Patrick Duvaut

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.

The first step of long-reach ADSL: smart DSL technology, READSL

Long-reach DSL (LDSL) is a project in the ITU, with the objective to increase the reach of DSL solutions. Range extended ADSL (READ-SL) is the first step in the LDSL project, driven primarily by requirements for North America. READSL extends the connectivity of G.992.1/G.992.3 based ADSL up to 18 kft, increasing the ADSL reach by more than 3 kft under the severe crosstalk conditions typically experienced in the North American access network. To cope with the multiple physical layer field conditions, READSL uses smart DSL technology based on a selectable mask procedure. READSL is spectrally compatible with the basis systems in the North American Spectrum Management Standard T1.417, and requires minimum changes to both the G.992.1/G.992.3 standards and product platforms. This article reviews READSL as a subset of the LDSL project.

DMT performance improvement based on clustering modulation, applications to ADSL

Dynamic bit-loading for ADSL-DMT is driven by the signal-to-noise ratio (SNR) achieved per tone. 14.3 dB of minimum SNR is required to load a tone with the smallest square 4QAM constellation, for a fixed 10/sup -7/ bit error rate (BER). In the presence of physical layer impairments such as loop attenuation, bridge taps or crosstalks, many tones cannot be used because their individual SNR values are too small. On the other hand the associated total unloaded bandwidth exhibits a significant capacity. To benefit from parts of this unloaded bandwidth capacity, trellis coding with one bit or fractional bit loading has been proposed, unfortunately requiring some trellis decoding modifications. This paper introduces a new method called clustering modulation (CM) that does not require any trellis change and which clusters unloaded tones together to increase the SNR after maximum likelihood cluster payload demodulation in the frequency domain. Clustering modulation achieves up to 424 kbits/s of rate improvement on a 2.8 km (9 kfeet) 26AWG loop and up to 200 m of reach increase against SDSL noise B crosstalks.

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.

Enhanced TCM based on a multilevel hierarchical trellis coded modulation (HTCM)

This paper introduces a new multilevel coding scheme, identified as hierarchical trellis coded modulation (HTCM), that significantly improves a given TCM, while maintaining a 100% compatible fallback mode into the original trellis structure. HTCM exhibits a reasonable complexity increase when compared to higher order trellises or full turbo schemes that could reach similar gains. HTCM combines a given TCM code and a very high protection code (HPC), such as turbo code or low-density parity-check (LDPC) code. The HPC encodes the least significant bits of a large constellation, thus increasing the minimum Euclidean distance between TCM coded signals. The paper derives the theoretical HTCM asymptotic gain. HTCM is then applied to enhance the 16-state 4D Wei scheme by 1.76 dB of net coding gain., making use of a rate 1/2 parallel concatenated turbo code for the HPC and a new DSQ (diagonally shifted QAMs) constellation mapping. HTCM modularity, efficiency and relative simplicity open new ways to TCM enhancement that would help many broadband communication fields including wireless, satellite and wired systems.

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.

New criteria for determining improved perforation patterns for variable-rate convolutional codes

This paper presents a new efficient algorithm for determining the perforation patterns yielding variable-rate convolutional coding schemes with improved performance. Given a particular code rate, the new criteria optimize the code for a specific bit error rate, compared to existing techniques searching codes with good asymptotical performance. As an example, the patterns to be used in ultra-wideband systems following the upcoming IEEE standard 802.15.3a were evaluated for different error rates. The new patterns improve the performance of the currently suggested patterns by up to 0.4 dB without changing the structure and complexity of the encoder-decoder.

A simple test for multistage MIMO detection

Maximum likelihood detection of multiple input multiple output (MIMO) signals requires computational capabilities that are challenging to obtain in most practical systems. Although many alternative schemes have been devised to reduce the operational burden, the complexity of algorithmic schemes remains much higher than that of equalizer-based techniques, which in turn provide poor performance in terms of error rate. In this paper, we derive a low-complexity test that allows to run a more efficient but complex detection scheme only on selected received symbols. The few symbols that are identified as erroneously detected by the low complexity method are the only ones to be reprocessed. The global complexity is hence reduced, typically by more than one order of magnitude, with very little loss in performance

New Tight Upper-Bounds on the Error Probability of Multilevel Coded Modulation Schemes

This paper presents two theoretical upper-bounds on the word and bit error probabilities of individual levels of multilevel coded modulation schemes with equal error protection and multistage decoding. Lattice-type or PSK modulation with Ungerboeck partitioning over an AWGN channel is assumed. Inspired by both Imais [1] and Wachsmanns [2] channel models, a new model is introduced, yielding the expression of two upperbounds tighter than those known so far [3][4]. The tightness of the bounds is illustrated with simulation results and compared to existing bounds. Finally, a method for approximating the simulated performance of individual levels is discussed.

8-ary Unbalanced PSK Turbo Coded Modulation with Low Complexity Soft-Demapping

The 8-ary phase shift keying (8-PSK) constellation remains today widely used in turbo coded modulation schemes transmitting 3 coded bits per signal waveform. The present paper investigates a binary turbo coded modulation scheme using unbalanced 8-PSK (8-UPSK) constellations. A 8-UPSK with a structure similar to that of quadrature amplitude modulation (QAM) is proposed, yielding a low complexity calculation of the soft-decision associated with each bit. We suggest an architecture for computing the exact logarithm of the maximum likelihood ratio with a complexity reduced by more than 50% compared to that required for 8-PSK demapping. In addition, Monte-Carlo simulations show that some UPSK turbo coded modulations outperform the regular PSK modulation, hence achieving improved error performance with a reduced computational complexity.