Gregory Alan Wright

Optical and mechanical design of the Antarctic Submillimeter Telescope and Remote Observatory

Antarctic Submillimeter Telescope and Remote Observatory (AST/RO), a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 3000 μm, was installed at the South Pole during the 1994–95 Austral summer. The optical design is Gregorian, offset in both azimuth and elevation, with the exit pupil at the chopping tertiary mirror: this arrangement provides for consistent illumination of the primary mirror even when the beam is thrown one degree or more on the sky. Aberrations are minimized by the choice of secondary mirror offset angle. Alignment is accomplished by mechanical means. There is a Coude focus in a warm, spacious receiver room and also a Nasmyth focus. Both the elevation and azimuth axes are driven by two pinion gears with opposed torques to eliminate backlash. The encoders are unusually robust but have high friction, necessitating a stiff coupling. The azimuth limit switch scheme permits 1.5 rotations, but the switches will operate under extreme conditions with no single point of failure. The instrument is now operational with four heterodyne receivers and three acousto-optical spectrometers.

AST/RO Observations of Atomic Carbon near the Galactic Center

We present a coarsely sampled map of the region |l| ≤ 2°, |b| ≤ 01 in the 492 GHz (3P1 → 3P0) fine-structure transition of neutral carbon, observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). The distribution of [C I] emission is similar on the large scale to that of CO J = 1 → 0. On average, the ratio of the integrated intensities, I[C I]/I12CO, is higher in the Galactic disk than in the Galactic center region. This result is accounted for by the absorption of 12CO within the clouds located in the outer Galactic disk. The ratio I[C I]/I12CO is surprisingly uniform over the variety of environments near the Galactic center. On average, [C I] is optically thin [or as optically thin as 13CO (J = 1 → 0)], even in the dense molecular clouds of the Galactic center region.

Design of Energy- and Cost-Efficient Massive MIMO Arrays

Large arrays of radios have been exploited for beamforming and null steering in both radar and communication applications, but cost and form factor limitations have precluded their use in commercial systems. This paper discusses how to build arrays that enable multiuser massive multiple-input-multiple-output (MIMO) and aggressive spatial multiplexing with many users sharing the same spectrum. The focus of the paper is the energy- and cost-efficient realization of these arrays in order to enable new applications. Distributed algorithms for beamforming are proposed, and the optimum array size is considered as a function of the performance of the receiver, transmitter, frequency synthesizer, and signal distribution within the array. The effects of errors such as phase noise and synchronization skew across the array are analyzed. The paper discusses both RF frequencies below 10 GHz, where fully digital techniques are preferred, and operation at millimeter (mm)-wave bands where a combination of digital and analog techniques are needed to keep cost and power low.

Atomic Carbon Observations of Southern Hemisphere H II Regions

We report observations of atomic carbon (C I [3P1→3P0]) for a sample of 49 southern hemisphere H II regions using the Antarctic Submillimeter Telescope and Remote Observatory. The sources are compact and isolated members of the Wilson et al. H109α radio recombination line (RRL) catalog. The fourth Galactic quadrant is well covered by the sample. Atomic carbon emission is detected toward all of the regions, with multiple C I emission components found toward most sources. The RRL velocity is used to identify the C I emission associated with the H II region. We measure the mean velocity difference between the C I and RRL emission to be 0.8 ± 2.8 km s-1. Within the measurement errors this is exact agreement in velocity; we conclude that all H II regions have associated C I emission. The mean C I line temperature of these components is 2.4 ± 1.8 K, compared with 0.7 ± 0.7 K for the C I emission components not associated with the H II region. This suggests that C I intensity is dominated by local heating. The FWHM line width of C I gas associated with H II regions also is marginally greater than that found for unassociated gas (6.7 ± 3.0, compared with 4.8 ± 2.4 km s-1).

LARGE-SCALE CO MAPS OF THE LUPUS MOLECULAR CLOUD COMPLEX

Fully sampled degree-scale maps of the 13CO 2-1 and CO 4-3 transitions toward three members of the Lupus Molecular Cloud Complex?Lupus I, III, and IV?trace the column density and temperature of the molecular gas. Comparison with IR extinction maps from the c2d project requires most of the gas to have a temperature of 8-10 K. Estimates of the cloud mass from 13CO emission are roughly consistent with most previous estimates, while the line widths are higher, around 2 km s?1. CO 4-3 emission is found throughout Lupus I, indicating widespread dense gas, and toward Lupus III and IV. Enhanced line widths at the NW end and along the edge of the B 228 ridge in Lupus I, and a coherent velocity gradient across the ridge, are consistent with interaction between the molecular cloud and an expanding H I shell from the Upper-Scorpius subgroup of the Sco-Cen OB Association. Lupus III is dominated by the effects of two HAe/Be stars, and shows no sign of external influence. Slightly warmer gas around the core of Lupus IV and a low line width suggest heating by the Upper-Centaurus-Lupus subgroup of Sco-Cen, without the effects of an H I shell.

A scalable massive MIMO array architecture based on common modules

Massive MIMO is envisioned as one of the key enabling technologies for 5G wireless and beyond. While utilizing the spatial dimension to reduce interference and increase capacity in multi-user scenarios, massive MIMO base stations present several unique implementation challenges due to their large physical size and the high datarate generated by all the elements. To be cost-effective and energy efficient, practical designs must leverage the particular characteristics of massive MIMO to ensure scalability. Here, we propose an array architecture based on a common module which serves a small number of antennas with RF transceivers, data converters, and several support functions. Multiple chips are tiled into a grid and interconnected through a digital nearest-neighbor mesh network, avoiding the severe problems associated with analog signal distribution. Scalability across a wide range of array sizes is achieved by using distributed beamforming algorithms. It is demonstrated that by using this approach, the maximum backhaul datarate scales as the number of users rather than the number of antennas. Finally, we present a detailed accounting of the power consumption of the array and use the resulting optimization problem to show that per-element overhead limits the minimum achievable power consumption.

13C I in high-mass star-forming clouds

We report measurements of the 12C/13C abundance ratio in the three galactic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the 12CI 3P2-3P1 transition and the hyperfine components of the corresponding 13CI transition near 809 GHz. These transitions were observed simultaneously with the CO 7-6 line emission at 806 GHz with the AST/RO telescope located at the South Pole. From a simultaneous fit to the 12CI 3P2-3P1 transition and the HF components of the corresponding 13CI transition and an independent estimate of an upper limit to the optical depth of the 12CI emission we determine intrinsic 12CI/13CI column density ratios of 23+-1 for G 333.0-0.4, 56+-14 for NGC 6334 A and 69+-12 for G 351.6-1.3. As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of 13C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of 13CO, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al. (1998).