Bandpass Channels Research Articles

The processing of temporal modulation at different levels of retinal illuminance

How does our temporal vision change as the mean illuminance reduces? We have examined the processing of near-threshold temporal information for a range of illuminance values (2850 - 0.15 phot td). At high illuminance, the modulation transfer function can be shown to be mediated via three underlying temporal filters that vary in sensitivity with spatial frequency. As the mean illuminance decreases these channels appear to change their sensitivity. Even at the lowest (scotopic) illuminance levels we were able to find evidence for at least two channels mediating detection threshold. There are also changes in the tuning properties of these channels such that the processing of high temporal frequencies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the band-pass channel. The slope of the fall-off in sensitivity at high temporal frequencies is unaffected by test spatial frequency at each illuminance level, suggesting its limiting factor is one that is insensitive to spatial frequency. We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) change in the response dynamics, or by interactions between photoreceptors, rather than changes at or beyond the level of the channel response.

Color and luminance spatial tuning estimated by noise masking in the absence of off-frequency looking.

We assessed the contribution of off-frequency looking for pattern detection and obtained bandwidths for chromatic and luminance mechanisms in conditions free from this effect. We used a simultaneous spatial masking technique with Gaussian enveloped sinusoidal test stimuli (0.5 cycle/deg) and filtered one-dimensional static-noise masks whose spectral power was uniformly distributed per octave. Stimuli were modulated in the chromatic (isoluminant red-green) or the luminance (yellow-black) domain. Color and luminance detection thresholds were compared for low-pass, high-pass, and notch- (band-stopped) filtered noise. We obtained the following results: (1) at high-noise spectral densities, masking by notched noise is greater than the summed masking of the high- and low-pass noise, indicating the presence of off-frequency looking for both color and luminance detection. There is no evidence for off-frequency looking at lower power densities. (2) Using notch-filtered noise, which avoids the problem of off-frequency looking, we found that color processing is subserved by bandpass channels with bandwidths similar to those revealed for luminance processing. (3) Both color and luminance mechanisms appear to have bandwidths proportional to their center frequency (constant in octaves). (4) The lower and upper sides of the color and luminance tuning functions were estimated individually by use of high-pass and low-pass noise of a low power density and are revealed to be asymmetric, with the lower side declining more steeply than the upper side.

A rapid atmospheric transmittance algorithm for microwave sounding channels

A rapid transmittance algorithm for NOAA's Advanced Microwave Sounding Units A and B and possible future instruments has been devised. Window channels, water vapor channels, and oxygen-band channels are considered separately; each uses tabular or polynomial approximations to line-wing or near-line absorption from water vapor or oxygen as appropriate. Absorption by cloud liquid water is also included. For oxygen-band channels that sound the atmosphere above 40 km, water vapor and clouds are not significant, but Zeeman splitting produced by the terrestrial magnetic field is. Because magnetic field strength varies over the Earth, channels within the Zeeman region use an algorithm in which transmittance is parameterized as a function of magnetic field strength B add angle /spl theta//sub B/ (with respect to the direction of propagation) at a frequency resolution of B 2.22 kHz//spl mu/T; then the average over channel passbands is done on-line. In tests, the rapid algorithm required thirty times less computation than a line-by-line algorithm, and reproduced the line-by-line calculation of brightness temperatures with accuracy comparable to or better than the channel sensitivities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Gating demultiplexer integrated with MSM detector array in InGaAs/AlGaAs/GaAs for WDM

The authors demonstrate the integration of a grating demultiplexer with curved output waveguides and a MSM photodetector array in InGaAs/AlGaAs/GaAs operating in the 1- mu m wavelength region. The structure provides 38 channels with 1-nm channel spacing. The total loss, including scattering from the unmetallized grating, is about 17 dB. A channel crosstalk of -11 dB is obtained. The FWHM of the channel pass band is 0.5 nm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Integrated grating demultiplexer and pin array for high-density wavelength division multiplexed detection at 1.5 μm

An integrated wavelength demultiplexer and pin detector array is reported for high density WDM detection at 1.5 μm. Signals spaced at 1 nm over the 1.48–1.55 μm wavelength range are separated and detected; channel passbands are 0.6–0.8 nm. Grown on a semi-insulating substrate, and providing a pseudoplanar device surface, the HD-WDM detector array is suitable for monolithic integration with high-speed multichannel receiver circuitry.

On the capacity penalty due to input-bandwidth restrictions with an application to rate-limited binary signaling

It is well known that in cases of unlimited channel bandwidth and unconstrained signal spectrum, a constant-envelope constraint on the signal does not reduce the capacity of an additive Gaussian channel, as compared to its capacity with the optimum signals of similar average power. For band-limited channels of bandwidth B, however, the constant-envelope constraints does reduce the capacity, even if the signals themselves are allowed arbitrarily rapid alternations (transitions). Here, using a recently introduced technique, it is shown that the capacity is further drastically reduced if, in addition to the channel limitation, the constant-envelope signal is itself restricted to a small fractional out-of-band power epsilon . Ideal lowpass and bandpass channel filters are considered, and upper bounds on the capacity are derived. The out-of-band power restrictions can be met, for example, by restricting the average rate of transitions rho of the binary input signal. It is shown that in this case, epsilon <or=0.573 rho /B, which is a result used to upper-bound the capacity for binary signaling under an average-transition-rate constraint.<<ETX>>

Digitate Basisbandübertragung über Bandpaßkanäle

The performance of the optimum receiver for digital transmission systems is derived, which uses M-ary PAM baseband signalling on bandpass channels. For the example of a data transmission over twisted pairs the results are compared with the performance of suboptimum receivers. It is shown that the application of a simple threshold detector with decision-feedback equalization (DFE) causes only a small loss of 1.5 dB as compared to a Maximum Likelihood Sequence Estimation (MLSE)

Adjustment off predetection filters in high-speed data transmission systems

The paper presents four schemes for the adaptive adjustment of a linear feedforward filter placed ahead of a conventional digital data detector. The filter is broadly adjusted according to two criteria, those of maximisation of the signal-tonoise ratio at the input of the detector where perfect equalisation of the detector input signal results in zero intersymbol interference, or minimising the mean-square error present in the equalised signal at the detector input. The paper presents the results of extensive computer simultions performed upon models of the four schemes when operating over four different voice-frequency bandpass channels. The results are arranged to show the relative accuracies and complexities (in terms of the number of arithmetic operations required to implement the algorithms) of the tested systems. Furthermore, the results have been arranged to show the behaviour of the four systems in the presence of additive white Gaussian noise and various amounts of distortion introduced by the channels, for different lengths of the linear pre-filter.

Comments on 'Semianalytic BER evaluation by simulation for noisy nonlinear bandpass channels'

Pent et al. (see ibid., vol.SAC-6, no.1, p.34-41, 1988) proposed a semianalytic method for performance evaluation of communication channels with a bandpass nonlinearity. This method is based on a decomposition of the composite signal at the output of the nonlinear device which leads to the definition of an equivalent nonlinearity. In Pent et al. such an equivalent model is evaluated via numerical integration. In the present comment it is shown that a suitable series expansion of the nonlinearity characteristic yields to an evaluation of the equivalent nonlinearity in closed form.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A programmable optical demultiplexer using a 32–element photodiode array

AbstractThis paper describes the design and performance of a programmable optical demultiplexer which can compensate the influence of: (1) scattering in the wavelength of an optical source; and (2) wavelength fluctuation of the optical source caused by temperature change, by controlling the branching characteristics in accordance with the wavelength of the optical source. In this multiplexer, a 32‐element Si photodiode array was used to carry out programmable control of the branching characteristics. This optical demultiplexer has the following merits: (1) a wide channel passband is not needed; and (2) since the passband‐center wavelength of this multiplexer is adjusted to the wavelength of the optical sources, fine tuning of the branching characteristic in fabrication is not needed. The fabricated programmable optical demultiplexer has a channel wavelength spacing of 4.0 nm, an insertion loss of 8.0 dB which includes a loss of 3.9 dB due to the detection loss caused by the gap between two photodiodes composing the PD array, and a passband width of 2.8 nm. It was confirmed also by a digital transmission experiment (32 mB/s, RZ signal) that this demultiplexer can maintain its optimum branching characteristics for any wavelength deviation in the optical sources.

Multispectral sensor data simulation modeling based on the multiple scattering LOWTRAN code

A method for simulating satellite channel radiances is described which would be useful for sensor response evaluation and channel optimization studies. Emergent radiance spectra are calculated for wavelengths between 0.4 and 13.0 μm using a modified version of the AFGL LOWTRAN transmittance/radiance code which includes the effects of multiple scattering. These radiance spectra, calculated for a sensor's specific solar illumination condition, are channel bandpass weighted to simulate sensor channel radiance sensitivities to surface—atmosphere properties. This method can produce channel radiances of most earth resources and cloud field multispectral imagers including the Landsat Thematic Mapper (TM) and Multispectral Scanner (MSS), the NOAA Advanced Very High Resolution Radiometer (AVHRR), the GOES Visible Infrared Spin Scan Radiometer (VISSR), the Defense Meteorological Satellite Program's (DMSP) Operational Linescan System (OLS), and Multispectral Operational Linescan System (MSOLS). A variety of realistic remote sensing scenarios are illustrated for selected instruments and fields-of-view containing hazes, fogs, and water and ice clouds observed against surface backgrounds including the ocean surface, land, vegetation, and snow cover. Actual Landsat TM and MSS visible and near infrared data obtained for clear and cloudy fields-of-view are compared to simulated channel radiances to assess the model's accuracy and representativeness.

Semianalytic BER evaluation by simulation for noisy nonlinear bandpass channels

The bit error rate (BER) of channels including a memoryless bandpass nonlinearity is evaluated by simulation. This would typically be an onboard travelling wave tube (TWT) amplifier in a satellite repeater, when the noise at the input of such a nonlinear element is non-negligible. The usual evaluation technique, error counting, requires a large amount of computer time if small error probabilities are to be estimated. It is shown that faster semianalytic procedures can be used, provided that a proper model for the nonlinear element is adopted. The output process is decomposed into a signal component plus an additional term representing an equivalent noise component, and an equivalent nonlinear transformation, relating the input useful signal to the output signal component, is derived. In addition, several modes for the probability density function (PDF) of the uplink noise component at the output of the transmission chain are discussed and compared. The procedure has been tested on a transparent satellite link using 4-CPSK modulation format. The results compare well with those of the error-counting technique if a composite rectangular PDF with exponential tails as adopted.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flat passband birefringent wavelength-division multiplexers

Birefringent filter synthesis techniques are demonstrated to achieve very flat channel passbands for wavelength-division multiplexers. The crystal thickness and birefringence can be used to scale a given design to very small channel separations, limited primarily by the spectra of the lasers and their drift. Experimental results are reported, with typical optical crosstalk values of –19 dB.

Multichannel infinite clipping as a form of sampling of speech signals

A remarkable improvement of both intelligibility and naturalness of infinitely clipped speech can be achieved by means of a multichannel system in which the speech signal is splitted into several band-pass channels before the clipping and successively reconstructed by summing up the clipped outputs of each channel.

A New Design of Impulses for Digital Signal Transmission over Band-pass Channels

Nouvelle conception de transmission en bande de base sans interference intersymbole sur le canal avec une fonction de transfert passe-bande. La transmission est faite avec des impulsions obeissant simultanement au 1er et 2eme critere, ou les transformations de Fourier correspondantes ont une forme de passe-bande dans la zone de transmission

Optimum FIR Transmitter and Receiver Filters for Data Transmission Over Band-Limited Channels

The paper deals with the design of digital transmitter and receiver filters with finite impulse response (FIR) for data transmission over band-limited channels. The filters are matched and satisfy a zero intersymbol interference constraint when cascaded. For baseband transmission, the filters achieve optimum spectral concentration in the frequency range [-(1+\beta)/2T, (1+\beta)/2T] . Mathematically, the filter design leads to a generalized eigenvalue problem which is solved numerically by a projected gradient Procedure. For transmission over bandpass channels by combined amplitude and phase modulation, the design technique is modified so that filters with complex-valued impulse response and optimum spectral concentration in the range of positive bandpass frequencies [f_{c} - (1+\beta)/2T, f_{c} + (1+\beta)/2T] are obtained. In addition, the complex formulation allows the design of impulse responses with enhanced spectral attenuation in the corresponding range of negative frequencies in order to minimize imageband interference. Results are shown in terms of filter coefficients, signal spectra, and spectral concentrations obtained. For example, filters designed for a voiceband data modem operating at a symbol rate of 2400 baud achieve a spectral concentration of 98.5 percent with 24 coefficients and \beta = 0.1 , and with only 0.001 percent of the total energy in the imageband region.

An instrument for DC electric field and AC electric and magnetic field measurements aboard ‘Intercosmos-Bulgaria-1300’ satellite

The instrument IESP-IPMP represents the complex unit measuring the vector of the DC electric field, the vectors of the electric and magnetic field in the frequency range of 0.2 – 6.5 Hz (wave form), autocorrelation functions of waves with frequencies of 0.1 – 5 kHz, and wave amplitudes in 8 bandpass channels. Some results are shown and compared in the various frequency ranges.

Identification of dispersive communication channels using Barker codes

The identification of dispersive sommunication channels using short duration binary signals in considered in this paper. It is shown that Barker codes are the best known finite length binary test signals for the identification of the impulse response of base-bond channels using the cross-correlation technique. The identification of RF passband channels is performed by using RF pulse with Barker code phase modulation as test signals. The described identification method is suitable for adaptive digital communication systems operating in time varying channels. It is also suitable for channels that may show non-linear effects when long duration test signals are used for identification.

Galactic and extragalactic contributions to the far-ultraviolet background

Data from the 1350-1550 A bandpass channel of the 2.5-deg angular resolution extreme-ultraviolet telescope on the Apollo-Soyuz mission have been analyzed for stellar and airglow radiation contributions for the galactic latitude range above 30 deg. At intermediate latitudes, the resulting diffuse background is found to be correlated with the neutral hydrogen column density as determined by 21 cm radio data. This result, together with the observed anisotropy and mean intensity of the radiation field, is consistent with the hypothesis that scattering of galactic-plane starlight off dust grains having an albedo of 0.5 and a phase function coefficient of 0.5 is the primary source of the observed radiation field at the intermediate latitudes. The background intensity near the poles is, however, higher than expected from the H I column density and extinction measurements.

A Design Procedure for Bandpass Channel Multiplexers Connected at a Common Junction

A new general design procedure is presented for multiplexers having any number of Chebyshev channel filters, with arbitrary degrees, bandwidths, and interchannel spacings. The design procedure is developed for bandpass channel filters connected in series at a common junction for narrow-band applications. Commencing with the closed-form expressions for element values in Chebyshev filters, the multiplexer design process modifies all of the elements in each channel filter and preserves a match at the two points of perfect transmission closest to the band edges of each channel filter, while taking into account the frequency dependence across each channel. Examples of several multiplexer are given indicating that the design process is valid for most combinations of contiguous and noncontiguous channels.