Beamforming for network-coded MIMO two-way relaying

We propose a practical MIMO precoding strategy for the multiple-access phase of the two-way relay channel with network coding and M-QAM modulation. The proposed scheme is a hybrid between three classes of constrained precoders, namely subspace alignment, subspace separation and maximum ratio transmission. The optimal solution within each class can be computed efficiently, and switching between the classes is based on the minimum distance. Numerical simulations show that the proposed scheme approaches ideal performance, thus providing a low-complexity and near-optimal solution to the beamforming problem.