FX Correlator Research Articles

Optimization and commissioning of the EPIC commensal radio transient imager for the long wavelength array

ABSTRACT Next-generation aperture arrays are expected to consist of hundreds to thousands of antenna elements with substantial digital signal processing to handle large operating bandwidths of a few tens to hundreds of MHz. Conventionally, FX correlators are used as the primary signal processing unit of the interferometer. These correlators have computational costs that scale as $\mathcal {O}(N^2)$ for large arrays. An alternative imaging approach is implemented in the E-field Parallel Imaging Correlator (EPIC) that was recently deployed on the Long Wavelength Array station at the Sevilleta National Wildlife Refuge (LWA-SV) in New Mexico. EPIC uses a novel architecture that produces electric field or intensity images of the sky at the angular resolution of the array with full or partial polarization and the full spectral resolution of the channelizer. By eliminating the intermediate cross-correlation data products, the computational costs can be significantly lowered in comparison to a conventional FX or XF correlator from $\mathcal {O}(N^2)$ to $\mathcal {O}(N \log N)$ for dense (but otherwise arbitrary) array layouts. EPIC can also lower the output data rates by directly yielding polarimetric image products for science analysis. We have optimized EPIC and have now commissioned it at LWA-SV as a commensal all-sky imaging back-end that can potentially detect and localize sources of impulsive radio emission on millisecond timescales. In this article, we review the architecture of EPIC, describe code optimizations that improve performance, and present initial validations from commissioning observations. Comparisons between EPIC measurements and simultaneous beam-formed observations of bright sources show spectral-temporal structures in good agreement.

FX correlation trading: theory and practice

FX correlation trading: theory and practice

Dynamic allocations for currency investment strategies

This study conducts out-of-sample tests for returns on individual currency investment strategies and the weights on the universe of these strategies. We focus on five investment strategies: carry, momentum, value, dollar carry, and conditional FX correlation risk. The performances of our predictive models are evaluated using both statistical and economic measures. Within a dynamic asset allocation framework, an investor adjusts investment strategy weights based on the results of the prediction models. We find that our predictive model outperforms our benchmark, which uses historical average information in terms of statistical and economic measures. When the Sharpe ratio of the benchmark model is 0.52, our predictive model generates an economic gain of approximately 1.16% per annum over the benchmark. These findings are robust to the changes in investors’ risk aversion and target volatility for portfolio optimization.

An Overview of CHIME, the Canadian Hydrogen Intensity Mapping Experiment

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a drift scan radio telescope operating across the 400–800 MHz band. CHIME is located at the Dominion Radio Astrophysical Observatory near Penticton, BC, Canada. The instrument is designed to map neutral hydrogen over the redshift range 0.8–2.5 to constrain the expansion history of the universe. This goal drives the design features of the instrument. CHIME consists of four parallel cylindrical reflectors, oriented north–south, each 100 m × 20 m and outfitted with a 256-element dual-polarization linear feed array. CHIME observes a two-degree-wide stripe covering the entire meridian at any given moment, observing three-quarters of the sky every day owing to Earth’s rotation. An FX correlator utilizes field-programmable gate arrays and graphics processing units to digitize and correlate the signals, with different correlation products generated for cosmological, fast radio burst, pulsar, very long baseline interferometry, and 21 cm absorber back ends. For the cosmology back end, the correlation matrix is formed for 1024 frequency channels across the band every 31 ms. A data receiver system applies calibration and flagging and, for our primary cosmological data product, stacks redundant baselines and integrates for 10 s. We present an overview of the instrument, its performance metrics based on the first 3 yr of science data, and we describe the current progress in characterizing CHIME’s primary beam response. We also present maps of the sky derived from CHIME data; we are using versions of these maps for a cosmological stacking analysis, as well as for investigation of Galactic foregrounds.

Local correlation of delta-DOR signals with low signal-to-noise ratio and severe radio frequency interference

The Chinese very long baseline interferometry net (CVN) has applied the delta differential one-way ranging (delta-DOR) radiometric tracking technique to determine the orbit of a lunar and a deep-space spacecraft. In the past observations, CVN had sometimes encountered a difficult observation situation that the DOR signals were extremely weak or severely interfered. The CVN FX correlator, usually cannot process these extremely weak DOR signals. In order to enhance the weak signal processing capability of CVN, here we report an alternative method called ‘local correlation’. By this method, the distinct DOR signals structure features of the narrow bandwidth sinusoidal tone and their coherent phase, are used to extract the exact frequency and phase of each tone. We have adopted the method on the weak signal case, then compared with the results obtained from CVN FX correlator. Analysis result shows the local correlation method can process DOR tone at lower signal-to-noise ratios such as 1 dBHz. The method also has advantages of high anti-interference, low computation cost and high efficiency. Furthermore, the delta-DOR delay obtained from local correlation can improve the orbit accuracy and reduce the arc length required by the orbit determination after the trajectory course maneuver.

A cuBLAS-based GPU correlation engine for a low-frequency radio telescope

Abstract A low-frequency array is being set up to observe the Deuterium hyperfine line at 327.4 MHz in Mauritius, in the southern hemisphere. The array will be used to measure the total power of the Deuterium emission in the local galaxy in order to determine the D/H abundance ratio. Radio astronomy is a compute intensive discipline. With the advent of large interferometers the digital processing is predominantly done using parallel computing hardware. So our project represents an opportunity to make use of off-the-shelf parallel hardware such as gaming GPUs to establish a low-cost high performance pipeline for a radio telescope. It also constitutes a test-bed for the development of interferometric imaging techniques on parallel computing platforms. For this purpose a parallel GPU FX correlator was designed using the cuBLAS library. We made use of a function performing the batched multiplication of matrices to compute the full correlation in the frequency domain. The cuBLAS implementation is compared with the xGPU implementation and Roofline graphs are plotted for both using different instrument characteristics. The cuBLAS-based correlator might not be faster but represents an optimised solution quickly implemented with a very short development time. The data rates and performance achieved will enable real-time processing for the telescope.

Results of work of new spectropolarimeter for solar radio emission observations in the range 50–500 MHz

Ground-based observations within meter radio range are of importance for understanding processes occurring in the solar corona.
 We present concepts, block diagram, and results of first observations of the Solar Spectropolarimeter of Meter Range (SSMD), launched for solar observations in the range 50–500 MHz in April 2016. The main purpose of this work was to develop an up-to-date digital radio spectropolarimeter able to record the full Stokes vector for sporadic solar phenomena taking place in the 50–500 MHz range.
 We use a crossed log periodic antenna to detect solar radio emission. This antenna can simultaneously obtain horizontal and vertical polarization components. The main part of SSMD is a digital receiver based on an FX correlator architecture. We use the Fast Fourier Transform (FFT) algorithm based on the real-time pipeline circuit to construct amplitude dynamic spectra (intensity vs frequency and time).
 SSMD has 4608 frequency channels with 97.66 kHz channel bandwidth and 97.66 kHz spacing. Time resolution is 1 s. The spectropolarimeter has a 50–500 MHz range. It can record the full Stokes vector. At present, SSMD observes two of four parameters in regular mode (I and V). We have observational data since 2016. The catalog development is in progress.
 We plan to improve time and frequency characteristics, record all Stokes parameters, and conduct a calibration. We are working on providing access to the data archive via the Internet.

Результаты работы нового спектрополяриметра для наблюдения солнечного радиоизлучения в диапазоне 50–500 МГц

Ground-based observations within meter radio range are of importance for understanding processes occurring in the solar corona.
 We present concepts, block diagram, and results of first observations of the Solar Spectropolarimeter of Meter Range (SSMD), launched for solar observations in the range 50–500 MHz in April 2016. The main purpose of this work was to develop an up-to-date digital radio spectropolarimeter able to record the full Stokes vector for sporadic solar phenomena taking place in the 50–500 MHz range.
 We use a crossed log periodic antenna to detect solar radio emission. This antenna can simultaneously obtain horizontal and vertical polarization components. The main part of SSMD is a digital receiver based on an FX correlator architecture. We use the Fast Fourier Transform (FFT) algorithm based on the real-time pipeline circuit to construct amplitude dynamic spectra (intensity vs frequency and time).
 SSMD has 4608 frequency channels with 97.66 kHz channel bandwidth and 97.66 kHz spacing. Time resolution is 1 s. The spectropolarimeter has a 50–500 MHz range. It can record the full Stokes vector. At present, SSMD observes two of four parameters in regular mode (I and V). We have observational data since 2016. The catalog development is in progress.
 We plan to improve time and frequency characteristics, record all Stokes parameters, and conduct a calibration. We are working on providing access to the data archive via the Internet.

The correlation process in Very Long Baseline Interferometry

The correlation process is an important element of geodetic Very Long Baseline Interferometry (VLBI). For many decades there has been constant progress in the capabilities of geodetic VLBI correlators following technical development and the need for ever higher precision of the results. In this publication, we describe the concept and some of the important practical aspects of VLBI correlation for a better understanding of the general geodetic readership. Time-lag correlators provide the easiest approach to the concept and to the individual steps necessary for the operations. More modern approaches then use the path where the data streams are Fourier transformed first before the cross correlation is actually performed. These correlators are called FX correlators, in contrast to XF correlators, where the cross-multiplications are done before the Fourier transform. Both concepts lead to the same results, the cross power spectrum. Following the correlation process proper, the VLBI observables, time delay and delay rate, have to be found in a fringe fitting process.

New optimized ASIC multiplier in 28 nm CMOS for processing the X-part of FX correlator in radio interferometry

The choice of multiplier and accumulation unit determines the performance of digital signal processors, particularly in Big-Data correlators employed in radio interferometers. This work is targeted towards reducing the multiplier-size and delay in order to achieve low-power and compact-area for interferometer correlators such as in SKA and VLBI. For this purpose different fast and hardware efficient algorithms are compared and combined to achieve a new optimized ASIC multiplier design for performing correlation in radio interferometry. Further, the optimized new multiplier design is incorporated into our previous correlation processing cell designs of CMAC, CoSMAC and PE in Balu and Hasan: IEEE. Access. 5 25353–25364, 2017. The improvement in performance is verified using the GlobalFoundries 28 nm HPP standard cell library.

A digital correlator upgrade for the Arcminute MicroKelvin Imager

The Arcminute Microkelvin Imager (AMI) telescopes located at the Mullard Radio Astronomy Observatory near Cambridge have been significantly enhanced by the implementation of a new digital correlator with 1.2 MHz spectral resolution. This system has replaced a 750-MHz resolution analogue lag-based correlator, and was designed to mitigate the effects of radio frequency interference, particularly from geostationary satellites that contaminate observations at low declinations. The upgraded instrument consists of 18 ROACH2 Field Programmable Gate Array platforms used to implement a pair of real-time FX correlators -- one for each of AMI's two arrays. The new system separates the down-converted RF baseband signal from each AMI receiver into two 2.3 GHz-wide sub-bands which are each digitized at 5-Gsps with 8 bits of precision. These digital data streams are filtered into 2048 frequency channels and cross-correlated using FPGA hardware, with a commercial 10 Gb Ethernet switch providing high-speed data interconnect. Images formed using data from the new digital correlator show over an order of magnitude improvement in dynamic range over the previous system. The ability to observe at low declinations has also been significantly improved.

Peculiarities of Maser Data Correlation / Postcorrelation in Radioastron Mission

AbstractWe discuss specific aspects of space-ground VLBI (SVLBI) data processing of spectral line experiments (H2O & OH masers) in Radioastron project. In order to meet all technical requirements of the Radioastron mission a new software FX correlator (ASCFX) and the unique data archive which stores raw data from all VLBI stations for all experiments of the project were developed in Astro Space Center. Currently all maser observations conducted in Radioastron project were correlated using the ASCFX correlator. Positive detections on the space-ground baselines were found in 38 sessions out of 144 (detection rate of about 27%). Finally, we presented upper limits on the angular size of the most compact spots observed in two galactic H2O masers, W3OH(H2O) and OH043.8-0.1.

Software Correlator for Radioastron Mission

In this paper, we discuss the characteristics and operation of Astro Space Center (ASC) software FX correlator that is an important component of space–ground interferometer for Radioastron project. This project performs joint observations of compact radio sources using 10[Formula: see text]m space radio telescope (SRT) together with ground radio telescopes at 92, 18, 6 and 1.3 cm wavelengths. In this paper, we describe the main features of space–ground VLBI data processing of Radioastron project using ASC correlator. Quality of implemented fringe search procedure provides positive results without significant losses in correlated amplitude. ASC Correlator has a computational power close to real time operation. The correlator has a number of processing modes: “Continuum”, “Spectral Line”, “Pulsars”, “Giant Pulses”,“Coherent”. Special attention is paid to peculiarities of Radioastron space–ground VLBI data processing. The algorithms of time delay and delay rate calculation are also discussed, which is a matter of principle for data correlation of space–ground interferometers. During five years of Radioastron SRT successful operation, ASC correlator showed high potential of satisfying steady growing needs of current and future ground and space VLBI science. Results of ASC software correlator operation are demonstrated.

Using multiple-accumulator CMACs to improve efficiency of the X part of an input-buffered FX correlator

This paper presents the approach of using complex multiplier-accumulators (CMACs) with multiple accumulators to reduce the total number of memory operations in an input-buffered architecture for the X part of an FX correlator. A processing unit of this architecture uses an array of CMACs that are reused for different groups of baselines. The disadvantage of processing correlations in this way is that each input data sample has to be read multiple times from the memory because each input signal is used in many of these baseline groups. While a one-accumulator CMAC cannot switch to a different baseline until it is finished integrating the current one, a multiple-accumulator CMAC can. Thus, the array of multiple-accumulator CMACs can switch between processing different baselines that share some input signals at any moment to reuse the current data in the processing buffers. In this way significant reductions in the number of memory read operations are achieved with only a few accumulators per CMAC. For example, for a large number of input signals three-accumulator CMACs reduce the total number of memory operations by more than a third. Simulated energy measurements of four VLSI designs in a high-performance 28 nm CMOS technology are presented in this paper to demonstrate that using multiple accumulators can also lead to reduced power dissipation of the processing array. Using three accumulators as opposed to one has been found to reduce the overall energy of 8-bit CMACs by 1.4% through the reduction of the switching activity within their circuits, which is in addition to a more than 30% reduction in the memory.

Computationally Minimized X-Part for FX Correlator in Big-Data Interferometers

Big-data is a challenging domain for high-throughput digital signal processing (DSP), especially in global-projects like the square kilometer array. The composite input data rate for correlator in this system is more than 11 Tb/s, which immensely increases the memory requirement, complexity of correlation implementation and the overall power dissipation. This paper is focused on computational minimization as well as the improvement of energy efficiency in the complex architectural X-part of an FX correlator employed in large array radio telescopes. A dedicated correlator-multiplier block termed, correlator-system-multiplier-and-accumulator (CoSMAC) cell architecture is proposed, which produces two 16-b integer complex multiplications within the same clock period. The novel hardware optimization is achieved by utilizing the flipped mirror relationship (conjugate complex symmetry) between discrete Fourier transform (DFT) samples owing to the symmetry and periodicity of the DFT coefficient vectors (twiddle factors). In addition, using the proposed CoSMAC architecture a new processing element (PE) is designed to calculate both the cross- and auto-correlation functions within the same clock period. This paper describes how the arithmetic processing of three baseline calculations will be minimized in the X-part using the proposed novel algorithm and hardware design (CoSMAC and PE). In addition to optimizing the core processing elements, it is possible to eliminate nearly 50% of the usual memory requirement. The design has been synthesized using global foundries 28-nm HPP CMOS standard cells.

EOVSA Implementation of a Spectral Kurtosis Correlator for Transient Detection and Classification

We describe in general terms the practical use in astronomy of a higher-order statistical quantity called spectral kurtosis (SK), and describe the first implementation of SK-enabled firmware in the Fourier transform-engine (F-engine) of a digital FX correlator for the Expanded Owens Valley Solar Array (EOVSA). The development of the theory for SK is summarized, leading to an expression for generalized SK that is applicable to both SK spectrometers and those not specifically designed for SK. We also give the means for computing both the [Formula: see text] estimator and thresholds for its application as a discriminator of RFI contamination. Tests of the performance of EOVSA as an SK spectrometer are shown to agree precisely with theoretical expectations, and the methods for configuring the correlator for correct SK operation are described.

MITRA 2.0

We give the rationale of building a new innovative radio telescope in Mauritius. The instrument will tackle important astrophysical areas, such as the detection and imaging of the Deuterium line and of recombination lines, as well as bridging a gap in the technological area. The telescope will be composed of low cost dipole antennas, arranged in planar stations with a bandwidth of ∼200MHz. The Front-End Electronics will have GaS-HEMT technology Low Noise Amplifiers to constrain the value of the system temperature to a minimum. The signals will be digitized and Fast Fourier Transformed. The post-processing used will be dependent on the science observed. It could be a case of either accumulation or FX correlation.

On the architecture for the X part of a very large FX correlator using two-accumulator CMACs

This paper presents an improved input-buffer architecture for the X part of a very large FX correlator that optimizes memory use to both increase performance and reduce the overall power consumption. The architecture uses an array of two-accumulator CMACs that are reused for different pairs of correlated signals. Using two accumulators in every CMAC allows the processing array to alternately correlate two sets of signal pairs selected in such a way so that they share some or all of the processed data samples. This leads to increased processing bandwidth and a significant reduction of the memory read rate due to not having to update some or all of the processing buffers in every second processing cycle. The overall memory access rate is at most 75 % of that of the single-accumulator CMAC array. This architecture is intended for correlators of very large multi-element radio telescopes such as the Square Kilometre Array (SKA), and is suitable for an ASIC implementation.

Implementation of a direct-imaging and FX correlator for the BEST-2 array

A new digital backend has been developed for the BEST-2 array at Radiotelescopi di Medicina, INAF-IRA, Italy which allows concurrent operation of an FX correlator, and a direct-imaging correlator and beamformer. This backend serves as a platform for testing some of the spatial Fourier transform concepts which have been proposed for use in computing correlations on regularly gridded arrays. While spatial Fourier transform-based beamformers have been implemented previously, this is to our knowledge, the first time a direct-imaging correlator has been deployed on a radio astronomy array. Concurrent observations with the FX and direct-imaging correlator allows for direct comparison between the two architectures. Additionally, we show the potential of the direct-imaging correlator for time-domain astronomy, by passing a subset of beams though a pulsar and transient detection pipeline. These results provide a timely verification for spatial Fourier transform-based instruments that are currently in commissioning. These instruments aim to detect highly-redshifted hydrogen from the Epoch of Reionization and/or to perform widefield surveys for time-domain studies of the radio sky. We experimentally show the direct-imaging correlator architecture to be a viable solution for correlation and beamforming.

A Single-FPGA Real-Time Dual Beam-Former With FX Correlation Capabilities: First Results at the Nançay Radio Telescope With the FAN Antenna Array

A full-time beam-former for two independent antenna groups, with visibility computation capabilities at a slower rate, has been formerly designed on a single FPGA for the BAO-radio instrument, a radio telescope demonstrator for the study of dark energy by the observation of Baryonic Acoustic Oscillations (BAO) spectrum with HI probing technique. On the same hardware platform, a firmware, dedicated for the Focal Array at Nançay (FAN) project, can provide a full-time dual beam-forming on a single antenna array. It can process an incoming data flow of twelve channels, each organized as complex spectrum (2 × 8 bits) of 4096 frequencies with a 4 GbPS effective rate. The dual-beam capability of the system has been successfully tested on the FAN array with transits on the radio sources CasA and 3C123, the beam-former being configured by complex coefficients computed from a previous off-line software correlation on CasA. Recently, another beam-forming on 3C123 has been done after a calibration on CasA performed by the beam-former itself, set in correlation mode, and a source-tracking.