3.1  What You Will Learn

There is a large number of modulation schemes described in the literature. This chapter briefly lists some of them and then prioritizes QAM (quadrature amplitude modulation). This focus on QAM is not only because of its usefulness when discussing passband signals, but also because multicarrier DMT and OFDM modulations rely on QAM to transport information.

This chapter also discusses some digital modulation schemes and key concepts regarding modulation. For example, it is important to distinguish linear and non-linear modulation and, also, memoryless and modulation with memory. It is also important to know the two major symbol detection schemes: coherent (synchronous) detection or non-coherent detection. The former exploits phase reference information while the latter has a simpler receiver, but worse performance. Basically, synchronous detection requires the regeneration, at the receiver, of the carrier signal with the correct frequency and phase. This is accomplished by circuits such as the phase-locked loop (PLL), which are more complex than the circuits used for envelope detection as discussed for AM in Section 1.3.

Some of the concepts are:

• Characteristics of modulation schemes such as memory and linearity.
• How a QAM passband signal can transport two independent signals using the same carrier frequency but with a difference of 90 degrees in phase (e. g. a sine and a cosine).
• The generation of a QAM signal and the recovery of the corresponding symbols using coherent detection.
• The importance of the Hilbert transform when describing the QAM demodulation.

First, an overview of modulation schemes is provided and some of their properties discussed.