High-speed Data Access Research Articles

60-GHz Millimeter-Wave over Visible Light Communications for Downlink Wireless Networks

Background and Objective: In this paper, we address potential solution for downlink wireless communications by integrating visible light communications (VLC) with the broadband radio frequency (RF) networks operating at 60 GHz-millimeter wave (mmWave) band. For this hybrid design, the outdoor 60 GHz-mmWave based RF link is utilized to ensure backhaul connectivity for VLC indoor system, while the VLC exploiting the lighting infrastructure that essentially used LEDs as optical source to provide low-cost and high-speed data access. Methods: The hybrid 60 GHz-mmWave/VLC system is analyzed by using 16-QAM OFDM signal, and its performance is investigated by evaluating the signal to noise ratio (SNR) and the received signal power distributions, for regular placement of LEDs and random location of receivers within the room. The impact of the transmitter-receiver parameters, and their orientations and directivities are also considered. Results: Numerical results show the efficiency of the proposed system, which can retransmit RF signals at 60 GHz-mmWave band using visible optical carriers in the indoor environment. Conclusion: The results suggest that the hybrid 60 GHz-mmWave/VLC system is able to provide reliable wireless data transmission, making it an attractive solution for downlink indoor communications.

RANCANG BANGUN ANTENA MONOPOLE MIKROSTRIP MENGGUNAKAN DEFECTED GROUND STRUCTURE BENTUK PERSEGI UNTUK OPTIMASI BANDWIDTH PADA APLIKASI KOMUNIKASI ULTRA-WIDEBAND

The development of UWB technology is very helpful in supporting high-speed data access. UWB offers a solution for large capacity for wide bandwidth. The frequency range for UWB systems between 3.1–10.6GHz will cause interference to wireless communication systems via wireless local area networks (WLANs) for IEEE 802.11a operating at 5.15–5.35GHz and 5.725–5.825GHz. This research aims to obtain a design of a rectangular printed monopole patch microstrip antenna and a slit using the Defected Ground Structure (DGS) technique to obtain bandwidth optimization on the UWB spectrum. For the implementation of the antenna design, the simulation method is used with the support of the RT/ Duroid substrate material. Realization of the design with the results of a manufactured antenna prototype with a minimalist size. The simulation and measurement results of the Absolute UWB bandwidth parameter at return loss < -10 dB (VSWR <2) the measurement value is 600MHz narrower than the simulation and for fractional bandwidth the measurement value is narrower by 0.21%. Resonance at the 5.2GHz frequency produces polarization with a gain value of 3.29dBi, omnidirectional and vertical polarization. As a prototype antenna to support the application of indoor wireless access communication devices that are low profile, compact with minimalist dimensions.

Enhancing security & QoS of trust-enabled wireless networks using machine learning powered transformable blockchain sharding

Trust enabled wireless networks use temporal behaviour information of nodes in order to classify them into different trust categories. This information is utilized by the router for high performance communication that is optimized in terms of end-to-end delay, energy consumption, throughput, packet delivery ratio, and other quality of service (QoS) parameters. Establishing security in trust enabled wireless networks is a difficult task, because high trust nodes might be compromised by external or internal attacks, thereby disrupting normal communication. In order to perform this task, blockchain based security models are deployed. These models provide high transparency, comprehensive traceability, distributed processing, and data immutability, which makes them highly deployable for trust enabled networks. Blockchain models enforce compulsive verification of data before communication, which makes them resilient to DDoS, MITM, denial of service, and other data-based attacks. In order to enforce these checks, each of the block is hashed, and the hash values are compared with every existing block in the chain. These checks include hash uniqueness, and hash pattern validations; the later of which is decided by the network designer(s). As the length of blockchain increases, computational complexity of adding a new block (a.k.a. blockchain mining) increases exponentially, which adds to the end-to-end delay, and energy consumption of wireless nodes, which is a drawback of these models. To avoid this, sidechains & blockchain sharding models are developed. These models work by dividing the existing blockchain into multiple parts (based on a certain pre-set criteria), and then use the parts for high speed and low power mining. But again, due to increase in number of sidechains, the computational complexity of managing these chains, and locating data blocks within them increases exponentially. Moreover, in any practical wireless network, there is a need to communicate modifiable data, which is not supported by current blockchain implementations. In order to resolve these issues, this text proposes a transformable blockchain sharding model, which is managed via a light weight meta heuristic method for high-speed data access. The proposed model aims at reducing computational complexity of sidechain maintenance with the help of directed acyclic graphs (DAGs) for storing of hash ranges. The model also incorporates a transformable blockchain solution, wherein the block structure is designed to incorporate selectively mutable as well as non-mutable information. Both the mutable and non-mutable information is encrypted using high performance elliptic curve cryptosystem, which makes it highly secure against network attacks. The proposed model showcases 15% improvement in network lifetime, 8% reduction in end-to-end delay, 22% reduction in computational complexity, and 18% improvement in network throughput when compared with various blockchain and sidechain based wireless networks, thereby assisting in development of a high QoS and highly secure wireless network.

LTE Optimization Using The Electrical Tilt Method At The Mandau Site

Long Term Evolution (LTE) has high-speed data access of 300 Mbps on the downlink and 75 Mbps on the uplink. The area of ??the Mandau site is located in the town of Duri-Bengkalis, Riau. In this study, a network optimization was carried out on one of the providers. From the results of the drive test conducted, a problem was found, namely throughput quality which was in very bad condition. Therefore, a network optimization simulation was carried out using the Electrical Tilt method in order to overcome the problem of unstable throughput conditions. Simulation of the application of this method is carried out using Software Atoll. The results obtained after optimization with the value on Jalan Obor 2 increased to 19,000 kbps (normal), on Jalan Obor 1 increased to 50,000 kbps (very good), on Jalan Bandes increased to 50,000 kbps (very good), on Jalan Swadaya increased to 50,000 kbps (very good) and on Jalan Hangtuah increased to 50,000 kbps (very good).

Design and Analysis of Commercially Viable Free-Space Optical Communication Link for Diverse Beam Divergence Profiles

With an exponential increase in the use of smart devices and with increasing demand for high-speed data applications, the Radio Frequency and Microwave links will find themselves almost incapable of meeting this deficit. Free-space optical (FSO) links are poised to take the leading role as communication systems for futuristic needs in providing cost-effective and high-speed connectivity to end-users. However, on the flip side, the reliability of FSO links has an intrinsic relationship with the characteristics of the atmospheric channel. During the propagation, the atmospheric adversities result in the geometrical spreading of the FSO beam which in turn has consequential degrading effects on the link performance. In this work, an FSO communication system based on practical, commercial, and realistic parameters has been proposed with the intent of accurately determining the co-relation between different beam divergence profiles and overall link performance. For different beam divergence profiles, the proposed link has been evaluated for reference bit error rate (BER) of 10–9 which is an ideal condition for delivering high-speed data access to the end-users. During the analysis, the practical constraints related to the trade-off between the maximum possible link range and beam divergence have also been optimized. The findings of this work will be crucial for engineers and designers in configuring FSO links for improved link range and reliability.

Call Model and Test-Verification Methods for PS-LTE Core Equipment

Existing technologies in disaster and safety communication networks (DSCNs) face problems such as narrow bandwidth, low transmission rate, and difficulty in securing channels. Conversely, public safety long-term evolution (PS-LTE) technology provides high-speed, low-latency, quick-setup, and high-security data access for multimedia-based mission-critical services. To perform tests on each unit of core equipment with PS-LTE technology, a new call model such as push-to-talk, which is a service representing PS-LTE, is required. Moreover, objective and consistent metrics and methods are required for administering tests on the unique functions of DSCNs newly constructed with PS-LTE technology. This paper proposes a call model for the core facilities of DSCNs established in Korea, and it also suggests test and verification methods for all the areas of PS-LTE, including mission-critical push-to-talk, evolved multimedia broadcast and multicast service, and coverage and quality assessment for wireless services.

Developing Cost-Effective and High-Speed 40 Gbps FSO Systems Incorporating Wavelength and Spatial Diversity Techniques

Free-space optical (FSO) communication systems are being anticipated to offer promising alternatives to existing radio networks in delivering high-speed data access to end-users. Ease of installation, robust features, and cost-effective operation have been the hallmark of FSO systems, and these features will play an obvious role in deciding the ways in which futuristic smart communication models will operate. Despite these arrays of features, FSO links suffer severe performance degradation due to channel-induced impairments caused by atmospheric effects such as rain, haze, and fog. In this work, we have investigated and compared the performance of 40 Gbps FSO links for different channel conditions ranging from clear weather to severe attenuation by incorporating spatial and wavelength diversity as performance booster techniques. The use of an erbium-doped fiber amplifier (EDFA) with FSO links has also been proposed here. Using performance metrics like bit error rate (BER) and eye patterns, it has been found that the use of EDFA not only helps in compensating for the link losses but also aids in realizing an all-optical processing based last-mile access system. The proposed FSO system will be capable of bridging the existing backbone fiber networks with end-users with minimal changes to the existing hardware regime, thereby proving to be extremely cost-effective in sharp contrast to radio-frequency generations which require major infrastructure overhaul.

Performance optimization method for distributed storage system based on hybrid storage device

In the era of big data and artificial intelligence, the storage data of data centers is growing rapidly. Big data storage systems with high-performance computing require high-speed data access and greater storage capacity. However, traditional storage technology based on hard disk media (HDDs) has been difficult to meet the current demand for high-performance storage, solid state drives (SSDs)’ cost will be a lot for storing massive amounts of data. Therefore, in response to the above problems, this paper proposes a hierarchical hybrid storage technology for distributed file system, including an adaptive file migration strategy and client caching mechanism. Through experimental verification, the proposed technology fully combines the high I/O throughput characteristics of SSDs and the large capacity characteristics of disks, thereby providing a cost-effective high-performance distributed storage solution, and when processing real-time data, it has higher access performance.

Modified Data Storage and Replication Mechanism with Frequent Use-Case Based Indexing

The real time applications demands high speed and reliable data access from the remote database. An effective logical data management strategy that handles simultaneous connections with better performance negotiation is inevitable. This work considers an e-health care application that proposes MongoDB based modified indexing and performance tuning methods. To cope with certain high frequency use case and its performance mandates, a flexible and efficient logical data management may be preferred. By analysing the data dependency, data decomposition concerns and the performance requirements of the specific use case of the medical application, a logical schema may be customized on an ala-carte basis. This work focused on the flexible logical data modeling schemes and its performance factors of the NoSql DB. The efficiency of unstructured data base management in storing and retrieving the e-health care data was analysed with a web based tool. To enable faster data retrieval and query processing over the distributed nodes, a Spark based storage engine was built on top of the MongoDB based data storage management. With Spark tool, the database has been made distributed as master–slave structures with suitable data replication mechanisms. In such distributed database the fail-over also implemented with the suitable replication mechanism. This work considered MongoDB based flexible schema modeling and Spark based distributed computation with multiple chunks of data. The flexible data modeling scheme with MongoDB with the on-demand Spark based computation framework was proposed. To facilitate the eventual consistency, scalability aspects of the e-health care applications, use case based indexing was proposed. With the effective data management, faster query processing the horizontal scalability has been increased. The overall efficiency and scalability of the proposed logical data management approach was analysed. Through the simulation studies, the proposed approach has been claimed to boost the performance of the bigdata based application to a considerable extent.

A Vision to Smart Radio Environment: Surface Wave Communication Superhighways

Complementary to traditional approaches that focus on transceiver design for bringing the best out of unstable, lossy fading channels, one radical development in wireless communications that has recently emerged is to pursue a smart radio environment by using software-defined materials or programmable metasurfaces for establishing favorable propagation conditions. This article portraits a vision of communication superhighways enabled by surface wave (SW) propagation on intelligent surfaces for future smart radio environments. The concept differs from the mainstream efforts of using passive elements on a large surface for bouncing off radio waves intelligently toward intended user terminals. In the proposed SW vision, energy efficiency can be much higher, due to more desirable propagation characteristics compared to open-space radio propagation. The fact that SW is inherently confined to the smart surface not only greatly simplifies the task of interference management, but also makes possible exceptionally localized high-speed interference-less data access. We shall outline the opportunities and challenges arising from the SW paradigm. We shall also attempt to shed light on several key enabling technologies that make this realizable. One important technology which will be discussed is a software-controlled fluidic waveguiding architecture that permits dynamic creation of high-throughput data highways.

Artificial-Intelligence-Driven Fog Radio Access Networks: Recent Advances and Future Trends

The articles in this special section focus on artificial intelligence-driven fog radio access networks. To satisfy the explosively increasing demands of highspeed data applications and access requirements from a massive number of Internet-of-Things (IoT) devices, a paradigm of fog computing-based radio access network (F-RAN) has emerged as a promising evolution path for the fifth generation (5G) radio access networks. By taking full advantage of distributed caching and centralized processing, F-RANs provide great flexibility to satisfy the quality-of-service requirements of various 5G services. With the rapid deployment of 5G communication networks, the application of F-RANs to the envisioned sixth-generation (6G) mobile network has attracted extensive attention from academia, industry, and government agencies. The 6G network progress in enhanced mobile broadband, massive machine-type communications and ultra-reliable and low-latency communications will lead to the fast development of new applications, including augmented reality (AR), virtual reality (VR), holographic communications, vehicle-to-everything (V2X), self-driving cars, massive sensors connective on the ground and several tens of thousands of satellites connective in the sky.

Gain analysis of high-speed DWDM link with different optical amplification configurations

Abstract Evolution of data-hungry devices has prompted the need for an exclusive communication infrastructure that can cater to the exponential increase in the need for high-speed data access. Since optical fiber links are inherently capable of supporting high-transmission data rates, fiber systems will be an integral part of the larger strategy to provide cost-effective high-speed data access to end users. This paper demonstrates the 100 × 40-Gbps dense wavelength division multiplexing (DWDM)-based 100-km-long fiber link whose performance has been analyzed using various configurations of a hybrid optical amplifier. The performance of the proposed link has been investigated using parameters like gain (dB) and bit error rate (BER). During the analysis, it has been observed that hybrid amplification, i.e., combination semiconductor optical amplifier (SOA), erbium-doped fiber amplifier (EDFA), and Raman amplifier, delivers overwhelming gain (dB) in comparison to the conventional stand-alone amplifier. Further, it has been seen that among different positional configurations of the amplifier, symmetrically positioned hybrid amplifiers achieved the gain of 18.6 dB, while the SOA link with similar configuration and parameters delivered link gain of merely −2.6 dB. In terms of BER performance, the symmetric hybrid configuration was outstanding with BER of 10−5, while preamplification link BER was merely 10−3, and post amplification being the most undesirable. The proposed link has been designed and investigated using OptiSystemTM 14.2.

A New Flexible Multi-flow LRU Cache Management Paradigm for Minimizing Misses

The Least Recently Used (LRU) caching and its variants are used in large-scale data systems in order to provide high-speed data access for a wide class of applications. Nonetheless, a fundamental question still remains: in order to minimize the miss probabilities, how should the cache space be organized to serve multiple data flows? Commonly used strategies can be categorized into two designs: pooled LRU (PLRU) caching and separated LRU (SLRU) caching. However, neither of these designs can satisfactorily solve this problem. PLRU caching is easy to implement and self-adaptive, but does not often achieve optimal or even efficient performance because its set of feasible solutions are limited. SLRU caching can be statically configured to achieve optimal performance for stationary workload, which nevertheless could suffer in a dynamically changing environment and from a cold-start problem. To this end, we propose a new insertion based pooled LRU paradigm, named I-PLRU, where the data flows can be inserted at different positions of a pooled cache. This new design can achieve the optimal performance of the static SLRU, but retains the adaptability of PLRU for resource sharing. Theoretically, we characterize the asymptotic miss probabilities of I-PLRU, and prove that, for any given SLRU design, there always exists an I-PLRU configuration that achieves the same asymptotic miss probability, and vice versa. We next design a policy to minimize the miss probabilities. However, the miss probability minimization problem turns out to be non-convex under the I-PLRU paradigm. Notably, we utilize an equivalence mapping between I-PLRU and SLRU to efficiently find the optimal I-PLRU configuration. We prove that I-PLRU outperforms PLRU and achieves the same miss probability as the optimal SLRU under a stationary request arrival process. From an engineering perspective, the flexibility of I-PLRU avoids separating the memory space, supports dynamic and refined configurations, and alleviates the cold-start problem, potentially yielding better performance than both SLRU and PLRU.

Memristive Devices Formed By ALD Metal Oxide Growth on a Hafnium Layer – Study of the Interfacial HfO2 Formation

Redox-based memristive devices (ReRAM) are considered for future energy-efficient non-volatile memory. Memristive cells enable high speed data access, low power consumption and high scalability, which is a key requirement for their use as artificial synapses in neuromorphic computing. Applications in networks of up to 106 devices require a scaling of the cell volume to about 103 nm3. For a typical valence-change-mechanism (VCM)-type switching cell with a metal oxide, like HfO2, sandwiched between two metal electrodes of different oxidation enthalpy, this means that the individual layer thickness is in the range of a few nanometers. Therefore, a control of the device properties demands for a control of interfacial reactions during device fabrication. Here, we are going to present a careful analysis of the oxidation of an Hf electrode layer during atomic layer deposition (ALD) of different metal oxides, these are Al2O3, TiO2, and HfO2. The in-situ encapsulation of a reactive metal electrode by means of an ALD metal oxide layer enables a better process control compared to stack structures formed by breaking the vacuum conditions.In this study, plasma and thermal ALD processes were used to grow ultrathin layers of Al2O3, TiO2, and HfO2 on top of Hf metal films, after transfer under UHV conditions. The growth of homogeneous metal oxide films of a few nanometer in thickness enabled a systematic analysis of the HfO2 formation at the Hf/metal oxide interface by means of angle dependent X-ray photoelectron spectroscopy (XPS). The results show that the properties of the interfacial HfO2 layer can be controlled by the metal oxide deposited on top as well as by the process chemistry and deposition temperature. In contrast to the TiO2 film, the Al2O3 layer clearly serves as oxidation barrier. In addition, the chemistry and structure of the HfO2 interfacial layer were analyzed from XPS and transmission electron microscopy analysis.Memristive device were fabricated from the Hf/HfO2/MOx stacks by depositing Pt top electrodes. The initial current of the devices scales with the pad size indicating the high quality of the insulating oxide layers. Consistent with VCM-type memristive devices the cells required an electroforming voltage to enable bipolar-type resistive switching. In this presentation, the trade-off between the two functionalities of oxidation protection versus enabling of a low electroforming voltage will be discussed.

A New Flexible Multi-flow LRU Cache Management Paradigm for Minimizing Misses

The Least Recently Used (LRU) caching and its variants are used in large-scale data systems in order to provide high-speed data access for a wide class of applications. Nonetheless, a fundamental question still remains open: in order to minimize miss probabilities, how should the cache space be organized to serve multiple data flows? Commonly used strategies can be categorized into two designs: pooled LRU (PLRU) caching and separated LRU (SLRU) caching. However, neither of these designs can satisfactorily solve this problem. PLRU caching is easy to implement and self-adaptive, but does not often achieve optimal or even efficient performance because its set of feasible solutions are limited. SLRU caching can be statically configured to achieve optimal performance for stationary workload, which nevertheless could suffer in a dynamically changing environment and from a cold-start problem. To this end, we propose a new insertion based pooled LRU paradigm, termed I-PLRU, where data flows can be inserted at different positions of a pooled cache. This new design can achieve the optimal performance of the static SLRU, and retains the adaptability of PLRU in virtue of resource sharing. Theoretically, we characterize the asymptotic miss probabilities of I-PLRU, and prove that, for any given SLRU design, there always exists an I-PLRU configuration that achieves the same asymptotic miss probability, and vice versa. We next design a policy to minimize the miss probabilities. However, the miss probability minimization problem turns out to be non-convex under the I-PLRU paradigm. Notably, we utilize an equivalence mapping between I-PLRU and SLRU to efficiently find the optimal I-PLRU configuration. We prove that I-PLRU outperforms PLRU and achieves the same miss probability as the optimal SLRU for stationary workload. Engineeringly, the flexibility of I-PLRU avoids separating the memory space, supports dynamic and refined configurations, and alleviates the cold-start problem, potentially yielding better performance than both SLRU and PLRU.

Broadband LEO Satellite Communications: Architectures and Key Technologies

This article aims to provide a comprehensive overview for key issues in broadband LEO satellite communication systems. First of all, the network architecture is introduced, which is the basis of the whole system. The space-based LEO system with ISL, which requires a small number of ground gateways, is the focus. In this system, the satellite constellation design is important with impact on key system performances such as coverage. Two popular LEO constellations, the walker Delta and Star constellations, are introduced. Given satellite constellations, proper beam coverage schemes should be employed at satellites to provide seamless coverage all over the world. A hybrid wide and spot beam coverage scheme is presented, where the LEO provides a wide beam for large area coverage and several steering spot beams for highspeed data access. Moreover, special coverage schemes should be designed in broadband LEO systems for the interference coordination between LEO and GEO. To protect GEO communications, LEO satellites should be turned off if they cause interference to GEO. In this case, to provide services for users covered by the turned-off LEO satellites, a progressive pitch method and a coverage expanding method can be employed. Finally, the coverage performance of LEO is also closely related to resource management schemes. The global resource management for broadband LEO systems is complicated, involving a large amount of data, and a two-level management structure should be employed. Using this structure, an NMC with powerful storage and processing capabilities is employed to carry out the first-level management, making strategies based on all information collected from the whole system. Then satellite base stations with limited capabilities are employed to respond to the strategies generated by NMC in real time.

Polar-coded forward error correction for MLC NAND flash memory

With the ever-growing storage density, high-speed, and low-cost data access, flash memory has inevitably become popular. Multi-level cell (MLC) NAND flash memory, which can well balance the data density and memory stability, has occupied the largest market share of flash memory. With the aggressive memory scaling, however, the reliability decays sharply owing to multiple interferences. Therefore, the control system should be embedded with a suitable error correction code (ECC) to guarantee the data integrity and accuracy.We proposed the pre-check scheme which is a multi-strategy polar code scheme to strike a balance between reasonable frame error rate (FER) and decoding latency. Three decoders namely binary-input, quantized-soft, and pure-soft decoders are embedded in this scheme. Since the calculation of soft log-likelihood ratio (LLR) inputs needs multiple sensing operations and optional quantization boundaries, a $2$-bit quantized hard-decision decoder is proposed to outperform the hard-decoded LDPC bit-flipping decoder with fewer sensing operations. We notice that polar codes have much lower computational complexity compared with LDPC codes. The stepwise maximum mutual information (SMMI) scheme is also proposed to obtain overlapped boundaries without exhausting search. The mapping scheme using Gray code is employed and proved to achieve better raw error performance compared with other alternatives. Hardware architectures are also given in this paper.

Interfaces Formed by ALD Metal Oxide Growth on Metal Layers

Redox-based resistive switching random access memory (ReRAM) has the potential to fulfill the demands of future Green IT solutions for non-volatile memory circuits with high speed data access, high reliability, and low power consumption. A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy. It is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Thus, understanding oxidation/reduction reactions taking place already during the growth process is of utmost importance. For future three-dimensional (3D) integrated arrays of ReRAM cells atomic layer deposition (ALD) will become the method of choice to grow ultra-thin, conformal, homogeneous and dense oxide films at a thermal budget below 450 °C. For todays’ two dimensional stacked crossbar arrays, the metal oxide films can be grown on nearly inert Pt electrodes, while the oxygen exchange layer built from a reactive metal is deposited on top of the metal oxide. However, in the 3D ReRAM arrays the metal oxide might need to be grown on the reactive metal as well. In this study ultrathin ALD metal oxide layers are deposited on various metal electrodes by means of in-situ transfer from the sputtered metal into the ALD chamber. For the metal layers we utilized Pt, Hf, and Ta while for the metal oxides Al2O3, TiO2, and HfO2 were chosen. ALD metal oxide processes were performed by plasma assisted ALD in an Oxford FlexAlâ system. Due to the passivation property of the dense ALD metal oxide layer the metal oxide / metal interfaces could be analyzed ex-situ if the vacuum break was kept short. Angle resolved X-ray photoemission spectroscopy studies combined with X-ray reflectivity analysis reveal a clear picture of the interface formation and the oxygen gradient formed at the interface already during growth. The influence of process parameters and stack sequences on this ‘built-in’ oxygen gradient at metal / ALD metal oxide interfaces will be covered in this presentation. Among others, the possibility for PtOx formation during remote oxygen plasma ALD processes will be discussed.

Two Stable Switching Modes with Opposite Polarity in Pt/TiO2/Ti Cells Based on Concurring Phenomena Close to the Pt/TiO2 Interface

Redox-based resistive switching random access memory (ReRAM) has the potential to fulfill the demands of future Green IT solutions for non-volatile memory circuits with high-speed data access, high reliability, and low power consumption. A single valence-change-mechanism (VCM)-type switching cell is typically built from a transition metal oxide layer sandwiched between metal electrodes of different oxidation enthalpy. It is accepted in the scientific community that oxygen exchange and drift/diffusion processes play a major role in the resistive switching process. Recently, the importance of oxygen exchange reactions has been demonstrated for Ta2O5 based devices,[1] and several studies reported on the coexistence of switching events of opposite polarity in cells built from heteroepitaxial SrTiO3 films.[2,3] Among various explanations for such diverse switching behaviors, the inhomogeneous dopant distribution was suggested as the main source.[4] For technically relevant devices based on amorphous metal oxide layers, a comparable phenomenon but with partly volatile resistance states was reported for symmetric Pt/TiO2/Pt cells.[5] For this study, we fabricated 100 nm x 100 nm size nano-crossbar devices built from a Pt bottom electrode, a 3- to 5-nm-thick TiO2 layer grown by thermal atomic layer deposition and a 10 nm thick Ti top electrode covered by Pt. Metal layers were grown by e-beam evaporation. Electroforming and subsequent reset at moderate voltages initiate a filamentary-type bipolar resistive switching behavior with counter-eightwise (c8w) switching direction. This means that an abrupt set event from the c8w high resistance state (HRS) to the c8w low resistance state (LRS) is observed for negative voltages applied to the Pt Schottky electrode. When the positive voltage signal applied to the Pt electrode is increased, the switching direction changes towards eightwise (8w). This means that the cell sets to conducting low resistance state (8w-LRS*) for positive voltages applied to the Pt electrode, and resets to an insulating (8w) high resistance state (HRS*). The c8w- and 8w- switching hysteresis share the c8w-HRS and the 8w-LRS* states. In this presentation, we will characterize the different stable states using the temperature dependence of the measured current-voltage characteristics. Based on recent findings for the 8w-switching Nb:STO/STO/Pt system [6] we will discuss arguments for an interplay of drift/diffusion and oxygen exchange reactions that can explain the different switching events happening at the same Pt/TiO2 interface. [1] W. Kim, S. Menzel, D. J. Wouters, Y. Guo, J. Robertson, B. Rösgen, R. Waser, V. Rana, Nanoscale 8, 17774-17781 (2016) [2] R. Muenstermann, T. Menke, R. Dittmann, R. Waser, Adv. Mater. 22, 4819-4822 (2010) [3] M. Kubicek, R. Schmitt, F. Messerschmitt, J. L. M. Rupp, ACS Nano 9, 10737-10748 (2015) [4] J. S. Lee, S. B. Lee, B. Kahng, T. W. Noh, Appl. Phys. Lett. 102, 253503/1-4 (2013) [5] F. Miao, J J. Yang, J. Borghetti, G. Medeiros-Ribeiro, R. S. Williams, Nanotechnology 22, 254007 (2011) [6] Cooper, C. Baeumer, N. Bernier, A. Marchewka, C. La Torre, R. E. Dunin-Borkowski, S. Menzel, R. Waser, R. Dittmann, Adv. Mater., 1700212 (2017)

A Reliable and Efficient Data Migration Method Based On GlusterFS

Big data storage and high speed data access have become the main performance bottleneck for many big data applications. The higher speed data access and lower cost for storage must be required than some applications having small-scale data. Distributed hierarchical storage provides a good storage way to speed data access and lower cost. But it a data migration method which you choose decide the performance of distributed hierarchical storage system because data migration occurs frequently in hierarchical storage systems. There are many data migration methods, which most of those cannot ensure data utterly integrity after data migration. In this paper, we invent a reliable and efficient data migration method to ensure the utterly integrity of migrated data by MD5 checksum and improve performance of data migration by the pipeline technology. Through adjusting parameters, we get the best performance of data migration by using pipeline in our storage system which is a hierarchical storage system based on GlusterFS.