Spectral Nulls Research Articles

Cognitive perception of biosonar echo delay, phase, and spectrum

FM echolocating big brown bats combine the acoustic delay, phase, and spectrum of echoes into a unitary cognitive attribute of perceived delay. In this talk, we will present psychophysical, neurophysiological, and modeling data showing how this might be accomplished. Delay accuracy measured psychophysically approximates coherent matched-filter accuracy. Psychophysical curves of echo-delay resemble pulse-echo crosscorrelation functions, with phase manifested directly within 20 μs. Computational modeling shows that low-pass time-frequency bandpass filtering at 10 kHz enables the contribution from phase. Neural responses from the auditory midbrain are phase-sensitive for tone-burst offsets up to15 kHz. Echo spectrum (spectral nulls and ripples) is transformed into psychophysical percepts of delay. The external-ear spectral null creates an elevation-dependent peak at about 35 μs for vertical target tracking. Off-axis clutter masking is suppressed by a Gestalt-like normalization process that operates within delay percepts. The organization of perceived delay and the appearance of phase shifts is facilitated by delay processing that begins at the low-frequency tail end of FM sweeps and the linking of 1st and 2nd harmonic frequencies. Surprisingly, this result requires the entire echo to be received before any delay is determined. The implications of this result will be discussed. [Work supported by ONR.]

Low-PAPR OFDM Waveform Design for Radar and Communication Systems

In this correspondence we address the problem of designing OFDM waveforms with low peak-to-average power ratio (PAPR) for joint radar and communication (JRC) applications. The design problem is formulated as the minimization of PAPR with the constraint that the integrated periodic autocorrelation sidelobe level is zero. We propose an iterative algorithm based on the majorization-minimization (MM) technique to solve this design problem. The proposed algorithm yields OFDM waveforms with PAPR values close to one. The algorithm can also be used to design waveforms with spectral nulls. In extensive numerical simulations we compare the performance of our algorithm with that of a recently proposed benchmark.

Enhanced Automotive Sensing Assisted by Joint Communication and Cognitive Sparse MIMO Radar

Automotive radar is mandated to provide high accuracy direction of arrival (DOA) estimation for safe driving, while remaining a low cost device for feasible mass production. Sparse multiple input multiple output (MIMO) arrays emerge as a primary candidate to meet these requirements. As DOA estimation accuracy is a main indicator of tracking performance, Cramer Rao Bound (CRB) is chosen as the goodness measurement for sparse MIMO array optimization, but its application requires prior information of the road environment. We propose a cognitive sparse MIMO array automotive radar, which “perceives” the road environment via automotive sensing supplemented by co-existed communication from roadside unit to vehicle (R2V). This information is used for co-designing a sparse MIMO array for enhanced automotive sensing and vehicle to roadside unit (V2R) communication. Note that both static RSU and dynamic RSU are usually deployed in Internet of Vehicles (IoV), which can provide continuous transmission coverage and permanent connectivity. The bi-directional communications are integrated into the automotive radar. This is achieved by joint transmit waveform design with spectral nulls for communications and with shared sparse MIMO array co-design for both sensing and high quality V2R communications. Simulation results validate the enhanced automotive sensing performance assisted by the integrated bi-directional communications in the cycle of cognitive-driven optimization.

Joint antenna selection and waveform design for coexistence of MIMO radar and communications

As the problem of spectral congestion is becoming severe, the coexistence between two primary spectrum users, radar and communications, has spurred extensive research interest. To reduce the mutual interferences between the two functions, MIMO radar waveform design needs to consider the compatibility in both spectral and spatial domains, where the former is achieved by null forming in the frequency domain and the latter is achieved by shaped beampattern synthesis. Additionally, high power efficiency and low system overhead are two desirable characteristics for MIMO radar system design. To this end, we first introduce a new realistic waveform constraint, peak-to-valley-power-ratio (PVPR) constraint per antenna to improve the power efficiency. Then, combined with PVPR constraint, we propose a switchable individual antenna power control scheme to jointly optimize waveforms and antenna locations. We adopt a max–min beampattern matching criterion and impose the ell _{2,1}-norm penalty on the waveform matrix to promote the sparsity of the array. To solve the resultant non-smooth and non-convex problem, we develop a modified alternating directions method of multipliers, where a surrogate subproblem over primal variables is solved instead of the original problem, and its local convergence is analyzed. Finally, numerical experiments demonstrate the effectiveness and superiority of the proposed method over counterparts, especially obtaining the lowest sidelobe level and deeper spectral nulls using much fewer antennas.

Unsupervised resolution-agnostic quantitative susceptibility mapping using adaptive instance normalization.

Quantitative susceptibility mapping (QSM) is a useful magnetic resonance imaging (MRI) technique that provides the spatial distribution of magnetic susceptibility values of tissues. QSMs can be obtained by deconvolving the dipole kernel from phase images, but the spectral nulls in the dipole kernel make the inversion ill-posed. In recent years, deep learning approaches have shown a comparable QSM reconstruction performance to the classic approaches, in addition to the fast reconstruction time. Most of the existing deep learning methods are, however, based on supervised learning, so matched pairs of input phase images and ground-truth maps are needed. Moreover, it was reported that the deep learning-based methods fail to reconstruct QSM when the resolution of test data is different from the trained resolution. To address this, here we propose an unsupervised resolution-agnostic QSM deep learning method. The proposed method does not require QSM labels for training and reconstructs QSM with various resolutions by using adaptive instance normalization. Experimental results and clinical validation confirm that the proposed method provides accurate QSM with various resolutions compared to other deep learning approaches, and shows competitive performance to the best classical approaches in addition to the ultra-fast reconstruction.

Multi-Rate Nyquist-SCM for C-Band 100 Gbit/s Signal Over 50 km Dispersion-Uncompensated Link

In this paper, to the best of our knowledge, we propose the first multi-rate Nyquist-subcarriers modulation (SCM) for C-band 100Gbit/s signal transmission over 50km dispersion-uncompensated link. Chromatic dispersion (CD) introduces severe spectral nulls on optical double-sideband signal, which greatly degrades the performance of intensity-modulation and direct-detection systems. Based on the prior knowledge of the dispersive channel, Nyquist-SCM with multi-rate subcarriers is proposed to keep away from the CD-caused spectral nulls flexibly. Signal on each subcarrier can be individually recovered by a digital signal processing, including the feed-forward equalizer with no more than 31 taps, a two-tap post filter, and maximum likelihood sequence estimation with one memory length. Combining with entropy loading based on probabilistic constellation shaping to maximize the capacity-reach, the C-band 100Gbit/s multi-rate Nyquist-SCM signal over 50km dispersion-uncompensated link can achieve 7% hard-decision forward error correction limit and average normalized generalized mutual information of 0.967 at received optical power of -4dBm and optical signal-to-noise ratio of 47.67dB. In conclusion, the multi-rate Nyquist-SCM shows great potentials in solving the CD-caused spectral distortions.

Direct detection transmission of a PAM signal with power fading mitigation based on Alamouti coding and dual-drive MZM.

Power-fading impairment induced by fiber dispersion and photodiode detection imposes a fundamental limitation on the intensity-modulation direct-detection (IM-DD) transmission systems. In this work, we propose a cost-effective pulse-amplitude modulation (PAM) signal transmission scheme with power-fading mitigation enabled by Alamouti coding and dual-drive Mach-Zehnder modulator (DDMZM). By interleaving the symbol blocks in the time domain for upper- and lower-arm of the DDMZM, flat end-to-end frequency response can be obtained without spectral nulls after combining the photocurrents at odd and even time slots. For single channel demonstration, we experimentally transmit up to 160Gb/s PAM-4, 140Gb/s PAM-6, and 108Gb/s PAM-8 signals over 80 km standard single-mode fiber (SSMF) with bit-error rates (BERs) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.0×10-2. For wavelength division multiplexing (WDM) transmission, 8λ×150Gb/s PAM-4 signals spacing at 100 GHz can also achieve 80 km reach. Moreover, we compare the optical signal-to-noise ratio (OSNR) sensitivity with single sideband (SSB) scheme and evaluate the tolerance of bias deviation in numerical simulation. The proposed Alamouti coding-based scheme provides a hardware-efficient and dispersion-tolerant candidate for high-speed inter-data center interconnect (DCI) applications.

Detection, Estimation and Radiation Formation Using Smart Antennas for the Spatial Location

The Electromagnetic (EM) waves are impinging on the base station from all the directions, Equally Spaced Uniform Linear Antenna Array (ESULA) are used to process these incoming EM waves to Detect and Estimate the directions of the mobile transmitters. After the process of Detection and Estimation, Electronic Beamforming is used to provide the narrow sharper beam towards the detected user. This Detection, Estimation and Beamforming plays a key role in variety of use cases like Radar, Wireless Communication and Sonar based systems. Smart Antenna Systems are implemented using two strategies namely Direction of Arrival (DoA) and Beamforming (BF). Direction of Arrival is a mechanism of Detecting and Estimating the directions of the mobile transmitters. Beamforming on the other hand is a process of transmission of the EM waves towards the source in a specific direction and providing the Spectral Nulls to other Interfering users. To increase the user capacity and to enhance the user experience Spatial Location based Spatial Division Multiple Access (SDMA) technology is used. To improve the overall performance of the smart antenna systems energy and packet delivery is majorly focused on specific source directions rather than using blind transmission strategy. In this paper performance analysis of algorithms for Direction of Arrival methods as well as the Beamforming methods have been performed. Experimental simulations are conducted and comparison is done with respect to Bias, Resolution and Time complexity for the Direction of Arrival methods. Noise Subspace Method (NSM) DoA algorithm consistently delivered the optimal bias, high resolution detection of the user location in spatial domain and provided lesser time complexity for both the scenarios which uses fewer antenna elements or larger number of antenna array elements at the base station. Similarly for the case of Beamforming methods the Mean Square Error and Beam-directions have been compared.

A Novel Scheme of Multicarrier Modulation With the Discrete Cosine Transform

In this work, we derive a novel multicarrier modulation based on the Type-I even discrete cosine transform (DCT1e), which includes new procedures to carry out both the channel estimation and the signal reconstruction. By using a small number of training symbols, we achieve an accurate estimation of the channel’s impulse response (CIR) using a novel mirror, replicate and add (MIRA) procedure. The proposed scheme does not require knowing the length of the CIR and is valid even in the presence of spectral nulls. We provide the theoretical results that guarantee the validity of the developed technique. After the estimation process, the transmitted symbols are also reconstructed by means of the DCT1e using the same novel MIRA scheme. The conditions that ensure a perfect reconstruction in the absence of noise are also provided in this case. Numerical simulations illustrate the excellent behaviour of the proposed approach, both in terms of channel estimation and recovery of the transmitted information.

Burst-Error-Propagation Suppression for Decision-Feedback Equalizer in Field-Trial Submarine Fiber-Optic Communications

In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8km submarine optical cable causes four spectral nulls on the 36GHz bandwidth of 72Gbit/s OOK signal, which is the main obstacle for achieving an acceptable bit-error-rate (BER) performance. Decision feedback equalizer (DFE) is effective to compensate for the spectral nulls. However, DFE has a serious defect of burst-error propagation when the burst errors emerge due to the unstable submarine environment. Weighted DFE (WDFE) can be used to mitigate the burst-error propagation, but it cannot fully compensate for the spectral nulls because only a part of feedback symbols is directly decided. Fortunately, maximum likelihood sequence estimation (MLSE) can be added after the WDFE to simultaneously eliminate the resisting spectral distortions and implement optimal detection. Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm can effectively suppress the burst-error propagation to obtain a maximum 2.9dB improvement of $\boldsymbol{Q}$ factor and eliminate the phenomenon of BER floor. In conclusion, the joint WDFE and MLSE algorithm can solve the burst-error propagation for the field-trial fiber-optic communications.

Injection Coding with Spectral Nulls for Even Codeword Lengths

With a view to adapting information signals to suit the frequency characteristics of digital transmission channels, it is pertinent to design channel coding schemes that can exhibit spectral shaping capabilities after being processed with a suitable modulation technique. This involves concentrating the frequency energy of the coded and modulated information signal towards a predetermined range of the frequency spectrum, or making it have low power content at such frequencies. One modulation scheme that is suitable for achieving this is pulse amplitude modulation (PAM). We thus present a permutation coding (PC) system with injections that can exhibit spectral nulls at rational sub-multiples of the symbol frequency. Such injections are achieved by strategically removing δ columns from the source PC. We also present a mathematical expression that allows for the prediction of zero energy positions in the codebook’s spectrum. Due to the way the injections are introduced, it gives the scheme an advantage of achieving higher symbol rate, when compared with conventional PC systems. The work is however limited to cases where δ =2 and the lengths of the codewords involved are even numbers.

New Complementary Sets With Low PAPR Property Under Spectral Null Constraints

Complementary set sequences (CSSs) are useful for dealing with the high peak-to-average power ratio (PAPR) problem in orthogonal frequency division multiplexing (OFDM) systems. In practical OFDM transmission, however, certain sub-carriers maybe reserved and/or prohibited to transmit signals, leading to the so-called spectral null constraint (SNC) design problem. For example, the DC sub-carrier is reserved to avoid the offsets in D/A and A/D converter in the LTE systems. While most of the current research focus on the design of low PAPR CSSs to improve the code-rate, few works address the aforementioned SNC in their designs. This motivates us to investigate CSSs with SNC as well as low PAPR property. In this article, we present systematic constructions of CSSs under SNCs and low PAPR. First, we show that mutually orthogonal complementary sets (MOCSs) can be used as seed sequences to generate new CSSs with SNC and low PAPR, and then provide an iterative technique for the construction of MOCSs which can be further used to generate complementary sets (CSs) with low PAPRs and spectral nulls at varying positions in the designed sequences. Next, inspired by a recent idea of Chen, we propose a novel construction of these seed MOCSs with non-power-of-two lengths from generalized Boolean functions.

Pseudonoise Waveform Design for Spectrum Sharing Systems

With an increasing demand for high-resolution radar and wideband communication systems, the frequency spectrum is becoming a limited resource. Many such systems must coexist in a given frequency band. In order to minimize interference among these systems, a radar waveform design is used. Without a priori knowledge of the transmission channel and communication waveform, one viable strategy is to use spectral nulls for the frequencies used by communication system. This article describes an improved method for generating pseudonoise waveforms with limited range and Doppler sidelobes. The algorithm permits adjustable power spectral density of the generated waveform. An additional parameter, “significance factor,” is a tradeoff between power spectral density shape and sidelobe suppression depth. The method also allows the generation of a set of mutually orthogonal waveforms for multitransmitter scenarios. Effectiveness of the algorithm is confirmed by both numerical simulation and field trials.

SOURCE DEPTH DETERMINATION OF THE DPRK’S SIX NUCLEAR TESTS (2006, 2009, 2013, 2016J, 2016S, AND 2017) USING REGIONAL AND TELESEISMIC DATA

North Korea conducted underground nuclear explosions on October 9, 2006 (mb 4.3), May 25, 2009 (mb 4.7), February 12, 2013 (mb 5,1), January 6, 2016 (mb 5,1), September 9, 2016 (mb 5,3) and September 3, 2017 (mb 6,3). We estimated source depths for the North Korean nuclear tests using regional and teleseismic data. We found the burial depth at around 2 km for all North Korean nuclear tests using spectral nulls using pP+P/sP+P and pPn + Pn/ sPn+Pn including spectral minima (holes) of the fundamental-mode Rayleigh wave amplitude spectra. It should be noted that utilizing azimuth averaged spectra from the observations is most appropriate to estimate depth for unknown sources in the nonlinear topographic region such as the North Korean nuclear test sites. It is also noticeable to have found spectral anomalies depending on not only source effects but also the site effects. We found higher spectral nulls at the Fennoscandian Shield stations like ARCES and FINES due to the higher crustal velocity resulting in the fast P-wave arrivals with high Q and low attenuation containing high frequencies which very fit to underground nuclear detection, whereas spectral nulls at ASAR are found to be much lower because there is the Great Artesian Basin beneath the array including the low velocity zone in the upper mantle as well. These phenomena are also observed from spectral nulls due to reflection from the bottom of the 660-km Discontinuity by a deep-focus earthquake. It is also notable that the possibility of the over-burial detonation would affect MS : mb and seismic yield for the North Korean underground nuclear tests [1].

Adaptive Channel-Matched Detection for C-Band 64-Gbit/s Optical OOK System Over 100-km Dispersion-Uncompensated Link

In this paper, we propose adaptive channel-matched detection (ACMD) for C-band 64-Gbit/s intensity-modulation and direct-detection (IM/DD) optical on-off keying (OOK) system over a 100-km dispersion-uncompensated link. The proposed ACMD can adaptively compensate most of the link distortions based on channel and noise characteristics, which includes a polynomial nonlinear equalizer (PNLE), a decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE). Based on the channel characteristics, PNLE eliminates the linear and nonlinear distortions, while the followed DFE compensates the spectral nulls caused by chromatic dispersion. Finally, based on the noise characteristics, a post filter can whiten the noise for implementing optimal signal detection using MLSE. To the best of our knowledge, we present a record C-band 64-Gbit/s IM/DD optical OOK system over a 100 km dispersion-uncompensated link achieving 7\% hard-decision forward error correction limit using only the proposed ACMD at the receiver side. In conclusion, ACMD-based C-band 64-Gbit/s optical OOK system shows great potential for future optical interconnects.

OFDM Signal Recovery in Deep Faded Erasure Channel

In high-speed digital wireless communication applications, the intersymbol interference channel may have spectral nulls or erasures, which may degrade the orthogonal frequency division multiplexing (OFDM) bit error rate (BER) performance. In this paper, we prove that the BER performance of the OFDM system is considerably refined if the erased symbols in erasure channel (or received symbols on the deep-faded subcarriers in Rayleigh fading channel) are estimated based on the rest symbols in each OFDM signal. Also, an oversampled OFDM scheme which is a particular form of the pre-coded OFDM schemes is considered to guarantee high enough sampling rate. Furthermore, a new iterative receiver is proposed for perfect symbol recovery. A comprehensive Monte Carlo simulation study is performed to validate the theoretical results and affirm the superior performance of the oversampled OFDM scheme with the iterative receiver, compared with the typical detector for the OFDM scheme.

Depth Calculation for the January 06, 2016, the September 09, 2016 and the September 03, 2017 Nuclear Tests of North Korea from Detailed Depth Phases Using Regional and Teleseismic Arrays

North Korea has conducted six underground nuclear explosions so far. In this study, we determined source depth and characterization for the 2016J, 2016S and 2017S tests which were conducted on January 6, 2016 (2016J), September 9, 2016 (2016S) and September 3, 2017 (2017S), respectively. It has been difficult to ascertain the accurate depth of North Korean nuclear explosions due to paucity of data and information. We explore the depth calculation for the North Korean nuclear tests based on the detailed depth phases using teleseismic and regional arrays. We present the coherent spectral nulls from the average spectra of pP + P/sP + P and pPn + Pn/sPn + Pn which correlate with the depth phases showing 180° phase reversals with the P-wave arrivals. We estimated the burial depths at 2.12, 2.10, 1.98 km for the 2016J, 2016S and 2017S nuclear tests, respectively, We anticipate our absolute findings to be significant since in the past depth estimates for North Korean nuclear tests have been inconclusive and unclear owing to not only paucity of data but also trade-offs of the relative assessment based on the satellite images between the true source location and the tunnel entrance for the 2006 nuclear test used as a reference event.

Depth estimate of the DPRK’s 2006-10-09, 2009-05-25 and 2013-02-12 underground nuclear tests using local and teleseismic arrays

The Democratic People’s Republic of Korea (DPRK aka North Korea) conducted underground nuclear explosions on October 9, 2006 (M 4.3), May 25, 2009 (M 4.7) and February 12, 2013 (M 5.1). We expanded standard depth calculation methods to encompass full depth utilizing spectral nulls of spectra using body waves and Rayleigh waves from local and teleseismic arrays. The depths of burial were calculated from the fundamental-mode Rayleigh wave spectra as well as the average spectra of depth phase (pP + P/sP + P, pPn + Pn/sPn + Pn) considering take-off angles based on the flat Earth model. The average depths of the 2006, 2009 and 2013 nuclear tests were thus estimated at 2.12 km, 2.06 km and 2.05 km, respectively. It should be noted that those depths are significantly greater than expected from the standard experiment practice. The possibility of the over-buried nuclear tests would affect the interpretation of MS-mb discriminants as well as the estimation of the yield.

Improved Chopping in Continuous-Time Delta–Sigma Converters Using FIR Feedback and <inline-formula> <tex-math notation="LaTeX">${N}$ </tex-math> </inline-formula>-Path Techniques

Chopping the operational transconductance amplifier (OTA) of the input integrator in a continuous-time delta–sigma modulator is an effective way of addressing ${1/f}$ noise. This, however, causes shaped noise to alias into the signal band. A solution to this problem is to chop at the sampling rate of the modulator. High-speed chopping reduces the OTA’s dc gain, and degrades its linearity. Alternatively, the OTA can be chopped at a low rate, and FIR feedback can be used to introduce spectral nulls in the feedback waveform, so that the components that alias into the signal band are attenuated. This brief combines chopping and FIR feedback with ${N}$ -path techniques, enabling a much lower chopping frequency than would otherwise be possible with a given number of FIR DAC taps. Simulation results confirming the theory are given.

On the Performance Analysis of One Tap Equalizers in Oversampled OFDM Systems

Oversampling is commonly encountered in orthogonal frequency division multiplexing (OFDM) systems to ease various performance characteristics. In this paper, we investigate the performance and complexity of one tap zero-forcing (ZF) and minimum mean-square error (MMSE) equalizers in oversampled OFDM systems. Theoretical analysis and simulation results show that oversampling not only reduces the noise at equalizer output but also helps mitigate ill effects of spectral nulls. One tap equalizers therefore yield improved symbol-error-rate (SER) performance with the increase in oversampling rate, but at the expense of increased system bandwidth and modest complexity requirements.