Relay communication with hierarchical modulation

We consider a relay communication with distributed channel coding. The source broadcasts channel encoded and modulated information to the relay and to the destination. In a second time slot, the relay sends additional redundancy to the destination. The broadcast from the source leads to the dilemma of adjusting the modulation scheme to the relay or to the destination link. We propose to use hierarchical modulation to solve this dilemma. We show with simulation results that the proposed system achieves a significant gain compared to reference systems.