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Due to congestion in wireless frequency use, it is difficult to assign an independent frequency bandwidth to broadband wireless communication systems based on the DS-CDMA (Direct Sequence Code Division Multiple Access) system. The...
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Due to congestion in wireless frequency use, it is difficult to assign an independent frequency bandwidth to broadband wireless communication systems based on the DS-CDMA (Direct Sequence Code Division Multiple Access) system. Therefore, operation of the DS-CDMA system on the existing narrowband wireless system is contemplated. However, the interference of the transmitted power of the existing narrowband wireless system on the DS-CDMA system has not been studied. There is a possibility that significant narrowband interference on the DS-CDMA system may occur. In this paper, a narrowband interference rejection system in DS-CDMA is proposed. By making use of prior information on the carrier frequency of the narrowband interference, a narrowband interference rejection process with little computational overhead is realized.
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More and more people are turning to narrowband imaging as a strategy to combat light pollution and moonlight. The traditional method for utilising narrowband filters is to use a set of individual filters and a monochrome camera to...
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More and more people are turning to narrowband imaging as a strategy to combat light pollution and moonlight. The traditional method for utilising narrowband filters is to use a set of individual filters and a monochrome camera to capture a series of subs for each filter. These are then stacked and combined to produce a colour image. New filters are now being marketed that pass light at multiple emission wavelengths. For example, the STC Duo filter passes hydrogen-alpha and oxygen-Ill simultaneously, while the OPT Triad filter passes hydrogen-alpha, oxygen-Ill and hydrogen-beta simultaneously. The idea is to use these filters with a colour camera, such as a DSLR or a one-shot-colour astro-camera, to capture emission wavelengths simultaneously without the need for multiple individual filters or a filter-wheel.
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In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband systems are not necessar...
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In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband systems are not necessarily transmitted with low power level enough not to give damage to DS-CDMA signals. Therefore, an efficient rejection technique for narrowband interferers is required for successful CDMA overlay. Furthermore, it is necessary to cope with sudden changes of interference power levels associated with their appearance/disappearance. In this paper, we propose a chancellor to reject such high power narrowband interference for the DS-CDMA system and show the rejection performance obtained from the computer simulation.
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摘要 :
In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband systems are not necessar...
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In order to improve spectral efficiency, CDMA overlay, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered. However, signals from existing narrowband systems are not necessarily transmitted with low power level enough not to give damage to DS-CDMA signals. Therefore, an efficient rejection technique for narrowband interferers is required for successful CDMA overlay. Furthermore, it is necessary to cope with sudden changes of interference power levels associated with their appearance/disappearance. In this paper, we propose a chancellor to reject such high power narrowband interference for the DS-CDMA system and show the rejection performance obtained from the computer simulation.
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This paper presents a simple feedback methodology that uses second order filters to control narrowband resonant and non-resonant vibration of a structural system. In particular, a single degree-of-freedom system is studied through...
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This paper presents a simple feedback methodology that uses second order filters to control narrowband resonant and non-resonant vibration of a structural system. In particular, a single degree-of-freedom system is studied throughout the paper. The idea of the methodology is based on the fact that direct feedback is effective for in-phase vibration control. Thus, the position, velocity and acceleration are respectively fed back to control the low, resonant and high frequency vibration of the system. Each of these is passed through a band pass filter of second order that is inserted to extract and feed back the in-phase signal component only. This is called narrowband feedback. It is demonstrated with experiments that narrowband feedback is useful for narrowband control of resonant and non-resonant vibration.
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Recently, narrowband emissions ordered near the H~+ (proton) gyrofrequency (f_(cH)) were reported in the stimulated electromagnetic emission (SEE) spectrum during active geomagnetic conditions. This work presents new observations ...
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Recently, narrowband emissions ordered near the H~+ (proton) gyrofrequency (f_(cH)) were reported in the stimulated electromagnetic emission (SEE) spectrum during active geomagnetic conditions. This work presents new observations and theoretical analysis of these recently discovered emissions. These emission lines are observed in the stimulated electromagnetic emission (SEE) spectrum when the transmitter is tuned near the second electron gyroharmonic frequency (2f_(ce)) during recent ionospheric modification experiments at the High Frequency Active Auroral Research (HAARP) facility near Gakona, Alaska. The spectral lines are typically shifted below and above the pump wave frequency by harmonics of a frequency roughly 10% less than f_(cH) (≈ 800 Hz) with a narrow emission bandwidth less than the O~+ gyrofrequency (≈ 50 Hz). However, new observations and analysis of emission lines ordered by a frequency approximately 10% greater than f_(cH) are presented here for the first time as well. The interaction altitude for the heating for all the observations is in the range of 160 km up to 200 km. As described previously, proton precipitation due to active geomagnetic conditions is considered as the reason for the presence of H+ ions known to be a minor background constituent in this altitude region. DMSP satellite observations over HAARP during the heating experiments and ground-based magnetometer and riometer data validate active geomagnetic conditions. The theory of parametric decay instability in multi-ion component plasma including H~+ ions as a minority species described in previous work is expanded in light of simultaneously observed preexisting SEE features to interpret the newly reported observations. Impact of active geomagnetic conditions on the SEE spectrum as a diagnostic tool for proton precipitation event characterization is discussed.
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Very low frequency (VLF, 3-30 kHz) transmitter remote sensing has long been used as a simple yet useful diagnostic for the D region ionosphere (60-90 km). All it requires is a VLF radio receiver that records the amplitude and/or p...
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Very low frequency (VLF, 3-30 kHz) transmitter remote sensing has long been used as a simple yet useful diagnostic for the D region ionosphere (60-90 km). All it requires is a VLF radio receiver that records the amplitude and/or phase of a beacon signal as a function of time. During both ambient and disturbed conditions, the received signal can be compared to predictions from a theoretical model to infer ionospheric waveguide properties like electron density. Amplitude and phase have in most cases been analyzed each as individual data streams, often only the amplitude is used. Scattered field formulation combines amplitude and phase effectively, but does not address how to combine two magnetic field components. We present polarization ellipse analysis of VLF transmitter signals using two horizontal components of the magnetic field. The shape of the polarization ellipse is unchanged as the source phase varies, which circumvents a significant problem where VLF transmitters have an unknown source phase. A synchronized two-channel MSK demodulation algorithm is introduced to mitigate 90° ambiguity in the phase difference between the horizontal magnetic field components. Additionally, the synchronized demodulation improves phase measurements during low-SNR conditions. Using the polarization ellipse formulation, we take a new look at diurnal VLF transmitter variations, ambient conditions, and ionospheric disturbances from solar flares, lightning-ionospheric heating, and lightning-induced electron precipitation, and find differing signatures in the polarization ellipse.
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In this paper, we present an overview of research in the area of narrow-bandwidth generation and detection of ultrasound using a technique referred to as frequency domain photoacoustics. An intensity-modulated continuous wave lase...
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In this paper, we present an overview of research in the area of narrow-bandwidth generation and detection of ultrasound using a technique referred to as frequency domain photoacoustics. An intensity-modulated continuous wave laser is used for narrowband ultrasound generation, and an interferometer coupled to a radio frequency lock-in amplifier is used for detection. Excellent sensitivity is achieved using sinusoidal modulation of the excitation laser over long time scales, thereby focusing the acoustic energy at the excitation frequency. We describe an experimental approach for the direct detection of narrowband signals at the ultrasound excitation frequency, and a superheterodyne technique in which the signal is optically down-converted to a low and fixed intermediate frequency prior to detection. Two approaches for materials characterisation using frequency domain photoacoustic measurements are presented. In the first approach, the modulation frequency of the excitation laser is scanned over the bandwidth of interest, and a transient sample response is constructed from the frequency-domain data. Ultrasound arrivals that are separated in the time domain can then be time gated for further analysis. In the second approach, the modulation frequency of the excitation laser is fixed, but the source-to-receiver distance is varied. The spatial frequencies of the ultrasound modes generated by the laser are detected by analysing the spatial variation of the phase, allowing for the velocity of each mode at a given frequency to be determined. Experimental measurements in thin films, plates and nanomechanical structures are presented.
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Spectrally-narrow-(~0.003 cm~(-1)) and broadband(>1 cm~(-1)) fluorescence excitation of the A~1A_2-X~1A_1, 4_0~1 electronic transition of formaldehyde (CH_2O) in laminar premixed and non-premixed flames is investigated using the t...
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Spectrally-narrow-(~0.003 cm~(-1)) and broadband(>1 cm~(-1)) fluorescence excitation of the A~1A_2-X~1A_1, 4_0~1 electronic transition of formaldehyde (CH_2O) in laminar premixed and non-premixed flames is investigated using the third-harmonic output from a tunable, injectionseeded Nd:YAG laser. Spectrally-resolved, CH_2O fluorescence excitation spectra are examined over a broad range of conditions including room-temperature vapor cells and lean-to-rich premixed methane/air and dimethyl ether/air flames in order to understand the origin of the fluorescence using both narrowband and broadband excitation strategies. The measured CH_2O excitation spectra are nearly identical in all conditions considered which cover a broad range of composition and temperature conditions. These results imply that the predominant emission signature is CH_2O and suggest the potential for quantitative in-flame CH_2O LIF measurements using room-temperature calibration and existing fluorescence models. A specific emphasis of this study is on CH_2O isolation and potential fluorescence interference in the context of single-shot planar laserinduced fluorescence (PLIF) imaging in flames. The PLIF results indicate that for the premixed flames investigated, both narrowband (Nd:YAG laser operating in single mode) and broadband (no injection seeding) excitation yield a reliable marker of the CH_2O distribution, with no indication of major interference from additional species. However, frequency-tuned narrowband excitation resulted in a collected fluorescence emission signal that increased by a factor of two as compared to broadband excitation. In the methane-based non-premixed flames, evidence of the excitation of additional species (such as PAH) was noted; however, the impact of this interference is reduced when using narrowband excitation. Similar to the premixed flames, the CH_2O fluorescence emission signal increased by approximately a factor of two when using spectrally tuned, narrowband excitation from the third-harmonic output of an injection-seeded Nd:YAG laser. The current results indicate that narrowband excitation of CH_2O near 355 nm using the third-harmonic output of an injectionseeded Nd:YAG laser results in increased fluorescence emission signal and hence a reduced effect of interference from additional flame-generated species as compared to conventional broadband excitation using a frequency-tripled Nd:YAG laser
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Narrowband whistler mode waves with frequencies near 1Hz have been observed near the Moon. We reveal that the narrowband spectra, the frequency concentration near 1Hz, and the relations between the wave vector, magnetic field vect...
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Narrowband whistler mode waves with frequencies near 1Hz have been observed near the Moon. We reveal that the narrowband spectra, the frequency concentration near 1Hz, and the relations between the wave vector, magnetic field vector, and sunward directions can be explained by a condition in which the group velocity vector is almost canceled by the solar wind velocity vector in the spacecraft frame. Hereafter, we refer to this condition as the group-standing condition. The spectral density is modified and has a peak at the frequency satisfying the group-standing condition because of the difference of the frequency width between the solar wind plasma frame and the spacecraft frame. In addition, if the waves were decelerated to be group-standing, the conservation of the energy flux results in the intensification of the wave amplitude at that frequency. We also derive the analytical expression of the amount of the modifications, which depend on the group velocity. These effects can explain the narrowband spectra near 1Hz and support the relations between the wave vector, magnetic field vector, and sunward directions. The estimated frequency which satisfies the group-standing condition is in good agreement with the observed frequency within error bars of the estimation. Considering the group-standing condition,we suggest that the narrowbandwaves observed in the spacecraft frame are originated from oblique whistler mode waves in the frequencies near the lower hybrid frequency, which are possibly generated by reflected ions from the lunar magnetic anomalies.
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