Performance Analysis of rotated-QPSK in Physical-Layer Network Coding

Two wireless nodes outside each others radio range can communicate via a node in between, known as a relay node. A relay node may be a simple repeater or a fully functional entity depending upon the deployment scenario. In a two-way relay communication (TWRC), Physical-Layer network coding (PNC) exploits the superimposed nature of electromagnetic waves at the relay node to improve the throughput performance. Synchronization of information from two end users is critical when exchanging information via a relay node. Traditionally in PNC, modulation constellations sent from the two end users are chosen to be the same. In this paper, we investigate the performance of a PNC-based TWRC system in a Rayleigh fading scenario when the modulation constellation of one end user is rotated with respect to the modulation constellation at the other end user. The analysis shows that the rotated-QPSK gives distinct decision regions when the modulation constellation of QPSK and rotated-QPSK are superimposed, unlike the traditional (QPSK)(QPSK) transmission. Simulation results for the symbol error rate (SER) at the relay node shows the loss in performance of (QPSK)-(rotated-QPSK) scheme compared to the (QPSK)(QPSK) scheme due to phase rotation.