Task-Oriented Over-the-Air Computation for Multi-Device Edge AI

Departing from the classic paradigm of data-centric designs, the 6G networks for supporting edge AI features task-oriented techniques that focus on effective and efficient execution of AI task. Targeting end-to-end system performance, such techniques are sophisticated as they aim to seamlessly integrate sensing (data acquisition), communication (data transmission), and computation (data processing). Aligned with the paradigm shift, a task-oriented over-the-air computation (AirComp) scheme is proposed in this paper for multi-device split-inference system. In the considered system, local feature vectors, which are extracted from the real-time noisy sensory data on devices, are aggregated over-the-air by exploiting the waveform superposition in a multiuser channel. Then the aggregated features as received at a server are fed into an inference model with the result used for decision making or control of actuators. To design inference-oriented AirComp, the transmit precoders at edge devices and receive beamforming at edge server are jointly optimized to rein in the aggregation error and maximize the inference accuracy. The problem is made tractable by measuring the inference accuracy using a surrogate metric called discriminant gain, which measures the discernibility of two object classes in the application of object/event classification. It is discovered that the conventional AirComp beamforming design for minimizing the mean square error in generic AirComp with respect to the noiseless case may not lead to the optimal classification accuracy. The reason is due to the overlooking of the fact that feature dimensions have different sensitivity towards aggregation errors and are thus of different importance levels for classification. This issue is addressed in this work via a new task-oriented AirComp scheme designed by directly maximizing the derived discriminant gain

Channel Evolution Triggered by Large Flash Flood at an Earthquake-Affected Catchment

Earthquakes–induced landslides generally provide abundant loose materials at hillslopes, possibly triggering morphological reshaping processes at river channels and riverbeds during the large flash flood hydrograph and bringing huge risk downstream. Therefore, in a Wenchuan earthquake-affected catchment, the collected hydro-meteorological data and high-precision small Unmanned Aerial Vehicle (sUAV) data were used to quantitatively analyze channel evolution by a large flash flood event on 25 and 26 June 2018. It was found that the stable riverbed structure formed by the armour layer appeared in the tenth year after the Wenchuan earthquake. In a confined channel, the layer can protect the channel and resist the drastic change after the flash flood event with only a small bed elevation from 0.2 m to 2 m. Without the protection of the armour, the change could reach 6 m in the unconfined channel. Meanwhile, more materials with a deposition volume of about 7450 m3 from tributaries were generally taken to the main channel, and more intense erosion with a volume of 105 m3 mostly occurred downstream of tributaries. It was noted that, in the cross-section, the increased channel width could lead to a significant change with the large volume of 35 m3. Additionally, a conceptual diagram of the generalized channel response to large flash floods was provided during multi-stage periods after the Wenchuan earthquake. It determined the rebalance processes of channel evolution in the tenth year after the earthquake. This study will contribute to understanding the post-earthquake long-term channel evolutions and could provide decision-makers of assessing the mitigation strategies for higher-magnitude flood disasters triggered by channel change in earthquake-affected watersheds

Communication and Energy Efficient Decentralized Learning over D2D Networks

Device-to-device (D2D)-assisted decentralized learning has been proposed for mobile devices to collaboratively train artificial intelligence networks without the centralized parameter server. However, a densely connected network will cause large learning latency and energy consumption due to the limited computation and communication resources. In addition, link selection and aggregation weight have a significant impact on the learning performance. To cope with these challenges, we propose a joint computing power adjustment, wireless resource allocation, link selection, and aggregation weight adaptation mechanism to improve both communication and energy efficiencies. Specifically, the learning performances including the convergence rate, per-iteration learning latency, and per-iteration energy consumption are first analyzed. Then, an optimization problem is formulated to minimize the total learning cost, which is defined as the weighted sum of total learning latency and energy consumption. Given a network topology, the computing power and wireless resource allocation are optimized by the alternating optimization algorithm. Moreover, the optimal aggregation weight is obtained by semidefinite programming. With respect to link selection, we propose a tabu search based meta-heuristic algorithm to approximately achieve feasible solutions with a low computational complexity. Finally, extensive experiments demonstrate that the proposed link selection algorithm can significantly reduce the learning cost under the given learning accuracy requirement.</p

A New Method for Wet-Dry Front Treatment in Outburst Flood Simulation

When utilizing a finite volume method to predict outburst flood evolution in real geometry, the processing of wet-dry front and dry cells is an important step. In this paper, we propose a new approach to process wet-dry front and dry cells, including four steps: (1) estimating intercell properties; (2) modifying interface elevation; (3) calculating dry cell elevations by averaging intercell elevations; and (4) changing the value of the first term of slope limiter based on geometry in dry cells. The Harten, Lax, and van Leer with the contact wave restored (HLLC) scheme was implemented to calculate the flux. By combining the MUSCL (Monotone Upstream&ndash;centred Scheme for Conservation Laws)-Hancock method with the minmod slope limiter, we achieved second-order accuracy in space and time. This approach is able to keep the conservation property (C-property) and the mass conservation of complex bed geometry. The results of numerical tests in this study are consistent with experimental data, which verifies the effectiveness of the new approach. This method could be applied to acquire wetting and drying processes during flood evolution on structured meshes. Furthermore, a new settlement introduces few modification steps, so it could be easily applied to matrix calculations. The new method proposed in this study can facilitate the simulation of flood routing in real terrain

A New Method for Wet-Dry Front Treatment in Outburst Flood Simulation

When utilizing a finite volume method to predict outburst flood evolution in real geometry, the processing of wet-dry front and dry cells is an important step. In this paper, we propose a new approach to process wet-dry front and dry cells, including four steps: (1) estimating intercell properties; (2) modifying interface elevation; (3) calculating dry cell elevations by averaging intercell elevations; and (4) changing the value of the first term of slope limiter based on geometry in dry cells. The Harten, Lax, and van Leer with the contact wave restored (HLLC) scheme was implemented to calculate the flux. By combining the MUSCL (Monotone Upstream–centred Scheme for Conservation Laws)-Hancock method with the minmod slope limiter, we achieved second-order accuracy in space and time. This approach is able to keep the conservation property (C-property) and the mass conservation of complex bed geometry. The results of numerical tests in this study are consistent with experimental data, which verifies the effectiveness of the new approach. This method could be applied to acquire wetting and drying processes during flood evolution on structured meshes. Furthermore, a new settlement introduces few modification steps, so it could be easily applied to matrix calculations. The new method proposed in this study can facilitate the simulation of flood routing in real terrain

Outburst floods in China: A review

Outburst floods can have disastrous impacts on people, and are an important driving force in landscape change and have been studied widely on Earth. In China, although outburst floods have occurred frequently, there has been relatively little systematic investigation of the controlling factor. Here, we review outburst floods in China in terms of the characteristics, distribution, causes of dams and outburst floods. In terms of natural dams, landslides accounted for the majority (287 cases), followed by moraine dams (33 cases), which are mainly found on and around the Tibetan Plateau, and although other types (such as glacier and volcanic dams) were historically rare, many examples may be preserved in the geological record. In addition, there have been thousands of outburst floods from artificial-constructed dams, the majority of which were from small earth dams. The largest reliably recorded peak discharge for an outburst flood was 1.24 × 105 m3/s, which occurred in Yigong, Tibet. The peak discharge of the 1975 Banqiao collapse was 7.9 × 104 m3/s; the largest outburst flood of a man-made dam. Our recent investigations on the Yarlung Tsangpo in Southeast Tibet have identified gravel deposits that probably record megafloods and offer great potential for paleoflood analysis

Case Study: Effects of a Partial-Debris Dam on Riverbank Erosion in the Parlung Tsangpo River, China

This paper examines two successive debris flows that deposited a total of 1.4 million m3 of sediment into the Parlung Tsangpo River in China in 2010. As a result of these deposits, a partial-debris dam was formed in the river. This dam rerouted the discharge in the river along one of the riverbanks, which supported a highway. The rerouted discharge eroded the riverbank and the highway eventually collapsed. To enhance our understanding of the threat posed by partial-debris dams, a field investigation was carried out to measure the discharge in the river and to collect soil samples of the collapsed riverbank. Findings from the field investigation were then used to back-analyze fluvial erosion along the riverbank using a combined erosion framework proposed in this study. This combined framework adopts a dam-breach erosion model which can capture the progressive nature of fluvial erosion by considering the particle size distribution of the soil being eroded. The results from the back-analysis were then evaluated against unique high-resolution images obtained from satellites. This case study not only highlights the consequences of the formation of partial-debris dams on nearby infrastructure, but it also proposes the use of a combined erosion framework to provide a first-order assessment of riverbank stability. Unique high-resolution satellite images are used to assess the proposed erosion framework and key challenges in assessing erosion are discussed

The morphology and stratification of giant eddy bars reflect the variation in shape of palaeoflood hydrographs: a flume study.

Evidence for ungauged large freshwater palaeofloods in valley-confined landscapes frequently includes giant flow-eddy bars, deposited in alcoves along the floodway margins. Elevations of the bar tops commonly are used to define the minimum water level for computational flood simulations. Field study has shown that giant bar stratigraphy and sedimentology can be distinctive; identifying the alluvial signature of large palaeofloods where the morphological evidence may be less clear. However, whether the shape and stratigraphy of eddy bars provide indicators as to the nature of the floodwaves has not been considered widely. Flood hydrographs might be dam-break surge waves, gradually varied, or steady flows, for example. Yet, if bar form and stratigraphy vary systematically with the nature of the flood wave, then bars have additional value in defining the style of unrecorded floodwaves for environmental interpretation and flood modelling purposes. An experimental water flume was used to reproduce three styles of scaled floodwave that might be associated with sudden and more protracted releases of water from upstream reservoirs. Discharge was through a channel consisting of a series of contractions and expansions. Eddy bars were deposited within the flow separation zones that formed at each flow expansion. The basic hydraulic characteristics of the floodwaves were recorded and the form of the bars and the stratigraphy were examined. The results indicate that each style of flood deposited a distinctive barform and related stratigraphy. In this manner, we demonstrate that the examination of the form and stratification of giant bars in the natural environment can provide information on the style of the palaeoflood – sudden release or protracted flow – that was responsible for the deposition of the bars. Such information assists with the identification and interpretation of the nature and source of the floodwater as well as informing attempts to model hydrograph shapes.<br/