A complex modulo-and-forward strategy for two-way relay systems

Relays have become increasingly important components in modern wireless communication systems. We consider an amplify-and-forward-based two-way relay scenario where the two end terminals employ two-dimensional modulations. We propose a complex modulo-and-forward strategy to improve the energy efficiency of the relay station. This improvement comes about by the ability of the complex modulo operation to reduce the relay transmission power while maintaining the error performance at the terminal nodes. The amount of reduction in relay transmission power is evaluated for QAM modulations. When the two channel coefficients between the two terminal nodes and the relay have equal magnitude, a reduction close to 3 dB can be realized, depending on the modulation order. The transmission performance is verified in terms of QAM symbol error rates.