I. Introduction
Relaying is an emerging technique in the third/fourth generation (3G/4G) high bandwidth networks for its ability to enhance the coverage and capacity of edge nodes in a cellular system. Furthermore, relay cooperative communication is one of the promising paradigms to realize green-broadband communication in the future. In relay networks, as the power of mobile users is often scarce and constrained, energy efficiency is one of the most important issues to consider, especially for the scenarios where multiple users compete for the limited power provided by some assistant nodes, such as relay. In fact, as relay-cooperation concept is combined with other emerging technologies, such as multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) and cognitive radio, optimal power allocation in relay systems is always an important topic to study [1]–[3].