Per-line power controlled lattice-reduction aided zero-forcing precoding for G.fast downstream

Linear precoding for Far-End CrossTalk (FEXT) cancellation is supported by the first release of the next generation DSL standard G.fast utilizing bandwidth up to 106 MHz. The cancellation precoding for the second release (bandwidth up to 212 MHz with much stronger FEXT) has not been standardized yet. Non-linear Tomlinson-Harashima Precoding (THP) is a promising candidate as it outperforms the linear precoding. Nevertheless, sequential processing of THP introduces unfavorable additional delay proportional to the number of lines. In this paper, we focus on non- linear precoding based on Lattice Reduction aided Zero-Forcing (ZF-LR) which can be implemented in parallel without any sequential processing delay. ZF-LR precoding has been proposed in wireless scenario using scalar scaling to adjust average transmit power. We modify this scheme to comply with per-line per-carrier power constraint and enable power control of each line individually (e.g., to adapt power for different line lengths). Optimized Power Allocation (OPA) for the modified scheme maximizing weighted sum-rate leads to the signomial optimization problem. Similarly to THP, ZF-LR precoding with OPA outperforms the linear precoding as well.