Function Of Signal Bandwidth Research Articles

Improved Indoor Range Measurements at Various Signal Bandwidths

This paper investigates range error measurements over multiple paths in an indoor nonline-of-sight environment as a function of signal bandwidth. The raw range measurements were obtained by transmitting and receiving a signal of 4 GHz bandwidth. Then, through suitable processing of the received pulses, signals with an effective bandwidth from 60 MHz to 4 GHz were generated. The range errors had mean value of about 0.2 m for bandwidths greater than 300 MHz, but peak errors increased to 4–5 m with a 60-MHz bandwidth using a standard leading-edge threshold time-of-arrival (TOA) technique. These latter errors were observed to be related to distortions in the leading edge of the pulse due to multipath propagation. Based on the observations, a new enhanced leading-edge TOA algorithm is developed. The enhanced algorithm uses the initial estimate from the standard leading-edge threshold algorithm, but then uses back-projection to correct the initial estimate. Experimental results demonstrate that the proposed algorithm reduces the mean errors by about 50% at lower bandwidths. As a consequence, TOA positioning systems using the enhanced leading-edge algorithm and operating in the 2.4-GHz and especially the 5.8-GHz Industrial, Scientific and Medical band have position errors only about twice that of ultrawideband systems, despite the much smaller signal bandwidth.

The effect of signal bandwidth on partially saturated broadband ocean acoustic transmission scintillation in a shallow water waveguide

The acoustic field in a shallow water waveguide at small source-receiver separations <3 km are partially saturated. Here, we examine the scintillation statistics of partially saturated broadband ocean acoustic transmissions in the low to mid frequency range collected using a high-resolution towed horizontal coherent hydrophone array system. The standard deviation of intensity fluctuations and scintillation index are quantified as a function of signal bandwidth and source-receiver separation. The amplitude and intensity distributions are compared with corresponding distribution models for the partially saturated field to determine the number of independent statistical fluctuations present in the broadband data.

RF Monte Carlo calculation of power amplifier efficiency as function of signal bandwidth

We propose a definition of the efficiency bandwidth for a power amplifier (PA) using a modulated signal as the signal bandwidth at which the amplifier efficiency has dropped to a level of 90% of the maximum efficiency at small bandwidth. We introduce a Monte Carlo method to calculate the efficiency bandwidth for some popular PA architectures. The method assumes a given modulated signal at the load. From this load signal the wave forms at the drains of the transistors are derived and the energy dissipation in the transistors can be calculated. Using idealized transistors with no output capacitance the maximum realizable efficiency bandwidth of an asymmetrical Doherty amplifier is 60%, which is much larger than that of an outphasing amplifier, which is 14%. More realistic transistors that include output capacitances, need a matching circuit with a high Q-value which decreases the efficiency bandwidth. Using output capacitance values typical for LDMOS transistors, the asymmetrical Doherty amplifier shows an efficiency bandwidth of about 400 MHz for a signal centered at 1 GHz. For a signal at 2 GHz the efficiency bandwidth is found to be 520 MHz. Due to the fixed values of the output capacitances the efficiency bandwidth does not scale with frequency.

Empirical dependence of acoustic transmission scintillation statistics on bandwidth, frequency, and range in the Gulf of Maine.

The scintillation statistics of broadband acoustic transmissions are determined as a function of signal bandwidth B, center frequency fc, and range with experimental data in Georges basin in the Gulf of Maine. The received signal intensity is shown to follow the gamma distribution implying that the central limit theorem has led to a fully saturated field from independent multi-modal propagation contributions. The gamma distribution depends on the mean intensity and the number of independent statistical fluctuations or coherent cells in the received signal. The latter is calculated for the matched filter, the Parseval sum, and the band-passed center frequency, all of which are standard ocean acoustic receivers. The number of fluctuations in the received signal is found to be an order of magnitude smaller than the time-bandwidth product TB of the transmitted signal, and to increase monotonically with relative bandwidth B/fc. A computationally efficient numerical approach is developed to predict the mean intensity and the corresponding broadband transmission loss of a fluctuating, range-dependent ocean waveguide by range and depth averaging the output of a time-harmonic stochastic propagation model. This model enables efficient and accurate estimation of transmission loss over wide areas, which has become essential in wide-area sonar imaging applications.

Empirical dependence of acoustic transmission scintillation statistics on bandwidth, frequency, and range in New Jersey continental shelf

The scintillation statistics of broadband acoustic transmissions are determined as a function of signal bandwidth B, center frequency f(c), and range with experimental data in the New Jersey continental shelf. The received signal intensity is shown to follow the Gamma distribution implying that the central limit theorem has led to a fully saturated field from independent multimodal propagation contributions. The Gamma distribution depends on the mean intensity and the number of independent statistical fluctuations or coherent cells micro of the received signal. The latter is calculated for the matched filter, the Parseval sum, and the bandpassed center frequency, all of which are standard ocean acoustic receivers. The number of fluctuations mu of the received signal is found to be an order of magnitude smaller than the time-bandwidth product TB of the transmitted signal, and to increase monotonically with relative bandwidth Bfc. A computationally efficient numerical approach is developed to predict the mean intensity and the corresponding broadband transmission loss of a fluctuating, range-dependent ocean waveguide by range and depth averaging the output of a time-harmonic stochastic propagation model. This model enables efficient and accurate estimation of transmission loss over wide areas, which has become essential in wide-area sonar imaging applications.

Responses of ventral cochlear nucleus onset and chopper units as a function of signal bandwidth.

1. The responses of units in the ventral cochlear nucleus in anesthetized guinea pigs have been measured to best-frequency tones, noise bands geometrically centered around the unit best frequency, and noise bands asymmetrically positioned around the best frequency. 2. Each unit isolated was characterized using peristimulus time histograms (PSTHs) to best-frequency tones at 20 and 50 dB suprathreshold, frequency-intensity response areas and rate-versus-level functions in response to best-frequency tones and wideband noise. The data reported here are derived from full analyses of 5 chopper units and 17 onset units. The onsets were divided into onset-I (OnI), onset-L (OnL), and onset-C (OnC) by the criteria described by Winter and Palmer: the PSTHs of OnI units show only an onset response, OnL units respond with a single spike at onset followed by a low level of sustained activity, and OnC units have PSTHs with one to four onset peaks and low levels of sustained discharge. 3. In response to geometrically centered noise bands of constant spectral density, the discharge of chopper units and one OnI unit increased over a relatively narrow range of bandwidths, corresponding to the equivalent rectangular bandwidth calculated from their response area, and then became constant. In contrast, OnL and OnC units showed increases in discharge rate with noise bandwidth over very wide ranges of bandwidth. The growth of the discharge rate with noise bandwidth was approximately linear on double logarithmic axes and therefore could be described by a power function with an exponent of 0.37. This relation held even for noise levels near threshold. 4. When noise bands with constant spectral density (at the input to the earphone) were presented with one edge fixed at the unit's best frequency, the discharge rate of most chopper units and the one OnI unit increased over a narrow range of bandwidths and then became constant. This pattern was observed irrespective of whether the second edge of the noise was progressively increased above, or decreased below, the best frequency. For two of the chopper units, in which lateral inhibitory sidebands could be demonstrated, increasing the noise bandwidth led first to increases and then to decreases in the discharge rate as the noise energy impinged upon the sideband. The chopper units act like energy detectors with a filter corresponding to their single tone response area, but, for some units, with the addition of inhibitory sidebands. 5. For the OnL and OnC units, increasing the noise bandwidth above or below best frequency caused progressive increases in the discharge rate over wide ranges of bandwidth. These increases occurred even for low noise spectral densities. The growth in discharge rate for these onset units was well fitted at all spectral density levels by power functions: one above best frequency and one below. At levels of the noise 40 dB above the unit threshold, the point at which the discharge rate reached 90% of its maximum was, on average, about 2 octaves below best frequency and 1 octave above. For some onset units, changes in the discharge rate were seen as the noise bandwidth was varied over about 14 kHz, which is about one-third of the total frequency hearing range of the guinea pig. 6. The data for onset units is consistent with the hypothesis that onset units in the ventral cochlear nucleus achieve their precision in the temporal domain by integration of the inputs from auditory nerve fibers with a wide range of best frequencies. The range of frequency over which onset units integrate frequency matches that of the inhibitory input to dorsal cochlear nucleus neurons, suggesting a possible role as an inhibitory interneuron.

Homophasic and antiphasic masking as a function of signal bandwidth and frequency region

Thresholds for homophasic and antiphasic multicomponent tonal complexes were measured as a function of bandwidth in broadband noise to replicate and to extend the classic masking experiment [G. Gassler, Acoustica 4, 408–414 (1954)]. Gassler reported that masked thresholds for tonal complexes were nearly constant for bandwidths less than a nominal “critical bandwidth,” beyond which thresholds increased 3 dB/oct. Three experiments were conducted in broadband noise employing (a) homophasic signals similar to Gassler's (1 to 20 sinusoids spaced 20 Hz apart extending downward from 1100 Hz); (b) homophasic and antiphasic signals composed of 1 to 23 sinusoids centered at 500, 1000, and 2000 Hz; and (c) homophasic and antiphasic signals centered at 500 Hz with bandwidths to 800 Hz (71 sinusoids). The slope of the threshold/signal bandwidth function beyond 100 Hz was < dB/oct at each center frequency, ranging from 0.3 dB/oct (2000 Hz) to 1.9 dB/oct (1000 Hz). The data indicate that the integration of signal energy continued beyond the so‐called “critical band” estimates reported by Gassler. There were no substantial differences between the slopes of the homophasic and antiphasic conditions at 500, 1000, and 2000 Hz. [Work supported by VA Medical Research Service.]

Detection of temporal gaps in bandlimited noise: effects of variations in bandwidth and signal-to-masker ratio.

Thresholds were measured for the detection of a temporal gap in a bandlimited noise signal presented in a continuous wideband masker, using an adaptive forced-choice procedure. In experiment I the ratio of signal spectrum level to masker spectrum level (the SMR) was fixed at 10 dB and gap thresholds were measured as a function of signal bandwidth at three center frequencies: 0.4, 1.0, and 6.5 kHz. Performance improved with increasing bandwidth and increasing center frequency. For a subset of conditions, gap threshold was also measured as bandwidth was varied keeping the upper cutoff frequency of the signal constant. In this case the variation of gap threshold with bandwidth was more gradual, suggesting that subjects detect the gap using primarily the highest frequency region available in the signal. At low center frequencies, however, subjects may have a limited ability to combine information in different frequency regions. In experiment II gap thresholds were measured as a function of SMR for several signal bandwidths at each of three center frequencies: 0.5, 1.0, and 6.5 kHz. Gap thresholds improved with increasing SMR, but the improvement was minimal for SMRs greater than 12-15 dB. The results are used to evaluate the relative importance of factors influencing gap threshold.

Binaural Critical Bandwidth as a Function of Signal Bandwidth

There are data in the literature which suggest that the critical band, as measured in masking experiments, may be wider for short signals than for long signals. Our data agree with this result. However, we suggest that it is the bandwidth of the signal and not only its duration that determines the critical bandwidth. Three types of signals were used: a 100-msec burst of 500-Hz sinusoid; a 100-msec burst of noise, bandpassed at 500 Hz; and a 0.1-msec click, bandpass filtered at 500 Hz. Signals were presented in both the S0N0 (signal and masker interaurally in-phase) and SπN0 (masker in-phase, signal inverted at one ear) configurations. The critical bandwidth for each signal was estimated by measuring its masked threshold in bands of noise of various widths. The important results are as follows: in S0N0 conditions the critical bandwidth for an octave-band filtered click is almost an order of magnitude larger than for a 100-msec sinusoid. Moreover, the critical bandwidth for a 100-msec octave-band filtered noise is about the same as for an octave-band click. This suggests that it is signal bandwidth that determines the width of the critical band, not its duration. In SπN0 conditions none of the above effects is observed. The critical bandwidth appears to be nearly independent of both signal duration and bandwidth.

Auditory thresholds during visual stimulation as a function of signal bandwidth.

A Béḱsy-type tracking procedure was employed with eight male subjects to obtain auditory thresholds for pulsing wide-band. low-band, mid-band, and high-band noise during nonvisual and visual stimulation periods. Thresholds and attenuator pen-excursion sizes obtained for each bandwidth auditory stimulus were not significantly different between nonvisual and visual condition. The data indicated that psychophysical sensory interaction between the auditory and visual systems did not occur for the stimulus used, that is, auditory thresholds were neither facilitated nor inhibited during presentation of a time-locked stroboscopic visual stimulus.

Front-Back Discrimination in Free-Field Sound Localization

When localizing sounds in a free-field environment, listeners frequently confuse front and back. Careful study of these front-back reversals reveals a dependence on factors other than the lack of a reverberant sound field. In the experiments described here, median-plane localizations were produced by single and symmetrically disposed pairs of loudspeakers. Front-back discrimination was examined as a function of source position, head movements, signal type, frequency, and bandwidth. Subjects differed in their sensitivity to each of these factors, but it was clear that source position and signal bandwidth had the most pronounced effects. Some subjects demonstrated front-back localizations that were almost entirely dependent on source position; others exhibited localizations that altered as a function of signal bandwidth but that appeared to be substantially independent of source position. Experiments in which head movements were progressively restricted have shown that small involuntary head movements cannot be of more than slight importance in front-back discrimination.

Detection of Complex Signals as a Function of Signal Bandwidth and Duration

A study is reported which examined the efficiency of human observers in the detection of a stimulus waveform which is a train of damped sinusoids. The signal duration and degree of damping (or spectral bandwidth) were varied, with the energy of the signal held constant. Increased bandwidth is shown to decrease detection of relatively long duration signals. A negligible effect of bandwidth was found for short durations. Detection of an amplitude difference, in contrast to detection of the presence of weak signal in noise, is shown to be relatively unaffected either by signal duration or bandwidth over the range of values studied.