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1. A VOEvent-based automatic trigger system for the Murchison Widefield Array

   https://doi.org/10.1017/pasa.2019.40  

   Hancock, P. J.; Anderson, G. E.; Williams, A.; Sokolowski, M.; Tremblay, S. E.; Rowlinson, A.; Crosse, B.; Meyers, B. W.; Lynch, C. R.; Zic, A.; et al (January 2019, Publications of the Astronomical Society of Australia)

   Abstract The Murchison Widefield Array (MWA) is an electronically steered low-frequency (<300 MHz) radio interferometer, with a ‘slew’ time less than 8 s. Low-frequency (∼100 MHz) radio telescopes are ideally suited for rapid response follow-up of transients due to their large field of view, the inverted spectrum of coherent emission, and the fact that the dispersion delay between a 1 GHz and 100 MHz pulse is on the order of 1–10 min for dispersion measures of 100–2000 pc/cm 3 . The MWA has previously been used to provide fast follow-up for transient events including gamma-ray bursts (GRBs), fast radio bursts (FRBs), and gravitational waves, using systems that respond to gamma-ray coordinates network packet-based notifications. We describe a system for automatically triggering MWA observations of such events, based on Virtual Observatory Event standard triggers, which is more flexible, capable, and accurate than previous systems. The system can respond to external multi-messenger triggers, which makes it well-suited to searching for prompt coherent radio emission from GRBs, the study of FRBs and gravitational waves, single pulse studies of pulsars, and rapid follow-up of high-energy superflares from flare stars. The new triggering system has the capability to trigger observations in both the regular correlator mode (limited to ≥0.5 s integrations) and using the Voltage Capture System (VCS, 0.1 ms integration) of the MWA and represents a new mode of operation for the MWA. The upgraded standard correlator triggering capability has been in use since MWA observing semester 2018B (July–Dec 2018), and the VCS and buffered mode triggers will become available for observing in a future semester. 

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2. A Digital Non-Foster VHF Radio Approach for Enabling Low-Power Internet of Things

   https://doi.org/10.1109/ISCAS45731.2020.9180893  

   Weldon, Thomas P. (October 2020, 2020 IEEE International Symposium on Circuits and Systems (ISCAS))

   A digital non-Foster radio approach is proposed to mitigate Wheeler-Chu limits of electrically-small antennas, with significant potential to significantly reduce energy consumption in the VHF (very high frequency) band, where radio propagation losses below 200 MHz are 100 times less than losses above 2 GHz. Operation at lower frequency could greatly extend lifetimes of small low-power Internet-of-Things devices such as battery-powered sensors operating primarily as transmitters. Unfortunately, physical size constraints and the Wheeler-Chu limit have greatly hindered utilization of VHF bands for mobile devices, where even a 200 MHz half-wave dipole is an unwieldy 0.75 m. However, recent advances in non-Foster impedance matching methods have overcome these limits. In addition, recent digital non-Foster methods are shown to closely resemble digital radio architectures, suggesting that these newer digital non-Foster methods can be readily adopted in new digital radio designs. Therefore, a novel digital non-Foster radio architecture is proposed, where digital non-Foster methods enable small devices in energy-efficient VHF bands while overcoming Wheeler-Chu antenna-size limits. 

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3. GaLactic and Extragalactic All-sky Murchison Widefield Array survey eXtended (GLEAM-X) I: Survey description and initial data release

   https://doi.org/10.1017/pasa.2022.17  

   Hurley-Walker, N.; Galvin, T. J.; Duchesne, S. W.; Zhang, X.; Morgan, J.; Hancock, P. J.; An, T.; Franzen, T. M.; Heald, G.; Ross, K.; et al (January 2022, Publications of the Astronomical Society of Australia)

   Abstract We describe a new low-frequency wideband radio survey of the southern sky. Observations covering 72–231 MHz and Declinations south of $$+30^\circ$$ have been performed with the Murchison Widefield Array “extended” Phase II configuration over 2018–2020 and will be processed to form data products including continuum and polarisation images and mosaics, multi-frequency catalogues, transient search data, and ionospheric measurements. From a pilot field described in this work, we publish an initial data release covering 1,447 $$\mathrm{deg}^2$$ over $$4\,\mathrm{h}\leq \mathrm{RA}\leq 13\,\mathrm{h}$$ , $$-32.7^\circ \leq \mathrm{Dec} \leq -20.7^\circ$$ . We process twenty frequency bands sampling 72–231 MHz, with a resolution of 2′–45 ′′ , and produce a wideband source-finding image across 170–231 MHz with a root mean square noise of $$1.27\pm0.15\,\mathrm{mJy\,beam}^{-1}$$ . Source-finding yields 78,967 components, of which 71,320 are fitted spectrally. The catalogue has a completeness of 98% at $${{\sim}}50\,\mathrm{mJy}$$ , and a reliability of 98.2% at $$5\sigma$$ rising to 99.7% at $$7\sigma$$ . A catalogue is available from Vizier; images are made available via the PASA datastore, AAO Data Central, and SkyView. This is the first in a series of data releases from the GLEAM-X survey. 

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

   https://doi.org/10.1093/mnras/stad263  

   Krishnan, Harihanan; Beardsley, Adam P.; Bowman, Judd D.; Dowell, Jayce; Kolopanis, Matthew; Taylor, Greg; Thyagarajan, Nithyanandan (January 2023, Monthly Notices of the Royal Astronomical Society)

   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. 

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5. An Outdoor Experimental Study of Many Antenna Full-Duplex Wireless

   Hosseini, H.; Almutairi, A.; Hashir, S. M.; Aryafar, E.; Camp, J. (October 2023, International Teletraffic Congress)

   Full-duplex (FD) wireless communication refers to a communication system in which both ends of a wireless link transmit and receive data simultaneously and on the same frequency band. One of the major challenges of FD communication is self-interference (SI), which refers to the interference caused by transmitting elements of a radio to its own receiving elements. Fully digital beamforming is a technique used to conduct beamforming and has been recently repurposed to also reduce SI. However, the cost of fully digital systems (e.g., base stations) dramatically increases with the increase in the number of antennas as these systems use a separate Tx-Rx RF chain for each antenna element. Hybrid beamforming systems use a much smaller number of RF chains to feed the same number of antennas, and hence can significantly reduce the deployment cost. In this paper, we aim to quantify the performance gap between these two radio architectures in terms of SI cancellation and system capacity in FD multi-user MIMO setups. We first obtained over-the-air channel measurement data on two outdoor massive MIMO deployments over the course of three months. We next study two state-of-the-art transmit beamforming based FD systems for fully digital and hybrid architectures. We show that the hybrid beamforming system can achieve 80-97% of the fully digital system capacity, depending on the number of clients. 

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