On Superposition Multilevel Coding for the 3-Node Relay Channel Exploiting Decode and Forward

Relaying techniques are a well investigated subject in literature. The simplest relay channel is a 3-node network and the communication is split into broadcast phase and Multiple Access (MAC) phase. The focus of this paper lies on the MAC part of the relay channel which is a superposition of the source´s and the relay´s signals. From information theory it is known that a trade-off between distributed Beamforming (dBF) and the classical MAC has to be found. Generally, the source transmits a superposition of a beamforming part and new information, where the power distribution between both parts has to be optimized. In this work, this conventional approach will be reinterpreted from a Multilevel Coding/Code (MLC) point of view, where the new information can be seen as a level on top of the information transmitted by the relay. In addition, Superposition Modulation (SPM) will be used as a joined mapping scheme to ensure coherent superposition of the dBF part. In this MLC view, the conventional scheme is extended to a more generalized scheme which allows to achieve the theoretical Decode and Forward (DF) rate of Gaussian relay channels. Furthermore, it is possible to realize a low complexity joint demapping of both signals at the destination with only minor performance degradation. Indeed, results show that the new scheme will outperform practical mapping schemes like Quadrature Amplitude Modulation (QAM).