9.1. Introduction9.2. Crest factor of the signals to be amplified9.2.1. Characteristic parameters of the crest factor9.2.1.1. Crest factor definition9.2.1.2. Definition of continuous and finite PR: PRc,f9.2.1.3. Definition of discrete and finite PR: PRd,f9.2.2. Relationship with the literature notations9.2.2.1. PAPR case9.2.2.2. Relationship between PAPR and PMEPR9.2.3. Distribution function of PR9.3. Variation of crest factor in different contexts9.3.1. Single-carrier signals' context9.3.1.1. Influence of Nyquist filter9.3.1.2. Influence of square-root Nyquist filter9.3.2. Multicarrier signals' context9.3.3. Software radio signals' context9.3.4. Context of cognitive radio9.3.4.1. Introduction9.3.4.2. Variations in crest factor on spectrum access by using carrier-by-carrier vision9.3.4.3. Influence of spectrum access on PAPR in the CR context9.4. Methods for reducing nonlinearities9.4.1. Introduction9.4.2. PAPR reduction methods9.4.2.1. Methods with modifications of the receiver9.4.2.2. Methods without modifying the receiver9.4.2.3. Synthesis of the PAPR reduction methods9.4.3. Methods working on the linearity of the amplifier9.4.3.1. Methods to change the function of amplification9.4.3.2. Methods without changing the amplification function9.5. Conclusion