An Amplify-and-Forward architecture for low spectral efficiencies

This paper proposes a simple architecture for half-duplex cooperative systems which use amplify-and-forward (AF) as a relay strategy and one-dimensional modulations for source messages. The proposed solution uses the two orthogonal channels of quadrature modulation in order to allow a node to behave simultaneously as a source and a relay. We prove that the new scheme has similar performance to the conventional AF approach, and avoids error propagation problems of previously reported AF superposition schemes. The proposed technique is suitable for applications with low spectral efficiencies and practical adaptive systems where real modulations are implemented based on a quadrature modulation core.