| 1. |
- Wilson, Samuel T., et al.
(author)
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Short-term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA : A case study of summer 2012
- 2015
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In: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 29:8, s. 1145-1164
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Journal article (peer-reviewed)abstract
- Time-series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time-series program has maintained near-monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near-monthly time-series observations, an extensive field campaign was conducted during July-September 2012 at Station ALOHA with near-daily sampling of upper water-column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea-surface salinity minimum feature which was prominent in the upper water column (0-50 m) for a period of approximately 30 days. The shipboard observations during July-September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea-surface salinity minimum feature in the NPSG.
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| 2. |
- Kumar, A., et al.
(author)
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A Scalable Feedback Mechanism for Distributed Nullforming with Phase-Only Adaptation
- 2015
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In: IEEE Transactions on Signal and Information Processing over Networks. - : Institute of Electrical and Electronics Engineers Inc.. - 2373-776X. ; 1:1, s. 58-70
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Journal article (peer-reviewed)abstract
- This paper considers a problem of distributed nullforming, in which multiple wireless transmitters steer a null toward a designated receiver by only adjusting their carrier phases. Since each transmitter transmits at full power, the system maximizes 'power pooling' gains for cooperative communication or jamming, while simultaneously protecting a designated receiver. Analysis in a noiseless setting shows that, while the received power at the designated receiver, as a function of the transmitted phases, is nonconvex with multiple critical points, all of its local minima are also global minima. This implies that a null can be formed using a distributed, scalable protocol based on gradient descent: each transmitter adapts its phase based only on aggregate feedback broadcast by the receiver (so that feedback overhead does not increase with the number of transmitters), along with an estimate of its own channel gain (which can be obtained, e.g., via reciprocity). Simulations show that the convergence rate actually improves with the number of transmitters, and that the algorithm is robust to noise, substantial channel estimation errors, and oscillator drift.
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