• nonlinear channel
• Multicarrier
• MIMO
• MC-CDMA
• adaptive signal design
• adaptive modulation and coding
• OFDM
• PAPR
• power allocation

The exponential growth of voice-oriented mobile communications during the 90s and their evolution toward data services such as short messaging, email and internet accessing, paved the way to current wireless systems supporting a number of different applications and media. The vision of seamless and pervasive wireless communication systems providing even more advanced multimedia services (video streaming, video broadcasting,
high speed internet, etc.) is the research frontier of the next few decades. However, the achievement of the high data rates, typically expected to support these services, is mainly hampered by the harsh multipath propagation conditions encountered in typical urban outdoor and indoor environments and by the shortage of available bandwidth. An effective response to overcome these weighty obstacles is the design of flexible radio terminals that adjust their configuration on the fly in order to take the best from the actual channel/system conditions. The notion of flexibility encompasses different features such as adaptivity, reconfigurability, scalability and so on. This work focuses on the adaptivity property, which is commonly defined as the capability of varying the numerical value of a set of parameters in response to channel/system changes. However, the terminology in this field has a certain degree of arbitrariness and is not universally agreed. For instance, the term adaptivity is sometimes used also for adaptive switching between different channel coding schemes, or different multiple antenna techniques, which implies a circuit-design change and not just a numeric parameters change. Therefore, in this work we use the terminology adaptive signal design (ASD) to collect all the actions aimed at modifying the transmitted waveform according to the current channel/system status. In order to support flexible and robust multimode terminals, multicarrier (MC) techniques,
has gained a considerable attention as the corner stone of the next generation wireless systems. It is well-known that MC modulation, in the form of Orthogonal Frequency Division Multiplexing (OFDM), is very robust to typical mobile radio multi-path environments and, efficiently uses the available spectrum. Moreover, OFDM makes possible
to exploit channel state information at the transmitter side to adapt link resources, such as transmitted power, modulation size/scheme, channel coding rate, etc. Another powerful technology fitting in the broad MC modulation concept is multicarrier code division multiple access (MC-CDMA), which combines the strong points of multicarrier (MC) and code division multiple access (CDMA) techniques. In this work, the goal of providing significative contributions to the ASD framework in the context of MC transmission schemes, is pursued through different routes, specifically tailored to OFDM and MC-CDMA systems.
The work is organized into two main parts, related to different ASD issues, namely:
i) adaptive modulation and coding for OFDM systems, and ii) adaptive techniques to nonlinear distortion compensation in MC-CDMA systems. For the sake of clarity, each part contains its own introduction and bibliography.