Amitkumar Mahadevan

Construction of a DSL-MIMO channel model for evaluation of FEXT cancellation systems in VDSL2

Of growing interest in the field of VDSL2 transmission is the evaluation of algorithms for improved performance using far-end crosstalk (FEXT) cancellation pre-coding. In order to perform fair evaluations of different algorithms, it is necessary to define a multiple input multiple output (MIMO) channel model that realistically represents the nature of the FEXT coupling dispersion that may be encountered in a multi-pair cable. This paper describes a general method for generating a MIMO channel model for a twisted wire pair cable that is extended from the currently defined 1% worst case FEXT coupling model. This extended model defines varying levels of FEXT cross-coupling values across the channel matrix and still preserves the 1% worst case crosstalk levels. Based on cable measurements, we determine a best fit probability density function to model the FEXT coupling dispersion; then for an example 25-pair 26-gauge (AWG) cable, we define four reference 25times25 channel matrices that contain varying degrees of channel dispersion: specifically, we define a 25times25 dispersion matrix for each of dasiazero,psila dasialow,psila dasiamedium,psila and dasiahighpsila dispersion. Finally, we compute rate vs. reach performance of FEXT Cancellation Pre-coding with cancellation of 5 dominant disturbers for each channel matrix.

Co-operative Alien Noise Cancellation in Upstream VDSL: A New Decision Directed Approach

Alien noise in the vectored very-high-speed digital subscriber line (VDSL) system is part of the additive noise at the receiver and exhibits strong correlation among users. We present a per-tone co-operative alien noise cancellation (CoMAC) algorithm for the upstream (US) VDSL that can be applied subsequent to any self far-end-crosstalk (FEXT) mitigation strategy. CoMAC operates by predicting the noise seen by a given user based on the error samples from the remaining users. These errors are conveniently obtained after slicing the self-FEXT canceled signal of all the vectored users. We show that if the estimation of these errors is accurate, the proposed alien canceler achieves the Cramer-Rao lower bound (CRLB). In practice, the seamless rate adaptation (SRA) operation, which enables increased bit rate by increasing the bit-loading per-tone, can cause decision errors in any decision directed strategy. We also analyze the impact of these decision errors - an issue not addressed in the literature. We propose a strategy for bit-loading during the SRA operation by formulating a max-min optimization problem and demonstrate a possibility of a guaranteed (minimum) improvement in the per-user rate. Simulations indicate that performance of the algorithm can exceed the minimum value significantly in practical situations.

Cooperative MIMO for alien noise cancellation in upstream VDSL

We present cooperative MIMO for alien noise cancellation (CoMAC): a per-tone, blind, low-complexity, linear, and adaptive noise whitening algorithm for alien crosstalk mitigation in upstream vectored VDSL systems. CoMAC directly acts on the residual errors of the vectored users after self-FEXT cancellation and frequency domain equalization, and thus, leverages the inherent alien-crosstalk-induced spatial correlation across users. CoMAC employs a lowcomplexity recursion scheme derived from the optimal MMSE noise whitener to non-disruptively initialize, engage, and adapt the noise canceller while the vectored users operate in data mode. Assuming reliable transmit symbol estimation at its input, we show that CoMAC achieves the Cramer-Rao lower bound. Further, the SNR improvements accruing from CoMAC can be translated into substantial rate improvements for upstream vectored VDSL.

Performance and Design of an Impulse Noise Detector for OFDM Systems with Reed-Solomon Erasure-Decoding

We develop an analytical framework to evaluate the probability-of-miss and probability-of-false-alarm performance of an impulse noise (IN) detection algorithm for orthogonal frequency-domain multiplexing (OFDM) symbols based on monitoring the post-frequency-domain-equalizer (FEQ) instantaneous error at the output of the QAM constellation demapper over a range of tones (sub-carriers). Using the VDSL system as a reference, we further analyze the impact of impulse-noise detection error events at the scale of the OFDM symbol (miss and false alarm) on the bit-error-rate (BER) performance of interleaved Reed-Solomon (RS) codes with erasure-decoding. The analysis culminates with a sequence of steps to design the parameters of the frequency-domain IN detector based on the design BER of the communication system and practical transceiver features such as noise-margin and seamless-rate-adaptation (SRA). We conclude that a tone-error based frequency-domain IN detector monitoring 100 tones is capable of providing IN detection performance that does not cause a deterioration of the average BER below the VDSL system design BER of 10-7 when interleaved RS coding with erasure-decoding is employed.

Self-Protection of Turbo- and LDPC-Coded OFDM Systems against Non-Stationary Disturbances

This paper presents a method to protect turbo- and low density parity check (LDPC)-coded OFDM systems against non-stationary disturbances without using external codes. The technique does not alter the structure of the original code and the constellation mapper/demapper, but requires the insertion of a so-called self-protection unit (SPU) between the coding and constellation mapping units. The SPU enhances the burst-error-correction capabilities of turbo and LDPC codes by combining erasure-decoding, subframing (i.e., reordering the coded data into subframes), and the optional use of subframe interleaving to spread error bursts. Self-protected systems exhibit a reduced latency and storage capacity compared to classical channel interleaving schemes with similar protection level.