I. Introduction
Being considered one of the potential enabling techniques for future communication networks, relaying gives rise to a plethora of interesting applications and new business opportunities. The relay model was first introduced by Van Der Meulen [1] and substantially developed by Cover and El Gamal [2]. Common relaying schemes including amplify-and-forward (AF), decode-and-forward (DF), and compress-and-forward (CF) have been investigated for single-relay systems [3]– [8].