Improved linear crosstalk precompensation for DSL

Abstract

Crosstalk is the major source of performance degradation in next generation DSL systems such as VDSL. In downstream communications transmitting modems are co-located at the central office. This allows crosstalk precompensation to be employed. In crosstalk precompensation the transmitted signal is pre-distorted such that the pre-distortion destructively interferes with the crosstalk introduced by the channel.Existing crosstalk precompensation techniques either give poor performance or require modification of customer premises equipment (CPE). This is impractical since there are millions of legacy CPE modems already in use.We present a novel crosstalk precompensation technique based on a diagonalization of the crosstalk channel matrix. This technique does not require modification of CPE. Furthermore, certain properties of the DSL channel ensure that this diagonalizing precompensator achieves near-optimal performance.

Finite Word Length Effects on Transmission Rate in Zero Forcing Linear Precoding for Multichannel DSL

“…Let H(f ) be the channel matrix of p twisted pairs at frequency f . Let r(f ) = r(H(f )) as in (6). The transmission rate loss of the i-th user at frequency f due to quantization is bounded by …”

“…In [2], [3] it is shown that optimal cancelation achieves capacity boost ranging from 2× to 4×, and also substantially reduces per-loop capacity spread and outage, which are very important metrics from an operator's perspective. References [5], [6] advocate the use of a diagonalizing precompensator, and demonstrate that, without modification of the Customer Premise Equipment (CPE), one can obtain near optimal performance. Recent work in [7], [8] has shown that a low-order truncated series approximation of the inverse channel matrix affords significant complexity reduction in the computation of the precoding matrix.…”

Experimental Evaluation of Capacity Statistics for Short VDSL Loops

“…For the loop lengths considered here (up to 590 m), IL is at least 30 dB less than FEXT attenuation. The matrix is therefore diagonally dominated, and it is possible to pre-equalize it at the transmitter's side without a significant penalty 3 in terms of transmission power [3]. Thus, upon premultiplication by (5) the effective MIMO channel-transfer matrix becomes (6) Note that we do not invert the direct IL channels; that would entail a significant power penalty, especially at higher frequencies.…”

Analysis of Inverse Crosstalk Channel Estimation Using SNR Feedback

“…The normalized receiver error signal can be expressed by (4) For a given line , a key performance measurement obtained via standardized mechanisms is an SNR estimate , where (5) Since the transmit power is known precisely, an estimate of is equivalent to an estimate of . In order to drive the residual matrix to zero, we must manipulate the precoder matrix , without a priori knowledge of the channel matrix .…”