MIMO Per-Tone Equalizer Design for Long Reach xDSL

Abstract

Recently, long reach x-digital subscriber line (LR-xDSL) has been proposed to extend the reach of conventional DSL systems. The extended loop lengths are characterized by a longer channel impulse response (CIR), which requires a similarly longer cyclic prefix (CP) to successfully eliminate the inter-symbol interference (ISI) between successive time-domain discrete multi-tone (DMT) symbols and inter-carrier interference (ICI) between the carriers or tones of the same DMT symbol. This adds a large overhead to the transmitted symbols and results in throughput loss. A per-tone equalizer (PTEQ) is an attractive alternative to deal with extended loop lengths. However, it imposes a large initialization computational complexity and memory requirement, hindering the use of a PTEQ in practical multiple-input multiple-output (MIMO) scenarios. To tackle this problem, a specific structure in the MIMO DSL channel, namely that the combined ISI and ICI signal power from the crosstalk channels is significantly lower than the desired and combined ISI and ICI signal power from the direct channels, may be exploited in deriving a novel low complexity/memory solution, here referred as sparse MIMO PTEQ, with negligible impact (≈ 0.5% drop) on performance compared to a full MIMO PTEQ. For a conventional DSL binder size of 16 lines and a PTEQ order of 3, the proposed sparse MIMO PTEQ performs at 42% of the initialization computational complexity and 29.7% of the memory requirement, with negligible performance degradation, compared to a full MIMO PTEQ. The initialization computational complexity and memory requirement is further reduced by the proposed diagonal MIMO PTEQ which operates at 0.4% of the initialization computational complexity of a full MIMO PTEQ and requires 6.25% memory compared to a full MIMO PTEQ, with some degradation in the performance compared to the full MIMO PTEQ. The diagonal MIMO PTEQ has the additional benefit that it can be applied in both upstream and downstream scenarios, in contrast to the full and sparse MIMO PTEQ which can be used only in upstream scenarios.

The Perspective of Metallic Access Networks Based on MELT Parameter Measurements on Homogeneous Symmetric Lines

“…The recent use case proposed under [4] is for G.mgfast, which is also likely the latest DSL standard. In G.mgfast deployments, one of the proposed scenarios is a dual DPU mode to serve G.mgfast for shorter loop lengths and G.fast and LR-G.fast for longer loop lengths [5].…”