MIMO Per-Tone Equalizer Design for Long Reach xDSL

Recently, long reach x-digital subscriber loop (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 longer cyclic prefix to successfully eliminate the inter-symbol interference (ISI) between subsequent time-domain discrete multi-tone (DMT) symbols and inter-carrier interference (ICI) between the tones of the same DMT symbol. The conventional way to deal with extended loop lengths is the use of a channel shortening filter; commonly referred as a time domain equalizer (TEQ). However, the performance of TEQ is restricted, since it treats all the tones jointly. A per-tone equalizer (PTEQ) is an attractive alternative to TEQ and provides an upper bound to the achievable performance of TEQ. Despite the advantages posed by a PTEQ, it suffers from large memory and computational complexity requirements, hindering the use of a PTEQ in practical multiple-input multiple-output (MIMO) scenarios. This paper proposes two new MIMO PTEQ models, which show a significant reduction in complexity and memory requirement with little or no performance degradation compared to the existing full MIMO PTEQ. The diagonal MIMO PTEQ allows the use of MIMO PTEQ in upstream as well as downstream scenarios.