Coordination Clusters Research Articles

Cluster-Based Coordination Polymers of Mn/Fe-Oxo Pivalates and Isobutyrates

Polynuclear coordination clusters can be readily arranged in cluster-based coordination polymers (CCPs) by appropriate bridging linkers. This review provides an overview of our recent developments in exploring structurally well-defined Mn/Fe-oxo pivalate and isobutyrate building blocks in the formation of CCPs assemblies with an emphasis on synthetic strategies and magnetic properties.

Identifying Challenges and Improvement Approaches for More Efficient Procurement Coordination in Relief Supply Chains

Coordinating humanitarian organizations in the procurement of urgently needed relief items is complex and characterized by decentralization and isolated decision-making. The decentralized coordination of associated tasks often results in the duplication of efforts and redundant/incorrect relief items supplies into single disaster regions, having negative impacts on the most vulnerable ones. This paper devotes attention to the challenges that exist in the coordination of procurement activities in relief supply chains and asks for improvement approaches to facilitate more efficient demand satisfaction in disaster situations. Therefore, the authors apply multiple case study research including expert interviews with procurement agents and heads of logistics from international non-governmental organizations (NGOs), humanitarian start-ups and military logistics centers. Results indicate that a lack of incentives to cooperate, poor communication, unclear division of competences and noncompliance to standards and regulations constitute substantial challenges in procurement coordination. Moreover, expert interview results show that social media integration, procurement flexibility, cluster and collaborative coordination have potential to support improving the coordination of NGOs’ procurement activities. By presenting several theoretical propositions, this paper complements already existing literature and provides a reference point for future research. Practitioners can benefit from findings as they are provided with a guide that allows redesigning certain processes in procurement coordination.

A pentanuclear Er (III) coordination cluster as a catalyst for selective synthesis of 1,2‐disubstituted benzimidazoles

A new tridentate ligand (H3L) was prepared from the reaction of 6‐formyl‐2‐(hydroxymethyl)‐4‐tert‐butylphenol and 2‐amino‐4‐nitrophenol. The ligand H3L and acetylacetone were treated with Er (NO3)3·5H2O, which resulted in the formation of a pentanuclear coordination cluster [Er5(LH)4(acac)4(μ3‐O)(μ3‐OH)(H2O)2].5H2O (1) (acac = acetylacetonate). Five Er (III) ions are arranged in a nonlinear fashion in 1. Complex 1 was utilized as a catalyst towards the selective synthesis of 1,2‐disubstituted benzimidazole derivatives involving o‐phenylenediamine and different aldehydes. Yields of 1,2‐disubstituted benzimidazole derivatives were in the range of 66%–91%. This study demonstrates the first‐ever approach to employ a homo‐ and pentanuclear lanthanide coordination cluster for catalyzing the synthesis of 1,2‐disubstituted benzimidazoles.

Key Drivers to Implement an Evidence-based Tobacco Control Programme in Schools of India: A Mixed-Methods Study.

Background:Adolescence is an influential stage in students’ lives when lifelong behaviours such as tobacco use are formed. During these years, school teachers are important role models for tobacco control among students. A study was conducted among school personnel and administrators to understand the key drivers for implementing an evidence-based school tobacco control program. Methodology:A cross-sectional, mixed-method study was conducted in five districts of Assam, India. The quantitative study was conducted among 565 school personnel across 40 Government-aided schools. Data was collected by means of an anonymous, self-administered questionnaire. Qualitative data was generated from 15 focus group discussions (FGDs) among 146 participants - District Program Officers, Block Education Officers, Cluster Coordinators, Headmasters and Teachers. Results:While the prevalence of smoked tobacco was low (3%), the use of smokeless tobacco was higher (40%), and the prevalence of use of areca nut without tobacco (65%) was still higher among school personnel. They were aware of the school policies prohibiting the use of tobacco among students within or outside school buildings or during school-sponsored activities (81%); they had rather limited knowledge about policy for themselves (58%). There was lack of access to training materials about prevention of tobacco use among youth. The FGDs amongst school personnel resulted in several constructive suggestions on tobacco control in schools mainly in training school teachers, monitoring the program and incentives for execution of the program. However, there was a reluctance to implement a smokeless tobacco control programme since many were current users of smokeless tobacco and areca nut. Conclusion:Tobacco control policies as well as training school personnel in schools need to improve and further measures must be taken to prohibit use of areca nut, which contains carcinogens. The existing system of the education department can be utilised to implement tobacco control programmes effectively.

Cluster Coordination between High-speed Rail Transportation Hub Construction and Regional Economy Based on Big Data

As people’s lives get better and better, more and more people choose to travel and with that comes the demand for more transportation. For now, traditional transportation hubs can temporarily meet people’s travel needs. If driven by big data concepts and methods, the various capabilities of high-speed rail transportation hubs will be sublimated, and the regional economy will be in line with the prosperity of this place. Proportionally, railway hubs are extremely attractive to the rapid growth of the regional economy. This paper takes the high-speed railway hub construction model under big data as the research object and verifies the reliability of the research model and the development of economic regions based on the high-speed railway data in recent years as reference parameters. This article selects the panel data of railway transportation and regional economy in China’s provinces for 10 consecutive years from 2011 to 2020. Among them, seven indicators were selected for railway transportation: passenger volume, freight volume, passenger turnover, cargo turnover, number of railway employees, railway transportation industry fixed asset investment and construction scale, and per capita railway network density. In terms of regional economy, six indicators were selected: regional GDP, per capita GDP, per capita investment in fixed assets, per capita total retail sales of consumer goods, per capita investment in imports and exports, and the proportion of the added value of the tertiary industry in GDP. The experimental results prove that each sample is tested in pairs, the standard error level of the mean is 0.002, which is less than 0.05, and high-speed railway construction can finally achieve economic integration. By improving the development of high-speed railways, continuously shortening the distance between time and space, breaking regional trade barriers, and reducing the cost of commodity circulation, industrial interaction and coordinated development between different regions can be effectively promoted.

Axial Piston Pump Fault Diagnosis Method Based on Symmetrical Polar Coordinate Image and Fuzzy C-Means Clustering Algorithm

In this paper, a fault diagnosis method based on symmetric polar coordinate image and Fuzzy C-Means clustering algorithm is proposed to solve the problem that the fault signal of axial piston pump is not intuitive under the time-domain waveform diagram. In this paper, the sampled vibration signals of axial piston pump were denoised firstly by the combination of ensemble empirical mode decomposition and Pearson correlation coefficient. Secondly, the data, after noise reduction, was converted into images, called snowflake images, according to symmetric polar coordinate method. Different fault types of axial piston pump can be identified by observing the snowflake images. After that, in order to evaluate the research results objectively, the obtained images were converted into Gray-Level Cooccurrence Matrixes. Their multiple eigenvalues were extracted, and the eigenvectors consisting of multiple eigenvalues were classified by Fuzzy C-Means clustering algorithm. Finally, according to the accuracy of classification results, the feasibility of applying the symmetric polar coordinate method to axial piston pump fault diagnosis has been validated.

A {Na2Fe10} isobutyrate cluster, interlinked into 1D chains

The coordination cluster [Na2Fe10O6(OH)4(isobutyrate)14(MeO)2(MeCN)2], derived from a {Fe3(μ3-O)(isobutyrate)6} precursor, features a cluster shell of corner-sharing {Fe3(μ3-O)} motifs, capped by two sodium ions.

Solvent-induced structural transformation from heptanuclear to decanuclear [Co-Ln] coordination clusters: trapping of unique counteranion and understanding of aggregation pathways.

Five new cobalt(ii/iii)-lanthanide(iii)-based coordination aggregates, [LnIII3CoII2CoIII2(L1)2(O2CCMe3)8(OH)4(OMe)2(H2O)4]·Ln(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·2MeOH·2H2O (where Ln = Tb (1), Ho (3), and H2L1 = N-(2-hydroxyethyl)-salicylaldimine), TbIII3CoII3CoIII4(L1)4(O2CCMe3)9(OH)10(H2O) (4) and LnIII3CoII2CoIII5(L1)4(O2CCMe3)10(OH)10 (Ln = Dy (5), Ho (6)) have been synthesized and characterized, including structural analysis via single-crystal X-ray diffraction. The dysprosium analogue (2) of 1 and 3 was previously reported by us. The heptanuclear monocationic clusters in 1 and 3 were formed by placement of seven metal ions (4 Co and 3 Ln) in a vertex shared dicubane structure from the control of two Schiff base anions and crystallized in the presence of in situ generated and literature unknown counter anions Tb(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2- and Ho(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2-. Interesting solvent-induced cluster structure transformation was observed on dissolving the heptanuclear aggregates in MeCN for the formation of decanuclear clusters 4-6. These high nuclearity clusters consist of a vertex shared heptanuclear dicubane part and a curved trinuclear chain linking the two cubic halves. The dicubane unit differs from that of the heptanuclear precursors in the presence of CoII/III at the shared vertex as opposed to LnIII and the absence of OMe- bridges. HRMS (+ve) analysis shed light on the pathway of formation of these heptanuclear molecules, while at the same time revealing a different aggregation process for the decanuclear clusters.

Further synthetic investigation of the general lanthanoid(iii) [Ln(iii)]/copper(ii)/pyridine-2,6-dimethanol/carboxylate reaction system: {CuLn} coordination clusters (Ln = Dy, Tb, Ho) and their yttrium(iii) analogue.

In addition to previously studied {CuGd6}, {CuGd4}, {CuLn7} and {CuLn8} coordination clusters (Ln = trivalent lanthanide) containing pdm2- or Hpdm- ligands (H2pdm = pyridine-2,6-dimethanol) and ancillary carboxylate groups (RCO2-), the present work reports the synthesis and study of three new members of a fifth family of such complexes. Compounds [Cu5Ln4O2(OMe)4(NO3)4(O2CCH2But)2(pdm)4(MeOH)2] (Ln = Dy, 1; Ln = Tb, 2; Ln = Ho, 3) were prepared from the reaction of Ln(NO3)3·xH2O (x = 5, 6), CuX2·yH2O (X = ClO4, Cl, NO3; y = 6, 2 and 3, respectively), H2pdm, ButCH2CO2H and Et3N (2 : 2.5 : 2 : 1 : 9) in MeCN/MeOH. Rather surprisingly, the copper(ii)/yttrium(iii) analogue has a slightly different composition, i.e. [Cu5Y4O2(OMe)4(NO3)2(O2CCH2But)4(pdm)4(MeOH)2] (4). The structures of 1·4MeCN·1.5MeOH and 4·2MeOH were solved by single-crystal X-ray crystallography. The five CuII and four DyIII centres in 1 are held together by two μ5-O2-, four μ-MeO-, two syn,synη1:η1:μ ButCH2CO2-, four η2:η1:η2:μ3 pdm2- (each of these groups chelates a CuII atom and simultaneously bridges two DyIII atoms through its two -CH2O- arms) and two μ-MeOH ligands. The four terminal nitrato groups each chelate (η1:η1) a DyIII centre. The five CuII atoms are co-planar (by symmetry) forming a bow-tie arrangement; the four outer CuII atoms form a rectangle with edges of 3.061(1) and 6.076(1) Å. The four DyIII centres also form a rectangle that lies above and below the plane of the CuII centres, with edges of 3.739(1) and 5.328(1) Å. The two strictly planar rectangles are almost perpendicular. Two trigonal bipyramidal μ5-O2- groups link the perpendicular Cu5 and Dy4 frameworks together. The molecule 4 has a very similar structure to that of 1, differences being the replacement of the two chelating nitrato groups of 1 by two chelating ButCH2CO2- ligands in 4 and the coordination polyhedra of the LnIII and YIII atoms (Snub diphenoids in 1 and biaugmented trigonal prisms in 4). Dc magnetic susceptibility data (χM) on analytically pure samples of 1-3, collected in the 300-2 K range, indicate that ferromagnetic exchange interactions dominate leading to large spin ground states. The χMT vs. T data for 4 suggest moderately strong antiferromagnetic CuIICuII exchange interactions. Studies of the dynamic magnetic properties of the {Cu5Ln4} clusters show that 1 behaves as a SMM at zero field and 2 is a very weak field-induced SMM, while 3 exhibits only weak tails in the χ''Mvs. T plots at various ac frequencies at zero dc field.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration.

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

Coupling coordination of industrial parks and industrial clusters in Hexi Corridor and their spatial correlation analysis

With Hexi Corridor as the study case, this study established an evaluation index system for the development of the industrial parks and industrial clusters, and used the coupling coordination model and exploratory spatial data analysis method (ESDA) to explore the evolution and spatial correlation of the coupling coordination between the industrial park and industrial clusters in Hexi Corridor from 2005 to 2017. It is found that from 2005 to 2017, the industrial parks and industrial clusters has shown closer correlations; despite the high level of coordination, the degree of coupling remains low; the Hexi Corridor has not yet developed a growth engine for the coordinated development of the industrial parks and the clusters, and there is no county-level industrial entities that have strong stimulating power. Given the requirements of the current industrial agglomeration model for regional innovation capabilities, this study proposes suggestions for coordinated development of industrial parks and industrial clusters in Hexi Corridor.

Recent advances in lanthanide coordination polymers and clusters with magnetocaloric effect or single-molecule magnet behavior.

Molecular magnetorefrigerant materials for low-temperature magnetic refrigeration and single-molecule magnets for high-density information storage and quantum computing have received extensive attention from chemists and magnetic experts. Lanthanide ions with unique magnetic properties have always been considered as ideal candidates for the construction of such materials. This frontier article focuses on GdIII-based molecular magnetorefrigerants and lanthanide-based single-molecule magnets and highlights the most significant advances.

Resolvable Cluster Target Tracking Based on the DBSCAN Clustering Algorithm and Labeled RFS

When a sensor can resolve the members in a cluster, it is difficult to accurately track each target due to cooperative interaction among the targets. In this paper, we research the tracking problem of resolvable cluster targets with cooperative interaction. Firstly, we use the stochastic differential equation to model the cluster coordination rules, and the state equation of the single target in the cluster is derived. On this basis, a Bayes recursive filter tracking method based on the combination of the DBSCAN clustering algorithm and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> -GLMB filter is proposed. In the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> -GLMB filter prediction stage, the DBSCAN algorithm is used to determine the cluster where the target is located in real time. Then, the collaborative noise of the target is estimated, which will be used as the input to correct the prediction state of the target. The simulation and experiment results demonstrate the effectiveness of the proposed algorithm when the cluster is splitting, merging, and in reorganization.

Mobile Sink Based Improved Energy Efficient Routing Algorithm Using Cluster Coordinator Node in Wireless Sensor Network

It is imperative to perform energy-effective data transmission over a Wireless Sensor Network (WSN) to maintain the efficacy of the network; the sensor nodes in the network are responsible for relaying the data from the source to the destination node. To this end, a Mobile sink is introduced by the researchers to enhance the transmission ability. Though it is proved to be efficacious still energy consumption of the nodes is higher that in turn affects the reliability of the network. Researchers have developed various methods in which they have used the mobile sink that work in a static method. Thus, a novel approach is designed in which the network is evenly divided into five sections in which sensor nodes are equally distributed. The data transmission occurred through the Cluster Coordinator node (CCO) as they are rechargeable and assist in passing data to the mobile sink in each cluster with high efficacy. The simulation of the model is performed in MATLAB software for different network areas and different radius of sink mobility to evaluate the efficacy of the network in energy consumptions. The comparison with the traditional method demonstrated that incorporation of the CCO enabled the network to consume less energy and increase the network life span of the WSN area for data communication.

A Nanozyme‐Based Artificial Peroxisome Ameliorates Hyperuricemia and Ischemic Stroke

AbstractArtificial peroxisome has drawn a lot of attentions for its usefulness in fabricating protocell system and great potential in treating diseases. However, it is still a significant challenge to prepare a practicable artificial peroxisome to complement multiple and stable functions under physiological condition. Herein, a novel strategy is reported to design an artificial peroxisome using a nanozyme to accommodate multiple enzyme‐like activities that mimics those enzymes in natural peroxisome. The enzymatic active sites are introduced into graphitized moieties on the shell of a hollow carbon nanozyme by doping iron and nitrogen to form Fe–N4 coordination and atomic Fe cluster. With Fe clusters as reversible cofactors and Fe–N4 as prosthetic group, the resulted carbon nanozyme exhibits stable and multiple peroxisomal‐like activities, including catalase, uricase, superoxide dismutase, peroxidase, and oxidase, which is referred as nanozyme‐based artificial peroxisome (pero‐nanozysome). This pero‐nanozysome shows therapeutic effect for treating hyperuricemia and protecting neurons from free radicals generated during ischemic stroke by employing the tandem activities of uricase‐catalase and superoxide‐dismutase‐catalase, respectively. This study indicates that the pero‐nanozysome is a promising candidate to design artificial peroxisome performing in vivo functions.

Heme biosynthesis depends on previously unrecognized acquisition of iron-sulfur cofactors in human amino-levulinic acid dehydratase

Heme biosynthesis and iron-sulfur cluster (ISC) biogenesis are two major mammalian metabolic pathways that require iron. It has long been known that these two pathways interconnect, but the previously described interactions do not fully explain why heme biosynthesis depends on intact ISC biogenesis. Herein we identify a previously unrecognized connection between these two pathways through our discovery that human aminolevulinic acid dehydratase (ALAD), which catalyzes the second step of heme biosynthesis, is an Fe-S protein. We find that several highly conserved cysteines and an Ala306-Phe307-Arg308 motif of human ALAD are important for [Fe4S4] cluster acquisition and coordination. The enzymatic activity of human ALAD is greatly reduced upon loss of its Fe-S cluster, which results in reduced heme biosynthesis in human cells. As ALAD provides an early Fe-S-dependent checkpoint in the heme biosynthetic pathway, our findings help explain why heme biosynthesis depends on intact ISC biogenesis.

Redox Active Hexanuclear Mixed Valence Dicationic Ce(III)/Ce(IV) Coordination Clusters

A series of mixed valence hexanuclear dicationic coordination clusters containing two µ4‐O2– bridges with general formula M2+(X–)2 [M = CeIV2CeIII4(µ4‐O)2(L–R)4(val)6(H2O)2; 12+, 32+: R = H, 22+: Me; 12+, 22+: X = CeIII2(val)3(NO3)4, 32+: X = NO3] were synthesized, isolated and characterized. The reaction of cerium(III)nitrate hexahydrate with Schiff base ligand (H2L–R) and o‐vanillin (val‐H) under basic condition led to the formation of M2+(X–)2 and the reaction progress was monitored by mass spectrometric studies. The molecular structures of all complexes were unambiguously characterized by single‐crystal X‐ray diffraction. The magnetic susceptibility measurements of M2+(X–)2 showed that the Ce(III) ions are weakly antiferromagnetically coupled. Thermal stability and molar conductivity of M2+(X–)2 were also studied. Complex 32+(X–)2 was further studied by X‐ray photoelectron spectroscopy to confirm the oxidation states of Ce(III/IV) ions. The M2+ cation was shown to catalyze the TEMPO‐free oxidation of functionalized benzyl alcohols quantitatively to the corresponding benzaldehyde derivatives at 100 °C in presence of aerial O2 using DMF as the solvent.

Assessment of viral assemblages in different types of wetland water in northeast China using RAPD-PCR

Viruses are the most abundant and ubiquitous biological entities in various ecosystems, yet few investigations of viral communities in wetlands have been performed. To address this data gap, water samples from 6 wetlands were randomly collected across northeast China; viruses in the water were concentrated by sequential tangential flow filtration, and viral communities were assessed through randomly amplified polymorphic DNA-PCR (RAPD-PCR) with 4 decamer oligonucleotide primers. Principal coordinate analysis and hierarchical clustering analysis of the DNA fingerprints showed that viral community compositions differed among the water samples: communities in the 2 coastal wetlands were more similar to each other than to those in the 4 freshwater wetlands. The Shannon-Weaver index (H) and evenness index (E) of the RAPD-PCR fingerprint also differed among the 6 wetlands. Mantel test revealed that the changes in viral communities in wetland water were most closely related to the water NH4+-N and inorganic C content, followed by total K, P, C and NO3--N. DNA sequence analysis of the excised bands revealed that viruses accounted for ~40% of all sequences. Among the hit viral homologs, the majority belonged to the Microviridae. Moreover, variance partitioning analysis showed that the viral community contributed 24.58% while environmental factors explained 30.56% of the bacterial community variation, indicating that the bacterial community composition was strongly affected by both viral community and water variables. This work provides an initial outline of the viral communities from different types of wetlands in northeast China and improves our understanding of the viral diversity in these ecosystems.

A new Zn(II)-cluster coordination polymer: selective detection of Al(III) ion and treatment activity on hypotension after anesthesia by increasing α receptor expression

In this research, a novel coordination polymer (CP) based on Zn(II) ions as nodes [Zn2(1,4-NDC)2(py)2(H2O)] (1, py = pyridine and 1,4-NDCH2 = 1,4-naphthalenedicarboxylic acid) has been produced under hydrothermal reaction condition and then structurally characterized. Furthermore, Zn-complex reveals high sensitivity (with the detection limit of 1.97 × 10−6 M) and fast test time for the Al3+ in the water phase through the fluorescence enhancement method which is rare example in the reported Al3+ detection sensors. The compound was designed for hypotension treatment after anesthesia. First of all, the hypotension animal model was constructed; then, the compound’s protective was evaluated. The real-time reverse transcription-polymerase chain reaction (RT-PCR) was performed, and the α receptor expression on the vascular endothelial cells was determined. In addition to this, the western blot was also conducted for the α receptor expression assessment.

Adaptive variable-size virtual clustering for control channel assignment in dynamic access networks: Design and simulations

Cognitive Radio (CR) technology was introduced to effectively exploit the under-utilization portions of the licensed spectrum. A primary challenge in enabling CR networking is how to coordinate the CR communications, in which CR users need to exchange their control information before data exchange can take place. Providing a reliable coordination mechanism in CR networks (CRNs) is not an easy task because of the dynamic nature of primary radio (PR) activities and the heterogeneity of both spectrum availability and CR users’ distribution. In this paper, we propose a dynamic-size clustering with CSMA/CA-based algorithm to overcome the challenge of achieving reliable coordination in CR networks (CRNs). CR users in our scheme are organized into dynamic size clusters and cooperate in selecting common control channels (CCCs) and use them for cluster coordination. The proposed scheme called Dynamic-hop virtual clustering distributed coordination (DH-VCDC). This scheme provides a dynamic-hop virtual clustering based on the traffic condition of both PR and CR users while achieving a minimum number of common available data channels per cluster, in which a better trade-off between the number of common idle channels per cluster and the cluster size is achieved. DH-VCDC scheme exploits the advantages of using Multi-packet Multi-channel Discontinuous Orthogonal Division Multiplexing (DOFDM) to reduce neighbor discovery time, allow CR users to have several cluster memberships, select multiple CCCs and simplify the inter-cluster coordination. CR users in our scheme are grouped to form virtual clusters each contains at least 2 CCCs (primary and backup CCCs). Three modes of DH-VCDC are presented: one-, two- and three-hop modes. Each CR must continuously monitor both PR and CR traffic loads and accordingly chooses the appropriate DH-VCDC mode. The performance of DH-VCDC is compared with reference schemes, in which significant performance improvement is reported.