Optimal training for wireless energy transfer

Energy beamforming (EB) is a key technique for achieving efficient radio-frequency (RF) transmission enabled wireless energy transfer (WET). By optimally designing the waveforms from multiple energy transmitters (ETs) over the wireless channels, they are constructively combined at the energy receiver (ER) to achieve an EB gain that scales with the number of ETs. However, the optimal design of transmit waveforms requires accurate channel state information (CSI) at the ETs, which is challenging to obtain in practical WET systems. In this paper, we propose a new channel training scheme to achieve optimal EB gain in a distributed WET system, where multiple separated ETs adjust their transmit phases to collaboratively send power to a single ER in an iterative manner, based on one-bit feedback from the ER per training interval which indicates the increase/decrease of the received power level from one particular ET over two preassigned transmit phases. The proposed EB algorithm can be efficiently implemented in practical WET systems even with a large number of distributed ETs, and is analytically shown to converge quickly to the optimal EB design as the number of feedback intervals per ET increases. Numerical results are provided to evaluate the performance of the proposed algorithm as compared to other distributed EB designs.

This paper proposes a novel channel estimation method and a cluster-based opportunistic scheduling policy, for a wireless energy transfer (WET) system consisting of multiple low-complex energy receivers (ERs) with limited processing capabilities. Firstly, in the training stage, the energy transmitter (ET) obtains a set of Received Signal Strength Indicator (RSSI) feedback values from all ERs, and these values are used to estimate the channels between the ET and all ERs. Next, based on the channel estimates, the ERs are grouped into clusters, and the cluster that has its members closest to its centroid in phase is selected for dedicated WET. The beamformer that maximizes the minimum harvested energy among all ERs in the selected cluster is found by solving a convex optimization problem. All ERs have the same chance of being selected regardless of their distances from the ET, and hence, this scheduling policy can be considered to be opportunistic as well as fair. It is shown that the proposed method achieves significant performance gains over benchmark schemes.

PurposeThis paper aims to develop mathematical models of variously compensated wireless energy transfer (WET) systems. Attention is primarily paid to the derivation of the most important energy transfer characteristics such as efficiency and amount of transferred power. This paper discusses the main advantages and disadvantages of various compensation techniques to show their possible application areas. On the basis of these results, a designer will be able to quickly identify which compensation type suites as the best solution to fulfill a given system’s requirements.Design/methodology/approachFirst, the current state in the field of mathematical modeling of WET systems is introduced. Next, the non-resonant magnetic-coupled circuit together with four most common resonant magnetic-coupled circuits is analyzed. The equivalent circuit models using loop currents methodology is applied to the analyses. The proposed methodology is experimentally verified by the laboratory measurement of selected circuit topology. The main contribution of the proposed methodology lies in its quick applicability on more complicated or extended systems while keeping a relatively good match with the real system’s behavior.FindingsThe authors have presented the usage of a simple and accurate methodology for investigating variously compensated WET systems. Electrical engineers who require effective and powerful tools for the identification of basic WET systems properties will find this methodology to be of extensive help.Research limitations/implicationsThe analyses consider only the sinusoidal type of supply voltage; so, it is valid mainly for the close range of the resonant state. Nonlinearities cannot be taken into account.Practical implicationsThis research may be applied in the field of WET systems.Originality/valueResearch in the area of power electronic systems, which provides a clear and straightforward procedure for WET system identification, will be helpful to most practical technicians who are not well versed in areas of physical-based phenomena.

This letter considers spectrum sharing between a multiuser multiple-input multiple-output (MIMO) wireless energy transfer (WET) system and a coexisting point-to-point MIMO wireless information transmission (WIT) system, where WET generates interference to WIT and degrades its throughput performance. We show that due to the interference, the WIT system suffers from a loss of the degrees of freedom (DoF) proportional to the number of energy beams sent by the energy transmitter (ET), which, in general, needs to be larger than one to optimize the multiuser WET with user fairness consideration. To minimize the DoF loss in WIT, we further propose a new single-beam energy transmission scheme based on the principle of time sharing, where the ET transmits one of the optimal energy beams at each time. This new scheme achieves the same optimal performance for the WET system, and minimizes the impact of its interference to the WIT system.

The current trend puts emphasis on the efficiency of the all electronics devices. For this reason, it is necessary to maximize the efficiency of the wireless energy transfer system as well as on other electronic devices. Therefore, this article deals with the most suitable working point analysis of the serial–serial WET system. The core of the article contains the simulation analysis at the various loads and various coupling coefficients. Experimental measurements were made on the existing prototype and the results were compared with the results from the simulation analysis. The main outcome of this paper is thus detailed investigation of operational point of WET system in order to meet the highest possible system efficiency together with the highest possible level of transferred power.

The performance of two wireless power transfer scheduling schemes, time sharing (TS) and spatial multiplexing (SM), in terms of provisioning fairness among energy receivers (ERs) is studied and compared while taking into account the nonlinearity of the energy harvesters circuits. In the network, the multiple-antenna energy transmitter (ET) attempts to maximize the harvested energy by the ER which has accumulated the minimum amount of energy among all single-antenna ERs during the wireless power transfer block T-hence the max-min fairness criterion. Two network scenarios are studied: homogeneous, where the channels power coefficients are assumed to be the same for all the well-apart ERs, and heterogeneous, where the said coefficients can take arbitrary values. It is proven analytically that in the homogeneous scenario, the optimal scheduling policy is to allocate each ER the full transmit power P with uniform distribution of charging times among ERs, rather than to allocate the full power transfer block T with uniform distribution of the power among ERs. Generalization of the network to the heterogeneous scenario aims to find the optimal beamforming vector for the SM scheme and the optimal time sharing vector for the TS scheme. It is shown through extensive Monte Carlo simulations that again TS outperforms SM in terms of max-min fairness as a result of taking into account the inherent non-linear characteristics of the harvesting devices.

The effect of various parameters on the resonance-based wireless energy transfer system has been investigated to obtain the design steps for efficient wireless energy transfer. Both theoretically and experimentally, it has been observed that every system has its own strongly coupled region that is an optimum frequency band for which maximum energy transfer occurs. The frequency band of the strongly coupled region changes with the coil size, structure, physical spacing, relative location, and material properties of the wire. Based on the theoretical results, experiments have been carried out to demonstrate the effectiveness and characteristics of the resonant wireless energy transfer method. It is found that the energy transfer efficiency reaches maximum at resonant frequency. The energy transfer efficiency is further enhanced by changing the configuration of coil and through low-resistance wire with high-quality factor. The analyses will give future guidelines for designing a compact efficient wireless energy transfer system to energize macro as well as micro devices used for various applications.

Consider a multiple-input multiple-output (MIMO) beamforming system, where a multi-antenna base station transmits information and energy simultaneously to a multi-antenna information receiver (IR) and a number of multi-antenna energy receivers (ERs). The ERs are assumed to possess dual functionality such that they can also decode information. This gives rise to the physical layer security issue of providing confidential information to the IR while ensuring efficient energy harvesting for the ERs. Hence, we employ an artificial noise (AN)-aided secrecy approach at the transmitter, where the AN improves the security by interfering with the information decoding at the ERs while, at the same time, providing them with wireless energy to harvest. In this paper, we address the problem of jointly optimizing the beamforming matrix for the IR and the covariance matrix of the AN such that the secrecy rate is maximized subject to energy harvesting constraints and a total power constraint. The corresponding optimization problem is a difference of convex functions (DC) programming problem, which is generally non-convex. Nevertheless, we propose an alternating optimization (AO) strategy to tackle this problem for the cases of a square and a non-square IR channel. The performance of the proposed algorithms is demonstrated by simulation results.

This chapter provides an overview of the main energy harvesting (EH) technologies and their relevant role in present and future internet of things (IoT) deployments. EH technologies for wireless machine-type devices can be mainly classified into the following two categories: ambient EH, and dedicated EH. The chapter emphasizes on RF–EH, and highlights its advantages compared to competing EH technologies reduced form-factor implementation and native multi-user support. RF–wireless energy transfer (WET) systems consist of three key components: RF transmitters, medium, and EH node. It focuses on the non-decreasing EH transfer function with the incident RF waveform power as sole input. The chapter describes the general architecture of an RF–EH IoT network, and the main layouts for wireless information transfer (WIT) and WET along with potential applications. WET, as a pivotal component of RF–EH networks, is fundamental for realizing the Internet of Energy paradigm.

In this paper, we investigate the outage performance of an amplify-and-forward (AF) multiple input multiple output (MIMO) opportunistic relay selection system under time-varying channels. A practical scenario is considered, where both the relay selection and the beamforming transmission are performed with outdated channel state information (CSI), due to channel fluctuations and delays involved in relay selection and CSI feedback. To provide efficient means of evaluating the outage performance, a closed-form expression for the outage probability is given. The effects of parameters, such as the correlation between the outdated and current CSI, the number of antennas are also analyzed. All the analytical results are validated by Monte Carlo simulations.

This paper investigates the multi-cell coordinated beamforming (MCBF) design for secure simultaneous wireless information and power transfer (SWIPT) in both centralized and distributed manners. In each cell, one transmitter serves multiple information receivers (IRs) and energy receivers (ERs) with the non-linear energy harvesting (EH) model. Meanwhile, several eavesdroppers (Eves) intend to intercept the confidential information transmitted for IRs. To achieve a secure transmission, the artificial noise (AN) is embedded in the transmit signals of each transmitter. The proposed design is formulated into a power-minimization problem to guarantee the IRs’ information and ERs’ energy requirements while avoiding the information being intercepted by Eves. Since the problem is non-convex and not easy to solve, a solution method based on semi-definition relaxation (SDR) is proposed and the global optimum is proved to be guaranteed with full channel state information (CSI). We further present a distributed AN-aided MCBF for the system by using alternating direction method of multipliers (ADMM), with which each transmitter is able to calculate its own beamforming vectors and AN covariance matrix based on its local CSI. Simulation results show that our proposed distributed design converges to the global optimum obtained by the centralized one. It is also shown that by employing AN, the total required power of the system is reduced and the effect of AN on the system performance decreases with increment of transmit antennas. Compared with traditional linear EH model, optimizing the system under the non-linear EH one avoids false output power at the ERs and saves power at the transmitter.

In the near future, the Wireless Energy Transfer (WET) will be widely used, especially for electronics recharging in the human organ implantations. WET has been researched intensively by some researchers, but the low efficient energy transfer is still a problem. In order to figure out the efficiency of the energy transfer, the experiment had been conducted in Laboratory of Sensor and Telecontrol Systems. The complexities of the measurement system setup and assessment of the sufficient energy both transmitter and receiver will also be discussed. The experiment results deliver the new design of the WET, empiric data of the energy transfer, and the sufficient antenna model as well. In this research, The UHF transmitter generated radio frequency with output energy of 39.00-watt energy source. The range of various frequencies in this paper is 400 MHz to 489 MHz and the SWR is 1.2. The transmitter unit and the antenna were connected with the 50 ohm-coaxial cable. In order to transfer the energy, the transmitter system consisting Yagi-Uda model antenna was applied. The results show that the efficiency maximum of the measured energy transfer is 0.36%. In our investigations, the efficiency of the result also depends on the polarity of the Yagi-Uda Antenna used in the experiment. According to the results, the angle that the maximum energy transferred is 60°, on the contrary, the angle of 150° gives the lowest efficiency results. We found that the distance between the transmitter and receiver antenna contribute significantly to the amount of energy transferred from the system. In this case, the energy can be calculated from the current and voltage measurement. The energy transfer versus distance relationship shows that a specific trend was obtained. This specific trend seems are affected by the antenna characters used in the experiment. Based on the experiment results, the wireless energy transfer could be done by utilizing UHF radio wave. Even though the efficiencies were still relatively low, but it can be used for some specific purposes in the example, in the electronic human organ implant recharging and applicable for the general electronic devices in the special areas. This experiment data are very useful for the scientists, practicing who work in the WET research field.

Wireless charging systems are critical for safely and efficiently recharging mobile electronic devices. Current wireless charging technologies involving inductive coupling, magnetic resonance coupling, and microwave transmission are bulky, require complicated systems, expose users to harmful radiation, and have very short energy transmission distances. Herein, we report on a long-distance optical power transmission system by optimizing the external cavity structure of semiconductor lasers for laser charging applications. An ultra-long stable oscillating laser cavity with a transmission distance of 10 m is designed. The optimal laser cavity design is determined by simulating the structural parameters for stable operation, and an improved laser cavity that produces an output of 2.589 W at a transmission distance of 150 cm is fabricated. The peak power attenuation when the transmission distance increases from 50 to 150 cm is only approximately 6.4%, which proves that this wireless power transfer scheme based on a vertical external cavity surface-emitting laser can be used to realize ultra-long-distance power transmission. The proposed wireless energy transmission scheme based on a VECSEL laser is the first of its kind to report a 1.5 m transmission distance output power that exceeds 2.5 W. Compared with other wireless energy transmission technologies, this simple, compact, and safe long-distance wireless laser energy transmission system is more suitable for indoor charging applications.

The robot development is actively done as a very hopeful tool in many disciplines, however, right now, the robot is not used actually for practical use. It is clear that once it is actually used, it becomes very important how to keep supplying energy. The wireless energy supply is mandatory. In most cases, it is considered that the rechargeable battery is most easy to solve it, however, for example, in nuclear power plant accident or chemical weapon disaster due to terrorism, once the robot enters into the contaminated area, it is almost impossible to return for the battery recharging. The wireless energy transmission is mandatory. Even for those robots working in the area where it is impossible or very difficult for a man to access but the robot can return for the refueling, the wireless energy transmission is required when, for example, the robot malfunctions and can not return. A long time operation is required for the trouble shooting and for repair. Moreover, some important activities urgently needed may not allow the robot to return for the refueling, then the wireless energy transmission is also very useful. To realize the wireless energy supply, either the microwave or the laser energy transmission is considered most hopeful. The microwave energy transmission has an advantage of higher energy conversion efficiency and can be used through the cloud, but it is not easy to concentrate the power in a small region. It is considered as a useful tool for the power transmission from the space power station (SPS) to the earth. Laser is easy to focus the beam in a small area and the mirror can be used to transmit energy behind a blind corner. Due to the development of laser diodes (LD), a very compact laser transmitter system can be constructed. For the robot use, the laser is more advantageous over the microwave system.

This paper studies a wireless-energy-transfer (WET) enabled massive multiple-input-multiple-output (MIMO) system (MM) consisting of a hybrid data-and-energy access point (H-AP) and multiple single-antenna users. In the WET-MM system, the H-AP is equipped with a large number $M$ of antennas and functions like a conventional AP in receiving data from users, but additionally supplies wireless power to the users. We consider frame-based transmissions. Each frame is divided into three phases: the uplink channel estimation (CE) phase, the downlink WET phase, as well as the uplink wireless information transmission (WIT) phase. Firstly, users use a fraction of the previously harvested energy to send pilots, while the H-AP estimates the uplink channels and obtains the downlink channels by exploiting channel reciprocity. Next, the H-AP utilizes the channel estimates just obtained to transfer wireless energy to all users in the downlink via energy beamforming. Finally, the users use a portion of the harvested energy to send data to the H-AP simultaneously in the uplink (reserving some harvested energy for sending pilots in the next frame). To optimize the throughput and ensure rate fairness, we consider the problem of maximizing the minimum rate among all users. In the large-$M$ regime, we obtain the asymptotically optimal solutions and some interesting insights for the optimal design of WET-MM system. We define a metric, namely, the massive MIMO degree-of-rate-gain (MM-DoRG), as the asymptotic UL rate normalized by $\log(M)$. We show that the proposed WET-MM system is optimal in terms of MM-DoRG, i.e., it achieves the same MM-DoRG as the case with ideal CE.