Communication over a wireless channel is subject to large and small scale propagation effects including path loss, shadow fading, and small scale multipath fading. These effects are the most important sources of performance degradation in communication systems. Various diversity techniques such as time, frequency, and spatial diversity are proposed to mitigate the adverse effects of fading. Among all diversity techniques more attention has been given to spatial diversity. Cooperative diversity is an innovative approach to improve reliability of communication by exploiting spatial diversity. However, this technology is challenging in practice. In some multi relay cooperative diversity protocols that are based on simultaneous transmission, time and frequency synchronization of relay nodes are very important. However, realization of such requirement is difficult. The imperfect synchronization in destination is the main reason of performance degradation for simultaneous transmission based protocols such as distributed space-time coding (DSTC). Although separation in transmission time of terminals in traditional time division multiple access (TDMA) method can solve the synchronization problems, but it leads to lower spectral efficiency. In addition to these problems, asymmetric nature of wireless networks is another issue that must be considered in cooperative diversity protocols. In practical wireless networks different source nodes are distributed at different positions. Therefore, various terminals experience different channel conditions and thus some nodes are disadvantageous in their transmission in comparison with others. Communication in asymmetric networks applying non-cooperative protocols results in high aggregate transmit power and uneven power distribution among various terminals. These issues cause performance degradation of communication. Cooperative communication is an approach to improve the performance of communication in asymmetric wireless networks. In this thesis after reviewing some of the most important cooperative diversity schemes, a cooperative strategy is proposed for asymmetric wireless networks that improves overall wireless communication performance (in terms of reducing total power consumption, improving error performance and increasing the network lifetime) in comparison with non-cooperative protocols on one hand, and solves some problems of current cooperative diversity schemes such as synchronization and low spectral efficiency on the other hand. In our proposed scheme, we use a combination of superposition coding and network coding to achieve incremental diversity in asymmetric wireless communication networks, hence the name superposition network coding. This technique solves the synchronization problems by single transmission of terminals in TDMA scheme. Also, applying superposition coding improves spectral efficiency in comparison with traditional TDMA cooperative communication strategies. In this thesis, it is tried to implement the receiver structure as simple as possible. In addition, the error performance of superposition network coding is analyzed by appropriate approximations and the analytical results are compared with simulation results. Excellent agreement between the derived analytical results and simulation results demonstrates the accuracy of the proposed scheme in improving system performance. Keywords: Cooperative diversity, relay terminal, Symbol Error Rate Selection Combining (SER-SC), Interference Ignorant Detector (IID), Network coding, Superposition coding