Showing papers on "Relay published in 2014"

Power Allocation Strategies in Energy Harvesting Wireless Cooperative Networks

Abstract: In this paper, a wireless cooperative network is considered, in which multiple source-destination pairs communicate with each other via an energy harvesting relay. The focus of this paper is on the relay's strategies to distribute the harvested energy among the multiple users and their impact on the system performance. Specifically, a non-cooperative strategy that uses the energy harvested from the i-th source as the relay transmission power to the i-th destination is considered first, and asymptotic results show that its outage performance decays as log SNR/SNR. A faster decay rate, 1/SNR, can be achieved by two centralized strategies proposed next, of which a water filling based one can achieve optimal performance with respect to several criteria, at the price of high complexity. An auction based power allocation scheme is also proposed to achieve a better tradeoff between system performance and complexity. Simulation results are provided to confirm the accuracy of the developed analytical results.

Multipair Full-Duplex Relaying With Massive Arrays and Linear Processing

Abstract: We consider a multipair decode-and-forward relay channel, where multiple sources transmit simultaneously their signals to multiple destinations with the help of a full-duplex relay station. We assume that the relay station is equipped with massive arrays, while all sources and destinations have a single antenna. The relay station uses channel estimates obtained from received pilots and zero-forcing (ZF) or maximum-ratio combining/maximum-ratio transmission (MRC/MRT) to process the signals. To significantly reduce the loop interference effect, we propose two techniques: i) using a massive receive antenna array; or ii) using a massive transmit antenna array together with very low transmit power at the relay station. We derive an exact achievable rate expression in closed-form for MRC/MRT processing and an analytical approximation of the achievable rate for ZF processing. This approximation is very tight, particularly for a large number of relay station antennas. These closed-form expressions enable us to determine the regions where the full-duplex mode outperforms the half-duplex mode, as well as to design an optimal power allocation scheme. This optimal power allocation scheme aims to maximize the energy efficiency for a given sum spectral efficiency and under peak power constraints at the relay station and sources. Numerical results verify the effectiveness of the optimal power allocation scheme. Furthermore, we show that, by doubling the number of transmit/receive antennas at the relay station, the transmit power of each source and of the relay station can be reduced by 1.5 dB if the pilot power is equal to the signal power, and by 3 dB if the pilot power is kept fixed, while maintaining a given quality of service.

Wireless Information and Power Transfer With Full Duplex Relaying

Abstract: We consider a dual-hop full-duplex relaying system, where the energy constrained relay node is powered by radio frequency signals from the source using the time-switching architecture, both the amplify-and-forward and decode-and-forward relaying protocols are studied. Specifically, we provide an analytical characterization of the achievable throughput of three different communication modes, namely, instantaneous transmission, delay-constrained transmission, and delay tolerant transmission. In addition, the optimal time split is studied for different transmission modes. Our results reveal that, when the time split is optimized, the full-duplex relaying could substantially boost the system throughput compared to the conventional half-duplex relaying architecture for all three transmission modes. In addition, it is shown that the instantaneous transmission mode attains the highest throughput. However, compared to the delay-constrained transmission mode, the throughput gap is rather small. Unlike the instantaneous time split optimization which requires instantaneous channel state information, the optimal time split in the delay-constrained transmission mode depends only on the statistics of the channel, hence, is suitable for practical implementations.

Simultaneous Information and Energy Transfer in Large-Scale Networks with/without Relaying

Abstract: Energy harvesting (EH) from ambient radio-frequency (RF) electromagnetic waves is an efficient solution for fully autonomous and sustainable communication networks. Most of the related works presented in the literature are based on specific (and small-scale) network structures, which although give useful insights on the potential benefits of the RF-EH technology, cannot characterize the performance of general networks. In this paper, we adopt a large-scale approach of the RF-EH technology and we characterize the performance of a network with random number of transmitter-receiver pairs by using stochastic-geometry tools. Specifically, we analyze the outage probability performance and the average harvested energy, when receivers employ power splitting (PS) technique for "simultaneous" information and energy transfer. A non-cooperative scheme, where information/energy are conveyed only via direct links, is firstly considered and the outage performance of the system as well as the average harvested energy are derived in closed form in function of the power splitting. For this protocol, an interesting optimization problem which minimizes the transmitted power under outage probability and harvesting constraints, is formulated and solved in closed form. In addition, we study a cooperative protocol where sources' transmissions are supported by a random number of potential relays that are randomly distributed into the network. In this case, information/energy can be received at each destination via two independent and orthogonal paths (in case of relaying). We characterize both performance metrics, when a selection combining scheme is applied at the receivers and a single relay is randomly selected for cooperative diversity.

Low-Complexity End-to-End Performance Optimization in MIMO Full-Duplex Relay Systems

Abstract: In this paper, we deal with the deployment of full-duplex relaying in amplify-and-forward (AF) cooperative networks with multiple-antenna terminals. In contrast to previous studies, which focus on the spatial mitigation of the loopback interference (LI) at the relay node, a joint precoding/decoding design that maximizes the end-to-end (e2e) performance is investigated. The proposed precoding incorporates rank-1 zero-forcing (ZF) LI suppression at the relay node and is derived in closed-form by solving appropriate optimization problems. In order to further reduce system complexity, the antenna selection (AS) problem for full-duplex AF cooperative systems is discussed. We investigate different AS schemes to select a single transmit antenna at both the source and the relay, as well as a single receive antenna at both the relay and the destination. To facilitate comparison, exact outage probability expressions and asymptotic approximations of the proposed AS schemes are provided. In order to overcome zero-diversity effects associated with the AS operation, a simple power allocation scheme at the relay node is also investigated and its optimal value is analytically derived. Numerical and simulation results show that the joint ZF-based precoding significantly improves e2e performance, while AS schemes are efficient solutions for scenarios with strict computational constraints.

Throughput and ergodic capacity of wireless energy harvesting based DF relaying network

Abstract: In this paper, we consider a decode-and-forward (DF) relaying network based on wireless energy harvesting. The energy constrained relay node first harvests energy through radio-frequency (RF) signals from the source node. Next, the relay node uses the harvested energy to forward the decoded source information to the destination node. The source node transfers energy and information to the relay node through two mechanisms, i) time switching-based relaying (TSR) and ii) power splitting-based relaying (PSR). Considering wireless energy harvesting constraint at the relay node, we derive the exact analytical expressions of the achievable throughput and ergodic capacity of a DF relaying network for both TSR and PSR schemes. Through numerical analysis, we study the throughput performance of the overall system for different system parameters, such as energy harvesting time, power splitting ratio, and signal-to-noise-ratio (SNR). In particular, the throughput performance of the PSR scheme outperforms the throughput performance of the TSR scheme for a wide range of SNRs.

Relay Selection for Two-Way Full Duplex Relay Networks With Amplify-and-Forward Protocol

Abstract: The full duplex (FD) technique, which allows the communication node to transmit and receive signals over the same frequency band simultaneously, has the potential to double the spectral efficiency in comparison with the traditional half duplex (HD) technique. However, self-interference, leaking from the FD node's transmission to its own reception, has the detrimental impact on the performance of FD communication. In this paper, we analyze and optimize the two-way FD relay system using amplify-and-forward protocol, when the multi-relay scenario is considered. The optimal relay selection scheme in maximizing the effective signal-to-interference and noise ratio is proposed, which significantly improves the system performance than a single relay network. Furthermore, to facilitate the comparisons with the traditional two-way HD relay, the analytical expressions of the two-way FD relay are derived in a closed form, including bit error rate (BER), ergodic capacity, and outage probability. Based on the analytical expressions, the optimal power allocation and the optimal choice of duplex mode, i.e., FD and HD, are obtained by minimizing the outage probability. Monte-Carlo simulations are fulfilled to verify the analytical expressions. The results reveal that the residual self-interference after interference suppression limits the performance of two-way FD relay: when the residual interference is small, the FD mode has lower BER/outage probability and higher ergodic capacity since it utilizes the resources effectively; otherwise, the HD mode achieves lower BER/outage probability and higher ergodic capacity since it can completely cancel the self-interference at the cost of lower resource utilization.

A Differential Sequence Component Protection Scheme for Microgrids With Inverter-Based Distributed Generators

Abstract: The protection of a microgrid containing inverter- based distributed generators (IBDGs) presents several problems if traditional techniques which rely on the current (fuses and overcurrent relays) are used. A possible solution to these problems is the use of a new type of the relay which takes advantage of the enhanced processing techniques and communication infrastructure, both of which are recently becoming available for power networks application. This paper proposes a new communication-based protection scheme for isolated microgrids where a data mining approach is used to identify the relay settings and parameters. A feature selection technique is implemented to help identify the most relevant electrical features required for the fault detection and to establish the best communication strategy to use between relays. The proposed approach is tested using a MATLAB simulation of a facility scale isolated microgrid embedded with IBDGs. The results show that a differential protection scheme that relies on symmetrical components is the most effective strategy for protecting microgrids with IBDGs.

Distributed Power Splitting for SWIPT in Relay Interference Channels using Game Theory

Abstract: In this paper, we consider simultaneous wireless information and power transfer (SWIPT) in relay interference channels, where multiple source-destination pairs communicate through their dedicated energy harvesting relays. Each relay needs to split its received signal from sources into two streams: one for information forwarding and the other for energy harvesting. We develop a distributed power splitting framework using game theory to derive a profile of power splitting ratios for all relays that can achieve a good network-wide performance. Specifically, non-cooperative games are respectively formulated for pure amplify-and-forward (AF) and decode-and-forward (DF) networks, in which each link is modeled as a strategic player who aims to maximize its own achievable rate. The existence and uniqueness for the Nash equilibriums (NEs) of the formulated games are analyzed and a distributed algorithm with provable convergence to achieve the NEs is also developed. Subsequently, the developed framework is extended to the more general network setting with mixed AF and DF relays. All the theoretical analyses are validated by extensive numerical results. Simulation results show that the proposed game-theoretical approach can achieve a near-optimal network-wide performance on average, especially for the scenarios with relatively low and moderate interference.

System and method of relay communication with electronic beam adjustment

Abstract: The invention relates to millimeter-wave point-to-point communication systems. A system comprises two separated millimeter-wave transceivers which provide high throughput data transmission and reception in frequency duplex mode and use high gain antennas capable of electronic scanning in some continuous angle range provided by the control module that implement control algorithms for antenna radiation pattern. Also a method based on the exploitation of scanning antennas of initial beam directions fine adjustment and subsequent beam directions tracking and readjustment when needed is proposed. The proposed system and method provide automatic recovery of failed connection in case of relay station orientation change due to influence of various external factors (wind, vibration, different intensity of heating of the bracing mountings at different time of a day and others) and can be used in backhaul systems of base station sites for mobile networks.

A Low Complexity Antenna Switching for Joint Wireless Information and Energy Transfer in MIMO Relay Channels

Abstract: In this paper, we investigate a low-complexity technique for simultaneous wireless information and energy transfer in multiple-input multiple-output relay channels. The proposed technique exploits the array configuration at the relay node and uses the antenna elements either for conventional decoding or for rectifying (rectennas). In order to keep the complexity low, a dynamic antenna switching between decoding/rectifying is proposed based on the principles of the generalized selection combiner (GSC); the L strongest paths are allocated for decoding while the remaining channel paths for rectifying (and vice versa). The optimal L as well as the allocation strategy that minimizes the outage probability are investigated via theoretical and numerical results. In addition, two performance bounds that provide the optimal performance without the limitation of GSC are proposed by solving a linear programming and a binary knapsack problem, respectively. The proposed technique is extended to scenarios with multi-user interference, where a zero-forcing receiver is used at the relay node; closed-forms expressions for the outage probability are also derived.

Wireless information and power transfer in two-way amplify-and-forward relaying channels

Abstract: Various wireless networks have made the ambient radio frequency signals around the world. Wireless information and power transfer (WIPT) enables the devices to recycle energy from these ambient radio frequency signals and process information simultaneously. In this paper, we develop a WIPT protocol in two-way amplify-and-forward relaying channels, where two sources exchange information via an energy harvesting relay node. The relay node collects energy from the received signal and uses it as the transmission power to forward the received signal. We analytically derive the exact expressions of the outage probability, the ergodic capacity and the finite-SNR diversity-multiplexing trade-off (DMT). Furthermore, the tight closed-form upper and lower bounds of the outage probability and the ergodic capacity are then developed. Moreover, the impact of the power splitting ratio is also evaluated and analyzed. Finally, we show that compared to the non-cooperative relaying scheme, the proposed protocol is a green solution to offer higher transmission rate and more reliable communication without consuming additional resource.

Security Versus Reliability Analysis of Opportunistic Relaying

Abstract: Physical-layer security is emerging as a promising paradigm of securing wireless communications against eavesdropping between legitimate users, when the main link spanning from a source to a destination has better propagation conditions than the wiretap link from a source to an eavesdropper. In this paper, we identify and analyze the tradeoffs between the security and reliability of wireless communications in the presence of eavesdropping attacks. Typically, the reliability of the main link can be improved by increasing the source's transmit power (or decreasing its date rate) to reduce the outage probability (OP), which unfortunately increases the risk that an eavesdropper succeeds in intercepting the source message through the wiretap link, since the OP of the wiretap link also decreases when a higher transmit power (or lower date rate) is used. We characterize the security-reliability tradeoffs (SRT) of conventional direct transmission from the source to the destination in the presence of an eavesdropper, where the security and reliability are quantified in terms of the intercept probability (IP) by an eavesdropper and the OP experienced at the destination, respectively. To improve the SRT, we then propose opportunistic relay selection (ORS) and quantify the attainable SRT improvement upon increasing the number of relays. It is shown that given the maximum tolerable IP, the OP of our ORS scheme approaches zero for N → ∞, where N is the number of relays. Conversely, given the maximum tolerable OP, the IP of our ORS scheme tends to zero for N → ∞.

Multi-Pair Two-Way Amplify-and-Forward Relaying with Very Large Number of Relay Antennas

Abstract: In this paper, we investigate the performance of multi-pair two-way relaying, in which multiple pairs of users exchange information within pair, with the help of a shared relay. Each user has a single antenna, and the relay is equipped with very large number of antennas. The relay adopts the amplify-and-forward protocol, and the beamforming matrixes of maximum-ratio combining/maximum ratio transmission and zero-forcing reception/zero-forcing transmission are both considered. Due to array gain of antenna array, the power of each user or the relay (or both) can be made inversely proportional to the number of relay antennas, without compromising the performance. Thus, three power-scaling schemes are studied. Furthermore, the asymptotic spectral and energy efficiencies of the system are obtained analytically, when the number of relay antennas approaches to infinity. The asymptotic results are beneficial to provide more insightful understandings for the fundamental limits of the very large antenna system, and verified by the Monte-Carlo simulations. The analytical and simulation results reveal that very large antenna arrays in such system can average the small-scale fading, eliminate the inter-pair interference, and reduce the total power consumption.

Max-Ratio Relay Selection in Secure Buffer-Aided Cooperative Wireless Networks

Abstract: This paper considers the security of transmission in buffer-aided decode-and-forward cooperative wireless networks. An eavesdropper which can intercept the data transmission from both the source and relay nodes is considered to threaten the security of transmission. Finite size data buffers are assumed to be available at every relay in order to avoid having to select concurrently the best source-to-relay and relay-to-destination links. A new max-ratio relay selection policy is proposed to optimize the secrecy transmission by considering all the possible source-to-relay and relay-to-destination links and selecting the relay having the link which maximizes the signal to eavesdropper channel gain ratio. Two cases are considered in terms of knowledge of the eavesdropper channel strengths: exact and average gains, respectively. Closed-form expressions for the secrecy outage probability for both cases are obtained, which are verified by simulations. The proposed max-ratio relay selection scheme is shown to outperform one based on a max-min-ratio relay scheme.

Method of relay discovery and communication in a wireless communications system

Abstract: A method of relay discovery and communication for a user equipment (UE) in a wireless communications system, including: receiving a first relay discovery request, from a remote UE which announces relay discovery requests; transmitting a relay discovery response including an identification of the UE, to the remote UE; receiving a second relay discovery request including the identification of the UE, from the remote UE; and activating a relay function to start relaying traffic of Proximity-based Service (ProSe) communication for the remote UE.

Joint Optimization of Cooperative Beamforming and Relay Assignment in Multi-User Wireless Relay Networks

Abstract: This paper considers joint optimization of cooper- ative beamforming and relay assignment for multi-user multi- relay wireless networks to maximize the minimum of the received signal-to-interference-plus-noise ratios (SINR). Separated contin- uous optimization of beamforming and binary optimization of relay assignment already pose very challenging programs. Cer- tainly, their joint optimization, which involves nonconvex objec- tives and coupled constraints in continuous and binary variables, is among the most challenging optimization problems. Even the conventional relaxation of binary constraints by continuous box constraints is still computationally intractable because the relaxed program is still highly nonconvex. However, it is shown in this paper that the joint programs fit well in the d.c. (difference of two convex functions/sets) optimization framework. Efficient optimization algorithms are then developed for both cases of orthogonal and nonorthogonal transmission by multiple users. Simulation results show that the jointly optimized beamforming and relay assignment not only save transmission bandwidth but can also maintain well the network SINRs.

SEECH: Scalable Energy Efficient Clustering Hierarchy Protocol in Wireless Sensor Networks

Abstract: The energy efficiency is an important issue for employ distributed wireless sensor networks in smart space and extreme environments. The cluster-based communication protocols play a considerable role for energy saving in hierarchical wireless sensor networks. In most of traditional clustering algorithms, a cluster head (CH) simultaneously serves as a relay sensor node to transmit its cluster/other clusters data packet(s) to the data sink. As a result, each node would have CH role as many as relay role during network lifetime. In our view, this is inefficient from an energy efficiency perspective because in lots of cases, a node due to its position in the network comparatively is more proper to work as a CH and/a relay. This paper proposes a new distributed algorithm named scalable energy efficient clustering hierarchy (SEECH), which selects CHs and relays separately and based on nodes eligibilities. In this way, high and low degree nodes are, respectively, employed as CHs and relays. In only a few past researches, CHs and relays are different, but their goal was mainly mitigation of CHs energy burden which is intrinsically satisfied by the proposed mechanism. To consider uniformity of CHs to balance clusters, SEECH uses a new distance-based algorithm. Comparisons with LEACH and TCAC protocols show obvious better performance of SEECH in term of lifetime. To evaluate the scalability of SEECH strategy, simulations are conducted in three different network size scenarios.

Secure Transmission with Optimal Power Allocation in Untrusted Relay Networks

Abstract: We consider the problem of secure transmission in two-hop amplify-and-forward untrusted relay networks. We analyze the ergodic secrecy capacity (ESC) and present compact expressions for the ESC in the high signal-to-noise ratio regime. We also examine the impact of large scale antenna arrays at either the source or the destination. For large antenna arrays at the source, we confirm that the ESC is solely determined by the channel between the relay and the destination. For very large antenna arrays at the destination, we confirm that the ESC is solely determined by the channel between the source and the relay.

Multiuser Relaying over Mixed RF/FSO Links

Abstract: A multiuser dual-hop relaying system over mixed radio frequency/free-space optical (RF/FSO) links is investigated. Specifically, the system consists of m single-antenna sources, a relay node equipped with n≥ m receive antennas and a single photo-aperture transmitter, and one destination equipped with a single photo-detector. RF links are used for the simultaneous data transmission from multiple sources to the relay. The relay operates under the decode-and-forward protocol and utilizes the popular V-BLAST technique by successively decoding each user's transmitted stream. Two common norm-based orderings are adopted, i.e., the streams are decoded in an ascending or a descending order. After V-BLAST, the relay retransmits the decoded information to the destination via a point-to-point FSO link in m consecutive timeslots. Analytical expressions for the end-to-end outage probability and average symbol error probability of each user are derived, while closed-form asymptotic expressions are also presented. Capitalizing on the derived results, some engineering insights are manifested, such as the coding and diversity gain of each user, the impact of the pointing error displacement on the FSO link and the V-BLAST ordering effectiveness at the relay.

Resource Allocation Under Channel Uncertainties for Relay-Aided Device-to-Device Communication Underlaying LTE-A Cellular Networks

Abstract: Device-to-device (D2D) communication in cellular networks allows direct transmission between two cellular devices with local communication needs. Due to the increasing number of autonomous heterogeneous devices in future mobile networks, an efficient resource allocation scheme is required to maximize network throughput and achieve higher spectral efficiency. In this paper, performance of network-integrated D2D communication under channel uncertainties is investigated where D2D traffic is carried through relay nodes. Considering a multi-user and multi-relay network, we propose a robust distributed solution for resource allocation with a view to maximizing network sum-rate when the interference from other relay nodes and the link gains are uncertain. An optimization problem is formulated for allocating radio resources at the relays to maximize end-to-end rate as well as satisfy the quality-of-service (QoS) requirements for cellular and D2D user equipments under total power constraint. Each of the uncertain parameters is modeled by a bounded distance between its estimated and bounded values. We show that the robust problem is convex and a gradient-aided dual decomposition algorithm is applied to allocate radio resources in a distributed manner. Finally, to reduce the cost of robustness defined as the reduction of achievable sum-rate, we utilize the chance constraint approach to achieve a trade-off between robustness and optimality. The numerical results show that there is a distance threshold beyond which relay-aided D2D communication significantly improves network performance when compared to direct communication between D2D peers.

Secure Multiuser Communications in Multiple Amplify-and-Forward Relay Networks

Abstract: This paper proposes relay selection to increase the physical layer security in multiuser cooperative relay networks with multiple amplify-and-forward relays, in the presence of multiple eavesdroppers. To strengthen the network security against eavesdropping attack, we present three criteria to select the best relay and user pair. Specifically, criteria I and II study the received signal-to-noise ratio (SNR) at the receivers, and perform the selection by maximizing the SNR ratio of the user to the eavesdroppers. To this end, criterion I relies on both the main and eavesdropper links, while criterion II relies on the main links only. Criterion III is the standard max-min selection criterion, which maximizes the minimum of the dual-hop channel gains of main links. For the three selection criteria, we examine the system secrecy performance by deriving the analytical expressions for the secrecy outage probability. We also derive the asymptotic analysis for the secrecy outage probability with high main-to-eavesdropper ratio. From the asymptotic analysis, an interesting observation is reached: for each criterion, the system diversity order is equivalent to the number of relays regardless of the number of users and eavesdroppers.

Energy Efficiency Optimization in Relay-Assisted MIMO Systems With Perfect and Statistical CSI

Abstract: A framework for energy-efficient resource allocation in a single-user, amplify-and-forward (AF), relay-assisted, multiple-input-multiple-output (MIMO) system is devised in this paper. Previous results in this area have focused on rate maximization or sum power minimization problems, whereas fewer results are available when bits/Joule energy efficiency (EE) optimization is the goal. Here, the performance metric to optimize is the ratio between the system's achievable rate and the total consumed power. The optimization is carried out with respect to the source and relay precoding matrices, subject to quality-of-service (QoS) and power constraints. Such a challenging non-convex optimization problem is tackled by means of fractional programming and alternating maximization algorithms, for various channel state information (CSI) assumptions at the source and relay. In particular the scenarios of perfect CSI and those of statistical CSI for either the source-relay or the relay-destination channel are addressed. Moreover, sufficient conditions for beamforming optimality are derived, which is useful in simplifying the system design. Numerical results are provided to corroborate the validity of the theoretical findings.

Secure Beamforming for MIMO Two-Way Communications With an Untrusted Relay

Abstract: This paper studies the secure beamforming design in a multiple-antenna three-node system where two source nodes exchange messages with the help of an untrusted relay node. The relay acts as both an essential signal forwarder and a potential eavesdropper. Both two-phase and three-phase two-way relay strategies are considered. Our goal is to jointly optimize the source and relay beamformers for maximizing the secrecy sum rate of the two-way communications. We first derive the optimal relay beamformer structures. Then, iterative algorithms are proposed to find source and relay beamformers jointly based on alternating optimization. Furthermore, we conduct asymptotic analysis on the maximum secrecy sum-rate. Our analysis shows that when all transmit powers approach infinity, the two-phase two-way relay scheme achieves the maximum secrecy sum rate if the source beamformers are designed such that the received signals at the relay align in the same direction. This reveals an important advantage of signal alignment technique in against eavesdropping. It is also shown that if the source powers approach zero, the three-phase scheme performs the best while the two-phase scheme is even worse than direct transmission. Simulation results have verified the efficiency of the proposed secure beamforming algorithms as well as the analytical findings.

An Iterative Hungarian Method to Joint Relay Selection and Resource Allocation for D2D Communications

Abstract: The joint relay selection and related subchannel and power allocation problem is investigated for relay-aided device-to-device (D2D) communications underlying cellular networks. We show the optimal power allocation problem can be solved in a closed-form. Considering that the associated relay selection and subchannel assignment problem is NP-complete, we devise an iterative technique, the iterative Hungarian method (IHM). Interestingly, numerical results show that the proposed technique can offer near-optimal performance with polynomial complexity.

Energy-efficient maneuvering and communication of a single UAV-based relay

Abstract: We focus on energy-efficient circular maneuvering and communication for a small unmanned aerial vehicle (UAV)-based relay, acting as a communication relay for connecting two stationary ground nodes. Based on an energy-efficiency metric, which is defined as the ratio of network capacity to the power consumption of both maneuvering and communication, we formulate an energy efficiency maximization problem with two variables: speed and load factor, of the UAV-based relay, and then obtain a closed-form suboptimal solution.

Optimal Power Allocation and Capacity of Full-Duplex AF Relaying under Residual Self-Interference

Abstract: This paper investigates the optimal power allocation scheme and corresponding capacity limit of a full-duplex (FD) dual-hop amplify-and-forward relay system under residual self-interference whose variance is proportional to the λ'th power of the transmitted power (0≤λ≤1). At first, the related optimization problems are shown to be quasiconcave under both per-node and sum power constraints. Given the non-linearity of the derivative, bisection is then proposed to obtain the optimal power strategies. The capacity and optimal schemes are then analyzed in different high power regions. Specifically, we apply the dominant balance method to show that full power at the relay is suboptimal when its power constraint approaches a large value. Following a similar approach, we then show that the multiplexing gain of the FD scheme with the optimal allocation is 1/[1+λ]. Comparisons between the half-duplex and FD systems are finally carried out, where analytical and simulation results reveal that the FD system is superior in high source power regions with either fixed or large power constraints at the relay.

Secure Relay Beamforming for Simultaneous Wireless Information and Power Transfer in Nonregenerative Relay Networks

Abstract: For simultaneous wireless information and power transfer (SWIPT), secure communication is an important issue. In this correspondence, we study the secure relay beamforming (SRB) scheme for SWIPT in a nonregenerative multiantenna relay network. We propose a constrained concave convex procedure (CCCP)-based iterative algorithm that is able to achieve a local optimum, where the secrecy rate is maximized, and the relay transmit power and energy harvesting constraints are satisfied. Simulation results have shown that our proposed CCCP-based iterative algorithm achieves a larger secrecy rate and lower computational complexity than the convectional SRB schemes. Since the CCCP-based iterative algorithm is still complex, we propose a semidefinite programming (SDP)-based noniterative suboptimal algorithm and a closed-form suboptimal algorithm. It is shown that when the maximum transmit power of the relay to noise power ratio is high, the SDP-based noniterative suboptimal algorithm performs close to the CCCP-based iterative algorithm.

Energy Efficient MIMO Relay Transmissions via Joint Power Allocations

Abstract: The energy efficient joint source-relay power allocation problem is studied for the multi-input multi-output (MIMO) amplify-and-forward two-hop relaying system, in which the objective of the optimization is the number of the bits per second per hertz per Joule with the guarantee of the minimum spectral efficiency (SE). By assuming perfect channel state information at the transmitter, the MIMO channel is divided into several single-input single-output (SISO) subchannels via singular value decomposition (SVD). The problem is first established as an optimization subject to the minimum SE constraint, in which it is observed that both of the cost function and the constraint are not convex. By employing the high signal-to-noise ratio (SNR) approximation, the problem becomes suddenly a pseudo-convex optimization problem. Yet, the difficulty comes from solving the final Lagrangian equation, which is tackled by our proposed relaxation method according to the Jensen inequality. Finally, the analytical expression is derived for the energy efficient power allocations of the multiple antennas at the source and at the relay. Simulations demonstrate the effectiveness of the proposed method.

Two-way relaying networks with wireless power transfer: Policies design and throughput analysis

Abstract: This paper exploits an amplify-and-forward (AF) two-way relaying network (TWRN), where an energy constrained relay node harvests energy with wireless power transfer. Two bidirectional protocols, multiple access broadcast (MABC) protocol and time division broadcast (TDBC) protocol, are considered. Three wireless power transfer policies, namely, 1) dual-source (DS) power transfer; 2) single-fixed-source (SFS) power transfer; and 3) single-best-source (SBS) power transfer are proposed and well-designed based on time switching receiver architecture. We derive analytical expressions to determine the throughput both for delay-limited transmission and delay-tolerant transmission. Numerical results corroborate our analysis and show that MABC protocol achieves a higher throughput than TDBC protocol. An important observation is that SBS policy offers a good tradeoff between throughput and power.