Baseband Data Research Articles

Tied-array beam localization of radio transients and pulsars

Abstract Multi-element interferometers such as MeerKAT, which observe with high time resolution and have a wide field of view, provide an ideal opportunity to perform real-time, untargeted transient and pulsar searches. However, because of data storage limitations, it is not always feasible to store the baseband data required to image the field of a discovered transient or pulsar. This limits the ability of surveys to effectively localize their discoveries and may restrict opportunities for follow-up science, especially of one-off events like some fast radio bursts. Here, we present a novel maximum-likelihood estimation approach to localizing transients and pulsars detected in multiple MeerKAT tied-array beams at once, which we call tied-array beam localization, as well as a Python implementation of the method named SeeKAT. We provide real-world examples of SeeKAT’s use as well as a Monte Carlo analysis to show that it is capable of localizing single pulses detected in beamformed MeerKAT data to (sub)arcsec precision.

Research on a Coherent Dedispersion Algorithm for Pulsar Baseband Data

When the pulsar signal propagates in the interstellar medium (ISM), the high frequency and low frequency components of the signal reach the radio telescope with a certain delay. Therefore, the pulsar signal will appear energy dispersion, which will broaden the pulse profile, decrease the signal to noise ratio, and even lead to the disappearance of the pulse signal. In this paper, we analyze the sampling, polarization and arrangement of baseband data based on the coherent dedispersion algorithm for the problem of pulsar baseband data dedispersion. We systematically study the coherent dedispersion data processing procedure, and test the pulse profile changes under different FFT block sizes. An optimal selection strategy of FFT block sizes is proposed for reducing the operation time and obtaining a better pulse profile. We propose two methods, one is the generation of ISM transfer function, the other is the pulsar period and phase prediction method at a certain time, and discuss integral and folding strategies. We test the algorithm based on the baseband data of CASPSR and Medusa terminals observed by the Parkes 64 m radio telescope, and analyze the reading and processing methods of baseband data of different terminals. The experimental results show that the phase and amplitude information of the pulse profile processed by our algorithm is basically consistent with the results obtained by DSPSR.

Broadband Optical Heterodyne Millimeter-Wave-over-Fiber Wireless Links Based on a Quantum Dash Dual-Wavelength DFB Laser

We demonstrate real-time broadband multi-Gb/s electrical RF synthesizer-free millimeter-wave (MMW) signals generation and wireless transmission at the 5G new radio (NR) frequency band of 47 GHz based on analog radio-over-fiber (A-RoF) fronthaul. This is enabled by a low noise, highly correlated, monolithic C-band semiconductor InAs/InP quantum-dash (QDash) dual-wavelength distributed feedback (DW-DFB) laser. One laser mode is encoded using 4-/6-GBaud multilevel quadrature amplitude modulation (M-QAM) (16-/32-/64-QAM) baseband data signals, the other lasing mode is used as an optical local oscillator for optical-heterodyne remote up-conversion to a MMW carrier of 47.27 GHz. Consequently, optical baseband modulated data signals with data capacity up to 36 Gb/s (6-GBaud × 64-QAM) are transmitted through back-to-back (BtB) and 25-/50-km of standard single mode fiber (SSMF) before the MMW carrier is optically synthesized remotely for free space wireless data transmission and detection over up to 9-m. The end-to-end MMW-over-fiber (MMWoF) wireless link is thoroughly characterized exhibiting promising error-vector-magnitude (EVM) and bit-error-rate (BER) values. The 4-/6-GBaud 16-QAM MMWoF wireless links achieve EVMs down to 6.32%/7.33%, 6.71%/7.78%, and 7.35%/8.91% through BtB, 25-km, and 50-km SSMF, respectively. Similarly, the EVM for 32-QAM and 64-QAM links is observed to be 5.56%/6.56% and 6.05%/6.62%, respectively. Moreover, in each case, the calculated BER is below the forward error correction (FEC) limit of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . The results corroborate the potential and viability of the QDash DW-DFB laser as a simple, efficient and cost-effective alternative to individual laser sources for deployment in broadband photonic MMWoF fronthaul systems of 5G wireless networks.

High Performance Real-time Odd-even Channels Combination Algorithm for Transmission Baseband Data Using Dual-polarized Technology

数传基带奇偶合路实时处理，是极化复用方式下卫星过境弧段即时判读星载设备状态及获取空间科学探测成果的前提和基础。面向空间科学卫星高速数传实时处理需求，针对传统合路处理方法受限于内存拷贝与排序操作导致处理性能较低的问题，提出一种使用环形队列对数据进行内存管理并基于各虚拟信道奇偶单路传输帧计数自然有序特征进行合路处理的新方法，有效规避频繁的内存移动与复杂耗时的排序操作，显著提升奇偶合路实时处理性能。空间科学卫星任务的工程实践结果表明，所提方法的实时合路性能满足ASO-S卫星1 Gbit·s^–1和CASEarth卫星1.6 Gbit·s^–1的高速数传实时处理需求。

FRB 190520B – An FRB in a young supernova remnant?

ABSTRACT FRB 190520B, a repeating FRB near-twin of FRB 121102, has a dispersion measure excess over the intergalactic and Galactic contributions of about 900 pc cm−3, attributable to its host galaxy or near-source environment. This excess varies on a time-scale of $\sim30$ yr and might be explained by a supernova remnant no more than a few decades old. A magnetic field in equipartition with the remnant’s expansion would be $\sim1\, \rm{G}$. I suggest the use of baseband data to measure very large rotation measures.

Design of a multi-function high-speed digital baseband data acquisition system

A multi-function digital baseband data acquisition system is designed for the sampling, distribution and recording of wide-band multi-channel astronomical signals. The system hires a SNAP2 board as a digital baseband converter to digitize, channelize and packetize the received signal. It can be configured dynamically from a single channel to eight channels with a maximum bandwidth of 4096 MHz. Eight parallel HASHPIPE instances run on four servers, each carrying two NVMe SSD cards, achieving a total continuous write rate of 8 GB s−1. Data are recorded in the standard VDIF file format. The system is deployed on a 25-meter radio telescope to verify its functionality based on pulsar observations. Our results indicate that during the 30-minute observation period, the system achieved zero data loss at a data recording rate of 1 GB s−1 on a single server. The system will serve as a verification platform for testing the functions of the QTT (QiTai radio Telescope) digital backend system. In addition, it can be used as a baseband/VLBI (Very Long Baseline Interferometry) recorder or D-F-engine of correlator/beamformer as well.

Transparent multichannel wireless bridge for optical fiber links based on a two-stage upconversion

A bridge architecture enabling seamless wireless interconnection within an optical fiber link is studied. Optical baseband data is remodulated by local oscillators using two stages to allow efficient and cost-effective upconversion. Proof-of-concept experiments show the feasibility of the approach. Direct operation with standard optical signals and ease of integration in the conventional optical fiber infrastructure define the transparency property of this subsystem, which represents a potential solution to restore data traffic in scenarios where fiber deployment is uneconomical or a disaster has taken place.

Profile changes associated with dispersion measure events in PSR J1713+0747

ABSTRACT Propagation effects in the interstellar medium and intrinsic profile changes can cause variability in the timing of pulsars, which limits the accuracy of fundamental science done via pulsar timing. One of the best timing pulsars, PSR J1713+0747, has gone through two ‘dip’ events in its dispersion measure (DM) time series. If these events reflect real changes in electron column density, they should lead to multiple imaging. We show that the events are well fitted by an underdense corrugated sheet model, and look for associated variability in the pulse profile using principal component analysis. We find that there are transient pulse profile variations, but they vary in concert with the DM, unlike what is expected from lensing due to a corrugated sheet. The change is consistent in shape across profiles from both the Green Bank and Arecibo radio observatories, and its amplitude appears to be achromatic across the 820-MHz, 1.4-GHz, and 2.3-GHz bands, again unlike expected from interference between lensed images. This result is puzzling. We note that some of the predicted lensing effects would need higher time and frequency resolution data than used in this analysis. Future events appear likely, and storing baseband data or keeping multiple time–frequency resolutions will allow more in-depth study of propagation effects and hence improvements to pulsar timing accuracy.

Development of Large Capacity FM Telemetry System for Long March 5 Launch Vehicle

The telemetry system is responsible for obtaining the telemetry data of the whole rocket flight, which is the key to evaluate the comprehensive performance of the rocket afterwards. This paper introduces the overall scheme and composition of a new generation of large capacity FM telemetry system applied to Long March 5 carrier rocket. Focusing on the problems faced by the application of the key technology of 10 Mbps high bit rate FM telemetry, the technical breakthrough and flight test result are given from the aspects of the application of FM telemetry enhancement technology, the synthesis and transmission of hierarchical baseband data and the high gain antenna feed. Combined with the development process, the development experience and application of a new generation of large capacity FM telemetry system for rockets are summarized.

10-Gbps real-time wireless link over 1.5 km at 220-GHz band based on Schottky-diode transceiver and 16-QAM modulation

A wireless link working at 220-GHz band is proposed for point-to-point high-speed communication. The system is primarily composed of sub-terahertz transceivers and baseband data processing platforms. Subharmonic mixers based on Schottky diode are developed and measured with a double-sideband (DSB) noise temperature of lower than 1100 K from 205 GHz to 230 GHz. High-selectivity waveguide bandpass filters are designed to reject the image sideband noise from entering the receiver. Two 58-dBi Cassegrain antennas are used to compensate for the large energy losses contributed by free space path and atmospheric absorption. Besides, sub-THz amplifiers based on InP HEMT technology are developed to further increase the transmitting power. To improve spectrum efficiency, 16 quadrature amplitude modulation is adopted. Forward error correction is implemented by using Reed-Solomon codes to reinforce the reliability of signal transmission. An outdoor wireless link is finally established within 1.5 km, and a data rate of 10 Gbps is achieved in real-time mode with an EVM of 9.53% (BER 1.4×10-9). For demonstration, uncompressed ultra-HD videos are successfully transmitted. Note that any auxiliary electronic testing and measurement equipment is not required while communicating. It is believed that this article provides an effective and practical solution for future long-distance high-speed communication.

An encrypted multitone modulation method for physical layer security based on chaotic cryptography

This paper proposes an encrypted multitone modulation method to achieve low probability of being deciphered for physical layer security (PLS) of communication based on chaotic cryptography. The main objective of the proposed method is to map random, unpredictable, many-to-one relations between a multitone group (MTG) and baseband data. After the baseband data are mapped to several types of MTGs within the available frequency band with the mapping relation, the transmitter first determines the MTG type according to the data to be sent, and then the definite MTG is selected according to the MTG index sequence. Obviously, under this modulation scheme, the mapping relationship and MTG index sequence determine the distribution of frequency points. Inspired by the characteristics of this modulation, the long-term statistical undifferentiated features of different types of MTGs are obtained by encrypting the mapping relationships, and then the index sequence is designed to realize a roughly equal probability of being used for each MTG type. To accurately decipher the baseband data information, the cooperative receiver needs to master the mapping relationship and related secret keys. After establishing the communication system model under this modulation mode, the anti-non-cooperative reception deciphering ability of the system and its performance are analyzed theoretically. The reliability and concealment of the proposed method are experimentally verified and compared with the reliability and security of the orthogonal frequency-division multiplexing- (OFDM) quadrature phase-shift keying (QPSK) and differential frequency hopping (DFH) modulation methods under the same number of subcarriers and channel environments.

Considerations of VLBI transmitters on Galileo satellites

For directly linking the dynamical reference frame of satellite orbits to the quasi-inertial reference frame of extra-galactic radio sources, observations of satellites with the Very Long Baseline Interferometry (VLBI) technique are the only conceivable method. Hence, the satellite observations should be embedded in VLBI network sessions during which also natural radio sources are observed. For this reason, it would be most practical if the artificial signal generated at the satellite for VLBI observations covers the same frequency bands as regularly observed by VLBI radio telescopes and should have a similar flux density across the observed bandwidth as these natural sources. The use of satellites of Global Navigation Satellite Systems (GNSS) such as the Galileo system is advisable because they are well monitored in terms of precise orbit determination and the altitude allows common visibilities of many VLBI telescopes. So far, signal generation on a GNSS satellite dedicated to VLBI observations has not been realized yet, partly because suitable signal generation equipment has not been considered in depth. In addition, many aspects, such as legal implications and technical complications, have not yet been addressed. In this publication, we compiled various aspects of generating an artificial VLBI signal on a GNSS satellite. We describe the legal and technical aspects of generating and emitting an artificial signal on a Galileo satellite suitable for VLBI observations including a design study for the necessary equipment on the satellite. Since geodetic VLBI is currently in a transition period from traditional observations at S and X band to the broadband VLBI Global Observing System (VGOS), the proposed equipment generates a signal suitable for both frequency setups. We have also considered the restrictions for installation on a satellite, such as power consumption, weight, and size. The equipment mainly consists of three devices: noise source, amplifier, and antenna. A diode is used as the noise source. This noise is amplified by a set of low noise amplifiers and then radiated by a spiral antenna. The diode and the amplifiers were chosen from the market, but the antenna was newly designed and simulated. The output signal of this chain was tested using a VLBI baseband data simulator, then correlated and fringe-fitted for validation. The instrumentation proposed here is easy to be constructed, but will still have to be tested in the laboratory together with the instruments on the actual satellite.

A Novel Baseband Generation Method for Modeling RF Power Amplifiers for Bit Error Rate Computations

In this article, we present a novel method for modeling radio frequency (RF) power amplifiers (PAs). The proposed method for baseband data generation introduced in this paper, enables us to specify operational power range and bandwidth criteria of the PA, yielding a generic power amplifier model, which includes both memory effects as well as non-linearity of the PA. Further, we show that, the modeling approach is useful for bit error rate (BER) computations with PA distortions for any digitally modulated signal. Estimated BER using the proposed PA model for the test case of QPSK and 16-QAM, shows a significant difference in BER at high input back-offs due to memory effects, compared to the conventional non-linear PA models.

Channel Sounding and Scene Classification of Indoor 6G Millimeter Wave Channel Based on Machine Learning

Millimeter wave, especially the high frequency millimeter wave near 100 GHz, is one of the key spectrum resources for the sixth generation (6G) mobile communication, which can be used for precise positioning, imaging and large capacity data transmission. Therefore, high frequency millimeter wave channel sounding is the first step to better understand 6G signal propagation. Because indoor wireless deployment is critical to 6G and different scenes classification can make future radio network optimization easy, we built a 6G indoor millimeter wave channel sounding system using just commercial instruments based on time-domain correlation method. Taking transmission and reception of a typical 93 GHz millimeter wave signal in the W-band as an example, four indoor millimeter wave communication scenes were modeled. Furthermore, we proposed a data-driven supervised machine learning method to extract fingerprint features from different scenes. Then we trained the scene classification model based on these features. Baseband data from receiver was transformed to channel Power Delay Profile (PDP), and then six fingerprint features were extracted for each scene. The decision tree, Support Vector Machine (SVM) and the optimal bagging channel scene classification algorithms were used to train machine learning model, with test accuracies of 94.3%, 86.4% and 96.5% respectively. The results show that the channel fingerprint classification model trained by machine learning method is effective. This method can be used in 6G channel sounding and scene classification to THz in the future.

An Analysis Pipeline for CHIME/FRB Full-array Baseband Data

Abstract The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has become a leading facility for detecting fast radio bursts (FRBs) through the CHIME/FRB backend. CHIME/FRB searches for fast transients in polarization-summed intensity data streams that have 24 kHz spectral and 1 ms temporal resolution. The intensity beams are pointed to predetermined locations in the sky. A triggered baseband system records the coherent electric field measured by each antenna in the CHIME array at the time of FRB detections. Here we describe the analysis techniques and automated pipeline developed to process these full-array baseband data recordings. Whereas the real-time FRB detection pipeline has a localization limit of several arcminutes, offline analysis of baseband data yields source localizations with subarcminute precision, as characterized by using a sample of pulsars and one repeating FRB with known positions. The baseband pipeline also enables resolving temporal substructure on a microsecond scale and the study of polarization including detections of Faraday rotation.

Training Images Generation for CNN Based Automatic Modulation Classification

Convolutional neural network (CNN) models have recently demonstrated impressive classification and recognition performance on image and video processing scope. In this paper, we investigate the application of CNN to identifying modulation classes for digitally modulated signals. First, the received baseband data samples of modulated signal are gathered up and transformed to generate the constellation-like training images for convolutional networks. Among the resulting training images, the proposed convolutional gray image is preferred for network training and inference because of the lower computational burden. Second, we propose to use a multiple-scale convolutional neural network (MSCNN) as the classifier. The skip-connection technique is deployed for mitigating the negative effect of vanishing gradients and overfitting during the network training process. Numerical simulations have been carried out to validate the effectiveness of the proposed scheme, the results show that the proposed scheme outperforms the traditional algorithms in terms of classification accuracy.

Modeling and simulation methods of sea clutter based on measured data

The modeling and simulation of sea clutter are important in detecting radar targets in sea backgrounds. Because the nonstationary property of sea clutter is ignored in traditional statistical models, a new method based on measured sea clutter is proposed in this paper. First, we convert the measured sea clutter data under different sea conditions [[Formula: see text]] into real amplitude [Formula: see text]. Instantaneous phase [Formula: see text] is then extracted from the coherent radar’s baseband data. Second, we select a candidate statistic model and estimate its parameters based on [Formula: see text] by utilizing maximum likelihood estimation. Finally, we generate random series [Formula: see text] using corresponding random data generator and then add instantaneous phase [Formula: see text] into [Formula: see text], i.e., [Formula: see text], to obtain simulated sea clutter series. Through a comparison of simulated sea clutter and measured sea clutter data via histogram, the validity of the proposed method is proved.

The Array of Long Baseline Antennas for Taking Radio Observations from the Sub-Antarctic

Measurements of redshifted 21[Formula: see text]cm emission of neutral hydrogen at [Formula: see text][Formula: see text]MHz have the potential to probe the cosmic “dark ages,” a period of the universe’s history that remains unobserved to date. Observations at these frequencies are exceptionally challenging because of bright Galactic foregrounds, ionospheric contamination, and terrestrial radio-frequency interference. Very few sky maps exist at [Formula: see text][Formula: see text]MHz, and most have modest resolution. We introduce the Array of Long Baseline Antennas for Taking Radio Observations from the Sub-Antarctic (ALBATROS), a new experiment that aims to image low-frequency Galactic emission with an order-of-magnitude improvement in resolution over existing data. The ALBATROS array will consist of antenna stations that operate autonomously, each recording baseband data that will be interferometrically combined offline. The array will be installed on Marion Island and will ultimately comprise 10 stations, with an operating frequency range of 1.2–125[Formula: see text]MHz and maximum baseline lengths of [Formula: see text][Formula: see text]km. We present the ALBATROS instrument design and discuss pathfinder observations that were taken from Marion Island during 2018–2019.

A Millimetre-Wave Radar-Based Fall Detection Method Using Line Kernel Convolutional Neural Network

Fall accidents are significant threats to the health and life of older people. When a millimetre-wave (mmWave) frequency modulated continuous wave (FMCW) radar is used for fall detection, the selected features for further classification can determine the detection performance. In this paper, a line kernel convolutional neural network (LKCNN) is proposed to process the baseband data directly to detect fall motions. This method utilizes the characteristic of a convolutional neural network (CNN) that it can learn to extract useful features during the training process. A data sample generation method is also proposed to generate multiple samples for the training process by utilizing the multiple receiving channels and sufficiently small pulse repetition time (PRT). The experiment results show that the proposed method can detect fall motions with high accuracy, sensitivity and specificity with fewer network parameters and less computation cost, which is meaningful in realizing an all-time indoor fall detection system.

Decentralized Signal Detection via Expectation Propagation Algorithm for Uplink Massive MIMO Systems

The existing centralized antenna array architecture and the corresponding centralized baseband processing based signal detection algorithms incur huge burden on the raw baseband data rates and chip input/output bandwidth for massive multiple-input multiple-output (MIMO) systems. In this paper, we propose a novel decentralized baseband processing based signal detection under the framework of the expectation propagation algorithm (EPA) for uplink massive MIMO systems. This architecture partitions the base station antennas into multiple independent antenna clusters, each associated with analog and digital modulation circuitry, independent radio-frequency chains, and computing hardwares. The antenna clusters only access local channel state information and require low communication bandwidth. In the proposed scheme, the decentralized processing unit executes local channel estimation and signal detection, and delivers the partial message to the centralized processing unit parallelly, where the message contains the local means and variances iteratively updated by the EPA detector from each antenna cluster. The centralized processing unit operates the message fusion based on the proposed message fusion rule for decoding operation. Simulation results illustrate that the proposed decentralized EPA signal detection approaches the conventional centralized one with marginal computational complexity overhead when the number of antenna clusters is small, and outperforms the state-of-the-art decentralized counterparts.