In this PhD thesis defense, Jesús Gómez presents his work, which aims the study of cooperative communications in wireless networks. In cooperative networks, each user transmits its own data and also aids the communication of other users. User cooperation is particularly attractive for the wireless medium, where every user listens to the transmission of other users. The main benefit of user cooperation in wireless networks is, probably, its efficacy to combat the wireless channel impairments.

Cooperation changes the classical idea of the channel as a simple link between a source and a destination. If users cooperate, then any node that serves as a relay becomes an element of the channel, just as reflecting obstacles that cause signal fading. Thus, the channel is no more one link but the network itself. Due to these new possibilities, the design of cooperative networks has motivated new problems at all the levels of the communication protocol stack.

In this dissertation, we address these problems by analyzing the energy efficient regime of different cooperative communication systems. We consider three basic cooperative channels: the single relay channel, the two-user cooperative multiple access channel, and the multi-hop multiple relay channel. These channels capture the essence of user cooperation and serve as primary building blocks for cooperation on a larger scale.

For these channels, we are interested in studying, the spectral efficiency as a function of the transmitted power per information bit relative to the noise spectral level. Obtaining the spectral efficiency for all values of energy per bit is usually unfeasible. Instead, if the energy efficient regime coincides with the low power regime, the communication strategy can be well characterized by computing two fundamental metrics: the minimum energy that we need to dedicate to each transmitted bit to have a reliable communication or, equivalently, the maximum rate that can be achieved per unit energy (RPE) and also the slope of the spectral efficiency at the point of minimum energy per bit. This slope indicates the bandwidth efficiency.