Signal alignment for multicarrier code division multiple user two-way relay systems

Two-way relay (TWR) transmission provides high spectral efficiency when one-pair of two users exchange information via a single relay. However, in multiuser relay systems where multi-pair of users exchange messages, if the relay does not have sufficient degrees of freedom, the gain of TWR communication may vanish. Fortunately, signal alignment (SA) signaling can recover the spectral efficiency of TWR transmission in multiuser scenarios, which is originally proposed for multiple-input-multiple-output (MIMO) systems. In this paper we investigate signal alignment for multicarrier code division multiple access (MC-CDMA) TWR systems. Due to the difference in channel characteristics and degrees of freedom, the existing SA signalings designed for MIMO TWR systems do not always perform well in MC-CDMA TWR systems. By exploiting the unique features of both TWR systems and MC-CDMA channels, we propose a spectral-efficient SA signaling for MC-CDMA TWR systems, where each pair of users employ a maximal ratio transmitter of its counterpart to align their signals at the relay. We then analyze the spectral efficiency of the designed SA signaling, compare it with non-SA (NSA) signaling, and optimize the power allocation among the relay and users. It is shown from asymptotic analysis and simulation results that the proposed SA signaling can support more users and achieve higher spectral efficiency than NSA signaling.