Abstract-An alternative receiver structure is presented for discrete multitone-based systems. The usual structure consisting of a (real) time-domain equalizer in combination with a (complex) 1-tap frequency-domain equalizer (FEQ) per tone, is modified into a structure with a (complex) multitap FEQ per tone. By solving a minimum mean-square-error problem, the signal-to-noise ratio is maximized for each individual tone. The result is a larger bit rate while complexity during data transmission is kept at the same level. Moreover, the per tone equalization is shown to have a reduced sensitivity to the synchronization delay.
Crosstalk is a major problem in modern DSL systems such as VDSL. Many crosstalk cancellation techniques have been proposed to help mitigate crosstalk, but whilst they lead to impressive performance gains, their complexity grows with the square of the number of lines within a binder. In binder groups which can carry up to hundreds of lines, this complexity is outside the scope of current implementation. In this paper, we investigate partial crosstalk cancellation for upstream VDSL. The majority of the detrimental effects of crosstalk are typically limited to a small subset of lines and tones. Furthermore, significant crosstalk is often only seen from neighbouring pairs within the binder configuration. We present a number of algorithms which exploit these properties to reduce the complexity of crosstalk cancellation. These algorithms are shown to achieve the majority of the performance gains of full crosstalk cancellation with significantly reduced run-time complexity
Very high bit-rate digital subscriber line (VDSL) is the latest generation in the ongoing evolution of DSL standards. VDSL aims at bringing truly broadband access, greater than 52 Mbps in the downstream, to the mass consumer market. This is achieved by transmitting in frequencies up to 12 MHz. Operating at such high frequencies gives rise to crosstalk between the DSL systems in a binder, limiting achievable data-rates. Crosstalk is typically 10-15 dB larger than other noise sources and is the primary limitation on performance in VDSL. In downstream transmission several crosstalk precompensation schemes have been proposed to address this issue. Whilst these schemes lead to large performance gains, they also have extremely high complexities, beyond the scope of current implementation.In this paper we develop the concept of partial crosstalk precompensation. The majority of the crosstalk experienced in a DSL system comes from only a few other lines within the binder. Furthermore its effects are limited to a small subset of tones. Partial precompensation exploits this by limiting precompensation to the tones and lines where it gives maximum benefit. As a result, these schemes achieve the majority of the gains of full crosstalk precompensation at a fraction of the run-time complexity. In this paper we develop several partial precompensation schemes. We show that with only 20% of the run-time complexity of full precompensation it is possible to achieve 80% of the performance gains. r
) is an intracellular second messenger used by many cells to release Ca 2ϩ from their internal stores (6). IP 3 acts on the IP 3 receptor, which is an important Ca 2ϩ release channel in the endoplasmic reticulum (7). There is, however, increasing evidence that IP 3 receptors are also localized in the Golgi apparatus (8 -12). The role of this latter organelle in the generation of cytosolic Ca 2ϩ signals is not well known, partly because the mechanism by which the Golgi complex takes up Ca 2ϩ is less well understood compared with the Ca 2ϩ transport into the endoplasmic reticulum mediated by the sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA). The Golgi apparatus expresses a specific Ca 2ϩ pump belonging to the Pmr1 family of Ca 2ϩ -transport ATPases (13). Pmr1 is insensitive to the inhibitor thapsigargin (14), in contrast to the SERCA Ca 2ϩ pumps in the sarco-and endoplasmic reticulum (15).We expressed the Pmr1 Ca 2ϩ pump of Caenorhabditis elegans (12) in COS-1 cells and pretreated the cells with thapsigargin to prevent SERCA-mediated Ca 2ϩ uptake in the endoplasmic reticulum. We demonstrate that a Ca 2ϩ store filled by the Pmr1 Ca 2ϩ pump and probably belonging to the Golgi compartment can set up baseline Ca 2ϩ spiking. EXPERIMENTAL PROCEDURESCell Culture and Transfection-COS-1 cells and HeLa cells were cultured as described previously (12, 16). The culture medium was replaced every 2 to 3 days. The cells were plated at a density of 2500 cells/cm 2 in Coverglass Chambers (Nunc Inc., Naperville, IL) or at a density of 10,000 cells/cm 2 in 12-well clusters (Costar, MA). Four days after plating, COS-1 cells were transiently transfected with Pmr1 from C. elegans or with the rabbit SERCA1a in pMT2 vector and investigated 3 days later (12). The full-length rabbit SERCA1a clone was kindly provided by Drs. J. P. Andersen and B. Vilsen (Department of Physiology, University of Aarhus, Denmark). Some cells were transfected with pig SERCA2a or SERCA2b in pSV57 (17).Immunofluorescence-Immunofluorescence microscopy was done as described previously (12). The primary antiserum Celpmrloop against Pmr1 was raised against a recombinant protein corresponding to the putative large cytoplasmic loop between transmembrane segments 4 and 5 of Pmr1 from C. elegans (12). The primary antiserum 809-27 against rabbit SERCA1 was kindly provided by Dr. J. Møller (Department of Biophysics, University of Aarhus, Denmark) (18). The antiserum against SERCA2a was described previously (19). The antibody against the 58K protein of the Golgi apparatus (clone 58K-9) was purchased from Abcams (Cambridge, UK).45 Ca 2ϩ Uptake-Fluxes were performed on saponin-permeabilized cells at 25°C as described previously (20). The cells were permeabilized by treating them for 10 min with 20 g/ml saponin at 25°C in a medium containing 120 mM KCl, 30 mM imidazole-HCl (pH 6.8), 2 mM MgCl 2 , 1 mM ATP and 1 mM EGTA. The non-mitochondrial Ca 2ϩ stores were loaded for 45 min in loading medium containing 120 mM KCl, 30 mM imidazole-HCl (pH 6.8), 5 mM MgCl 2 , 5 mM ATP, ...
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