High SNR Conditions Research Articles

High-Precision Position Detection and Communication Fusion Technology Using Beacon Spread-Spectrum Modulation with Four-Quadrant Detector

In space laser communication, the wide divergence angle of beacon light leads to substantial spatial losses, compounded by background light and detector noise; this results in compromised precision in the detection of the beacon light position. To solve this problem, a high-precision detection technique and communication composite technology employing a four-quadrant detector (QD) with beacon spread-spectrum modulation are proposed. Pseudo-random sequences (PRNs) are employed to spread the beacon communication spectrum, with the spread-spectrum signal utilized to modulate the intensity of the transmitted beacon light at the transmitter end. At the receiver, QD photocurrent signals are cross-correlated with an identical PRN that is used for modulation. The strong auto-correlation properties of PRNs, which are uncorrelated with noise, enhance the output signal-to-noise ratio (SNR), enabling precise position detection and beacon communication under high-SNR conditions. Theoretical analysis is used to explore the effects of spreading gain on the sensitivity of system detection and the precision of position detection. The experimental results demonstrate that the beacon spread-spectrum modulation scheme effectively detects the position of the light spot. At a received optical power of −37 dBm and spreading sequence PRN depths of 1023, 127, and 31, the root-mean-square error (RMSE) values are 0.983 μm, 2.876 μm, and 7.275 μm, respectively. This corresponds to improvements of 14.96 dB, 10.29, dB, and 6.26 dB compared to direct detection precision (30.811 μm). Additionally, under an identical signal bandwidth, the sensitivity improves by 14.6 dB, 10.1 dB, and 6.4 dB, respectively. The proposed beacon spread-spectrum scheme mitigates the limitations of hardware reception sensitivity and position-detection precision, demonstrating its potential application in high-precision detection in long-distance interstellar laser communication.

Deep Learning at the Physical Layer for Adaptive Terahertz Communications

Wireless communications in the terahertz (THz) band will become a cornerstone of sixth-generation (6G) networks. The THz channel, however, presents several challenges, such as distance-dependent absorption coefficients that can change the bandwidth significantly in case of mobility. Thus, future THz transmitters will have to switch modulation and bandwidth almost continuously. Moreover, using the same transmission scheme can enable adversaries to leverage smart interfering to inflict more damage with less energy expense. To help enable adaptive and secure THz communications, this article presents the first ever experimental study of modulation and bandwidth classification (MBC) at THz frequencies through deep learning (DL) techniques. We have performed an extensive experimental data collection campaign at 120 GHz with different modulation schemes, signal bandwidth (up to 20 GHz), and different signal-to-noise ratio (SNR) levels. We prove for the first time the feasibility and effectiveness of MBC at THz frequencies, with our DL models reaching accuracy up to 78% and 90% in low- and high-SNR conditions. Furthermore, we investigate the memory and latency constraints that need to be satisfied as a function of the signal bandwidth, and propose a boosting technique to improve the inference quality by trading off latency for accuracy. Finally, we experimentally evaluate the latency of our CNN models through FPGA implementation.

Exploiting hybrid non‐orthogonal multiple access in satellite–terrestrial systems with randomly deployed users

In this letter, a hybrid non-orthogonal multiple access (NOMA) scheme for satellite–terrestrial systems is proposed, which is capable of integrating both power domain NOMA (PD-NOMA) and code domain NOMA (CD-NOMA) design modes. The stochastic geometry method is considered to model the position of the terrestrial NOMA user. Specifically, the closed-form expressions of exact outage probability with imperfect interference cancellation are derived. In order to get more insights, the asymptotic outage probabilities for high SNR conditions are also discussed. Numerical results are provided to confirm the validity of theoretical analysis and reveal the effects of critical parameters on system performance.

Fairness-Oriented Transmission Schemes for Multiuser MISO Broadcast Channels

With the increasing number of Internet of Things (IoT), Industry 4.0 (I4.0), and mobile devices, it can be expected that base stations will have to serve more and more clients with a limited number of antennas. For their broadcast channels, nonorthogonal multiple access (NOMA) and blind interference alignment (BIA) are two efficient and commonly adopted transmission schemes. This paper conducts a comparison study on these techniques on a 3-user 2 × 1 multiple-input single-output (MISO) broadcast channel with a limited number of transmit antennas. Specifically, space-time block coding based NOMA (STBC-NOMA) and NOMA-assisted beamforming (NOMA-BF) are compared with BIA. Both perfect and imperfect successive interference cancellation (SIC) have been considered for NOMA-based schemes, and the theoretical achievable rates of all schemes have been derived. Furthermore, with a given fairness constraint among end users, the power allocation (PA) problems have been solved for cases when accurate channel state information is available at the transmitter (CSIT) as well as when only path loss information is available. Numerical results show the following: (1) none of the schemes under this study can always outperform the others under different SNR regions. (2) With imperfect SIC, NOMA-BF, and STBC-NOMA both suffer from a significant performance loss under a high SNR condition. (3) Fairness PA with only path loss information provides similar performance as that with perfect CSIT, thus partial CSIT is adequate for system or scheme designs in practice.

Optical OFDM Error Floor Estimation by Means of OTDR Enhanced by Front-End Optical Preamplifier.

Optical time-domain reflectometer (OTDR) enables simple identification and localization of a plethora of refractive and reflective events on a fiber link, including splices, connectors and breaks, and measuring insertion/return loss. Specifically, large enough OTDR dynamic range (DR) and thus high signal-to-noise-ratio (SNR) enable clear far-end visibility of longer fibers. We point out here that, under such conditions, the optical bit-error-rate (BER) floor is dominantly determined by reflective events that introduce significant return loss. This complements the OTDR legacy tests by appropriate optical BER floor estimation in the field. As high SNR implies inter-symbol interference as dominating error generating mechanism, we could apply the classical time-dispersion channel model for the optical BER floor determined by the root-mean-square (rms) delay spread of the actual fiber channel power-delay profile. However, as the high-SNR condition is not always fulfilled mostly due to insufficient DR, we propose here inserting a low-noise optical preamplifier as the OTDR front-end to reduce noise floor and amplify the backscattered signal. In order to verify the model for the exemplar test situation, we measured BER on the same fiber link to find very good matching between the measured BER floor values and the ones predicted from the OTDR trace.

A Simulation Study for Three Dimensional Tomographic Field Free Line Magnetic Particle Imaging.

Magnetic Particle Imaging (MPI) is an emerging modality that images the magnetic nanoparticle distribution inside the body. The method is based on the non-linear response of the magnetic nanoparticles to an applied magnetic field. In this study, we present simulation results for three-dimensional (3D) tomographic imaging using an open-bore MPI system that can electronically scan a field free line (FFL). A field of view with 26×26×10 mm3 volume is imaged with a relatively low gradient field of 0.5 T/m. Imaging results for two 3D phantoms are presented: a letter phantom and a vessel phantom with stenosis regions. Using the system-matrix based reconstruction approach, the images were obtained with the Algebraic reconstruction technique (ART) and alternating direction method of multipliers (ADMM) methods. The stenosis regions were visually recognizable in high SNR conditions with ADMM. The effect of low gradient strength became prominent with increasing noise level, resulting in interlayer coupling artifacts.Clinical relevance- Magnetic Particle Imaging (MPI) is a new tracer-based imaging modality with high-spatiotemporal resolution. MPI can map quantitative distribution of super-paramagnetic iron oxide nanoparticles introduced inside the body. A field free line scanning MPI system with an open configuration can enable imaging of human-size volumes for interventional operations. In this study, we present simulation results for an FFL scanning open MPI system configuration to scan 3D field of view volume electronically. We analyze 3D imaging performance for various SNR levels at a low gradient value of 0.5 T/m that is relevant for clinical-size systems.

Multiple sub filter based proportionate filtering for nonlinear acoustic echo cancellation

This paper presents a multiple sub-filter-based improved nonlinear acoustic echo cancellation (NAEC) framework to enhance the echo cancellation performance of the NAEC in the presence of nonlinear distortion. The proposed algorithm uses a novel combination of adaptive multiple sub-filter approach and proportionate filtering to enhance the convergence rate of the existing proportionate functional link-based NAEC algorithm by reducing the adaption time needed for updating the coefficients of the adaptive filter. In addition to that, the convergence and the steady-state analysis of the proposed algorithm are presented. The proposed NAEC framework is subjected to speech signal input corrupted with both white as well as colored noise as background noise and the colored noise input at low-to–high SNR conditions for a comprehensive analysis of the echo cancellation performance. The experimental results comprising of the echo return loss enhancement, spectrograms, and the perceptual evaluation of speech quality demonstrate the improvements brought by the proposed algorithm. At all signal-to-noise ratio conditions, the proposed algorithm has shown a 4 dB improvement in mean echo return loss enhancement compared to the existing algorithms validating the improvement in the proposed NAEC scheme’s performance.

Aging effects on categorical perception of Mandarin tones in noise

The current study aimed at investigating the aging effect on the categorical perception of Mandarin lexical tones with varied fundamental frequency (F0) contours in noise. Mandarin tone identification and tone discrimination in quiet and noise were measured for younger and older listeners in a categorical perception paradigm where the stimuli continua varied their F0 contour systematically from the level to the rising/falling tones. Results reported that older listeners, in contrast with their younger counterparts, performed with less stimulus-tuned changes in both identification and discrimination functions and smaller peakedness in the discrimination function for both level-rising and level-falling tones. The aging effects on Mandarin tone categoricality were observed in both quiet and noise. Moreover, noise aggravated the aging effects, especially with the high SNR condition. Plus, older listeners’ identification and discrimination functions in CP paradigm positively correlated with their performance in general speech identification in noise; such correlation was not found with younger listeners. Our study suggested that older listeners had less categoricality in both identification and discrimination functions for the level-rising and level-falling tones, probably due to the aging-related decline in temporal processing.

A Survey on Multi-Layer Mimo for Lte/Lte-Advanced

The primary intention of the paper is to explore on various Multi-Layer MIMO approaches for the higher system performance. MIMO technique is said to be the key technology in LTE-A to achieve the spatial diversity in the system. Multi-Layer MIMO is the enhancement to the MIMO technology where multiple streams of data could be transferred to different layers for optimization with increased capacity in the network in high SNR condition. In a massive MIMO system, the cell interference hinders the system performance due to high channel dimensionality. The main design objective of the Multi-Layer MIMO techniques studied is to reduce the Cell Interference with lower complexity.

A Bayesian Approach to Adaptive RAKE Receiver

The RAKE receivers are widely used in code division multiple access communication systems to achieve anti-multipath fading. However, a traditional RAKE receiver requires pilot data stream inserted into the sequence, which occupies channel resources and limits its applications. In this paper, a new adaptive RAKE receiver based on Bayesian theory is reported that only uses received signals to estimate its channel parameters. Observed data are used to obtain the information of the channel impulse response. Next, the prior information is used. In the iterative process, a priori information is accumulated to improve the receiver performance. Thus, the mean and covariance of the channel impulse response that is modeled as a complex and uncertain Gaussian random vector are recursively estimated using Bayesian theory. Finally, the RAKE weights are obtained using the mean and covariance. As shown in the simulation results, the bit error rate (BER) decreases as the number of fingers increases. The performance of the new RAKE receiver has been greatly improved compared with the all-RAKE receiver with maximal ratio combining, RAKE receiver with singular value decomposition, and RAKE receiver with fast approximated power iteration. Under medium to high SNR conditions (i.e., ≥−5 dB), the BER performance of the new RAKE receiver provides at least $3\times 10^{-4}$ less than that of the other receiver tested.

Calibration of non-Common path aberration using multi-Channel phase diversity technique

This paper provides a method of using multi-channel Phase Diversity technique for online calibration of non-common path aberration of AO system. The PD technique estimate the phase aberration by collecting one or more frames images with short time of exposure at focal plane and out-of-focus plane, meanwhile reconstruct the target image. Under the working condition of closed-loop AO system, the static aberration of imaging path can be detected online by PD algorithm and transfer the wavefront aberration coefficient into the initial form of the deformable mirror, thus compensate for the static aberration of the non-common optical path. The experiment result shows that the imaging quality is improved by a large margin after calibration, the FWHM of the point source target decreased from 8.90 to 7.64; the wavefront RMS of imaging path decreased from 0.133 λ to 0.033 λ, and PV deceased from 0.79 λ to 0.17 λ, in accordance with the wavefront aberration measured by the closed-loop. This fully testified the ability of the application in optical detection of the Phase Diversity. There is no need to modify the original adaptive optical path, and it can precisely solve the system aberration in the condition of high SNR, and is one of the ideal optical detection techniques of large-diameter optical imaging system.

Identification of radionuclides for the spectroscopic radiation portal monitor for pedestrian screening under a low signal-to-noise ratio condition

The spectroscopic radiation portal monitor (SPM) is widely used for homeland security. Many research groups are studying the radionuclide identification method which is one of the most important factors in the performance of the SPM using the large size of a thallium activated sodium iodide (NaI(Tl)) detector. In this study, we developed the radionuclide identification method for the SPM for pedestrian screening using a single NaI(Tl) detector that is small in size (2in.), which is much smaller than those in the existing studies under the low signal-to-noise-ratio (SNR) condition. From the anomalous radionuclide spectrum, the noise component was effectively reduced by the wavelet decomposition and the proposed background subtraction method, and the signal component was enhanced by the principal component analysis. Finally, peak locations which have been determined by the peak search algorithm with a valley check method were compared with a pre-calibrated and constructed radionuclide database. To verify the radiation identification performance of the proposed method, experiments with various kinds of sources (137Cs, 133Ba, 22Na, and 57Co) and different SNR values (from distances of 10–150cm and for scan times of 1–5s) were performed. Although the high-SNR condition was explored as well, most experiments were conducted under the low-SNR condition to verify the robustness and reproducibility of the proposed algorithm. The results showed that over 98.3% of the single radionuclide detection rate was achieved regardless of which radionuclides were used, up to 50cm under the worst SNR condition (1s of scan time) and up to 90cm under the best SNR condition (5s of scan time). Furthermore we achieved accurate identification of multiple radionuclides at 40cm with only 1s of scan time. The results show that the proposed algorithm is competitive with the commercial method and our radionuclide identification method can be successfully applied to the SPM for pedestrian monitoring, with a small detector size and a short scan time.

Research on Relay Selection Based on Two-Hop Multi-Relay Networks

A relay selection criteria was proposed based on time division non-orthogonal cooperation, aiming at solving the problem of relay selection in the two-hop multi-relay cooperative communication to predigest the system design. The system BER (bit error rate) expression was obtained with its performance analysis under the condition of high SNR. Furthermore, computer simulations was conducted to show that the system diversity order is increased with the increase of the relay number under the condition of the distributed relay selection criterion, and diversity gain can be obtained significantly with the BER of the system significant decreasing.

On the Robustness of Lattice Interference Alignment

A static (constant channel gains) real K-user interference channel is considered, where all interference (cross) channel gains are integers. For such channels, previous results demonstrate that the number of degrees of freedom is very sensitive to slight variations in the direct channel gains. In this paper, we derive an achievable rate region for such channels that is valid for finite SNR. At moderate values of SNR, the derived rate region is robust to slight variations in the direct channel gains. At asymptotic high SNR conditions, known results on the degrees of freedom are recovered. The new rate region is based on lattice interference alignment. The result is established via a new coding theorem for the two-user Gaussian multiple-access channel where both users use a single linear code.

Proportional Fair Scheduling Gains for AMC-Aware Systems under Heterogeneous Radio Conditions

This letter provides exact analytical expressions and simpler, but still effective, approximations for the multiuser diversity gains associated with Proportional Fair (PF) opportunistic scheduling. The impact of Adaptive Modulation & Coding (AMC) and of heterogeneous user populations with different radio conditions on the downlink are taken into account. The analysis is applicable to all major fading models and is based on the notion of the scheduling gain, a quantity that captures throughput and fairness aspects relevant to a user served under a considered scheduling policy as part of a heterogeneous group of users. Exploitation of relevant characteristics in the AMC rate function gives rise to two distinct regimes, corresponding to regular and very high average SNR conditions. All users in each of these regimes have approximately equal PF scheduling gains.

Multihop Analog Network Coding via Amplify-and-Forward: The High SNR Regime

In the simplest relaying strategy, a network node amplifies and forwards a received signal over a wireless channel. Multihop amplify-and-forward allows for a (noisy) linear combination of signals simultaneously sent from multiple sources to be propagated through the network over multiple layers of relays. The performance of multihop amplify-and-forward is limited by noise propagated to the destination over multiple hops, and we expect this strategy to perform well only in high SNR. In this paper, this intuition is formalized and high-SNR conditions under which multihop amplify-and-forward approaches capacity in a layered relay network are determined. By relating the received signal power and the received power of the propagated noise at the nodes, the rate achievable with multihop amplify-and-forward is determined. In particular, when all received powers are lower bounded by 1/δ, the noise power propagated to the destination over L layers is of the order Lδ. The result demonstrates that multihop amplify-and-forward approaches the cut-set bound as received powers at relays increase. As all powers in the network increase at the same rate, the multihop amplify-and-forward rate and the upper bound are within a gap that is independent of channel gains. This gap grows linearly with the number of nodes.

An Efficient Packet Detection Method with a Verification Mode in MB-OFDM UWB Systems

In this paper, we propose a novel packet detection method using a cross-correlation scheme with a verification mode in ultra wide-band (UWB) systems adopting multi-band OFDM (MB-OFDM) techniques. In the packet detection process, a cross-correlation scheme has the advantage of performing the acquisition of transmitted time-frequency code (TFC) for frequency hopping as well as packet detection with low hardware costs; however, it suffers from performance degradation caused by an imperfect decision process using a threshold value compared with an auto-correlation method. By using a verification mode, the proposed method improves detection performance of a cross-correlation technique considerably. This is because the verification mode can reduce a false alarm probability on packet detection by reconfirming whether or not the desired signal is received and can effectively reject temporal small correlation peaks due to the noise or sub-path signals having smaller energies. For the verification of proposed algorithm, we employ CM1 and CM4 channel models given in IEEE802.15.3a and compare the detection performance between conventional algorithms as well as the proposed one by using computer simulations. Experimental results show that the proposed scheme can reduce a packet detection error probability (PDER) to about 10?3 in CM1 channel and about 10?2 in CM4 channel at SNR = 9 dB compared with conventional methods having more than 10?1 PDER. From the experimental results, we can find that the proposed algorithm can compensate for performance degradation under high SNR condition caused by using a fixed single threshold value. We also estimate the power consumption of algorithms by utilizing both the total number of multiplications and additions employed in the algorithms and the consuming power of hardware elements under 0.13 um CMOS technology. Next, we discover that the proposed method can reduce computational complexity by a minimum of 24.5% compared with conventional cross-correlation algorithms. From the experimental and power estimation results, we can see that the proposed method is very useful for packet detection in UWB MB-OFDM systems, in which low-power implementation is an important issue.

Phase-coded pulse aperiodic transmitter coding

Abstract. Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.

Steady-state performance analysis of HOS-based blind adaptive multiuser detection

The blind adaptive multiuser detections based on higher-order statistics (HOS) can obtain higher steady-state decorrelating performance than the conventional linear algorithm under the high SNR condition. However, the closed-form analysis for this steady-state performance is scarce due to the complication of analyzing the nonlinear updates of the adaptive algorithm. An analysis approach based on ordinary differential equation (ODE) method is proposed to get the closed-form excess mean-square error (EMSE) expression of the HOS-based multiuser detections. The simulation and the comparison verify the results of the analysis.

A Modified Decorrelated Delay Lock Loop for Synchronous DS-CDMA Systems

This paper presents a modified decorrelated delay lock loop (MD-DLL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environments. The structure of the MD-DLL is similar to that of a traditional delay lock loop (T-DLL), aside from the fact that the despreading signals in T-DLL are replaced by a linear combination of all the users' code sequences with various weightings and timing offsets. The basic principles of MD-DLL are given as follows: 1) to use the decorrelating technique of a multiuser detector to remove the multiple-access interference at the on-time code position instant and 2) to choose a proper timing offset for the code sequences to reduce the weighting imbalance between the two arms of the tracking loop and suppress noise enhancement. The proposed MD-DLL scheme solves the tracking bias problems in a decorrelated delay lock loop under a low-signal-to-noise-ratio (SNR) condition. Computer simulations based on Gold code sequences are used to verify the findings. It is shown that MD-DLL achieves a better and stable S-curve with negligible tracking bias, better rms tracking error and mean-time-to-lose-lock performance under high-SNR conditions, and stronger near-far resistance than T-DLL.