Time Switching Ratio Research Articles

Full-duplex wireless information and power transfer in two-way relaying networks with self-energy recycling

In this paper, we consider full-duplex (FD) two-way relaying networks, where wireless power transfers from hybrid power access-point, H to user and wireless information transmits from user to H via relay. We propose FD-based power splitting (PS) and time-switching (TS) protocols which enables the energy harvesting (EH) and information processing at relay. These protocols overcome the limitation of spectral efficiency and in band self-interference caused by half-duplex relaying and operation of FD respectively. The energy constraint nodes such as relay and user harvest energy from H by radio-frequency signal, where relay also harvests energy by self-energy recycling. The optimal PS ratio and the optimal TS ratio are investigated and expressed in closed-form. With Rayleigh fading channel, we derive the end-to-end ergodic capacity for PS and TS protocols. Numerical results show that our proposed FD-based EH schemes outperform the benchmark schemes significantly. In particular, the PS protocol is superior to TS protocol at high transmit power of H while at low transmit power of H, TS protocol is better than the PS protocol.

EPS-TRA: Energy Efficient Peer Selection and Time Switching Ratio Allocation for SWIPT-Enabled D2D Communication

This paper considers device-to-device (D2D) network with Simultaneous Wireless Information and Power Transfer (SWIPT) enabled devices to ensure self-sustained communication in situations like disasters. Such direct link networks can ensure connectivity with devices having drained back-up, when trapped in collapsed infrastructure, through mutual sharing of energy on RF link. To guarantee successful execution of SWIPT session for an isolated device in wake of disasters, it is pertinent to select a reliable peer with ultimate aim to maximize link Energy Efficiency (EE). In practice, Energy Harvesting (EH) is not achievable after Information Decoding (ID); however, it has been made possible through splitting the signal in the time domain. Selection of D2D peer for self-sustained communication with an objective to maximize EE through optimum time based splitting of signal has not been extensively studied. In this paper to manifest the aforesaid goal, we worked out a joint problem of peer association and time switching ratio allocation with an objective to maximize the EE for a device contained under collapsed infrastructure. We propose an Energy efficient Peer Selection and Time switching Ratio Allocation (EPS-TRA) algorithm to solve the proposed mixed integer problem. Numerical results validate our proposed approach in acquiring better EE when compared with Uniform Allocation Scheme of time slots for EH & ID. Furthermore, results explain how EE of the link varies with the choice of constrained variables i.e., data rate and harvested energy.

Adaptive time-switching and power-Splitting protocols for energy harvesting sensor networks with multiple relays

In the conventional energy-harvesting (EH) relaying protocols based on time switching (TS) and power splitting (PS), the system performance depends on the parameters such as TS and PS ratios. In order to achieve the optimum performance, these parameters need to be optimized. In the distributed cooperative networks, where relay operates independently, the optimization of PS and TS ratios becomes cumbersome and challenging. In this work, we overcome this issue by proposing two relaying protocols referred to as adaptive time-switching (ATS) and adaptive time-switching adaptive power-splitting (ATS-APS) protocols. In the proposed protocols, the relay can dynamically optimize their respective TS and PS ratios according to their local channel condition among relays, i.e., without any external help from the source or destination, which makes the proposed protocols promising for the distributed relaying network. Through rigorous numerical analysis, we show that the proposed protocols can outperform the conventional approach. We also derive mathematical expressions for the outage probability and the effective transmission rate in the framework of decode-and-forward (DF) relaying with partial relay selection (PRS) scheme based on various diversity combining techniques including maximum ratio combining (MRC), selection combining (SC), and selective relaying (SR). Through numerical analysis, we demonstrate that the proposed adaptive SR technique, where the source receives assistance from the relay only if the source-to-destination link is in deep fading, can achieve better performance than the other techniques compared here in terms of the achievable effective transmission rate. All the theoretical results developed are confirmed by numerical simulations.

Modeling and Performance Analysis for Ambient Backscattering Underlaying Cellular Networks

Ambient backscatter is an intriguing technology that enables ultra-low-power devices to communicate with each other by reflecting and modulating the existing pervasive RF signals, e.g., TV signals, WIFI signals, cellular signals, etc. This paper works on the mathematical modeling and performance analysis of ambient backscatter communications (AmBackComs) underlying cellular networks for Internet of Things applications. Based on the time switching (TS) and power splitting (PS) receiver architectures, two operating schemes are employed to enable energy harvesting and ambient backscattering at the ambient backscatter (AB) transmitter. In order to maximize the outage capacity for the AB transmitters, we formulate two optimization problems optimizing the TS and PS ratios. Two schemes with low complexity are proposed to obtain the near-optimal outage performance. Considering the coexistence of the AmBackComs and cellular downlink transmissions, successful transmission probabilities for AB transmitters and cellular downlink transmissions are both derived for the TS and PS schemes and provide insights on how various system parameters, such as the TS ratio, the PS ratio, the density of base stations (BSs)/AB transmitters, etc., impact the system performance. Numerical results show that the outage capacity for AmBackComs can be significantly improved by selecting a proper TS/PS ratio and our proposed schemes can achieve near-optimal outage capacities. The study also shows that many more ultra-low-power devices users can be connected via the proposed scheme at a cost of a slightly reduced overall network capacity.

Joint beamforming and time switching designs for energy-constrained cognitive two-way relay networks

In this paper, we investigate a joint beamforming and time switching (TS) design for an energy-constrained cognitive two-way relay (TWR) network. In the network, the energy-constrained secondary user (SU) relay employs TS protocol to harvest energy from the signals sent by the circuit-powered primary user (PU) transmitter, and then exploits the harvested energy to perform information forwarding. Our aim is to maximize the sum rate of SUs under the constraints of the data rate of PU, the energy harvesting and the transmit power of the SU relay. To determine the beamforming matrix and TS ratio, we decouple the original non-convex problem into two subproblems which can be solved by semidefinite relaxation and successive convex optimization methods. Furthermore, we derive closed form expressions of the optimal solutions for each subproblem, which facilitates the design of a suboptimal iterative algorithm to handle the original sum rate maximization problem. Simulation results are presented to illustrate the effectiveness and superior performance of the proposed joint design against other conventional schemes in the literature.

Energy-Constrained UAV-Assisted Secure Communications With Position Optimization and Cooperative Jamming

In this paper, we consider an energy-constrained unmanned aerial vehicle (UAV)-enabled mobile relay assisted secure communication system in the presence of a legitimate source-destination pair and multiple eavesdroppers with imperfect locations. The energy-constrained UAV employs the power splitting (PS) scheme to simultaneously receive information and harvest energy from the source, and then exploits the time switching (TS) protocol to perform information relaying. Furthermore, we consider a full-duplex destination node which can simultaneously receive confidential signals from the UAV and cooperatively transmit artificial noise (AN) signals to confuse malicious eavesdroppers. To further enhance the reliability and security of this system, we formulate a worst case secrecy rate maximization problem, which jointly optimizes the position of the UAV, the AN transmit power, as well as the PS and TS ratios. The formulated problem is non-convex and generally intractable. In order to circumvent the non-convexity, we decouple the original optimization problem into three subproblems; this facilitates the design of a suboptimal iterative algorithm. In each iteration, we propose a multi-dimensional search and numerical method to handle the subproblem. Numerical simulation results are provided to demonstrate the effectiveness and superior performance of the proposed joint design versus the conventional schemes in the literature.

Outage Analysis of RF Energy Harvesting Cooperative Communication Systems Over Nakagami-$m$ Fading Channels With Integer and Non-Integer $m$

The study of the effects of Nakagami- $m$ fading channels with both integer and non-integer $m$ plays an important role in the design of practical wireless systems. In this paper, we investigate a dual-hop cooperative communication system, where source node communicates with destination node through direct and relaying paths. Partial relay selection (PRS) scheme is used to select the best relay among $N$ relays which are capable of harvesting energy from radio frequency (RF) signals. At the destination, selection combining (SC) technique is employed to combine the signals of the direct path and the relaying path. We consider two relaying protocols, i.e. amplify-and-forward (AF) and decode-and-forward (DF). For both AF and DF protocols, we derive the tight lower-bound closed-form approximate expression and the exact closed-form expression of the outage probability (OP) over Nakagami- $m$ fading channels with both integer and non-integer $m$ . Moreover, the optimal energy harvesting duration which minimizes the OP is also determined. Numerical results show that, the OP of DF protocol and its minimum value is lower than those of AF protocol for the same parameter settings. Moreover, the time switching ratio which gives the smallest OP in the case without direct path is larger than the case with direct path.

Joint Precoder Design for SWIPT-Enabled MIMO Relay Networks With Finite-Alphabet Inputs

In this paper, the problem of mutual information maximization in a two-hop multiple-input multiple-output (MIMO) relay network with simultaneous wireless information and power transfer (SWIPT) is investigated, where the relay node, without constant power supply, harvests the energy for information forwarding. The goal is to maximize the mutual information by using the joint design of source and relay precoders, which is formulated as an optimization problem under the constraints of transmit power and harvested energy. Two scenarios with practical energy harvesting schemes employed at the relay, i.e., power splitting (PS) and time switching (TS) schemes, are considered. Although the formulated optimization problems in the two scenarios are both nondeterministic polynomial-time (NP)-hard, by exploiting the structure of the optimization problems and analyzing the characteristics of precoders, we develop near optimal joint source and relay precoding algorithms for them. Additionally, we analyze the feasible regions of PS and TS ratios, respectively, and propose a backtracking line search based method to find near optimal PS and TS ratios. The main contributions of this paper are as follows: 1) Unlike existing works based on the assumption of ideal Gaussian signals, this paper supposes the inputs to the network are finite-alphabet signals, which is a more practical scenario. 2) The high complexity of mutual information with finite-alphabet signals leads to an intractable optimization problem; however, an efficient solving framework based on semidefinite relaxation (SDR) and Karush-Kuhn-Tucker (KKT) theorem is proposed. Finally, simulation results verify the efficacy of the proposed joint precoding designs.

Exploiting Opportunistic Scheduling Schemes and WPT-Based Multi-Hop Transmissions to Improve Physical Layer Security in Wireless Sensor Networks.

This paper studies the secrecy performance of wireless power transfer (WPT)-based multi-hop transmissions in wireless sensors networks (WSNs), where legitimate nodes harvest energy from multiple power beacons (PBs) to support the multi-hop secure data transmission to a destination in the presence of an eavesdropper. Specifically, the PBs not only transfer radio frequency energy to the legitimate nodes but also act as friendly jammers to protect data transmission. To improve secrecy performance, we propose two secure scheduling schemes, named minimum node selection (MNS) scheme and optimal node selection (ONS) scheme. We then evaluate the performance of the proposed schemes in terms of the exact closed-form for secrecy outage probability (SOP) and asymptotic SOP. The developed analyses are verified by Monte-Carlo simulations. The numerical results show that the ONS scheme outperforms the MNS scheme emerging as an effective protocol for secure multi-hop transmission in WSNs. Furthermore, the effects of the number of PBs, number of hops, time switching ratio, and the secure target data rate on the system performance are also investigated.

Performance analysis of power beacon-assisted energy harvesting NOMA multi-user relaying system over Nakagami-m fading channels

In this paper, we propose and analyze a multi-user non-orthogonal multiple access (NOMA) relaying system under the effect of Nakagami-m fading. In this system, both source and the relay harvest the energy from a multiple-antenna power beacon (PB) in the first time duration by using the time switching protocol and then use the harvested energy to transmit and receive the signals in the remaining time duration. We derive the exact closed-form expressions of the outage probability in two cases of successive interference cancellation (SIC), i.e., perfect SIC and imperfect SIC. Moreover, we determine the optimal time switching ratio that maximizes the system throughput. We also compare the achievable sum rate of the proposed NOMA relaying system with that of the orthogonal multiple access (OMA) relaying system. The correctness of all derived mathematical expressions results in this paper is verified by Monte-Carlo simulations. Numerical results show that: (1) the fading severity of wireless channels has a significant impact on the outage probability and the diversity; (2) increasing the number of antennas of PB helps to lower the outage probability, decrease the energy harvesting time, and increase the throughput; (3) the sum rate of the proposed NOMA relaying system is proportional to the SIC quality and is higher than that of the OMA relaying system in the low and moderate signal-to-noise ratio (SNR) regimes.

Resource allocation for wireless information and power transfer based on WBAN

In this paper, a simultaneous wireless information and power transfer (SWIPT) system with a relay in wireless body area network (WBAN) is studied, where the relay harvests energy from the radio frequency (RF) signals sent by the access point (AP), then uses RF energy to transmit energy to the user and forward the information to the AP, respectively. Firstly, we propose a planar transmission model based on the time switching (TS) and power splitting (PS) protocols. The sensor nodes are placed on the human torso in this model. In order to explore the system performance, the optimal information throughput for the two protocols is given subject to the energy harvesting and consumption balance. Then, corresponding optimal power splitting and time switching ratios which maximize the information throughputs are obtained. Besides, the effects of the relay position on system performance are also discussed. Finally, while considering the relay transmission power constraint, we solve the dynamic power allocation problem by the Lagrange multiplier iteration method. Numerical results and analysis show that our proposed optimal strategy achieves the optimal throughput. Moreover, the TS protocol has better performance than the PS protocol in dynamic model.

Impact of Outdated CSI on the Secrecy Performance of Wireless-Powered Untrusted Relay Networks

We investigate the effect of outdated channel state information (CSI) on the secrecy performance of wireless-powered untrusted relay networks, in which the relay is a potential eavesdropper. To keep the information secret from this untrusted relay, the destination sends a jamming signal to the relay when the source transmits an information signal. At the same time, the relay harvests energy from the radio-frequency power, and forwards the received signals to the destination using this harvested energy. To determine the proportions of energy harvesting and information processing, the relay makes use of relaying based on power splitting or time switching policy. Although the destination tries to remove the jamming signal from the relaying signal, it cannot cancel out the jamming signal perfectly due to imperfect channel reciprocity caused by the outdated CSI; this residual jamming signal therefore has a negative impact on secrecy performance. In this scenario, we derive the closed-form expressions for the outage probability and average secrecy rate, and find the jamming power ratio, power splitting ratio, and time switching ratio to optimize these secrecy performance metrics. The numerical results demonstrate the accuracy of our analysis, and show that the proposed secure relaying protocols achieve a near-optimal secrecy performance, as well as outperforming the conventional scheme without the jamming power control.

SWIPT in mMIMO system with non-linear energy-harvesting terminals: protocol design and performance optimization

In this paper, we design the simultaneous wireless information and power transfer (SWIPT) protocol for massive multi-input multi-output (mMIMO) system with non-linear energy-harvesting (EH) terminals. In this system, the base station (BS) serves a set of uplink fixed half-duplex (HD) terminals with non-linear energy harvester. Considering the non-linearity of practical energy-harvesting circuits, we adopt the realistic non-linear EH model rather than the idealistic linear EH model. The proposed SWIPT protocol can be divided into two phases. The first phase is designed for terminals EH and downlink training. A beam domain energy beamforming method is employed for the wireless power transmission. In the second phase, the BS forms the two-layer receive beamformers for the reception of signals transmitted by terminals. In order to improve the spectral efficiency (SE) of the system, the BS transmit power- and time-switching ratios are optimized. Simulation results show the superiority of the proposed beam-domain SWIPT protocol on SE performance compared with the conventional mMIMO SWIPT protocols.

Beam-Domain SWIPT for mMIMO System With Nonlinear Energy Harvesting Legitimate Terminals and a Non-Cooperative Terminal

In this paper, we design the simultaneous wireless information and power transfer (SWIPT) protocol for massive multi-input multi-output (mMIMO) system in the beam-domain (BD). In this system, the base station (BS) serves a set of fixed uplink half-duplex legitimate terminals with nonlinear energy harvester. Meanwhile, a non-cooperative energy harvesting (EH) terminal, which is not pay for the energy transmission or untrusted, is also within the coverage of the BS. Considering the nonlinearity of practical EH circuits, we adopt the realistic nonlinear EH model rather than the idealistic linear EH model. The entire protocol can be divided into two phases. The first phase is designed for legitimate terminals EH and downlink training. A BD energy beamformer is employed for wireless energy transmission. Meanwhile, the received energy of the non-cooperative EH terminal is minimized. In the second phase, the BS forms the two-layer receive beamformers for the reception of signals transmitted by legitimate terminals. In order to improve the spectral efficiency (SE) and energy efficiency (EE) of the system, the BS transmit power and time switching ratio are optimized. Simulation results show the superiority of the proposed BD SWIPT protocol on SE and EE compared with the conventional mMIMO SWIPT protocols.

Secrecy outage analysis of energy harvesting two‐way relaying networks with friendly jammer

This study investigates two-way relaying where an energy harvesting capable relay exchanges information between two sources. Legitimate information is wire-tapped by an eavesdropper. To intercept wire-tapping, this study exploits a friendly jammer. Secrecy outage analysis of the energy harvesting two-way relaying network with the friendly jammer over Rayleigh fading channels is first proposed in terms of individual secrecy outage probabilities and then validated by computer simulations. Various results demonstrate the efficacy of the jammer in preventing the eavesdropper from stealing the source information and significant impacts of relay location, jammer location, jammer's transmit power, energy conversion efficiency, time switching ratio on the security performance.

Multipoint Wireless Information and Power Transfer to Maximize Sum-Throughput in WBAN With Energy Harvesting

Wireless body area networks (WBANs) are not only an extension and branch of wireless sensor networks (WSNs) but also a practical application area of Internet of Things (IoT). With the extensive development of IoT technology, WBAN can monitor human physiological parameters in real time. Reliable information transmission is an important factor limiting the development of WBAN owing to special path loss and shadowing effect. Therefore, maximizing throughput is a pivotal part of improving system performance. In this paper, a multipoint WBAN (MP-WBAN) with energy harvesting for normal and abnormal scenarios is studied. We propose two different protocols, including a time switching (TS) strategy and a hybrid TS and power splitting (PS) strategy, respectively. In the abnormal scenarios, the access point (AP) harvests independent command signals from sensor nodes in the uplink (UL) and broadcasts dedicated energy signals to all sensor nodes in the downlink (DL). At the same time, the AP simultaneously broadcasts wireless command and energy signals to all sensor nodes in the normal situation. After all sensors harvest energy from the radio frequency (RF) signals, physiological datas can be transfered to the AP in a specific time sequence. We optimize TS ratios to achieve the abnormal situation sum-throughput maximization by utilizing convex optimization techniques. For sum-throughput maximization in normal situation, a near-optimal solution can be acquired by iteratively updating TS ratios and PS ratios. Numerical simulation results show the system performances of sum-throughput can be significantly improved by the proposed algorithms.

Outage Performance Analysis of Non-Orthogonal Multiple Access with Time-Switching Energy Harvesting

In recent years, although non-orthogonal multiple access (NOMA) has shown its potentials thanks to its ability to enhance the performance of future wireless communication networks, a number of issues emerge related to the improvement of NOMA systems. In this work, we consider a half-duplex (HD) relaying cooperative NOMA network using decode-and-forward (DF) transmission mode with energy harvesting (EH) capacity, where we assume the NOMA destination (D) is able to receive two data symbols in two continuous time slots which leads to the higher transmission rate than traditional relaying networks. To analyse EH, we deploy time-switching (TS) architecture to comprehensively study the optimal transmission time and outage performance at D. In particular, we are going to obtain closed-form expressions for outage probability (OP) with optimal TS ratio for both data symbols with both exact and approximate forms. The given simulation results show that the placement of the relay (R) plays an important role in the system performance.

A high-throughput wireless-powered relay network with joint time and power allocations

This paper studies how to maximise the throughput for a wireless-powered relay network where an unreliable direct link exists between a source node and a destination node. Our idea is to optimally combine time allocations (for different relay activities) and power allocations (for the relay to forward different frames) based on a harvest-store-consume (HSC) model. With the HSC model, the network may operate in a direct transmission (DT) mode or a cooperative transmission (CT) mode to transmit one frame. Accordingly, the relay may either harvest energy from the radio-frequency signals in the direct link in DT mode, or harvest energy and forward information in CT mode, where the energy harvested in both modes is stored temporarily before it is consumed for information relaying. Moreover, in our CT mode, the relay distributes the energy to the transmissions of multiple consecutive frames by taking time-varying wireless channel conditions in a time period into account in order to achieve the best throughput performance. We formulate a deterministic optimization problem and a stochastic programming problem under the assumption of full channel state information (CSI) and causal CSI, respectively. Both problems optimally determine DT or CT modes for each frame transmission. In the CT mode, the problems also decide the time-switching (TS) ratios and the transmit power at the relay. Our formulated problems are intractable due to the energy distribution at the relay between multiple frames. This intractability is addressed by dynamic programming techniques which decompose the problems into two tractable subproblems: an outer problem, and an inner problem. By solving the two subproblems, we propose the joint TS operation and power allocation algorithms for the two original formulated problems. The proposed algorithm for causal CSI is an online algorithm with low complexity, while the proposed algorithm for full CSI is offline and provides a benchmark for the online algorithm.

Analysis and Optimization of Wireless-Powered Cooperative Jamming for Sensor Network Over Nakagami-$m$ Fading Channels

This letter studies the secrecy communication in a wireless-powered sensor network, where the access point (AP) first sends an energy signal to power the sensor node (S) and jammer (J) and, then, J exploits the harvested energy to send a jamming signal to interfere with the eavesdropper, in order to protect the uplink communication from S to the AP. Specifically, assuming that all the channels are subjected to Nakagami-m fading, we derive a closed-form expression of the exact secrecy outage probability (SOP). Furthermore, by exploiting the results about the approximations to the statistics of products of independent random variables, we provide approximate and asymptotic expressions of the SOP, and the latter enables us to obtain the secrecy diversity order (SDO), observe the impact of several parameters on the SOP, and provide a closed-form expression for the time switching (TS) ratio, which maximizes the secrecy throughput. Simulations present the accuracy of the derived SOP and SDO, the tightness of the approximation, and the effectiveness of the obtained TS ratio.

Improved Hybrid Relaying Protocol for DF Relaying in the Presence of a Direct Link

This letter proposes an improved hybrid relaying (i-HRe) protocol for a decode-and-forward relay system in the presence of a direct link between the source and the destination. A non-linear energy harvester is equipped at the relay to facilitate simultaneous wireless information and power transfer. We formulate an optimization problem to maximize the system capacity by optimizing the parameters of the proposed i-HRe protocol, i.e., time-switching ratio and power-splitting ratio. As this problem is non-convex, we convert it into two subproblems by means of the inequality relation between the signal noise ratios of source-relay link and relay-destination link to derive the optimal solution. Simulation study verifies the fast convergence of the proposed iterative algorithm. The results show that the i-HRe protocol achieves a higher system capacity than the existing protocols.