This chapter looks at three important modulation paradigms. Trellis-coded modulation (TCM) is considered first. Developed in the late 1970s as a method to conserve bandwidth without sacrificing error performance it has become an extremely important modulation.

The second technique is called code-division multiple access (CDMA). It falls within the broad class of multiple access methods and is an administrator's favorite. It makes the addition (or deletion) of new users essentially transparent, easing the administrator's work. However, the technique does have technical merits that warrant its study by communication engineers. It forms the basis for the so-called 3G (third generation) and beyond wireless communication systems. In contrast to TCM, CDMA is a very wideband modulation technique.

The last modulation method studied uses space-time codes which provide diversity gain for the fading channel through the use of multiple transmit antennas. The important Alamouti's space-time block code (STBC) is discussed in some detail. It was first described in 1998, almost at the same time Tarokh et al. published their paper on space-time trellis codes.

Trellis-coded modulation (TCM)

By now one can appreciate that over an AWGN channel, the basic idea in digital communications is to find signal constellations with as large a distance between signals as possible without increasing the energy Eb (joules) expended per bit inordinately and with as small a bandwidth as possible.