the
spectral efficiency of finite order constellations. Finally, in the last
chapters we will combine all the presented techniques, and we will consider
their application to satellite channels.
