Showing papers on "Bandwidth (signal processing) published in 1993"

Method and apparatus for adaptive, variable bandwidth, high-speed data transmission of a multicarrier signal over digital subscriber lines

Abstract: A method and apparatus for adaptive, variable bandwidth, high-speed data transmission of a multicarrier signal over digital subscriber lines wherein the initial optimal transmission bandwidth is identified based on initial signal-to-noise ratio (SNR) estimates of the orthogonal carriers of the multicarrier system. Maximum data throughput, or system performance margin, is achieved by assigning the total amount of information, or number of bits, to be transmitted in each multicarrier symbol to particular carriers through an initial bit allocation procedure, which is possibly subject to variable target bit error rates among the carriers. A transmit power mask, of any shape and level, is imposed upon the system by an initial energy allocation procedure that limits the maximum amount of power to be transmitted in each of the several carriers. Lastly, run-time adaptivity is achieved by monitoring the mean-squared-errors (MSE) of the orthogonal carriers, and the transmission bandwidth, as well as the bit allocation within the multicarrier symbol, is caused to change in real-time corresponding to changes in the channel characteristics in order to maintain optimal system performance.

A new bandwidth efficient transmit antenna modulation diversity scheme for linear digital modulation

Abstract: A system is proposed and analyzed, which uses several transmit antennas to achieve diversity in a flat fading mobile radio environment. The signals at different antennas carry the same digital information, but have different modulation parameters (modulation diversity). In contrast to other proposals, no bandwidth expansion is required. Modulation diversity for quadrature amplitude modulation (QAM) type modulation is achieved by using different partial response type base pulses. Simulation results confirm the expected gain. >

Spectrum-sliced fiber amplifier light source for multichannel WDM applications

Abstract: A potentially inexpensive light source for multichannel WDM applications is proposed The high-power amplified spontaneous emission (ASE) from an erbium-doped fiber amplifier (EDFA), which is already in the single-mode fiber, can be efficiently divided into many channels by using an integrated optic wavelength-division-multiplexing (WDM) demultiplexer This spectrum-sliced ASE can be used as light sources for WDM systems in place of several wavelength-selected DFB lasers To demonstrate the principle, the 40-nm-wide ASE spectrum of an EDFA was sliced using a narrow optical filter (3-dB bandwidth: 13 nm), and the resulting source was used for the transmission of up to 17 Gb/s of data The problem of spontaneous-spontaneous beat noise in these sources is dealt with It is estimated that the total capacity would be about 40 Gb/s, realistically, since the channel spacing should be at least three times the optical bandwidth of each channel to avoid crosstalk >

Virtual interconnections for reconfigurable logic systems

Abstract: A compilation technique overcomes device pin limitations using virtual interconnections. Virtual interconnections overcome pin limitations by intelligently multiplexing each physical wire among multiple logical wires and pipelining these connections at the maximum clocking frequency. Virtual interconnections increase usable bandwidth and relax the absolute limits imposed on gate utilization in logic emulation systems employing Field Programmable Gate Arrays (FPGAs). A "softwire" compiler utilizes static routing and relies on minimal hardware support. The technique can be applied to any topology and FPGA device.

Virtual wires: overcoming pin limitations in FPGA-based logic emulators

Abstract: Existing FPGA-based logic emulators only use a fraction of potential communication bandwidth because they dedicate each FPGA pin (physical wire) to a single emulated signal (logical wire). Virtual wires overcome pin limitations by intelligently multiplexing each physical wire among multiple logical wires and pipelining these connections at the maximum clocking frequency of the FPGA. A virtual wire represents a connection from a logical output on one FPGA to a logical input on another FPGA. Virtual wires not only increase usable bandwidth, but also relax the absolute limits imposed on gate utilization. The resulting improvement in bandwidth reduces the need for global interconnect, allowing effective use of low dimension inter-chip connections (such as nearest-neighbor). Nearest-neighbor topologies, coupled with the ability of virtual wires to overlap communication with computation, can even improve emulation speeds. The authors present the concept of virtual wires and describe their first implementation, a 'softwire' compiler which utilizes static routing and relies on minimal hardware support. Results from compiling netlists for the 18 K gate Sparcle microprocessor and the 86 K gate Alewife Communications and Cache Controller indicate that virtual wires can increase FPGA gate utilization beyond 80 percent without a significant slowdown in emulation speed. >

Locally Adaptive Bandwidth Choice for Kernel Regression Estimators

Abstract: Kernel estimators with a global bandwidth are commonly used to estimate regression functions. On the other hand, it is obvious that the choice of a local bandwidth can lead to better results, because a larger class of kernel estimators is available. Evidently, this may in turn affect variability. The optimal bandwidths depend essentially on the regression function itself and on the residual variance, and it is desirable to estimate them from the data. In this article, a local bandwidth estimator is studied. A comparison with its global bandwidth equivalent is performed both in theory and in simulations. As the main result it is shown that the possible gain in mean integrated squared error of the resulting regression estimator must be paid for by a larger variability of the estimator. This may lead to worse results if the sample size is small. An algorithm has been devised that puts special weight on stability aspects. Our simulation study shows that improvements over a global bandwidth estimator ...

Time-division optical communications in multiprocessor arrays

Abstract: An optical communication structure is proposed for multiprocessor arrays which exploits the high communication bandwidth of optical waveguides. The structure takes advantage of two properties of optical signal transmissions on waveguides, namely, unidirectional propagation and predictable propagation delays per unit length. Because of these two properties, time-division multiplexing (TDM) of messages has the same effect as message pipelining on optical waveguides. Two TDM approaches are proposed, and the combination of the two is used in the design of the optical communication structure. Analysis and simulation results are given to demonstrate the communication effectiveness of the system. A clock distribution method is proposed to address potential synchronization problems. Feasibility issues with current and future technologies are discussed. >

High efficiency chiral nematic liquid crystal rear polarizer for liquid crystal displays having a notch polarization bandwidth of 100 nm to 250 nm

Abstract: The efficiency of LC displays is improved by the use of a polymeric chiral nematic liquid crystal rear polarizer and reflective backlight housing which increases the net transmission of light and, at the same time, reduces the weight of the assemblage and increases battery life by utilizing lower levels of illumination The polymeric chiral nematic liquid crystal can be a freestanding film or films, or can be supported by a single glass substrate The liquid crystal polarizer can have a bandwidth sufficient to transmit electromagnetic radiation across the entire visible spectrum

CMOS continuous-time current-mode filters for high-frequency applications

Abstract: Design considerations for high-frequency CMOS continuous-time current-mode filters are presented. The basic building block is a differential current integrator with its gain constant set by a small-signal transconductance and a gate capacitance. A prototype fifth-order low-pass ladder filter implemented in a standard digital 2 mu m n-well CMOS process achieved a -3 dB cutoff frequency (f/sub 0/) of 42 MHz; f/sub 0/ was tunable from 24 to 42 MHZ by varying a reference bias current from 50 to 150 mu A. Using a single 5 V power supply with a nominal reference current of 100 mu A, the five-pole filter dissipated 25.5 mW. The active filter area was 0.056 mm/sup 2//pole. With the minimum input signal defined as the input-referred noise integrated over a 40 MHz bandwidth, and the maximum input signal defined at the 1% total intermodulation distortion (TIMD) level, the measured dynamic range was 69 dB. A third-order elliptic low-pass ladder filter was also integrated in the 2 mu m n-well CMOS process to verify the implementation of finite transmission zeros. >

Volume holographic narrow-band optical filter.

Abstract: An 0.0125-nm (FWHM) bandwidth optical filter with >10% throughput, clean sidebands, f/12 speed, and a response roll-off of −40 dB/decade has been developed by using volume holography in photorefractive materials. The performance of the Accuwave holographic filter is characterized and compared with that of a Lyot filter, which is the standard for solar astronomy today. Moreover, this device represents the first commercial application of volume holography in photorefractive crystals.

Narrowband interference cancellation through the use of digital recursive notch filters

Abstract: A digital notch filter apparatus for removing narrowband interference signals from a wideband communication signal. The apparatus includes a recursive digital passband interference acquisition filter, an acquisition mode center frequency control circuit, a recursive digital passband interference tracking filter, a tracking mode center frequency control circuit and an interference detection circuit. The tracking filter has a narrower bandwidth than the acquisition filter. Both the acquisition and tracking filters are gang tuned so that the center frequencies of their passbands are adjusted in unison. During interference acquisition mode, the acquisition mode center frequency control circuit is used is to adjust the center frequency of the filters over the bandwidth covered by the interference canceler. When the interference detection circuit detects an interference signal within the passband of the tracking filter, the tracking center frequency control circuit is used to adjust the center frequency of the acquisition and tracking filters to insure that the filters remain locked onto the narrowband interference signal. Upon detection of the interference signal, the portion of the communication signal which includes the narrowband interference signal that is passed through the narrow passband tracking filter is subtracted from the communication signal to remove the narrowband interference signal. When no interference signal is detected, the communication signal is permitted to pass to the output of the notch filter circuit without attenuation.

Optical collimating device employing cholesteric liquid crystal and a non-transmissive reflector

Abstract: An optical collimating device employs an optical (concave) mirror and cholesteric liquid crystal element to collimate and project image light into the line-of-sight of an observer in efficient manner. Image light is generated with light components within a predetermined bandwidth and with a predetermined rotary sense. The image light is directed so that it is reflected by the concave mirror, expanding (collimating) the image light, projecting in onto a cholesteric liquid crystal element that reflects those components of the image light within the bandwidth B and having the proper rotary sense of circular polarization. The cholesteric liquid crystal element substantially reflects the returned image to the observer.

A mode-filtering scheme for improvement of the bandwidth-distance product in multimode fiber systems

Abstract: A scheme in which the bandwidth-distance product of a multimode fiber is extended, so that it can nearly support the transmission rate of single-mode systems, is studied. It is based on selective launching of lower order modes into the fiber at the transmitting end and on filtering out at the receiver the fraction of the energy that was coupled into the higher order modes throughout the propagation in the multimode fiber. The power penalty and the dispersion performance of the scheme are investigated. In particular, it is shown that the scheme carries about 6.5 dB penalty, and doubling of the bandwidth*distance value is demonstrated. The effect of splices on the performance is also presented. It is envisioned that this approach may be used to upgrade existing multimode systems without the need for rewiring. For example, it can be used to replace the FDDI (fiber distributed data interface) installations in future high-speed networks, when transmission rates increase from megabits per second to gigabits per second. >

Efficiency in detection of isoluminant and isochromatic interference fringes

Abstract: We examined the limitations imposed by neural factors on spatial contrast sensitivity for both isochromatic and isoluminant gratings. We used two strategies to isolate these neural factors. First, we eliminated the effect of blurring by the dioptrics of the eye by using interference fringes. Second, we corrected our data for additional sensitivity losses up to and including the site of photon absorption by applying an ideal-observer analysis described by Geisler [ J. Opt. Soc. Am. A1, 775 ( 1984)]. Our measurements indicate that the neural visual system modifies the shape of the contrast-sensitivity functions for both isochromatic and isoluminant stimuli at high spatial frequencies. If we assume that the high-spatial-frequency performance of the neural visual system is determined by a low-pass spatial filter followed by additive noise, then the visual system has a spatial bandwidth 1.8 times lower for isoluminant red–green than for isochromatic stimuli. On the other hand, we find no difference in bandwidth or sensitivity of the neural visual system for isoluminant red–green and S-cone-isolated stimuli.

Analysis of the limit to superresolution in incoherent imaging

Abstract: Superresolution algorithms have demonstrated impressive image-restoration results in the space domain. We consider the limits on superresolution performance in terms of usable bandwidth of the restored frequency spectrum. On the basis of a characterization of the spectral extrapolation errors (viz., null objects), we derive an expression for an approximate bound on accurate bandwidth extension for the general class of superresolution algorithms that incorporate a priori assumptions of a nonnegative, space-limited object. It is shown that accurate bandwidth extension is inversely related to the spatial extent of the object and the noise level in the image. For superresolution of sampled data, we present preliminary results relating bandwidth extrapolation to the difference between sampling rate and the discrete optical cutoff frequency. Simulation results are presented that substantiate the derived bandwidth extrapolation bounds.

Tunable liquid-crystal Fabry-Perot interferometer filter for wavelength-division multiplexing communication systems

Abstract: We explain how to obtain the bandwidth and tunable range required for wavelength-division multiplexing communication systems and how to design tunable liquid-crystal Fabry-Perot interferometer filters. The main factors determining the performance are liquid-crystal loss, mirror loss, surface roughness, and parallelism, as well as mirror reflectivity and cavity gap. Experimental results closely agree with the designed performance. Temperature dependence, response time, acceptable input power, and reliability are investigated. Pigtailed polarization-independent filter modules with a Peltier controller are made and are shown to have low polarization dependence ( >

Individual characterization of broadband sampling oscilloscopes with a nose-to-nose calibration procedure

Abstract: A method is proposed to find the individual impulse response of broad-band sampling oscilloscopes (HP54124T, 50 GHz bandwidth). The method is based on the fact that, when the oscilloscope is sampling a dc voltage, pulses are launched from the sampler towards the input connector. These pulses contain information on the scope's characteristic and can be measured by a second oscilloscope. This type of measurement is called a "nose-to-nose" calibration. Applying deconvolution techniques to the result of this measurement, the characteristic of the two scopes can be found, assuming that the two scopes are identical. To avoid relying on this assumption, three oscilloscopes can be used. The nose-to-nose is then applied three times, with a different pair of scopes connected together each time. The individual characteristics of the three sampling oscilloscopes are then calculated. Both SPICE simulations and close correspondence between swept-sine measurements and this method indicate that it is probably the most accurate method available at this moment to calibrate broadband sampling oscilloscopes. Several measurement uncertainties and practical problems are identified. The phase contribution of the sampling aperture can never be determined by this method, but there is strong evidence that the effect is negligible. Practical measurement problems are related to linearity of the samplers, additive noise, time-base jitter, and time-base drift. >

Filter Propagation in Dissemination Trees: Trading Off Bandwidth and Processing in Continuous Media Networks

Abstract: We describe the concept of the relocatable continuous media filter. The novelty of these filters is how they can propagate over a dissemination tree in a network. We describe the filter propagation protocol to achieve this. Execution of filters inside a network allows the network to be viewed in a novel way, as a “processor” with its “instruction set” being the various types of available filters. Since filters generally modify the data rate of the continuous media stream, usually (but not necessarily) reducing it, filters allow the trading off of bandwidth and processing in a network.

On the density of phase-space expansions

Abstract: Phase-space decompositions for signals of finite energy have been used to formulate the intuitive but elusive idea that frequency content of a signal can vary with time as the signal evolves. The decompositions consist of subdividing the time and frequency axes into certain ranges and expanding signals stably in a two-parameter basis of fixed functions of time, in which the (k,m)th coefficient is viewed as describing the part of the signal that is concentrated in the kth time and mth frequency range. It is shown that in any such expansion, the time-frequency space must be subdivided, or, equivalently, the coefficients computed, at least at the Nyquist rate of two per unit of time and cycle of bandwidth. >

Amplitude modulation detection of narrow‐band noise: Effects of absolute bandwidth and frequency region

Abstract: Thresholds and psychometric functions for the detection of amplitude modulation were measured as a function of modulation frequency under several stimulus conditions. The first experiment investigated the relative importance of stimulus bandwidth and frequency region for amplitude‐modulation detection. The stimulus bandwidth was either 200, 400, 800, or 1600 Hz. The frequency region was varied by adjusting the high‐frequency cutoff of the noise to be either 600, 2200, or 4400 Hz. Temporal modulation transfer functions demonstrated the typical low‐pass filter characteristic, with sensitivity to modulation decreasing with increasing modulation frequency. Time constants associated with the transfer functions were derived from low‐pass filter functions fitted to the data. The time constants varied inversely with noise bandwidth (≤1600 Hz) and were independent of frequency region. These results are consistent with estimates of temporal acuity based on previous studies of gap detection for narrow‐band noise as well as estimates of temporal acuity using deterministic stimuli. In a second experiment, psychometric functions, plotted with modulation depth in dB, demonstrated somewhat steeper slopes as modulation frequency increased. The estimated slope values did not vary greatly with frequency region or noise bandwidth.

Joint source and channel coding using a non-linear receiver

Abstract: The joint optimization of the source and channel coders in a system consisting of a vector quantizer (VQ) whose output indices are mapped directly into points in the modulation signal space is considered. A decoder based on a nonlinear estimator is used to reconstruct the source signal. An iterative algorithm is introduced which jointly optimizes the VQ and the modulation signal set using as optimality criterion the minimum mean-square error (MSE) between the original and the reconstructed signals. It is shown that a jointly optimized system based on average channel characteristics significantly outperforms (by up to 5 dB) a reference system based on a VQ designed for the given source and a standard quadrature amplitude modulation (QAM) modulation signal set. >

Diversity radio receiver

Abstract: A plurality of reception branches receive digital modulated signals and convert received digital modulated signals into intermediate frequency signals having frequency differences having a predetermined relationship with a modulation baseband signal. An adder adds the intermediate frequency signals converted by the plurality of reception branches. A delayed and differential detection section delays and differentially detects a sum signal from the adder. A post detection filter filters a delayed and differentially detected signal from the delayed and differential detection section at a bandwidth higher than a Nyquist frequency of the received digital modulated signals, thereby outputting demodulated signals of the received digital modulated signals.

The capacity of PCM voiceband channels

Abstract: It is theoretically shown that the capacity of a voiceband channel operating through a digital network is at least 56 kbps (for a 3.5 kHz baseband bandwidth). The capacity may actually be higher but how close one can come to 64 kbps, the limit imposed by the digital network itself, is unknown. The transmission method is based on generating nonuniformly pulsed PAM (pulse amplitude modulation) signals which at the network sampling instants go through one of the slicing levels of the A/D converter of the network. To accomplish this, both the slicing levels and the sampling clock times must be known. This is a hard practical problem and one can only conjecture if a reasonable solution will be possible. The signal may be recovered using a Viterbi detector to reduce intersymbol interference effects, but at present, the number of states required for this detector is very large. >

Video telephone station having variable image clarity

Abstract: Video telephone apparatus for transmitting video information over a limited bandwidth communications channel includes a source of video frames which are compressed to reduce the amount of redundant information, and to reduce the level of detail according to a user-selected control signal. The video frames are presented to the compressor at a rate which is varied in order to maintain a constant rate data signal at the compressor's output. As the level of detail is decreased the frame rate is correspondingly increased, thus providing the user with the ability to trade off between image clarity and smoothness of motion. Such tradeoffs are useful when the bandwidth of the communications channel is severely constrained. The control signal is transmitted to the video telephone apparatus from a far-end video telephone over the same channel.

Advanced Modulation Techniques for V.Fast

Abstract: An overview is provided of the modulation techniques that are being considered for a new voiceband modem Recommendation, nicknamed V.fast. These techniques include adaptive bandwidth operation for automatically selecting the transmission band, multi-dimensional trollis coded modulation for higher coding gain, constellation shaping for higher shaping gain, precoding and pre-emphasis for advanced equalization, and warping for resistance against signal-dependent impairments. Simulation results are presented to illustrate performance. By adaptively selecting an appropriate combination of these modulation techniques based on line conditions, V.fast modems are expected to double the maximum data rate of standard modems.

Performance analysis of an all-digital BPSK direct-sequence spread-spectrum IF receiver architecture

Abstract: A VLSI architecture for an all-digital binary phase shift keying (BPSK) direct-sequence (DS) spread spectrum (SS) intermediate frequency (IF) receiver is presented, and an in-depth performance analysis is given. The all-digital architecture incorporates a Costas loop for carrier recovery and a delay-locked loop for clock recovery. For the pseudorandom noise (PN) acquisition block, a robust energy detection scheme is proposed to reduce false PN locks over a broad range of signal-to-noise ratios. The proposed architecture is intended for use in the 902-928 MHz unlicensed spread spectrum radio band. A 100 kbs information rate and a 12.7 Mchips/second PN code rate are assumed. The IF center frequency is 12.7 MHz and the IF sampling rate is 50.8 Msamples/second, which is the Nyquist rate for the 25.4 MHz bandwidth signal. Finite wordlength effects have been simulated to optimize the architecture, thereby minimizing the chip area, and results of the finite wordlength simulations demonstrate that the chip architecture achieves a bit error rate performance within 1 dB of theory in an additive white Gaussian noise channel. >

Bounds on the capacity of a spectrally constrained Poisson channel

Abstract: An upper bound is derived on the capacity of a Poisson channel that has a stationary input process of a given spectrum and is subjected to peak and average power constraints. The bound is shown to be asymptotically tight with the relaxation of the spectral constraints. Its maximization over a given set of admissible spectra is closely related to an analogous problem in the AWGN regime. The results are used for bounding the capacity of a Poisson channel under a second-moment-bandwidth constraint, as well as the capacity under a strict bandwidth constraint. Asymptotically tight lower bounds on the channel capacity for the above two cases are also presented. The approach for lower bounding the capacity for the latter case yields, as a by-product, improved bounds on the bit-error probability in uncoded amplitude shift keying (and on-off modulation as a special case) operating over a Poisson channel impaired by intersymbol interference. >

Radio receiver adjacent-channel interference suppression circuit

Abstract: A mobile radio receiver, for example, a car radio, needs a circuit to detect and suppress adjacent-channel interference. This can be done by using bandpass filters (14) of differing bandwidth. The present invention dynamically adjusts bandwidth to an optimum value by continuously comparing (13) a first signal level, upstream of the bandpass filter, to a second signal level, downstream of the bandpass filter, to derive a difference value representative of adjacent-channel interference, then uses an electronically controlled selector switch (6) to select a particular filter (14N) whose bandwidth is sufficiently narrow to cut out the interfering broadcast signal. A table associating each magnitude range of interference with a particular filter bandwidth assures that bandwidth is broadened as interference diminishes, thereby assuring sufficient reception signal strength.