Digital twin (DT) can help create a digital representation of a physical system, thereby reflecting its real-time status. The digital object, often called cyber twin (CT), facilitates real-time monitoring and control of the physical object, i.e., the so-called physical twin (PT). Owing to this ability, CTs can optimize the PTs and simulate their status, without interrupting the physical world. Given the various CT use cases, one can identify two distinct types of DT tasks: 1) update tasks for PT-CT synchronization and 2) inference tasks for obtaining real-time testing responses. The diverse real-time requirements for update/inference tasks raise the task scheduling problem that has been neglected in previous studies. In this article, the real-time DT task scheduling problem is investigated. In particular, a new approach for evaluating the performance of real-time scheduling of DT tasks is introduced considering the relationship between update/inference tasks and fairness among CTs. Moreover, offline and online DT task scheduling schemes are proposed with the goals of maximizing the DT freshness ratio and minimizing task rejections. In particular, the DT freshness ratio maximization problem is formulated as an offline task scheduling scheme. The proposed offline solution can significantly reduce the solution space without losing optimality. Furthermore, the scheduling policies for achieving the maximal DT freshness ratio are established using which an online scheduling algorithm is designed. Simulation results show that the proposed offline/online schemes increase the DT freshness ratio by at least 16% and 11%, respectively, compared to benchmarks. The results also show that the task rejection ratio of the proposed online algorithm is within 8% of the lower bound.

Reconfigurable intelligent surface (RIS) is a promising solution to support a large volume of data traffic and massive connectivity for future wireless mobile networks. Especially, RIS can mitigate the drawbacks in massive multiple-input multiple-output (MIMO) systems, such as the blockage caused by obstacles and the signal processing overhead by constructively and passively reflecting the incident wave toward the destination. In this paper, we provide an asymptotic analysis of the distribution of sum rate (SR) in RIS-aided massive MIMO systems. Using the asymptotic distribution of SR, the achievable scheduling gain and the optimal number of users are determined. In addition, we examined the channel hardening effect and outage probability through the achievable scheduling gain, and the optimal number of users is utilized to develop a low-complexity scheduling algorithm. Simulation results reveal that the SR obtained from our analysis closely aligns with the actual SR. The results also show that the channel hardening effect can vanish with many users thereby achieving the multiuser diversity gain, and an RIS-aided system is more reliable than a conventional massive MIMO system in terms of the outage probability. Furthermore, the proposed scheduling algorithm is shown to reduce computational complexity compared to the conventional scheduling algorithm.

The deployment of digital twins (DTs) at the edge of a wireless network can facilitate low-latency and high-throughput DT autonomous and real-time Internet of everything (IoE) applications. In such DT edge networks (DTENs), each DT has two types of real-time tasks that require timely processing: DT update tasks and DT inference tasks. However, the joint scheduling of these two types of tasks has been overlooked in prior works. In this paper, the first joint real-time scheduling scheme for DT update and inference tasks in a DTEN is proposed. Moreover, a novel performance metric called freshness is introduced to capture the effectiveness and synchronization performance of scheduling. Also, a new scheduling scheme is proposed to efficiently solve a freshness maximization problem for DTENs. Simulation results show that the performance of the proposed scheme is within 4% of the upper bound for DTENs with 20 physical objects, and within 12% of the upper bound in worst cases for DTENs with more than 30 physical objects. The results also show that the proposed approach reduces the maximum de-synchronization time by 63% compared to existing real-time scheduling algorithms.

In the Internet of Things (IoT) environment, edge computing can be initiated at anytime and anywhere. However, in an IoT environment, edge computing sessions are often ephemeral, i.e., they last for a short period of time and can often be discontinued once the current application usage is completed or the edge devices leave the system due to factors such as mobility. Therefore, in this paper, the problem of ephemeral edge computing in an IoT is studied by considering scenarios in which edge computing operates within a limited time period. To this end, a novel online framework is proposed in which a source edge node offloads its computing tasks from sensors within an area to neighboring edge nodes for distributed task computing, within the limited period of time of an ephemeral edge computing system. The online nature of the framework allows the edge nodes to optimize their task allocation and decide on which neighbors to use for task processing, even when the tasks are revealed to the source edge node in an online manner, and the information on future task arrivals is unknown. The proposed framework essentially maximizes the number of computed tasks by jointly considering the communication and computation latency. To solve the joint optimization, an online greedy algorithm is proposed and solved by using the primal-dual approach. Since the primal problem provides an upper bound of the original dual problem, the competitive ratio of the online approach is analytically derived as a function of the task sizes and the data rates of the edge nodes. Simulation results show that the proposed online algorithm can achieve a near-optimal task allocation with an optimality gap that is no higher than 7.1% compared to the offline, optimal solution with complete knowledge of all tasks.

In this paper, the achievable sum rate of multiuser scheduling in reconfigurable intelligent surface (RIS)-based mas-sive multiple-input multiple-output systems is investigated. Using asymptotic analysis under the generic condition of large numbers of base station antennas, RISs, and users, the RIS-based sum rate is proven to follow a Gaussian distribution. In addition, based on the characteristics of Gaussian distribution, the conditions for the occurrence of the channel hardening phenomenon and achievable scheduling gain are derived as a function of the number of RISs and users. Numerical results show that the derived RIS-based sum rate and the Monte Carlo simulation results are in close agreement as well as that the proposed achievable sum rate constitutes a meaningful bound to verify the performance of various multiuser scheduling algorithms.

To provide information dissemination among distributed wireless networks, mobile devices can be utilized as a relay node. Communications between mobile nodes and gateways require paradigms to deal with the temporal availability whose link occurs at a certain time interval and lasts for a short period. In addition, it should guarantee the data delivery due to timeout before entering the next coverage. We propose an information dissemination scheme among gateways with mobile relays based on the temporal graph approach. The proposed scheme models the network topology connected between gateways as a temporal graph and applies a minimum spanning tree algorithm to construct the shortest paths. Gateways opportunistically select a forwarding mobile relay whenever it needs lower cost than the pre-constructed paths. Simulation results have shown significant improvement in reducing the cost of traversal weight.

In wireless sensor networks, sink mobility support is one of the essential functionalities in many applications. With continuous advancement, future applications will require not only sink mobility support but also high-performance data delivery service. Multipath routing is one of the promising technologies for improving data delivery performance by collaboratively using alternative or redundant multiple routing paths. However, existing multipath routing protocols had not dealt with sink mobility. As a result, they lead to bad performance in terms of energy efficiency due to the end-to-end path reconstruction. Consequently, a novel multipath management scheme is required thereby supporting sink mobility without performance degradation. In this paper, we propose a multipath management scheme for supporting sink mobility. The proposed scheme dynamically constructs multipath along the moving path of a sink. In addition, the proposed scheme provides the path shortening schemes according to the sink’s movement for reducing energy consumption. Our simulation results show that the proposed scheme is superior to existing path management schemes in terms of reliability and energy efficiency.

In Mobile-Assisted Sensing, a number of mobile devices may be gateways. Thus, the gateway selection procedure should be performed whenever multiple gateways attempt to acquire data from a sensor. Owing to different capacity and status of each mobile gateway, a selected gateway has great impact on the data collection. In this paper, we present possible criteria for gateway selection and compare their performance.

Large-scale phenomena monitoring such as fire or toxic gas monitoring is one of major applications in industrial wireless sensor networks. A multitude of sources stemming from a large-scale phenomenon brings heavy communication overhead to report data to a sink. Many researches have focused on monitoring a large-scale phenomenon and reporting to a static sink with an energy-efficient way. Recently, large-scale phenomena monitoring needs a mobile sink for real-time response. However, the sink mobility support in large-scale phenomenon monitoring brings challenging issues. Given the existing approaches to support sink mobility in individual object detection, a mobile sink should establish one-to-many communication with all sources of a large-scale phenomenon for location update. The one-to-many communication between a mobile sink and all sources triggers early energy depletion and thus reduces the network lifetime. In this paper, we propose the origin-mediated communication scheme exploiting a hierarchy-based architecture to resolve the one-to-many communication problem. The origin node builds the virtual backbone network to propagate a location update message on behalf of the mobile sink. Simulation results show that the proposed scheme outperforms all the other work and exhibits significant amounts of savings in terms of the entire network's energy consumption.

Nowadays, mobile applications are engaged with and depended on the physical world more and more. Interaction with a certain proximity and thing is bringing smarter services into our daily lives through cyber-physical world collaboration. Push notification of meaningful content associated with the proximity or thing is one of key technologies of the cyber-physical services. For example, empty parking spot recommendation in a parking lot or promotion coupon notification of a shop at a shopping cent er should be delivered accordingly. This paper explores reliable push notification of content for novel interactive mobile applications over Bluetooth Low Energy (BLE) beacon infrastructure. Since mobile users move around with different speeds, this paper deals with such difference to relay right content in a reliable manner, called Reliable Push Notification (RPN). RPN dynamically clusters contents and links them to a beacon to achieve reliability, and then adaptively pre-allocate contents for transmission.

In geographical network systems, face routing is the unique recovery solution for local minimum problem. A node in local minimum could not choose its neighbor node closer to the destination. Since the face routing is based on novel graph named planar graph, the nodes search a link between adjacent nodes in the graph and send their data via the link. During the planarization of the topology, many links become hidden although the links are available for communication. Due to the hidden links, the graph leads too many number of data transmission in the face routing. With this reason, face routing leads lots of node energy and shortens the lifetime of sensor networks. This paper proposes the novel face routing which is able to reduce the data transmission. The proposed scheme searches the hidden links which are able to select the farthest neighbors at first. When the proposed scheme finds those neighbor links, it stops the searching behavior and transmits the data packets to them directly in order to reduce the data transmission. The simulation compares the proposed scheme with the previous face routing and shows that the proposed scheme is more energy efficiency.

Geographic greedy routing in Wireless Sensor Networks (WSNs) suffers from the local minimum problem, which cannot select the next forwarding node such as the void. Especially, the void, which encountered during data transmission, have a negative effect on the time-sensitive application by the time delay in order to detour the void. Further, it is caused the void diffusion problem to exploiting the same node repeatedly in order to detour the void. To solve these problems, we proposed a dynamic anchor based void avoidance scheme for real-time service in WSNs. The proposed scheme broadcast the information of the void in network initialization phase in order to reduce a disadvantage caused the shape and location of the void. Furthermore, the source node calculates the dynamic anchor points using the information in order to satisfy the requirement of the real-time application and transmits the data to the sink node. Simulation result shows the proposed scheme is suitable for time-sensitive application and alleviate the void diffusion problem.

The wireless sensor networks (WSNs) are special network which has purpose of gathering information in certain area. The reliability is an important factor in the WSNs for transferring the information to the destination. However, the reliability is decreased while multi-hop transmission of the information because a node which composes the WSNs has limited communication range. Also, the transmission process may suffer from the various factors including the faulty nodes and network congestion. To solve this, the multipath routing protocols, which transfer the information through a multipath to the destination at the same time, have been proposed for improving the reliability. In the existing multipath routing protocols, however, the network scale should be sufficient for constructing multipath to guarantee of the disjoint path and the energy is consumed for multipath construction/maintenance consistently. The energy consumption leads to the reduction of networks lifetime and the restricted network scale may not even ensure the enough reliability. Thus, we proposed Low-Cost Multipath Routing Protocol by Adapting Opportunistic Routing which can reduce consistent energy consumption and less affected by network scale. The proposed protocol exploits the opportunistic routing that improves the single-hop reliability in order to improve the reliability in each path. It reduces the number of paths, which constructed for ensuring the enough reliability. Also, a fewer path reduces the energy consumption and alleviates the restriction of network scale. Simulation results show that the proposed protocol provides the more reliable performance than the existing protocols in the network, which restricted scale, and consumes the less energy.

In wireless sensor networks, sink mobility support is one of the essential functionality in many applications. With continuous advancement, the future applications will require not only sink mobility but also high performance. Multipath routing is one of the promising technologies for improving network performance. However, existing multipath routing protocols have not dealt with the sink mobility. As a result, they lead to bad performance in terms of energy efficiency due to the end-to-end path reconstruction. Consequently, a novel path management scheme is required thereby supporting sink mobility without performance degradation. In this paper we propose a multipath management scheme for supporting sink mobility. The proposed scheme dynamically constructs multipath along the moving path of a sink. In addition, the proposed scheme provides the path shortening mechanism according to the sink's movement for reducing energy consumption. Our simulation results show that the proposed scheme is superior to existing path management schemes in terms of reliability and energy efficiency.

In wireless sensor networks, one of the most important application is tracking diffusive and large-scale phenomena, called continuous object. A number of sources from the large size of a continuous object bring huge communication overhead that former studies have to deal with. Meanwhile, recent application scenarios, such as forest fire suppression by firefighters, require sink mobility support. Most sink mobility support schemes have proposed for individual object tracking. They provide one-to-one connection between a mobile sink and a source. That is, too many connections resulting excess energy consumption are built if continuous object tracking directly adopts them. In this paper, we propose a solution to the problem with building a one-to-many connection between a sink and sources detecting a continuous object. The proposed scheme gradually extends the initial connection between a sink and the first source according to the spread of the continuous object. Our simulation results show that the proposed scheme is superior to existing schemes in terms of energy consumption.

The real-time data dissemination is based on a minimum delivery speed of each hop on the end-to-end shortest distance in order to ensure to reach the destination within the desired time deadline in hop-by-hop fashion. In other words, the schemes depend not only on single hop-delay, but also on the distance between the source and destination. However, the performance of the real-time data dissemination could suffered from some "Voids" in the irregular sensor network which blocks the data forwarding. Namely, the voids lengthen the delivery path and the time for detour the voids. These extended delivery path and time aggravate real-time data dissemination success ratio because the additional distance and time increment were not considered in the calculated minimum delivery speed at source node. To deal with this phenomenon, there are detouring methods of voids such as perimeter routing, back pressure. Although the methods enable a void detouring of data packets, the real-time data dissemination success ratio have not improved much because of a many control message to detour voids. To solve this problem, we propose void avoidance scheme for real-time data dissemination. In our scheme, nodes surrounding a void could detect. This void information are transmitted to whole sensors in the network by broadcasting. When a source transmits data to sink, the source uses the void information to find certain point for avoiding voids by the shortest distance. Thus, the source could set desired speed toward the destination via the point without facing a voids. Performance evaluation shows that our scheme provides better real-time dissemination success ratio in practical environments.

Opportunistic routing (OR) has been emerged as a promising routing strategy to improve transmission reliability for wireless networks. By taking broadcast nature of wireless medium, a set of forwarding candidate nodes receive a data packet simultaneously, so one of them can relay it unless they all fail to receive. This strategy can dramatically improve one-hop reliability even in unreliable wireless links, however, it cannot be directly applied into large-scale wireless multi-hop networks since multi-hop communication significantly decreases end-to-end reliability. In this letter, we propose a novel opportunistic routing scheme for large-scale wireless multi-hop networks, which adopts multipath based routing in order to improve end-to-end reliability. While taking advantages of opportunistic routing, each intermediate node can determine whether it branches into multiple paths or not without any network topology information, so that a desired delivery ratio from a source to a destination given by application demands can be achieved in a fully localized manner.

The adoption of multi-hop wireless networks has received attention from industrial cyber-physical system. In industrial applications, real-time multicasting is widely used for controlling and monitoring machines. In multi-hop wireless networks, the end-to-end delay depends on the end-to-end distance. Existing real-time multicasting studies use a fixed distance limitation to achieving real-time transmission without complexity from time factor. However, when given time factor changes, existing protocols might lead to real-time fails and inefficient multicasting because of the difference between the fixed distance limitation and the maximum transmittable distance under the given time. To solve the problems, in this paper, we propose a flexible real-time multicasting protocol based on given time factor. We first introduce a scheme for obtaining a maximum transmittable distance under a given time. Then we provide a real-time multicasting protocol based on the maximum transmittable distance. Our simulation results show that the proposed protocol achieves higher real-time delivery success ratio and energy efficiency than existing protocols.

Real-time applications in wireless sensor networks require that packets should reach to a sink within the time deadline. To satisfy the requirements, existing studies exploit nodes whose speed is higher than the desired delivery speed decided by the spatiotemporal approach. However, they fail real-time transmission in the area where there is no neighbor nodes which meet the requirements toward a sink since the desired delivery speed is fixed. This paper proposes a scheme to pass the area using time gain which occurs by the nature of the existing real-time transmission scheme. Thus, in the area, a sending node could make full use of nodes which do not satisfy the requirements. In the end, the packet passes across the area with a lower speed than the desired delivery speed. The simulation results show that the proposed scheme is superior in terms of real-time transmission success ratio to the existing schemes.