Spectral Nulls Research Articles

Sonar processing by the spectrogram correlation and transformation model of biosonar

Echolocating big brown bats emit frequency-modulated (FM) biosonar sounds and perceive target range from echo delays through spectrogram correlation (SC) and target shape from interference nulls in echo spectra through spectrogram transformation (ST). Combined, the SCAT model is a computationally unified auditory description of biosonar as a real-time process. We developed a Matlab implementation of SCAT and tested it with a succession of simulated bat-like FM signals (chirps), each followed by one or more FM echoes that have realistic delay and spectral characteristics. The model simulates neural response latencies in frequency-tuned delay-lines that use coincidence detections for target ranging by SC. For ST, a novel, deconvolution-like network transforms echo spectra into images of the target’s glints by detecting coincidences between spikes that represent spectral nulls in parallel channels tuned to null frequencies. Experiments show that dolphins likely separate ST into two operations—MaPS for short glint separations (macro power spectral features, 80 µs). The ST deconvolution network models MiPS. The highly distributed character of the model favors real-time operation, an important goal for bioinspired sonar development. [Work supported by ONR.]

Adaptively loaded IM/DD optical OFDM based on set-partitioned QAM formats.

We investigate the constellation design and symbol error rate (SER) of set-partitioned (SP) quadrature amplitude modulation (QAM) formats. Based on the SER analysis, we derive the adaptive bit and power loading algorithm for SP QAM based intensity-modulation direct-detection (IM/DD) orthogonal frequency division multiplexing (OFDM). We experimentally show that the proposed system significantly outperforms the conventional adaptively-loaded IM/DD OFDM and can increase the data rate from 36 Gbit/s to 42 Gbit/s in the presence of severe dispersion-induced spectral nulls after 40-km single-mode fiber. It is also shown that the adaptive algorithm greatly enhances the tolerance to fiber nonlinearity and allows for more power budget.

A New Method of Generating Spectral Nulls at the Transmitter in Cognitive Radio

In cognitive radio (CR), cognitive users can sense the wholes and white spectrum and generate spectrum notch in the spectrum bands occupied by primary users (PUs) or interference. Thus, the key tec...

DS-UWB pulse design in IEEE802.15.3a multipath fading channel for cognitive radio applications

This paper presents a novel spectrum shaping technique in direct sequence ultra-wideband (DS-UWB) communication systems. This proposed pulse introduces spectral nulls to limit the interference impact of high power coexisting narrowband signals and to elevate the applications of cognitive radio. The idea depends on the addition and subtraction of several UWB Gaussian derivative pulses in order to design a new pulse that has spectral null at the operating frequency of the narrowband interference (NBI) signal. The use of the Gaussian derivative ensures that the UWB spectrum mask established by the Federal Communications Commission (FCC) is met. The NBI signals are modelled as either the wireless local area network (WLAN) IEEE802.11n which operates in the 5 GHz frequency band or multi tone signals. The power spectral density of the proposed transmitted UWB pulse is derived analytically. The bit error rate performance of the proposed pulse is derived in the multi-path IEEE802.15.3a UWB fading channel models and validated with simulations. Results show that the impact of either the IEEE802.11n interference signal or the multi tone interference signals can be effectively suppressed by transmitting and matched filtering the new pulse with a single or multiple spectral nulls at the interference operating frequencies.

Spectral Modulation Effect in Teleseismic P-waves from North Korean Nuclear Tests Recorded in Broad Azimuthal Range and Possible Source Depth Estimation

Three underground nuclear explosions, conducted by North Korea in 2006, 2009 and 2013, are analyzed. The last two tests were recorded by the Israel Seismic Network. Pronounced coherent minima (spectral nulls) at 1.2–1.3 Hz were revealed in the spectra of teleseismic P-waves. For a ground-truth explosion with a shallow source depth, this phenomenon can be interpreted in terms of the interference between the down-going P-wave and the pP phase reflected from the Earth’s surface. This effect was also observed at ISN stations for a Pakistan nuclear explosion at a different frequency 1.7 Hz and the PNE Rubin-2 in West Siberia at 1 Hz, indicating a source-effect and not a site-effect. Similar spectral minima having essentially the same frequency, as at ISN, were observed in teleseismic P-waves for all the three North Korean explosions recorded at networks and arrays in Kazakhstan (KURK), Norway (NNSN), Australia (ASAR, WRA) and Canada (YKA), covering a broad azimuthal range. Data of 2009 and 2013 tests at WRA and KURK arrays showed harmonic spectral modulation with three multiple minima frequencies, evidencing the clear interference effect. These observations support the above-mentioned interpretation. Based on the null frequency dependency on the near-surface acoustic velocity and the source depth, the depth of the North Korean tests was estimated about 2.0–2.1 km. It was shown that the observed null frequencies and the obtained source depth estimates correspond to P-pP interference phenomena in both cases of a vertical shaft or a horizontal drift in a mountain. This unusual depth estimation needs additional validation based on more stations and verification by other methods.

Reduced Complexity Optimum Detector for Block Data Transmission System Using<i> A<sub>n</sub> </i>Lattice Sphere Detection Technique

Block Data Transmission Systems (BDTS) are used in high speed wireless communication systems with time dispersive channel characteristics. Since Maximum Likelihood Block Detection (MLBD) requires huge amount of computation, Sphere Detection (SD) technique has been introduced as an alternative. This paper proposes an An Lattice Sphere Detection (AnLSD) technique for detection in block data transmission systems (BDTS). The An lattice structure offers more dense constellation points than the customary Zn lattice in SD technique. Thus, the proposed AnLSD uses An lattice generator matrix as the channel matrix, and uses the Gram matrix which is obtained from the generator matrix as part of the detection process. Simulation results show that the proposed AnLSD performs very close to the Exhaustive search (ES) using block size of 20 bits. It is also performing better than the other renowned methods tested under a channel with spectral nulls. The proposed AnLSD technique offers significant reduction in term of objective functions evaluation as compared to the other renowned methods.

In-band insertion of RoF LTE Services in OOK based PON’s using line coding techniques

We demonstrate the creation of multiple spectral nulls, in a 10Gb/s On Off Keyed signal with the help of encoders with inherent DC-balanced and minimum-bandwidth properties. Within a hybrid wired/wireless PON architecture, these generated nulls are subsequently used to transmit a wireless signal with 20MHz of bandwidth carrying Long Term Evolution (LTE) service. The level of interference experienced on both the wired 10G signal and the wireless signal was studied for various wireless power levels and the encoding order. Results show significant system performance enhancements due to the reduction of interference, which had been achieved by encoding the baseband data signal. Finally numerical simulations are employed to elaborate on performance of the encoded system.

Low Complexity Discrete Hartley Transform Precoded OFDM System over Frequency-Selective Fading Channel

Orthogonal frequency-division multiplexing (OFDM) suffers from spectral nulls of frequency-selective fading channels. Linear precoded (LP-) OFDM is an effective method that guarantees symbol detectability by spreading the frequency-domain symbols over the whole spectrum. This paper proposes a computationally efficient and low-cost implementation for discrete Hartley transform (DHT) precoded OFDM systems. Compared to conventional DHT-OFDM systems, at the transmitter, both the DHT and the inverse discrete Fourier transform are replaced by a one-level butterfly structure that involves only one addition per symbol to generate the time-domain DHT-OFDM signal. At the receiver, only the DHT is required to recover the distorted signal with a single-tap equalizer in contrast to both the DHT and the DFT in the conventional DHT-OFDM. Theoretical analysis of DHT-OFDM with linear equalizers is presented and confirmed by numerical simulation. It is shown that the proposed DHT-OFDM system achieves similar performance when compared to other LP-OFDMs but exhibits a lower implementation complexity and peak-to-average power ratio.

The cepstral peak: A theoretic analysis and implementation comparison of a popular voice measure

Cepstral analysis of speech has traditionally provided two pieces of information: (1) the cepstral lag, which is a measure of fundamental frequency, and (2) the cepstral peak, which is a measure of the degree of periodicity. Cepstral analysis conveniently separates the effects of the vocal tract from the acoustic source without explicit vocal tract estimation. The cepstral peak of speech is a popular diagnostic tool for voice disorders and vocal treatment assessment, yet the expected value of the cepstral peak has not received sufficient theoretic treatment. Discrete-time analysis of the cepstrum of a periodic pulse train revealed that the cepstral peak is 1/2 for all pulse trains with integer fundamental period T samples. For the more general case of a non-integer period, a pulse train formed with sinc functions produces a cepstral peak of 1/2+ε where the magnitude of ε scales with 1/T. The cepstral peak of various test signals was compared to cepstral peak prominence [Hillenbrand et al., J. Speech Hear. Res. 37, 769–778 (1994)], and accuracy was improved by (1) zero-padding the log spectrum before inverse Fourier transformation, which provides cepstral interpolation, and (2) limiting spectral nulls, which trades off estimate variance and bias.

Investigation of PMD in direct-detection optical OFDM with zero padding

We investigate the polarization-mode dispersion (PMD) effect of zero padding OFDM (ZP-OFDM) in direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems. We first study the conventional equalization method for ZP-OFDM. Then an equalization method based on sorted QR decomposition is proposed to further improve the performance. It is found that the performance improvement of ZP-OFDM is due to the frequency domain oversampling (FDO) induced inter-carrier interference (ICI). Numerical simulation results show that compared with cyclic prefix OFDM (CP-OFDM), ZP-OFDM has a significantly higher tolerance to PMD in DDO-OFDM systems when the channel spectral nulls occur at certain differential group delay (DGD) values.

Estimating earthquake source depths by combining surface wave amplitude spectra and teleseismic depth phase observations

The ability to confidently estimate the depths of small-to-medium sized (3.5 ≤ mb ≤ 5.5) seismic disturbances is important both in plate tectonics studies, and when monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Seismic source depths can be determined by identification of the teleseismic depth phases pP and sP, and also by modelling surface wave amplitude spectra. The radiation pattern of the teleseismic depth phase pP and fundamental mode Rayleigh amplitudes show that the effectiveness of these methods of earthquake depth estimation is dependent on the orientation of the focal mechanism and the station locations. For some focal mechanisms, the predicted amplitude of the teleseismic depth phase pP will only be large for stations in certain locations, and the Rayleigh wave spectral nulls that tightly constrain the seismic source depth when modelling surface wave amplitude spectra often only occur for a limited range of azimuths. In this study, we show that for sources where Rayleigh wave spectral nulls are not observed and the source depth cannot be constrained using the surface wave amplitude spectra, the focal mechanism obtained by modelling Rayleigh and Love wave amplitude spectra can be used to identify the locations of stations where pP should have a large amplitude and hence be easiest for an analyst to identify. The increased global coverage of seismometer stations means that there is an increased likelihood that stations exist in the locations where the predicted amplitude of pP is large. As the identified depth phases are consistent with the focal mechanism this approach allows increased confidence to be placed in the identified depth phases and hence the estimated source depth. This approach could potentially be used with other methods of focal mechanism estimation provided that the method used to estimate the focal mechanism is independent of the source depth.

Generation of spectral nulls with permutation sequences

Abstract This paper describes the construction of classes of spectral null codes, which are obtained by shaping permutation sequences. The properties and the construction techniques of binary spectral null codes are used. The power spectral densities (PSD) of the proposed codes show nulls at the zero frequency and at submultiples of the frequencies of the transmitted symbols. The channel symbols or voltage levels mapped to each transmitted permutation symbol are chosen to satisfy the spectral null equation. Interesting results show the elimination of the DC component at the lower frequencies. The well-shaped power spectral densities of these codes may overcome some communications problem like zero frequency components. The proposed technique is generalized and the corresponding spectral null permutation sequences that generate spectral null codes are presented.

Coherence for perception of target vs clutter in bat sonar: The role of neuroethology as pathfinder for neuroscience

Coherence for perception of target vs clutter in bat sonar: The role of neuroethology as pathfinder for neuroscience

Multiple narrowband interference suppression for UWB communication systems in Nakagami fading channels

This paper investigates the performance of ultra-wideband (UWB) communication systems in the presence of multiple in-band narrowband interference signals in a Nakagami-m frequency selective fading channel. The UWB systems have a restraint on their transmission power levels in order to control its interference to other in-band operating services; however UWB systems unavoidably suffer from the interference caused by the coexisting systems such as the IEEE802.11a signal, making it extremely difficult to maintain adequate signal-to-noise-and interference ratio (SNIR) levels. In order to mitigate the impact of such strong interference the paper proposes a suppression technique which introduces spectral nulls at the operating frequencies of the narrowband interference (NBI) signals. The bit error rate performance of the proposed scheme is derived in a Nakagami multi-path frequency selective fading channel and validated with simulation. The results show that with the use of such proposed technique the NBI impact can be effectively suppressed leading to an improvement to the bit error rate performance of the UWB communication systems. A comparison with a notch filter-based case is also presented where it will be shown that the proposed technique out performs a perfectly tuned notch filter.

Lower Bounds on the Correlation Property for OFDM Sequences with Spectral-Null Constraints

Sequences with specific autocorrelation (AC) and cross-correlation (CC) properties are crucial components in radar and wireless communications. In this paper, we derive the theoretical bounds on the AC and CC for OFDM sequences with constraints of spectral nulls, e.g., the mandatory nulls on the DC sub-carrier and guardbands in OFDM systems. The bounds and trade-off limits are provided for the properties of sequences, including the peak AC and CC levels, the cardinality of the sequence set, the sequence length, and the temporal length of the low correlation zone. We also investigate the trade-offs of correlations among sequence sets. The presented trade-off limits can serve as guidelines for applications where the performance measures or design criteria are related to the peak AC and CC levels.

A novel Haar wavelet‐based vector BPSK–OFDM robust to channel spectral nulls and with reduced cyclic prefix length and PAPR

SUMMARYThe performance of OFDM systems may be degraded when intersymbol interference (ISI) channels have spectral nulls. The communication overhead because of the insertion of the cyclic prefix is high when ISI channels have many taps. Recently, the precoded OFDM systems and vector OFDM (VOFDM) systems were proposed to combat these two problems, respectively. This paper proposes a novel vector binary phase‐shift keying (BPSK)–OFDM system based on Haar wavelet transformation. The Haar wavelet transformation performs decomposition over the vector information symbols produced by a BPSK modulator. It shows that half of the information symbols are zero and the rest are or . Then, the vector BPSK–OFDM becomes the precoded OFDM, which inserts one or more zeros between two sets of K consecutive information symbols. This precoding scheme is able to remove the spectral nulls of an ISI channel without knowing the ISI (like conventional precoded OFDM systems), and it has the same cyclic prefix length as VOFDM systems. The proposed OFDM systems show better robustness to the spectral nulls channel, compared with the conventional VOFDM systems. Because of the insertion of (M −K) zeros between two sets of K consecutive information symbols, the conventional precoded OFDM systems have the data rate expanded and signal‐to‐noise power ratio reduced by 10 log 10(M∕K) dB compared with the proposed OFDM systems. Moreover, because half of the information symbols are zeros after Haar wavelet transformation, the proposed OFDM system can further reduce the transmitting peak‐to‐average power ratio. Finally, we compare the proposed OFDM systems with the precoded OFDM and VOFDM systems. Copyright © 2011 John Wiley & Sons, Ltd.

Shaping the Noise Spectrum in Power Electronic Converters

The operation of switch-mode converters results in electromagnetic noise propagating throughout the power electronic system. Pulsewidth modulation (PWM) strategies with fixed switching frequency generate harmonics in the spectra of converter voltages and currents. Random PWM techniques allow the elimination of the harmonics, resulting in a continuous spectrum of noise, i.e., a spectrum retaining all frequency components. As a next step in advanced spectral shaping, a method presented in this paper produces spectral nulls at selected frequencies. This allows the carving of communication channels in the noise and the removal of spectral power at frequencies potentially harmful for the system. The theoretical analysis, computer simulations, implementation details, and experimental results are presented.

Robust Speech Dereverberation Based on Blind Adaptive Estimation of Acoustic Channels

This paper addresses the problem of speech dereverberation considering a noisy and slowly time-varying environment. The proposed multimicrophone speech dereverberation model utilizes the estimated acoustic impulse responses (AIRs) to dereverberate the speech as well as improve the signal-to-noise ratio without a priori information about the AIRs, location of the source and microphones, or statistical properties of the speech/noise, which are some common assumptions in the related literature. The received noisy signals are filtered through an eigenfilter which improves the power of the speech signal as compared to that of the additive noise. The eigenfilter is efficiently computed avoiding the tedious Cholesky decomposition, solely from the estimates of the AIRs. The design of the eigenfilter also incorporates a frequency domain constraint that improves the quality of the speech signal, resists spectral nulls in addition to improving the signal-to-noise ratio (SNR). A zero-forcing equalizer (ZFE) is used to dereverberate the speech signal by eliminating the distortion caused by the AIRs as well as the eigenfilter. The ZFE is implemented in block-adaptive form which makes the proposed technique suitable for speech dereverberation in a time-varying condition. The simulation results verify the superior performance of the proposed method as compared to the state-of-the-art dereverberation techniques in terms of log-likelihood ratio (LLR), segSNR, weighted spectral slope (WSS), and perceptual evaluation of speech quality (PESQ).

Spectral Nulling on Transmit via Nonlinear FM Radar Waveforms

Modern radars operate in electromagnetic environments crowded with other RF users. Adaptive spectral nulling on transmit can reduce interference from and to these other RF users. An analytical theory is developed for spectral nulling by a minimal adjustment, or offset, to the phase of the radar pulse that maintains constant pulse amplitude. Numerical examples show that a small time-varying phase offset designed by the proposed method can produce deep spectral nulls at one or more chosen frequencies in the radar's spectral sidelobes with little other effect on the pulse spectrum or ambiguity function.

A Signal Subspace Detection Technique for Single Carrier Block Transmission with Unique Words

For frequency selective channels, conventional equalizations based on a whole vector space enhance the noise power at randomly distributed spectral nulls. A signal subspace detection technique is proposed for single carrier (SC) block transmission with unique words (UW). Different from conventional Fourier basis, a new orthogonal basis to orderly separate the space into signal subspace and noise subspace is adopted. In this way, known symbols of UW can compensate the noise subspace and facilitate data recovery by a maximum likelihood (ML) detector in the signal subspace, which is robust to the ill-conditioned channel matrix. The proposed technique does not require any feedback from the receiver to the transmitter. A near optimal performance is achieved with its computational complexity similar to the time domain equalization.