Relay User Equipment Research Articles

Performance evaluation of pair selection algorithms in device-to-device communication using relay-assisted techniques

Communication services that are dependable are crucial, particularly during emergencies when the regular infrastructure for communication may be disrupted or nonexistent. In such situations, device-to-device (D2D) communication can be a helpful choice since it allows user equipment (UE) that is close to one another to connect directly, bypassing the cellular network infrastructure. The primary focus of this thesis is the application of D2D communication in a decentralized emergency scenario with a damaged eNodeB. The main objective is to find an appropriate strategy for finding and selecting D2D couples by simulating several methods in MATLAB. This study compares three D2D pair selection algorithms: distance-based, Signal-to-Interference and Noise Ratio (SINR)-based, and data rate-based distance-based. The simulation results show that the data rate-based strategy is the most effective method for selecting D2D couples in emergency scenarios. In contrast to algorithms that rely on distance and SINR, this one reduces the chance of an outage by 20%. Bit error rate (BER), capacity, spectral efficiency, and energy efficiency are the three types of links that are assessed: direct links, relay links, and UE relay links. The results show that, with the lowest BER and maximum data throughput, the direct link is the most reliable and efficient communication option. However, the relay connection and the UE relay link show better overall spectral efficiency in comparison to the direct link, indicating their ability to transport more data per unit of bandwidth. The option that consumes the least energy among the three is the direct link. The study demonstrates the great potential of D2D communication in emergency scenarios where conventional communication infrastructure may not be available. The direct link is the most dependable and effective alternative for communication, according to the data, although the UE link can still function effectively in the event that the direct link is compromised. The data rate-based method is a useful strategy for finding and choosing D2D partners. The results of this study can direct the development of D2D emergency communication solutions in 5G networks.

Two-Way Massive MIMO Relaying Systems With Non-Ideal Transceivers: Joint Power and Hardware Scaling

Two-way massive MIMO amplify-and-forward relaying systems with non-ideal transceivers are investigated in this paper. To be general, multiple-antenna nodes and antenna correlation at both the user equipments (UEs) and the relay are considered, which differentiates the analysis from the prior ones. The achievable rate is analyzed and derived deterministically in closed-form. Joint scaling of the transmission powers and hardware impairments is then particularly investigated. Feasible scaling speeds for the transmission powers and hardware impairments are discovered when the number of relay antennas grows large. It is shown that down scaling of the transmission powers at the UEs and the relay and up scaling of the hardware impairment at the relay with the number of relay antennas are tolerable without reducing the expected rate. However, UE hardware impairment is a key limiting factor to the achievable rate and is not allowed to scale up with the number of relay antennas in order to achieve a non-vanishing rate. Moreover, ceiling effect on the achievable rate is still observable and the ceiling rate varies among different scaling cases. More interestingly, scalings of the UEs transmission power and the relay hardware impairment are found to be offsettable, which means that the relay hardware cost and the UE transmission power are tradable. It is found that the best tradeoff is achieved in the medium scalings of both the relay hardware impairment and UE transmission power. Numerical results are provided to verify the analysis and the tradability between the relay hardware cost and the UE transmission power. The analytical results thus provide solid foundation for flexible system designs under various cost and energy constraints.

On signal processing scheme based on network coding in relay-assisted D2D systems

Network coding (NC) can greatly improve a system’s performance on throughput and channel utilization via corresponding signal processing on the relay side. Implementation of NC in relay-assisted device-to-device (RA-D2D) communications is a promising way to further explore the advantages of RA-D2D communications in cellular systems. This paper proposes a signal processing scheme for RA-D2D which jointly takes into account NC and optimal relay selection. First, the optimal relay user equipment (UE) is selected by jointly considering the end-to-end data rate, end-to-end transmission delay, and relay survival time. Then, in the procedure of signal processing, the transmitted useful signal is combined with interference signal for NC operation, and finally, the original useful signal is recovered at the destination node. Simulation results show that the proposed scheme is able to not only eliminate the interference effectively but also has a superior performance on end-to-end transmission delay, reachability, and amount of transmitted information than that without NC. In particular, signal processing via NC is equivalent to encrypting the signal, which further enhances the information security in D2D communications.

MIMO Cellular Networks Performance Under User-Assisted Relaying

We design user-assisted relaying strategies and analyze their performance in a multiple-input-multiple-output uplink cellular network. In user-assisted relaying, the base station serves an active user equipment (UE) using a transmission scheme adaptive between direct transmission and relaying via another UE according to a cooperation strategy. Modeling the network based on stochastic geometry and Poisson point processes, we propose a practical, geometric-based cooperation strategy determining how to select the relaying UE and when to perform relaying transmission. The proposed relaying transmission employs a simple transmit beamforming and one of two optimized receive combining schemes depending on interference awareness. The extra interference generated by relaying UEs is captured in an interference model that accounts for the adaptive transmission and random locations of these UEs. Integrating the proposed cooperation strategy, transmit and receive beamforming design, and interference model, we analyze the network outage rate performance. Provided sufficient density of potential relay UEs, results demonstrate user-assisted relaying to be most beneficial for cell edge UEs when the relaying UEs are equipped with multiple antenna elements and their signals propagate in relatively high shadowing environments.

Energy- and Spectral-Efficient Adaptive Forwarding Strategy for Multi-Hop Device-to-Device Communications Overlaying Cellular Networks

Device-to-device (D2D) communications in cellular networks enable direct transmissions between user equipments (UEs). If the source UE (SUE) and the destination UE (DUE) are far away from each other or the channel between them is too weak for direct transmission, then multi-hop D2D communications, where relay UEs (RUEs) forward the SUE’s data packets to the DUE, can be used. In this paper, we propose an energy-efficient optimal adaptive forwarding strategy (OAFS) for multi-hop D2D communications. OAFS adaptively chooses between the best relay forwarding (BRF) mode and the cooperative relay beamforming (CRB) mode with the optimal number of RUEs, depending on which of them provides the higher energy efficiency (EE). To reduce the computational complexity for selecting the optimal RUEs for CRB mode, we propose a low-complexity sub-optimal adaptive forwarding strategy (SAFS) that selects between the BRF and the CRB with two RUEs by comparing their EE. Furthermore, a distributed forwarding mode selection approach is proposed to reduce the overhead for forwarding mode selection. The analytical and simulation results show that OAFS and SAFS exhibit significantly higher EE and spectral efficiency than BRF, CRB, direct D2D communications, and conventional cellular communications. SAFS is almost as energy- and spectral-efficient as OAFS.

D2D Relay Selection Based on Joint Fuzzy and Entropy Theories With Social Similarity

The employment of social similarity (SS) aware device-to-device (D2D) user equipment (UE)-to-network (UE-NW) relay helps to improve system capacity and UEs' accessability to the evolved Node B (eNB), where a crucial aspect is the design of the relay helper UE (HUE) selection process. However, few existing studies jointly consider the diverse impacts from various design criteria on the performance of HUE selection. In this paper, we formulate the SS-aware D2D UE-NW relay selection (RS) issue as a multiobjective binary integer linear programming (MOBILP) problem under the constraints of several criteria. Then, we propose a new two-stage D2D UE-NW selection scheme. Specifically, a joint algorithm exploiting the intuitionistic fuzzy analytic hierarchy process and the entropy weight generation method is designed for stage one, which provides both subjective and objective preferences for assisting decision making. In stage two, with the constructed decision weights, the MOBILP can be transformed to a simpler binary integer linear programming problem and thus be solved distributively through an efficient message passing method. Based on realistic social data traces, our simulation results validate the benefits of the proposed two-stage scheme, which ensures high system performances in terms of throughput, fairness and energy efficiency.

Information Processing and Wireless Energy Harvesting in Interference-Aware Public Safety Networks

In a public safety environment, user equipments (UEs) located within the coverage area of evolved NodeB, relay network services to out-of-coverage UEs. However, relay UEs in public safety environments are typically energy constrained and cannot operate indefinitely without recharging. Radio frequency energy harvesting has been proposed as a solution for recharging wireless UEs. In this paper, we propose a scheme for extending the lifetime of a public safety network by wirelessly charging relay UEs. In addition, we propose a relay selection method considering the battery status of relay UEs. The proposed relay selection is defined as a bipartite graph matching problem and the optimal relay is obtained through matching games technique. The proposed scheme not only improves the network lifetime but also extend the network coverage. We also conduct system level simulations to evaluate the performance of the proposed scheme. Simulation results show that the overall performance of the system is improved in terms of achievable throughput and network lifetime.

Narrowband Internet of Things Systems With Opportunistic D2D Communication

Narrowband Internet of Things (NB-IoT) is a new cellular technology introduced by the third generation partnership (3GPP) providing low-power and wide-area coverage for IoT. In this paper, we consider the scenario that NB-IoT is deployed in an heterogeneous network and the quality of the direct link from the NB-IoT user equipment (UE) to the serving base station (BS) cannot satisfy the quality of service requirement for transmission of vital sensing data. Thereupon, device-to-device (D2D) communication is adopted as a routing extension to NB-IoT systems, and thus, enables two-hop routes between NB-IoT UE and the serving BS via a set of D2D relays. As the candidate UE relays work in duty cycle to save energy, we derive a model to select a set of UE relays and perform opportunistic D2D communication according to a working schedule. Two optimization problems are formulated aiming at achieving optimal expected delivery ratio (EDR) and expected two-hop delay, respectively. Dynamic programming-based algorithms are proposed to solve the optimization problems and obtain the optimal working schedule of the relays. Simulation results demonstrate that our proposed maxEDR and minEED algorithms improves the system performance compared with other state-of-the-art algorithms.

Cooperative Device-to-Device Communication for Uplink Transmission in Cellular System

The rapid development of the Internet of Things has brought new challenges to cellular networks with super-dense devices and deep-fading channels. These challenges may substantially decrease the transmission efficiency and increase the device’s power consumption, especially in the uplink. A pressing issue is to improve enhanced Node B’s (eNB) scheduler considering a large number of users. In this paper, a semi-centralized cooperative control method is proposed for the cellular uplink transmissions, where the user equipment (UE) relays are randomly selected according to a certain density decided by the eNB. Two specific cooperative schemes based on device-to-device (D2D) communications are proposed, which are the random UE relay scheme and the one further applying network coding. The D2D interference is considered and modeled based on stochastic geometry. The proposed schemes are analyzed based on two distinct traffic models, i.e., the machine type communications traffic with the small-data feature and the full-buffer traffic. Extensive Monte Carlo simulations have been conducted for the small-data traffic and the closed-form theoretical results have been derived for the full-buffer traffic. Performance gains are achieved in various scenarios and the comparisons between two cooperative schemes are made as well. The results provide an important guideline for the eNB to determine how to select and configure cooperative D2D communication for uplink.

Iterative Relay Scheduling With Hybrid ARQ Under Multiple User Equipment (Type II) Relay Environments

In this paper, we propose an iterative relay scheduling with hybrid ARQ (IRS-HARQ) scheme, which realizes fast jump-in/successive relaying and subframe-based decoding under the multiple user equipment (UE) relay environments applicable to the next-generation cellular systems (e.g., LTE-advanced and beyond). The proposed IRS-HARQ aims to increase the achievable data rate by iteratively scheduling a relatively better UE relay closer to the end user in a probabilistic sense, provided that the relay-to-end user link should be operated in an open-loop and transparent mode. The latter is due to the fact that not only there are no dedicated control channels between the UE relay and the end user, but also a new cell is not created. Under this open-loop and transparent mode, our proposed protocol is implemented by partially exploiting the channel state information based on the overhearing mechanism of acknowledge/negative acknowledge for HARQ. Further, the iterative scheduling enables UE-to-UE direct communication with proximity that offers spatial frequency reuse and energy saving.

Disaster Management Using D2D Communication With Power Transfer and Clustering Techniques

Device-to-device (D2D) communications as an underlay to cellular networks can not only increase the system capacity and energy efficiency but also enable national security and public safety services. A key requirement for these services is to provide alternative access to cellular networks when they are partially or fully damaged due to a natural disaster event. In this paper, we employ energy harvesting (EH) at the relay with simultaneous wireless information and power transfer to prolong the lifetime of energy constrained network. In particular, we consider a user equipment relay that harvests energy from radio frequency signal via base station and use harvested energy for D2D communications. We integrate clustering technique with D2D communications into cellular networks such that communication services can be maintained when the cellular infrastructure becomes partially dysfunctional. Simulation results show that our proposed EH-based D2D clustering model performs efficiently in terms of coverage, energy efficiency, and cluster formation to extend the communication area. Moreover, a novel concept of power transfer in D2D clustering with user equipment relay and cluster head is proposed to provide a new framework to handle critical and emergency situations. The proposed approach is shown to provide significant energy saving for both mobile users and clustering heads to survive in emergency and disaster situations.

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

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.

Efficient sidelink re‐establishment scheme in device‐to‐device communications‐based public safety networks

In cellular-based public safety networks, user equipments (UEs) located in coverage area of active cells are able to act as relay UEs (RUEs). These RUEs provide cellular services to isolated UEs (IUEs). Due to mobility of both an IUE and a RUE, a link between them may be disconnected. Thus, it is necessary to newly establish a link between the IUE and another RUE. Unfortunately, it may cause that packets sent by previous RUE to be lost during the link re-establishment procedure. An efficient sidelink re-establishment procedure is proposed to reduce link re-establishment set-up delay. The performance of the proposed method compared with the conventional scheme used in 3rd Generation Partnership Project is evaluated. The proposed scheme shows better performance than the conventional scheme in terms of signalling overhead and set-up delay for sidelink re-establishment.

Modified Dynamic Decode-and-Forward Relaying Protocol for Type II Relay in LTE-Advanced and Beyond.

In this paper, we propose a modified dynamic decode-and-forward (MoDDF) relaying protocol to meet the critical requirements for user equipment (UE) relays in next-generation cellular systems (e.g., LTE-Advanced and beyond). The proposed MoDDF realizes the fast jump-in relaying and the sequential decoding with an application of random codeset to encoding and re-encoding process at the source and the multiple UE relays, respectively. A subframe-by-subframe decoding based on the accumulated (or buffered) messages is employed to achieve energy, information, or mixed combining. Finally, possible early termination of decoding at the end user can lead to the higher spectral efficiency and more energy saving by reducing the frequency of redundant subframe transmission and decoding. These attractive features eliminate the need of directly exchanging control messages between multiple UE relays and the end user, which is an important prerequisite for the practical UE relay deployment.

Heterogeneous Cellular Network With Energy Harvesting-Based D2D Communication

The concept of mobile user equipment (UE) relay (UER) has been introduced to support device-to-device (D2D) communications for enhancing communication reliability. However, as the UER needs to use its own power for other UE’s data transmission, relaying information in D2D communication may be undesirable for the UER. To overcome this issue, motivated by the recent advances in energy harvesting (EH) techniques, we propose a D2D communication provided EH heterogeneous cellular network (D2D-EHHN), where UERs harvest energy from an access point (AP) and use the harvested energy for D2D communication. We develop a framework for the design and analysis of D2D-EHHN by introducing the EH region (EHR) and modeling the status of harvested energy using Markov chain. The UER distribution is derived, and a transmission mode selection scheme including the efficient UER selection method is proposed. The network outage probability is derived in close form to measure the performance of D2D-EHHN. Based on our analysis results, we explore the effects of network parameters on the outage probability and the optimal offloading bias in terms of the outage probability. Particularly, we show that having a high EH efficiency enhances the performance of D2D-EHHN, but can also degrade, especially for dense network.

Energy-Efficient Resource Allocation for Massive MIMO Amplify-and-Forward Relay Systems

Energy-efficient resource allocation is investigated for multi-pair massive MIMO amplify-and-forward relay systems, where a dedicated relay assists pairwise information exchange among many pieces of single-antenna user equipment (UE). The system energy efficiency (EE) is theoretically analyzed by employing large system analysis and random matrix theory. This analytical result provides excellent approximation for the system with a moderate number of antennas, and it also enables several efficient algorithms, working with a different knowledge of channel state information (CSI), to maximize the system EE by scheduling the optimal numbers of relay antennas and UE pairs as well as the corresponding relay transmission power. In contrast to the conventional resource allocation schemes, the proposed algorithms avoid complicated matrix calculations and the instantaneous CSI of small-scale fading; therefore, they are computationally efficient with low CSI overhead. The proposed optimization framework sheds light on the optimized system configurations, and it also offers an efficient way to achieve EE-oriented resource allocation for the multi-pair massive MIMO relay systems.

UE Relaying Cooperation Over D2D Uplink in Heterogeneous Cellular Networks

We consider beamformer optimization for user equipment (UE) relaying cooperation in heterogeneous cellular networks (HCNs), where the interference from femto cells nearby aggravates the signal-to-interference-plus-noise ratio (SINR) of a macro UE (MUE). A femto UE acts as a relay over device-to-device (D2D) uplink and forward to the MUE, the composite of desired signal and interference to improve the post SINR of the MUE. The beamformers of the UE relay and MUE collaborate to make a balance between the two signals so that the output signal after multistage maximal-ratio combining (MS-MRC) yields a sufficient post SINR for the desired signal. We first consider the case with a single interfering femto access point (FAP) and show that the beamforming for dual-stage MRC can effectively handle the interference. Then, the idea is generalized to the case of two interfering FAPs with a three-stage MRC. A geodesic geometry view allows us to parameterize the BF design with the angles that the involved channel vectors make. This approach makes the optimization of the BF simpler, reduces the feedback overhead, and provides better insight into the problem at hand. Simulations are carried out to validate the performance of the UE relaying cooperation in HCN environments.

Efficient Resource Scheduling by Exploiting Relay Cache for Cellular Networks

In relay-enhanced cellular systems, throughput of User Equipment (UE) is constrained by the bottleneck of the two-hop link, backhaul link (or the first hop link), and access link (the second hop link). To maximize the throughput, resource allocation should be coordinated between these two hops. A common resource scheduling algorithm, Adaptive Distributed Proportional Fair, only ensures that the throughput of the first hop is greater than or equal to that of the second hop. But it cannot guarantee a good balance of the throughput and fairness between the two hops. In this paper, we propose a Two-Hop Balanced Distributed Scheduling (TBS) algorithm by exploiting relay cache for non-real-time data traffic. The evolved Node Basestation (eNB) adaptively adjusts the number of Resource Blocks (RBs) allocated to the backhaul link and direct links based on the cache information of relays. Each relay allocates RBs for relay UEs based on the size of the relay UE’s Transport Block. We also design a relay UE’s ACK feedback mechanism to update the data at relay cache. Simulation results show that the proposed TBS can effectively improve resource utilization and achieve a good trade-off between system throughput and fairness by balancing the throughput of backhaul and access link.

A Distributed Algorithm for Bandwidth Resource Sharing in Relay-Aided Wireless Cellular Networks: From the Perspective of Economic Equilibrium Theory

Relay selection, power control and bandwidth allocation for relay nodes are the key issues in exploiting the advantage of cooperative relaying in wireless networks. In this paper, an effective bandwidth resource sharing scheme is proposed for relay-aided cellular networks. Considering a user equipment (UE) can act as a data source as well as a potential relay for other UEs, and a selfish UE is willing to share its resource for relaying purpose only if the cost can be recovered by the payment of the source UEs, we can formulate this cooperative resource sharing problem as an economic market model. Then, the demand-and-supply theory is employed to derive the equilibrium point (EP, i.e., the optimal price charged by the relay UE and the optimal resource purchased by the source) of the market. To assist the UEs to achieve the EP in decentralized network environments, an effective distributed algorithm, based on the excess demand theory, is developed to approach the EP in an iterative manner. Simulation results show that the proposed algorithm is stability as it can converge to the EP after several ($$<$$<15) times iterative computations.

Multihop cellular: from research to systems, standards, and applications [Guest Editorial

It has been more than a decade since the multihop cellular network (MCN) architecture was first proposed and analyzed in 2000 [1]. As the transmission range decreases <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> times, the number of simultaneous transmissions and hop count increase by <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> times and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> times, respectively, which leads to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> times cellular capacity increase. Fundamental research projects have demonstrated the benefits of MCN in terms of system capacity, service coverage, and network connectivity. Many special issues have been devoted to this stream of research [2–6]. The actual concept behind the MCN architecture could be regarded as a hybrid of mobile ad hoc networks (MANETs) and cellular networks. This concept of "relaying within a cell" also pushed standard bodies to consider solutions embedded with mesh or ad hoc architectures, such as IEEE 802.11s [7], IEEE 802.15.5 [8], and IEEE 802.16j [9]. Now, in the recent standards of the Third Generation Partnership Project (3GPP), Proximity-Based Services (ProSe) [10, 11] related work items also cover the MCN concept. In addition to device-to-device (D2D) direct communications, user equipment (UE)–UE relay and UE–network relay are also supported features. Both infrastructural and infrastructureless architectures are considered. Among many use cases, the most urgent one is public safety. These show that MCN architecture realization is ongoing.