I. Introduction
Recently, cooperative diversity has been widely discussed in wireless communications. The basic idea is that single-antenna nodes in wireless networks share their antennas and transmit cooperatively as a virtual multi-input multi-output (MIMO) system, thus spatial diversity can be achieved and multi-path fading can be effectively mitigated. When one node helps other nodes to forward information, it serves as a relay, and in this process, amplify-and-forward (AF) or decode-and-forward (DF) relaying schemes can be utilized [2]. In DF relaying scheme, the relay detects and retransmits the received signal, while the relay just simply amplifies and retransmits the received noisy signal in AF relaying scheme. The DF relaying strategy can be further classified into two types [3]: 1) fixed DF relaying scheme (FDF): the relay always detects and forwards the signals it has received from the source and 2) adaptive DF relaying scheme (ADF): the relay decides to forward the received signal only when the evaluated signal quality satisfies the predefined metric (e.g., the measured SNR is higher than a certain threshold), otherwise, it keeps silent.