The future wireless networks need to improve spectrum and energy efficiency to satisfy the increasing demand for high data rate. Device-to device (D2D) communications have the ability to address this problem. This paper focuses on the underlay D2D relay function to improve cellular coverage quality. Although there are a few relevant works in this aspect, there is room for further improvement. For example, there is the constraint on the number of relays in a transmission path, which hardly meets the requirement of the cell-edge devices to fully improve their cellular throughput. Also, there is little energy constraint for underlay D2D relay selection, which is difficult to guarantee the service life of underlay D2D relaying links. Furthermore, without careful regulation of transmission power in terms of cellular coverage improvement, it is not conducive to the improvement of spectrum and energy efficiency in this aspect. Therefore, this paper proposes the improved scheme to deal with these problems, which can: 1) improve spectrum efficiency by using underlay spectrum sharing mode and alleviating its weakness (e.g., co-channel interference); 2) enhance comprehensive performance of underlay D2D relaying links by jointly considering multiple Quality-of-Service (QoS) metrics; 3) reduce overhead of relay selection by proposing a greedy algorithm based on a distributed local search; and 4) improve both energy efficiency and convergence time by designing a new power adjustment scheme based on the improved potential game decision algorithm. The theoretical analysis proves the existence of Nash equilibrium, and the simulation results show that the proposed game decision algorithm accelerates convergence and the proposed whole scheme improves cellular coverage quality.

Multi-Hop Relay-Aided Underlay D2D Communications for Improving Cellular Coverage Quality

In this paper, we propose a full-duplex orthogonal frequency-division multiplexing (OFDM) device-to-device (D2D) system in two-hop networks, where multiple full-duplex decode-and-forward relays assist the transmission from D2D user equipment (DUE) transmitter to DUE receiver. By such a transmission mechanism, the signal transmitted by the DUE transmitter does not need to go through a base station. Meanwhile, due to the adoption of underlay D2D communication protocol, power control mechanisms are thereby necessary to be applied to mitigate the interference to conventional cellular communications. Based on these considerations, we analyze the outage performance of the proposed system, and derive the exact expressions of outage probabilities when bulk and per-subcarrier relay selection criteria are applied. Furthermore, closed-form expressions of outage probabilities are also obtained for special cases when the instantaneous channel state information between BS and cellular user equipments is not accessible, so that a static power control mechanism is applied. Subsequently, we also investigate the outage performance optimization problem by coordinating transmit power among DUE transmitter and relays, and provide a suboptimal solution, which is capable of improving the outage performance. All analyses are substantiated by numerical results provided by Monte Carlo simulations. The analytical and numerical results demonstrated in this paper can provide an insight into the full-duplex relay-assisted OFDM D2D systems, and serve as a guideline for its implementation in next generation networks.

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Multicarrier Relay Selection for Full-Duplex Relay-Assisted OFDM D2D Systems

This paper looks into the problem of a decentralized data offloading within an edge unmanned aerial vehicle (UAV) swarm to mitigate the complexities of a single UAV continually generating and processing large application-specific data. The mobile edge UAVs considered here are multirotor types having constrained energy and processing power, which makes long-term handling of large data volumes impossible for standalone UAVs. The load mitigation is carried out by offloading data from a source UAV to other swarm members with sufficient energy and processing requirements. In this paper, we focus on selecting the most optimal multihop path through the UAVs concerning available energies and processing resources, which can survive the duration of the data offload between the source and a target UAV. We formulate a multiarmed bandit-based offload path selection scheme, which selects the most energy and processing optimized multihop path between a source and a target UAV. Upon comparison of our scheme against the naive shortest path approach, we observe that our approach results in significant savings of collective network energies, even for long operational durations.

Resource-Optimized Multiarmed Bandit-Based Offload Path Selection in Edge UAV Swarms

Device-to-device (D2D) communication is a key enabling technology to facilely realizing the Internet of Things (IoT) due to its spectral and energy efficiencies features. Exploiting the physical-layer network coding (PNC) and energy harvesting (EH) technology, two-way relaying (TWR) D2D communication can achieve significant performance for IoT in terms of data rate and energy efficiency (EE). In this article, we investigate the EH-aided TWR D2D communication sharing the uplink (UL) spectrum of the traditional cellular networks. We assume that the D2D transmitters, receivers, and participating relays can collect renewable energy (RE) from natural resources. Also, the relays are considered to be powered by radio-frequency (RF) signals utilizing the power splitting (PS) protocol. Subject to the Quality of Service (QoS), power, subchannel assignment, EH, and maximum practical power constraints, two nonconvex mixed-integer nonlinear programming (MINLP) problems are formulated. The two problems provide a tradeoff on either maximizing the TWR D2D link (TDL) rate or its EE depending on the IoT application needs. Based on the particle swarm optimization (PSO) algorithm, we propose the rate and EE tradeoff EH-based algorithm (REET-EH) to deal with these problems. The proposed algorithm can optimally perform the resource allocation (RA), PS factors determination, power allocation (PA), and relay selection processes. The numerical results investigate the performance of the REET-EH algorithm and show its consistency over several parameters. Also, the results illustrate that our proposed algorithm improves the system performance compared with other state-of-the-art algorithms with regard to the D2D link rate and EE.

Joint Optimization of Energy-Harvesting-Powered Two-Way Relaying D2D Communication for IoT: A Rate–Energy Efficiency Tradeoff

Device-to-device (D2D) communication is considered as a promising technology for improving both the spectral and energy efficiencies of cellular networks by reusing the resources of conventional cellular users (CUs) for direct communication of two nearby devices in a spatial manner. When the channel between the two D2D devices is highly attenuated, it is necessary to use an intermediate relay to achieve reliable and flexible relay-aided D2D communication. In order to motivate the cooperative relays to participate, it is assumed that they can harvest energy from radio frequency (RF) signals based on the power splitting (PS) protocol as well as renewable energy (RE) sources. However, resource sharing between the cellular and relay-aided D2D links leads to mutual interference that degrades their sum rate. Considering the energy-harvesting relays (EHRs) and downlink (DL) resource sharing, this paper aims to maximize the sum rate of both the links without degrading the quality of service (QoS) requirements of all users. Our maximization problem is formulated as a mixed-integer nonlinear programming (MINLP) problem that cannot be solved in a straightforward manner. Therefore, we propose a low complexity algorithm, namely the resource and power allocation with relay selection EH-aided algorithm (RPRS-EH), which determines the reuse partners, the PS factor sub-optimal value with optimal links power allocation, and provides two different strategies for optimal relay selection. The numerical results show the behavior of the proposed algorithm under various parameters as well as its considerable performance when compared to one of the most recent algorithms in terms of the links sum rate and relay energy efficiency.

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Optimal Resource and Power Allocation With Relay Selection for RF/RE Energy Harvesting Relay-Aided D2D Communication

Device-to-device (D2D) communication is a promising technique for traffic offloading in next-generation cellular systems. However, the use of only direct D2D communications limits the advantages of D2D communications due to relatively long separation distances or poor link quality between source and destination user equipments (UEs). Relay-assisted D2D communication was proposed as a supplement to direct D2D communications for enhancing traffic offloading capacity in long-term evolution–advanced (LTE-A) systems. This work aims to design a relay UE selection strategy for D2D communications, which improves D2D communication performance significantly. We propose a cross-layer relay selection scheme that considers several criteria jointly, including end-to-end data rate, relay-capable UE (RUE) remaining battery time, and end-to-end transmission delay on relay-assisted D2D path. We show how to leverage these criteria at an eNB performing relay selection in D2D communications. In particular, an end-to-end delay estimation model is established based on queuing theory. Simulation results validate the performance of the proposed scheme in terms of total amount of data transmitted under RUE remaining battery and end-to-end transmission delay.

On Relay Selection Schemes for Relay-Assisted D2D Communications in LTE-A Systems

The photon energy-dependent selectivity of photocatalytic CO2-to-CO conversion by CsPbBr3 nanocrystals (NCs) and CsPbBr3/g-C3N4 nanoheterostructures (NHSs) was demonstrated for the first time. The surficial capping ligands of CsPbBr3 NCs would adsorb CO2, resulting in the carboxyl intermediate to process the CO2-to-CO conversion via carbene pathways. The type-II energy band structure at the heterojunction of CsPbBr3/g-C3N4 NHSs would separate the charge carriers, promoting the efficiency in photocatalytic CO2-to-CO conversion. The electron consumption rate of CO2-to-CO conversion for CsPbBr3/g-C3N4 NHSs was found to intensively depend on the rate constant of interfacial hole transfer from CsPbBr3 to g-C3N4. An in situ transient absorption spectroscopy investigation revealed that the half-life time of photoexcited electrons in optimized CsPbBr3/g-C3N4 NHS was extended two times more than that in the CsPbBr3 NCs, resulting in the higher probability of charge carriers to carry out the CO2-to-CO conversion. The current work presents important and novel insights of semiconductor NHSs for solar energy-driven CO2 conversion.

Yao-Jen Chang

Papers

On Relay Selection Schemes for Relay-Assisted D2D Communications in LTE-A Systems

Charge Carrier Dynamics of CsPbBr3/g-C3N4 Nanoheterostructures in Visible-Light-Driven CO2-to-CO Conversion.