Ordered Tomlinson-Harashima Precoding in G.fast Downstream

TL;DR

This paper introduces an optimized non-linear Tomlinson-Harashima Precoding scheme for G.fast DSL, improving interference cancellation by dynamically ordering precoding to maximize minimum rates across tones.

Contribution

It proposes a novel dynamic ordering strategy for THP in G.fast, outperforming existing equal-rate THP by considering channel asymmetries for better interference mitigation.

Findings

The proposed scheme achieves higher minimum rates than traditional methods.

Dynamic ordering based on channel statistics enhances overall system fairness.

V-BLAST ordering maximizes per-tone rate but not the global minimum rate.

Abstract

G.fast is an upcoming next generation DSL standard envisioned to use bandwidth up to 212 MHz. Far-end crosstalk (FEXT) at these frequencies greatly overcomes direct links. Its cancellation based on non-linear Tomlinson-Harashima Precoding (THP) proved to show significant advantage over standard linear precoding. This paper proposes a novel THP structure in which ordering of successive interference pre-cancellation can be optimized for downstream with non-cooperating receivers. The optimized scheme is compared to existing THP structure denoted as equal-rate THP which is widely adopted in wireless downlink. Structure and performance of both methods differ significantly favoring the proposed scheme. The ordering that maximizes the minimum rate (max-min fairness) for each tone of the discrete multi-tone modulation is the familiar V-BLAST ordering. However, V-BLAST does not lead to the global maximum when applied independently on each tone. The proposed novel Dynamic Ordering (DO) strategy takes into account asymmetric channel statistics to yield the highest minimum aggregated rate.