Giovanni Cherubini

Filtered multitone modulation for very high-speed digital subscriber lines

A filter-bank modulation technique called filtered multitone (FMT) and its application to data transmission for very high-speed digital subscriber line technology are described. The proposed scheme leads to significantly lower spectral overlapping between adjacent subchannels than for known multicarrier techniques such as discrete multitone (DMT) or discrete wavelet multitone. FMT modulation mitigates interference due to echo and near-end crosstalk signals, and increases the system throughput and reach. Signal equalization in an FMT receiver is accomplished in the form of per-subchannel symbol-spaced or fractionally spaced linear or decision-feedback equalization. The problem of channel coding for this type of modulation is also addressed, and an approach that allows combined removal of intersymbol-interference via precoding and trellis coding is described. Furthermore, practical design aspects regarding filter-bank realization, initial transceiver training, adaptive equalization, and timing recovery are discussed. Finally, simulation results of the performance achieved by FMT modulation for very high-speed digital subscriber line systems, where upstream and downstream signals are separated by frequency-division duplexing, are presented and compared with DMT modulation.

Millipede: a MEMS-based scanning-probe data-storage system

Ultrahigh storage densities of up to 1 Tbit/in./sup 2/ or more can be achieved by local-probe techniques to write, read back, and erase data in very thin polymer films. The thermomechanical scanning-probe-based data-storage concept called Millipede combines ultrahigh density, small form factor, and high data rate. After illustrating the principles of operation of the Millipede, we introduce system aspects related to the read-back process, multiplexing, and position-error-signal generation for tracking.

Filter bank modulation techniques for very high speed digital subscriber lines

Multicarrier basis functions and related issues are overviewed in this article with an emphasis on filtered multitone (FMT) modulation for VDSL. The combination of filtering, small amounts of excess bandwidth, multicarrier channel partitioning, and some equalization are shown to allow a good match for spectrum management and performance challenges in VDSL transmission. The FMT basic functions are overviewed in the context of filter banks, with appropriate changes for nonzero excess bandwidth.

Probe-based ultrahigh-density storage technology

Ultrahigh storage densities can be achieved by using a thermomechanical scanning-probe-based data-storage approach to write, read back, and erase data in very thin polymer films. High data rates are achieved by parallel operation of large two-dimensional arrays of cantilevers that can be batch fabricated by silicon-surface micromachining techniques. The very high precision required to navigate the storage medium relative to the array of probes is achieved by microelectromechanical system (MEMS)- based x and y actuators. The ultrahigh storage densities offered by probe-storage devices pose a significant challenge in terms of both control design for nanoscale positioning and read-channel design for reliable signal detection. Moreover, the high parallelism necessitates new dataflow architectures to ensure high performance and reliability of the system. In this paper, we present a small-scale prototype system of a storage device that we built based on scanning-probe technology. Experimental results of multiple sectors, recorded using multiple levers at 840 Gb/in2 and read back without errors, demonstrate the functionality of the prototype system. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all building blocks of a storage device.

Filtered multitone modulation for VDSL

We present a filter-bank modulation technique called filtered multitone (FMT) and describe its application to data transmission for very-high-speed digital subscriber line (VDSL) technology. The proposed scheme leads to significantly lower spectral overlapping between adjacent subchannels than for known multicarrier techniques such as discrete multitone (DMT) or discrete wavelet multitone. This allows mitigating interference due to echo and near-end crosstalk signals, increasing system throughput and reach, and simplifying system operations for VDSL transmission. In the receiver of an FMT system, signal equalization is accomplished in the form of per-subchannel symbol-spaced or fractionally-spaced linear or decision-feedback equalization. We study by simulation the performance that can be achieved by FMT modulation for VDSL systems where upstream and downstream signals are separated by frequency-division duplexing. We also provide comparisons with DMT modulation for a typical transmission scenario.

Control of MEMS-Based Scanning-Probe Data-Storage Devices

Micro-electro-mechanical-system (MEMS)-based scanning-probe data-storage devices are emerging as potential ultra-high-density, low-access-time, and low-power alternative to conventional data storage. Nanoscale accuracy and short latency in the navigation of the probes are the primary control challenges in probe-storage applications. This paper focuses on the control design to address these challenges in a probe-based storage prototype using a micro-scanner as the nanopositioner for the storage medium. Experimental results demonstrate remarkably short seek times on the order of 1 ms for the worst-case seek operations. Moreover, a thermal-sensor-based approach is compared with a two-sensor-control configuration employing both the global-position information from the thermal sensors and the medium-derived position information. Drift and low-frequency noise can affect the performance of the thermal-sensor-based control scheme over long periods of operation. This is addressed by the second scheme, a novel control architecture based on the Hinfin control framework that uses the best measurement in each of the frequency regions.

A servomechanism for a micro-electro-mechanical-system-based scanning-probe data storage device

Micro-electro-mechanical-system (MEMS)-based scanning-probe data storage devices are emerging as potential ultra-high-density, low-access-time, and low-power alternatives to conventional data storage. One implementation of probe-based storage uses thermomechanical means to store and retrieve information in thin polymer films. One of the challenges in building such devices is the extreme accuracy and the short latency required in the navigation of the probes over the polymer medium. This paper focuses on the design and characterization of a servomechanism to achieve such accurate positioning in a probe-based storage prototype. In our device, the polymer medium is positioned on a MEMS scanner with x/y-motion capabilities of about 100 µm. The device also includes thermal position sensors that provide x/y-position information to the servo controller. Based on a discrete state-space model of the scanner dynamics, a controller is designed using the linear quadratic Gaussian approach with state estimation. The random seek performance of this approach is evaluated and compared with that of the conventional proportional, integrator, and derivative (PID) approach. The results demonstrate the superiority of the state-space approach, which achieves seek times of about 4 ms in a ± 50 µm range. Finally, the experimental results show that closed-loop track following using the thermal position-sensor signals is feasible and yields a position-error standard deviation of approximately 2 nm.

Scaling tape-recording areal densities to 100 Gb/in 2

We examine the issue of scaling magnetic tape-recording to higher areal densities, focusing on the challenges of achieving 100 Gb/in2 in the linear tape format. The current highest achieved areal density demonstrations of 6.7 Gb/in2 in the linear tape and 23.0 Gb/in2 in the helical scan format provide a reference for this assessment. We argue that controlling the head-tape interaction is key to achieving high linear density, whereas track-following and reel-to-reel servomechanisms as well as transverse dimensional stability are key for achieving high track density. We envision that advancements in media, data-detection techniques, reel-to-reel control, and lateral motion control will enable much higher areal densities. An achievable goal is a linear density of 800 Kb/in and a track pitch of 0.2 µm, resulting in an areal density of 100 Gb/in2.

Control Methods in Data-Storage Systems

The recording performance of data-storage devices, in which write/read elements move relative to a storage medium to reliably store and retrieve information, depends on the capability of servo mechanisms to provide the necessary positioning accuracy. The desired characteristics of servo mechanisms for data-storage systems include robustness against variations of environmental parameters, high resolution, accuracy, stability, and fast response. This paper presents a comprehensive overview of advanced servo-control methods for data storage. The applications are to well-established recording technologies, including magnetic tape and magnetic disk systems as well as CD/DVD/Blue-Ray optical data-storage systems. Moreover, newer holographic and near-field optical systems and the emerging probe-storage technology are also addressed. Emphasis is given to the potential exhibited by the technologies considered for achieving ultra-high storage capacity, as required by the exploding demand in data-storage capacity for archival systems and massive multimedia data storage.

Servo-Pattern Design and Track-Following Control for Nanometer Head Positioning on Flexible Tape Media

Achieving multi-Terabyte capacity in tape cartridges requires a substantially higher track density than that available in present systems, and hence a significantly higher positioning accuracy is required of the track-following servo in tape drives. In this paper, advanced concepts are considered for several elements of a tape system that enhance the track-following servo performance to reach nanometer positioning accuracy. We introduce a novel method for optimizing the geometry of servo patterns in a timing-based servo system. The design criterion aims to minimize the measurement error in the position-error signal (PES) yielded by a digital synchronous servo channel. A flangeless tape path is adopted to mitigate high-frequency components of the lateral tape motion. The track-following servo controller, which is designed based on the H∞ approach, takes into account the measured plant transfer function, the disturbance characteristics of the tape path, and the properties of servo channel. These elements are combined to investigate the track-following performance achievable with a new high-SNR magnetic tape based on perpendicularly-oriented BaFe particles. With this setup, a record closed-loop track-following performance of less than 14 nm PES standard deviation is demonstrated.