We consider the downlink of a wireless system with an M-antenna base station and K single-antenna users. A limited feedback-based scheduling and precoding scenario is considered that builds on the ...multiuser random beamforming (RBF). Such a scheme was shown to yield the same capacity scaling, in terms of multiplexing and multiuser diversity gain, as the optimal full CSIT-based (channel state information at transmitter) precoding scheme, in the large number of users K regime. Unfortunately, for more practically relevant (low to moderate) K values, RBF yields degraded performance. In this work, we investigate solutions to this problem. We introduce a two-stage framework that decouples the scheduling and beamforming problems. In our scenario, RBF is exploited to identify good, spatially separable, users in a first stage. In the second stage, the initial random beams are refined based on the available feedback to offer improved performance toward the selected users. Specifically, we propose beam power control techniques that do not change the direction of the second-stage beams, offering feedback reduction and performance tradeoffs. The common feature of these schemes is to restore robustness of RBF with respect to sparse network settings (low K), at the cost of moderate complexity increase.
During eruptive activity of andesitic stratovolcanoes, the extrusion of lava domes, their collapse and intermittent explosions are common volcanic hazards. Many lava domes grow in a preferred ...direction, in turn affecting the direction of lava flows and pyroclastic density currents. Access to active lava domes is difficult and hazardous, so detailed data characterizing lava dome growth are typically limited, keeping the processes controlling the directionality of extrusions unclear. Here we combine TerraSAR-X satellite radar observations with high-resolution airborne photogrammetry to assess morphological changes, and perform finite element modeling to investigate the impact of loading stress on shallow magma ascent directions associated with lava dome extrusion and crater formation at Volcán de Colima, México. The TerraSAR-X data, acquired in ~1-m resolution spotlight mode, enable us to derive a chronology of the eruptive processes from intensity-based time-lapse observations of the general crater and dome evolution. The satellite images are complemented by close-range airborne photos, processed by the Structure-from-Motion workflow. This allows the derivation of high-resolution digital elevation models, providing insight into detailed loading and unloading features. During the observation period from Jan-2013 to Feb-2016, we identify a dominantly W-directed dome growth and lava flow production until Jan-2015. In Feb-2015, following the removal of the active summit dome, the surface crater widened and elongated along a NE-SW axis. Later in May-2015, a new dome grew toward the SW of the crater while a separate vent developed in the NE of the crater, reflecting a change in the direction of magma ascent and possible conduit bifurcation. Finite element models show a significant stress change in agreement with the observed magma ascent direction changes in response to the changing surface loads, both for loading (dome growth) and unloading (crater forming excavation) cases. These results allow insight into shallow dome growth dynamics and the migration of magma ascent in response to changing volcano summit morphology. They further highlight the importance of detailed volcano summit morphology surveillance, as changes in direction or location of dome extrusion may have major implications regarding the directions of potential volcanic hazards, such as pyroclastic density currents generated by dome collapse.Introduction
Using radar satellite data from the Italian COSMO‐SkyMed (CSK) constellation and the German TanDEM‐X formation, we present comprehensive measurements of the biweekly grounding line dynamics of Pine ...Island Glacier, West Antarctica, from August to December 2015. The 1 day repeat cycle of CSK reveals tidally induced, grounding line migration on the scale of kilometers and extensive seawater intrusion within the grounding zone, which significantly exceeds that predicted for a stiff bed but are consistent with that calculated for a deformable bed. The deformable bed also explains the continuous draining/filling of subglacial lakes proximal to the grounding line. After correction for oceanic tides, we estimate a retreat rate for 2011–2015 of 0.3 km/yr at the glacier center and 0.5 km/yr on the sides, which is 3 times slower than for 1994–2011 (1.2 km/yr at the center). We attribute the decrease in retreat rate to colder ocean conditions in 2012–2013 relative to 2000–2011.
Plain Language Summary
Using short‐term repeat satellite data from the Italian CSK constellation, we document significant tidal migration of the junction between ice and the ocean on the Pine Island Glacier, West Antarctica, which is the largest contributor to sea level rise from Antarctica. The results imply that the junction is not fixed but corresponds to a zone several kilometers wide where ocean water melts ice from below. At present, numerical models of ice flow do not account for a flush zone. The existence of a flush zone should make the glacier more prone to retreat in response to warmer ocean waters.
Key Points
We observe significant sort‐term grounding line dynamics of a major glacier in west Antarctica
Faster retreat along shallow bed slopes helps validate bed topography models
Short‐term dynamics implies the presence of a soft bed, a high sensitivity to tides, and flushing of seawater within the grounding zone
The geometry of the volcanic conduit is a main parameter controlling the dynamics and the style of volcanic eruptions and their precursors, but also one of the main unknowns. Pre-eruptive signals ...that originate in the upper conduit region include seismicity and deformation of different types and scales. However, the locality of the source of these signals and thus the conduit geometry often remain unconstrained at steep sloped and explosive volcanoes due to the sparse instrumental coverage in the summit region and difficult access. Here we infer the shallow conduit system geometry of Volcán de Colima, Mexico, based on ground displacements detected in high resolution satellite radar data up to seven hours prior to an explosion in January 2013. We use Boundary Element Method modeling to reproduce the data synthetically and constrain the parameters of the deformation source, in combination with an analysis of photographs of the summit. We favour a two-source model, indicative of distinct regions of pressurization at very shallow levels. The location of the upper pressurization source coincides with that of post-explosive extrusion; we therefore attribute the displacements to transient (elastic) pre-explosive pressurization of the conduit system. Our results highlight the geometrical complexity of shallow conduit systems at explosive volcanoes and its effect on the distribution of pre-eruptive deformation signals. An apparent absence of such signals at many explosive volcanoes may relate to its small temporal and spatial extent, partly controlled by upper conduit structures. Modern satellite radar instruments allow observations at high spatial and temporal resolution that may be the key for detecting and improving our understanding of the generation of precursors at explosive volcanoes.
The surface deformation field measured at volcanic domes provides insights into the effects of magmatic processes, gravity‐ and gas‐driven processes, and the development and distribution of internal ...dome structures. Here we study short‐term dome deformation associated with earthquakes at Mount St. Helens, recorded by a permanent optical camera and seismic monitoring network. We use Digital Image Correlation (DIC) to compute the displacement field between successive images and compare the results to the occurrence and characteristics of seismic events during a 6 week period of dome growth in 2006. The results reveal that dome growth at Mount St. Helens was repeatedly interrupted by short‐term meter‐scale downward displacements at the dome surface, which were associated in time with low‐frequency, large‐magnitude seismic events followed by a tremor‐like signal. The tremor was only recorded by the seismic stations closest to the dome. We find a correlation between the magnitudes of the camera‐derived displacements and the spectral amplitudes of the associated tremor. We use the DIC results from two cameras and a high‐resolution topographic model to derive full 3‐D displacement maps, which reveals internal dome structures and the effect of the seismic activity on daily surface velocities. We postulate that the tremor is recording the gravity‐driven response of the upper dome due to mechanical collapse or depressurization and fault‐controlled slumping. Our results highlight the different scales and structural expressions during growth and disintegration of lava domes and the relationships between seismic and deformation signals.
Key Points
Gravity‐driven deformation of the dome occurs in association with large‐magnitude, low‐frequency earthquakes
The deformation is recorded as a localized, high‐frequency tremor‐like seismic signal
Extraction of 3‐D surface displacement fields reveal partition of motion along clearly defined internal dome structures
Bubble testing is often regarded, at best, as a poor method for locating gross leaks. However, it is demonstrated that, under proper conditions, bubble testing can be used to detect both gross and ...fine leaks, as well as their precise locations. Although not applicable in all situations, these leak-tests offer rapid and effective methods for identifying leak locations in components that are not adversely affected by the increased differential pressure. This method offers additional advantages such as not being fooled by virtual leaks, and indicating exact leak-location of single or multiple leaks in both the gross and fine leak regimes.
To investigate the influence of the acid-base reaction between acidic monomers and amines on the polymerization behavior of self-etching, self-curing adhesives, determine the effect of the ...application mode on the shear bond strength and morphology, and elucidate the adhesion performance of such systems by shear bond strength measurements.
The amine redox-initiator system N,N-di(2-hydroxyethyl)p-toluene (DEPT)/dibenzoyl peroxide (BPO) was selected to investigate the influence of the amine-base reaction on polymerization behavior. The PKa value of DEPT hydrochloride was measured by titration with NaOH. The influence of the pH value and DEPT concentration on the polymerization rate of methacrylates was investigated by exotherm time measurements. Three different application protocols of Multilink Primer (Ivoclar Vivadent) and Panavia 21 ED-Primer (Kuraray) were tested, 15 s passive vs 15 s agitation vs 60 s passive in combination with the corresponding resin luting material. The effects of these three application protocols were evaluated and monitored by both shear bond strength tests and SEM characterization of the surface morphology. The adhesion potential of these self-etching, self-curing luting systems was compared on enamel and dentin both directly after application and after 24 h.
The pKa of DEPT-HCl is 4.45. The polymerization rate of the DEPT-containing, self-etching, self-curing adhesive system is highly influenced by both the amine concentration and the pH value. In the case of Multilink, agitation of the primer mixture for 15 s, especially on dentin, resulted in a higher bond strength and a more pronounced removal of the smear layer. Multilink resulted in statistically higher bond strengths (p < 0.05) than Panavia 21 for both the enamel and dentin directly after application and for the dentin after 24 h.
Radical polymerization initiators used in self-curing systems are strongly adversely affected by acidic monomers incorporated in self-etching adhesives. However, if there is a good adjustment of these components and effects of adhesive application are taken in account, high bond strengths can be achieved.
In communication systems based on code-division multiple access, a proper assignment of spreading sequences can help in reducing the multiple access interference. In this paper, an optimized ...assignment of spreading sequences is proposed for downlink orthogonal frequency and code division multiplexing (OFCDM) systems applying transmit beamforming at the base station. Assignment involves a selection of the needed subset of spreading sequences and a distribution of these selected sequences according to the spatial signature of mobile terminals. A general assignment rule is exposed for any kind of sequence, which requires some computational effort. A very simple assignment rule is then proposed in the particular case of Walsh-Hadamard sequences. Simulation results over realistic transmission scenarios emphasize the necessity to optimize the assignment of spreading sequences as well as the efficiency of the proposed solution whatever the system load and the beamforming efficiency.
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