Showing papers on "Bandwidth (signal processing) published in 2000"

Ultra-wide bandwidth time-hopping spread-spectrum impulse radio for wireless multiple-access communications

Abstract: Attractive features of time-hopping spread-spectrum multiple-access systems employing impulse signal technology are outlined, and emerging design issues are described. Performance of such communications systems in terms of achievable transmission rate and multiple-access capability are estimated for both analog and digital data modulation formats under ideal multiple-access channel conditions.

Point-to-point connectivity between neuromorphic chips using address events

Abstract: This paper discusses connectivity between neuromorphic chips, which use the timing of fixed-height fixed-width pulses to encode information. Address-events (log/sub 2/(N)-bit packets that uniquely identify one of N neurons) are used to transmit these pulses in real time on a random-access time-multiplexed communication channel. Activity is assumed to consist of neuronal ensembles-spikes clustered in space and in time. This paper quantifies tradeoffs faced in allocating bandwidth, granting access, and queuing, as well as throughput requirements, and concludes that an arbitered channel design is the best choice. The arbitered channel is implemented with a formal design methodology for asynchronous digital VLSI CMOS systems, after introducing the reader to this top-down synthesis technique. Following the evolution of three generations of designs, it is shown how the overhead of arbitrating, and encoding and decoding, can be reduced in area (from N to /spl radic/N) by organizing neurons into rows and columns, and reduced in time (from log/sub 2/(N) to 2) by exploiting locality in the arbiter tree and in the row-column architecture, and clustered activity. Throughput is boosted by pipelining and by reading spikes in parallel. Simple techniques that reduce crosstalk in these mixed analog-digital systems are described.

High-Q HF microelectromechanical filters

Abstract: IC-compatible microelectromechanical intermediate frequency filters using integrated resonators with Q's in the thousands to achieve filter Q's in the hundreds have been demonstrated using a polysilicon surface micromachining technology. These filters are composed of two clamped-clamped beam micromechanical resonators coupled by a soft flexural-mode mechanical spring. The center frequency of a given filter is determined by the resonance frequency of the constituent resonators, while the bandwidth is determined by the coupling spring dimensions and its location between the resonators. Quarter-wavelength coupling is required on this microscale to alleviate mass loading effects caused by similar resonator and coupler dimensions. Despite constraints arising from quarter-wavelength design, a range of percent bandwidths is still attainable by taking advantage of low-velocity spring attachment locations. A complete design procedure is presented in which electromechanical analogies are used to model the mechanical device via equivalent electrical circuits. Filter center frequencies around 8 MHz with Q's from 40 to 450 (i.e., percent bandwidths from 0.23 to 2.5%), associated insertion losses less than 2 dB, and spurious-free dynamic ranges around 78 dB are demonstrated using low-velocity designs with input and output termination resistances of the order of 12 k/spl Omega/.

A review of wireless SAW sensors

Abstract: Wireless measurement systems with passive surface acoustic wave (SAW) sensors offer new and exciting perspectives for remote monitoring and control of moving parts, even in harsh environments. This review paper gives a comprehensive survey of the present state of the measurement systems and should help a designer to find the parameters required to achieve a specified accuracy or uncertainty of measurement. Delay lines and resonators have been used, and two principles have been employed: SAW one-port devices that are directly affected by the measurand and SAW two-port devices that are electrically loaded by a conventional sensor and, therefore, indirectly affected by the measurand. For radio frequency (RF) interrogation, time domain sampling (TDS) and frequency domain sampling (FDS) have been investigated theoretically and experimentally; the methods of measurement are described. For an evaluation of the effects caused by the radio interrogation, we discuss the errors caused by noise, interference, bandwidth, manufacturing, and hardware tuning. The system parameters, distance range, and measurement uncertainty are given numerically for actual applications. Combinations of SAW sensors and special signal processing techniques to enhance accuracy, dynamic range, read out distance, and measurement repetition rate (measurement bandwidth) are presented. In conclusion, an overview of SAW sensor applications is given.

Large system performance of linear multiuser receivers in multipath fading channels

Abstract: A linear multiuser receiver for a particular user in a code-division multiple-access (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. We study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant. We employ a random model for the spreading sequences and the limiting signal-to-interference ratio expressions are independent of the actual signature sequences, depending only on the system loading and the channel statistics: background noise power, energy profile of resolvable multipaths, and channel coherence time. The effect of channel uncertainty on the performance of multiuser receivers is succinctly captured by the notion of effective interference.

A generalized RAKE receiver for interference suppression

Abstract: Currently, a global third-generation cellular system based on code-division multiple-access (CDMA) is being developed with a wider bandwidth than existing second-generation systems. The wider bandwidth provides increased multipath resolution in a time-dispersive channel, leading to higher frequency-selectivity. A generalized RAKE receiver for interference suppression and multipath mitigation is proposed. The receiver exploits the fact that time dispersion significantly distorts the interference spectrum from each base station in the downlink of a wideband CDMA system. Compared to the conventional RAKE receiver, this generalized RAKE receiver may have more fingers and different combining weights. The weights are derived from a maximum likelihood formulation, modeling the intracell interference as colored Gaussian noise. This low-complexity detector is especially useful for systems with orthogonal downlink spreading codes, as orthogonality between own cell signals cannot be maintained in a frequency-selective channel. The performance of the proposed receiver is quantified via analysis and simulation for different dispersive channels, including Rayleigh fading channels. Gains on the order of 1-3.5 dB are achieved, depending on the dispersiveness of the channel, with only a modest increase in the number of fingers. For a wideband CDMA (WCDMA) system and a realistic mobile radio channel, this translates to capacity gains of the order of 100%.

First- and second-order characterization of direction dispersion and space selectivity in the radio channel

Abstract: We present analytical results relating to first- and second-moment characterization of direction dispersion and spatial selectivity in the radio channel as well as to the duality between these two effects. Dispersion in direction can be characterized either by the direction power spectrum or by a family of spatial Doppler power spectra at the reception site. Two measures called the direction spread and the maximum spatial Doppler spread are introduced which describe the extent of channel dispersion in direction and in spatial Doppler frequency, respectively. Both measures are analogous to the delay and Doppler spreads, which are commonly employed to describe the extent of dispersion in delay and Doppler frequency, respectively. The relationships between the two approaches for characterizing spatial dispersion and especially between the direction and maximum spatial Doppler spreads are analytically established. The coherence distance at a certain level summarizes certain features of space selectivity in the radio channel which impact on the performance of communication systems. Two uncertainty relations between the direction spread and the coherence distance as well as between the maximum spatial Doppler spread and the coherence distance express the duality between direction dispersion and space selectivity. These relations are analogous to those established previously between the delay spread and the coherence bandwidth and between the Doppler spread and the coherence time. Examples relevant to mobile communications in the case where the waves propagate only horizontally illustrate the theoretical results. An application of these results to the design of uniform linear antenna arrays is also discussed.

Characterizing frequency selectivity for envelope fluctuations.

Abstract: Three experimental paradigms were used to specify the auditory system's frequency selectivity for amplitude modulation (AM). In the first experiment, masked-threshold patterns were obtained for signal-modulation frequencies of 4, 16, 64, and 256 Hz in the presence of a half-octave-wide modulation masker, both applied to the same noise carrier with a bandwidth ranging from 1 to 4 kHz. In the second experiment, psychophysical tuning curves (PTCs) were obtained for signal-modulation frequencies of 16 and 64 Hz imposed on a noise carrier as in the first experiment. In the third experiment, masked thresholds for signal-modulation frequencies of 8, 16, 32, and 64 Hz were obtained according to the "classical" band-widening paradigm, where the bandwidth of the modulation masker ranged from 1/8 to 4 octaves, geometrically centered on the signal frequency. The first two experiments allowed a direct derivation of the shape of the modulation filters while the latter paradigm only provided an indirect estimate of the filter bandwidth. Thresholds from the experiments were predicted on the basis of an envelope power-spectrum model (EPSM) which integrates the envelope power of the modulation masker in the passband of a modulation filter tuned to the signal-modulation frequency. The Q-value of second-order bandpass modulation filters was fitted to the masking patterns from the first experiment using a least-squares algorithm. Q-values of about 1 for frequencies up to 64 Hz suggest an even weaker selectivity for modulation than assumed in earlier studies. The same model also accounted reasonably well for the shape of the temporal modulation transfer function (TMTF) obtained for carrier bandwidths in the range from 1 to 6000 Hz. Peripheral filtering and effects of peripheral compression were also investigated using a multi-channel version of the model. Waveform compression did not influence the simulated results. Peripheral bandpass filtering only influenced thresholds for high modulation frequencies when signal information was strongly attenuated by the transfer function of the peripheral filters.

The influence of carrier level and frequency on modulation and beat-detection thresholds for sinusoidal carriers

Abstract: This paper is concerned with modulation and beat detection for sinusoidal carriers. In the first experiment, temporal modulation transfer functions (TMTFs) were measured for carrier frequencies between 1 and 10 kHz. Modulation rates covered the range from 10 Hz to about the rate equaling the critical bandwidth at the carrier frequency. In experiment 2, TMTFs for three carrier frequencies were obtained as a function of the carrier level. In the final experiment, thresholds for the detection of either the lower or the upper modulation sideband (beat detection) were measured for “carrier” frequencies of 5 and 10 kHz, using the same range of modulation rates as in experiment 1. The TMTFs for carrier frequencies of 2 kHz and higher remained flat up to a modulation rate of about 100–130 Hz and had similar values across carrier frequencies. For higher rates, modulation thresholds initially increased and then decreased rapidly, reflecting the subjects’ ability to resolve the sidebands spectrally. Detection thresholds generally improved with increasing carrier level, but large variations in the exact level dependence were observed, across subjects as well as across carrier frequencies. For beat rates up to about 70 Hz (at 5 kHz) and 100 Hz (at 10 kHz), beat detection thresholds were the same for the upper and the lower sidebands and were about 6 dB higher than the level per sideband at the modulation-detection threshold. At higher rates the threshold for both sidebands increased, but the increase was larger for the lower sideband. This reflects an asymmetry in masking with more masking towards lower frequencies. Only at rates well beyond the maximum of the TMTF did detection for the lower sideband start to be better than that for the upper sideband. The asymmetry at intermediate frequency separations can be explained by assuming that detection always takes place in filters centered above the stimulus spectrum. The shape of the TMTF and the beat-detection data reflects a limitation in resolving fast amplitude variations, which must occur central to the inner-ear filtering. Its characteristic resembles that of a first-order low-pass filter with a cutoff frequency of about 150 Hz.

Multichannel CSMA with signal power-based channel selection for multihop wireless networks

Abstract: We describe a new carrier-sense multiple access (CSMA) protocol for multihop wireless networks, using multiple channels and a distributed channel selection scheme. The proposed protocol divides the available bandwidth into N channels where the transmitting station selects an appropriate channel for packet transmission. The selection criterion is based on the interference power measurements on the channels. We show via simulations that this multichannel CSMA protocol provides a higher throughput compared to its single channel counterpart by reducing the packet loss due to collisions.

A combined OFDM/SDMA approach

Abstract: Two major technical challenges in the design of future broadband wireless networks are the impairments of the propagation channel and the need for spectral efficiency. To mitigate the channel impairments, orthogonal frequency division multiplexing (OFDM) can be used, which transforms a frequency-selective channel in a set of frequency-flat channels. On the other hand, to achieve higher spectral efficiency, space division multiple access (SDMA) can be used, which reuses bandwidth by multiplexing signals based on their spatial signature. In this paper, we present a combined OFDM/SDMA approach that couples the capabilities of the two techniques to tackle both challenges at once. We propose four algorithms, ranging from a low-complexity linear minimum mean squared error (MMSE) solution to the optimal maximum likelihood (ML) detector. By applying per-carrier successive interference cancellation (pcSIC), initially proposed for DS-CDMA, and introducing selective state insertion (SI), we achieve a good tradeoff between performance and complexity. A case study demonstrates that, compared to the MMSE approach, our pcSIC-SI-OFDM/SDMA algorithm obtains a performance gain of 10 dB for a BER of 10/sup -3/, while it is only three times more complex. On the other hand, it is two orders of magnitude less complex than the ML approach, for a performance penalty of only 2 dB.

High-gain wide-band gyrotron traveling wave amplifier with a helically corrugated waveguide

Abstract: First bandwidth measurements of a novel gyrotron amplifier are presented The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface With a beam energy of 185 keV, the amplifier achieved a maximum output power of 11 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 84 to 104 GHz ( $21%$ relative bandwidth), and an efficiency of $29%$, in good agreement with theory

Antenna arrays in mobile communications: gain, diversity, and channel capacity

Abstract: Are antenna arrays in mobile communications different from arrays in other applications? The answer is yes, sometimes, and it is the purpose of this paper to explain, in a tutorial fashion, when this is the case, and what this means for path loss in link calculations One aspect is the classical gain of an antenna, which we have to understand in a new way Another aspect is the possibility for two arrays, in a scattering environment, to create parallel channels, and thus, in effect, act as many independent antennas at the same time, carrying much more traffic over the same bandwidth

Filter bank modulation techniques for very high speed digital subscriber lines

Abstract: 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.

Adaptive power control in a mobile radio communications system

Abstract: An efficient and effective power control in a mobile communications system is provided that adapts to rapidly changing radio transmission conditions in varying and often unpredictable situations The value of a signal parameter detected from a signal received by a radio transceiver is compared with a desired signal parameter value, and a difference is determined A transmit power control command is sent to the radio transceiver and may instruct, for example, an increase or decrease in the level of radio transmit power Included with the transmit power control command is a power control indicator indicating whether a first or a second type of power control adjustment should be used by the radio transceiver depending upon the determined difference In one example embodiment, the power control indicator is a single flag bit A first value indicates that the first type of power control adjustment should be used; the second value indicates that the second type of power control adjustment should be used In any event, the power control indicator itself does not include specific details of the first or second type of power control adjustment Because only the indicator is sent (and not the details), signaling overhead and bandwidth consumption related to frequently sent power control commands are kept to a minimum The specific details of the first and second power control adjustments are initially stored in to the radio transceiver Such details may be updated when desirable, but the frequency of such updating is likely to be infrequent Alternatively, a power control indicator may be communicated using techniques other than adding one or more flag bits to a fast transmit power control message to effect a change in power control type as long as signaling overhead is not significantly increased

Wideband code division multiple access

Abstract: To satisfy ever-increasing demands for higher data rates, as well as to allow more users to simultaneously access the network, interest has peaked in what has come to be known as wideband code division multiple access (WCDMA). We discuss those basic characteristics of WCDMA waveforms that make them attractive for high data rate transmission over wireless and mobile channels. We emphasize how the choice of spread bandwidth affects the bit error rate of the system, as well as how it affects the reliability of various subsystems, such as those that perform coarse acquisition and adaptive power control. We then discuss how some of the waveforms being considered for cellular WCDMA systems differ from those in use in the narrowband CDMA cellular system, and emphasize, as an example, multicarrier CDMA. Finally, we discuss other potential enhancements to WCDMA systems, such as the use of interference suppression at the receiver, or multiple antennas at the transmitter.

Dual-band dual-polarized perforated microstrip antennas for SAR applications

Abstract: For dual-band dual-polarized synthetic aperture radar (SAR) applications a compact low-profile design is investigated. The operating frequencies are in the L and C-bands, centered about 1.275 and 5.3 GHz, respectively. Since the C-band frequency is larger by a factor of four, its array elements and inter-element separations are smaller by the same ratio. Thus, to allow similar scan ranges for both bands, the L-band elements are selected as perforated patches to enable the placement of C-band elements within them. Stacked-patch configurations were used to meet the bandwidth requirements, especially in the L-band. The C-band element was designed numerically, but the perforated L-band one required final experimental optimization. Also, in the latter case of L-band, a balanced transmission line feed was used to minimize cross polarization. For the C-band elements, slot coupling was used and, to simplify the feed, symmetric parasitic slots were incorporated to minimize cross polarization. No vertical connections were utilized, and electromagnetic couplings resulted in a compact low-profile design, with an electrically and thermally symmetric geometry.

Dynamically adjusting multiprotocol label switching (MPLS) traffic engineering tunnel bandwidth

Abstract: A method and apparatus for an improved Multiprotocol Label Switching (MPLS) system for traffic engineering is described. The improved MPLS system determines the actual traffic flow within a traffic engineering (TE) tunnel and dynamically adjusts the bandwidth to reflect the actual traffic flow. The actual traffic flow may be ascertained by accessing an average byte counter, which keeps track of the traffic flowing through the TE tunnel. Once the actual traffic flow is known the bandwidth is updated in accordance with the actual traffic flow. This allows the MPLS system to automatically maximize the bandwidth resources while minimizing operator intervention.

Widely programmable high-frequency continuous-time filters in digital CMOS technology

Abstract: We present design considerations for programmable high-frequency continuous-time filters implemented in standard digital CMOS processes. To reduce area, accumulation MOS capacitors are used as integrating elements. The filter design problem is examined from the viewpoint of programmability. To allow frequency scalability without deterioration of noise performance and of the frequency response shape, we employ a technique called "constant-capacitance scaling," which assures that even parasitic capacitances remain invariant when transconductors are switched in and out of the filter. This technique is applied to the design of a programmable fourth order Butterworth continuous-time filter with a bandwidth programmable from 60 to 350 MHz implemented in a 0.25-/spl mu/m digital CMOS process. The filter has a dynamic range of 54 dB, dissipates 70 mW from a 3.3-V supply, and occupies an area of 0.15 mm/sup 2/.

A variable-frequency parallel I/O interface with adaptive power supply regulation

Abstract: This paper presents a low-power high-speed CMOS signaling interface that operates off of an adaptively regulated supply. A feedback loop adjusts the supply voltage on a chain of inverters until the delay through the chain is equal to half of the input period. This voltage is then distributed to the I/O subsystem through an efficient switching power-supply regulator. Dynamically scaling the supply with respect to frequency leads to a simple and robust design consisting mostly of digital CMOS gates, while enabling maximum energy efficiency. The interface utilizes high-impedance drivers for operation across a wide range of voltages and frequencies, a dual-loop delay-locked loop for accurate timing recovery, and an input receiver whose bandwidth tracks with the I/O frequency to filter out high-frequency noise. Test chips fabricated in a 0.35-/spl mu/m CMOS technology achieve transfer rates of 0.2-1.0 Gb/s/pin with a regulated supply ranging from 1.3-3.2 V.

Orthogonal frequency division multiplexing based spread spectrum multiple access

Abstract: In an orthogonal frequency division multiplexing (OFDM) based spread spectrum multiple access system the entire bandwidth is divided into orthogonal tones, and all of the orthogonal tones are reused in each cell. To reduce peak-to-average ratio at the mobile transmitter, each voice user is allocated preferably a single one, but no more than a very small number, of the orthogonal tones for use in communicating with the base station. Data users are similarly allocated tones for data communication, however, the number of tones assigned for each particular data user is a function of the data rate for that user. The tone assignment for a given user is not always the same within the available band, but instead the tones assigned to each user are hopped over time. More specifically, in the downlink, the tones assigned to each user are change relatively rapidly, e.g., from symbol to symbol, i.e., the user fast "hops" from one tone to another. However, in the uplink, preferably slow hopping is employed to allow efficient modulation of the uplink signal which necessitates the employing of additional techniques, such as interleaving, to compensate for the reduction in the intercell interference averaging effect. For data communication power control is employed in the uplink and/or downlink so that the transmission rate is increased, e.g., by increasing the number of symbols transmitted per tone per unit time or the coding rate, as a function of allocated power per tone and the corresponding channel attenuation.

The effect of filtering on the performance of OFDM systems

Abstract: Filtering is required in the practical implementation of orthogonal frequency division multiplex (OFDM) systems operating at radio frequencies. These filters smear the transmitted waveform, and cause inter-symbol interference (ISI) between the blocks, forcing a further extension to the cyclic prefix. The tighter the filtering, the smaller the frequency bandwidth (guard-band) needed between adjacent channel users and the longer the prefix extension. Equations are developed that relate the prefix extension to the guard-band given the required subchannel signal-to-noise ratio (SNR). Numerical evaluation is then used to show the trade off between time domain and frequency domain capacity loss. There is an optimum guard-band loss, that will minimize the total (time+frequency) capacity loss. The total loss reduces as the inverse fast Fourier transform (IFFT) size N increases and the required SNR reduces. The capacity loss exceeds 40% when N=32 and SNR is 33 dB (needed for 8DPSK with an symbol error rate (SER) of 3% and a margin of 10 dB). Some of the loss can be reduced by using better filters (Chebyshev II), but the general trend of the results imply that schemes with a low number of subchannels (N<16) will not work well with high order modulations.

Inserting local signals during MPEG channel changes

Abstract: An apparatus and a method for inserting and displaying one or more signals during processing and display delays encountered in digital STB-receivers, including those delays encountered during channel changes. Moreover, a means for increasing the channel change delay period, while maintaining subscriber satisfaction, and thereby freeing up bandwidth for other services, is provided. These signals can be advertisements, entertainment, or other messages. When a channel change request occurs on a digital set-top receiver, a local watchdog module detects the channel change event and causes to be generated a signal that is immediately displayed on the TV, while the MPEG-2 subsystem of the receiver acquires, stores, decodes and presents the new program to the television. Providing advertisements or entertainment during the delay period allows for increase in the delay period thus allowing for higher compression and increased availability of bandwidth.

Effect of Narrowband Interference on GPS Code Tracking Accuracy

Abstract: The effect of narrowband interference on GPS code tracking accuracy is important, but has not been thoroughly evaluated. Theory shows that the effect of interference depends on details of receiver design, specially front-end bandwidth and early-late spacing in the discriminator, and that it has a different effect on code tracking accuracy than it does on some other aspects of GPS receiver performance. This paper compares theoretical predictions with experimental results to examine the effect of Gaussian interference on code tracking accuracy. Experimental results are obtained using MITRE’s real-time hardware testbed, measuring the effect of different interference frequencies on code tracking accuracy. Additional experimental results, obtained using a Novatel 3951-R 12 channel L1 C/A code receiver, are provided. The close correspondence between experimental results and theoretical predictions validates the theory, leading to better understanding of this important phenomenon. The results show that, for C/A code receivers, narrowband interference at frequencies away from band center can degrade code tracking accuracy more than interference having the same power at band center, even though the receiver’s measured signal-to-noise ratio degrades more when narrowband interference frequency is located at band center.

Millimeter-wave modulated optical signal generation with high spectral purity and wide-locking bandwidth using a fiber-integrated optical injection phase-lock loop

Abstract: We report the first experimental demonstration of millimeter-wave modulated optical signal generation by an optical injection phase-lock loop. A 36-GHz signal was generated by combining optical sideband injection locking with optical phase-lock loop techniques for two fiber-coupled DFB lasers. Single sideband noise spectral density of -92 dBc/Hz at 10 kHz offset, and phase-error variance lower than 0.005 rad/sup 2/ in a 100 MHz bandwidth were measured. The locking bandwidth exceeded 30 GHz.

Apparatus and methods for cleaning and/or processing delicate parts

Abstract: The invention utilizes harmonics of certain clamped ultrasound transducers to generate ultrasound within the liquid of an ultrasonic tank and in a frequency range of between about 100 khz to 350 khz (i.e., “microsonic” frequencies). The application of microsonic frequencies to liquid preferably occurs simultaneously with a sweeping of the microsonic frequency within the transducer's harmonic bandwidth to reduce or eliminate (a) standing waves within the liquid, (b) other resonances, (c) high energy cavitation implosion, and (d) non-uniform sound fields, each of which is undesirable for cleaning and/or processing of semiconductor wafers and other delicate parts. The invention can also drive ultrasonic transducers such that the frequency of applied energy has a sweep rate within the ultrasonic bandwidth of the transducers; and that sweep rate is also varied so that the sweep rate is substantially non-constant during operation. This reduces or eliminates resonances which are created by transducers operating with a single sweep rate. An ultrasound generator of the invention sometimes utilizes amplitude modulation (AM), and the AM frequency is swept over time so as to reduce resonances. AM control is preferably provided by selecting a portion of the rectified power line frequency. In applications which utilize multiple generators, multiple transducers, and one or more tanks, simultaneously, the invention synchronizes the operation of the generators to a common FM signal to reduce beat frequencies between generators. Each such generator can also be adjusted, through AM, to control the process characteristics within the associated tank. Two or more transducers are sometimes used by the invention, in combination, to broaden the overall bandwidth of acoustical energy applied to the liquid around the primary frequency or one of the harmonics. The bandwidths of the transducers are made to overlap such that an attached generator can drive the transducers, in combination, to deliver ultrasound to the liquid in a broader bandwidth. In a single chamber ultrasound system, two or more generators, each operating or optimized to generate a different range of frequencies, are connected to a multiplexer; and the desired frequency range is selected, and hence the right generator, according to the cavitation implosion energy that is desired within the tank chemistry.

Effective interference and effective bandwidth of linear multiuser receivers in asynchronous CDMA systems

Abstract: The performance of linear multiuser receivers in terms of the signal-to-interference ratio (SIR) achieved by the users has been analyzed in a synchronous CDMA system under random spreading sequences. In this paper, we extend these results to a symbol-asynchronous but chip-synchronous system and characterize the SIR for linear receivers-the matched-filter receiver the minimum mean-square error (MMSE) receiver and the decorrelator. For each of the receivers, we characterize the limiting SIR achieved when the processing gain is large and also derive lower bounds on the SIR using the notion of effective interference. Applying the results to a power controlled system, we derive effective bandwidths of the users for these linear receivers and characterize the user capacity region: a set of users is supportable by a system if the sum of the effective bandwidths is less than the processing gain of the system. We show that while the effective bandwidth of the decorrelator and the MMSE receiver is higher in an asynchronous system than that in a synchronous system, it progressively decreases with the increase in the length of the observation window and is asymptotic to that of the synchronous system, when the observation window extends infinitely on both sides of the symbol of interest. Moreover, the performance gap between the MMSE receiver and the decorrelator is significantly wider in the asynchronous setting as compared to the synchronous case.

Perceptual user interfaces: perceptual bandwidth

Abstract: Perceptual interfaces defined from a human perspective suggest similar topics (for example, vision, hearing, speech, touch), but radically different ideas about how sensory experience works and about how machines might change or facilitate human perception. The goal of a human-centered look at perceptual interfaces is to transform the world of computers into features of sensory experience that determine how the interfaces work with people. We have emphasized the human-centered approach in our lab at Stanford University [10]. The results of our work, taken from the psychology of sensation and perception, offer some surprising insights when applied to the design and evaluation of interactive media, especially media that seek to increase perceptual bandwidth. We have used these insights to help with four important questions about perceptual interfaces:

Extending the capacity of multiple access channels

Abstract: Multiple access techniques which allow a communication medium to be shared between different users represent one of the most challenging topics in digital communications. In terms of the number of users that can be accommodated on a given channel, there are two distinct classes of multiple access techniques. The first class includes the well-known FDMA, TDMA, and OCDMA. On a channel whose bandwidth is N times the bandwidth of the individual user signals, these techniques can accommodate N users without any mutual interference, but not a single additional user can be supported beyond this limiting number. The second class includes CDMA with pseudo-noise spreading sequences (which we refer to as PN-CDMA) and some other related schemes. PN-CDMA does not have a hard limit on the number of users that can be accommodated, but is subject to multi-user interference which grows linearly with the number of users. In this article, after reviewing the capacity limits of existing multiple access techniques, we describe some newly introduced concepts which allow us to accommodate N users without any interference while also accommodating additional users at the expense of some SNR penalty.

Coplanar waveguide fed quasi-Yagi antenna

Abstract: A novel coplanar waveguide fed quasi-Yagi antenna is presented. A wide bandwidth is achieved by using a broadband coplanar waveguide to a slotline balun. An X-band prototype has been realised which demonstrates a broad bandwidth (30%), –19 dB front-to-back ratio, and cross-polarisation better than -17 dB at 10 GHz.