This paper studies the newly emerging wireless powered communication network in which one hybrid access point (H-AP) with constant power supply coordinates the wireless energy/information transmissions to/from a set of distributed users that do not have other energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users' UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. By applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocations to maximize the sum-throughput. Our solution reveals an interesting "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than a nearer user in the DL, has to transmit with more power in the UL for reliable information transmission. As a result, the maximum sum-throughput is shown to be achieved by allocating substantially more time to the near users than the far users, thus resulting in unfair rate allocation among different users. To overcome this problem, we furthermore propose a new performance metric so-called common-throughput with the additional constraint that all users should be allocated with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the common-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the new doubly near-far problem in wireless powered communication networks.

Throughput Maximization in Wireless Powered Communication Networks

Miniaturized energy storage is essential for the continuous development and further miniaturization of electronic devices. Electrochemical capacitors (ECs), also called supercapacitors, are energy storage devices with a high power density, fast charge and discharge rates, and long service life. Small-scale supercapacitors, or micro-supercapacitors, can be integrated with microelectronic devices to work as stand-alone power sources or as efficient energy storage units complementing batteries and energy harvesters, leading to wider use of these devices in many industries. In recent years, the research in this field has rapidly advanced and micro-supercapacitors with improved storage capacity and power density have been developed. The important factors affecting the performance of micro-supercapacitors are the intrinsic properties of electrode materials and electrolyte, architectural design of the device and the fabrication methods. This paper reviews the recent advances in fabrication of materials and devices and provides a critical analysis of reported performances of micro-supercapacitors.

Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors

This paper studies the newly emerging wireless powered communication network (WPCN) in which one hybrid access point (H-AP) with constant power supply coordinates the wireless energy/information transmissions to/from distributed users that do not have energy sources. A "harvest-then-transmit" protocol is proposed where all users first harvest the wireless energy broadcast by the H-AP in the downlink (DL) and then send their independent information to the H-AP in the uplink (UL) by time-division-multiple-access (TDMA). First, we study the sum-throughput maximization of all users by jointly optimizing the time allocation for the DL wireless power transfer versus the users' UL information transmissions given a total time constraint based on the users' DL and UL channels as well as their average harvested energy values. By applying convex optimization techniques, we obtain the closed-form expressions for the optimal time allocations to maximize the sum-throughput. Our solution reveals "doubly near-far" phenomenon due to both the DL and UL distance-dependent signal attenuation, where a far user from the H-AP, which receives less wireless energy than a nearer user in the DL, has to transmit with more power in the UL for reliable information transmission. Consequently, the maximum sum-throughput is achieved by allocating substantially more time to the near users than the far users, thus resulting in unfair rate allocation among different users. To overcome this problem, we furthermore propose a new performance metric so-called common-throughput with the additional constraint that all users should be allocated with an equal rate regardless of their distances to the H-AP. We present an efficient algorithm to solve the common-throughput maximization problem. Simulation results demonstrate the effectiveness of the common-throughput approach for solving the new doubly near-far problem in WPCNs.

This article describes a simple, cost-effective, and scalable approach to fabricate a triboelectric nanogenerator (NG) with ultrahigh electric output. Triggered by commonly available ambient mechanical energy such as human footfalls, a NG with size smaller than a human palm can generate maximum short-circuit current of 2 mA, delivering instantaneous power output of 1.2 W to external load. The power output corresponds to an area power density of 313 W/m2 and a volume power density of 54 268 W/m3 at an open-circuit voltage of ∼1200 V. An energy conversion efficiency of 14.9% has been achieved. The power was capable of instantaneously lighting up as many as 600 multicolor commercial LED bulbs. The record high power output for the NG is attributed to optimized structure, proper materials selection and nanoscale surface modification. This work demonstrated the practicability of using NG to harvest large-scale mechanical energy, such as footsteps, rolling wheels, wind power, and ocean waves.

/pdf/toward-large-scale-energy-harvesting-by-a-nanoparticle-182yluorew.pdf

Toward large-scale energy harvesting by a nanoparticle-enhanced triboelectric nanogenerator.

The ECG signal has been shown to contain relevant information for human identification. Even though results validate the potential of these signals, data acquisition methods and apparatus explored so far compromise user acceptability, requiring the acquisition of ECG at the chest. In this paper, we propose a finger-based ECG biometric system, that uses signals collected at the fingers, through a minimally intrusive 1-lead ECG setup recurring to Ag/AgCl electrodes without gel as interface with the skin. The collected signal is significantly more noisy than the ECG acquired at the chest, motivating the application of feature extraction and signal processing techniques to the problem. Time domain ECG signal processing is performed, which comprises the usual steps of filtering, peak detection, heartbeat waveform segmentation, and amplitude normalization, plus an additional step of time normalization. Through a simple minimum distance criterion between the test patterns and the enrollment database, results have revealed this to be a promising technique for biometric applications.

https://downloads.hindawi.com/journals/cin/2011/720971.pdf

Unveiling the biometric potential of finger-based ECG signals

More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC–DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.

/pdf/micropower-energy-harvesting-1l9ase94z1.pdf

Micropower energy harvesting

More than a decade of research in the field of thermal, motion, vibration and electromagnetic radiation energy harvesting has yielded increasing power output and smaller embodiments. Power management circuits for rectification and DC-DC conversion are becoming able to efficiently convert the power from these energy harvesters. This paper summarizes recent energy harvesting results and their power management circuits.

Power autonomy is an important requirement for wireless sensor nodes. Thermoelectric generators can produce sufficient power for low power applications. Due to varying temperature, a power management circuit will be required. This paper presents such a power management circuit, realized in the AMIS I3T80 CMOS technology. It contains a Dickson charge pump with a variable number of stages as a DC/DC converter. A controller optimizes the number of stages of the charge pump, consuming 1.4 muA. The maximum efficiency of the charge pump is 82%, leading to a total efficiency that can reach up to 58%.

Capacitive Power Management Circuit for Micropower Thermoelectric Generators With a 1.4 $\mu$ A Controller

Energy harvesters [1] power energy-autonomous wireless sensor systems by converting ambient environmental energy into electrical energy. Thermoelectric generators (TEG's) are a suitable compact power supply for both on-the-body and industrial sensors. Due to changing conditions, the generated power and voltage vary. Therefore a power management circuit (PMC) has to store the generated energy in a rechargeable battery or capacitor. It contains a DC/DC-converter with variable conversion factor and a controller that sets the conversion factor.

Integrated capacitive power-management circuit for thermal harvesters with output power 10 to 1000µW

Energy scavenging is an emerging method to power energy-autonomous wireless sensor systems by converting ambient environmental energy into electrical energy. Miniature as well as micro-machined thermoelectric generators (TEG) can become a suitable compact power supply for both on-the-body and industrial sensors. To minimize the overall size of the power supply, the generator has to work at its maximum power point. Due to varying thermal conditions, the generated power and voltage are not constant. Typically the output voltage is lower than the power supply voltage of the sensor. For these reasons, a power management circuit (PMC) with maximum power-point tracking capability and containing a DC/DC-converter with variable conversion factor is required that stores the generated energy in a rechargeable battery or capacitor.

I. Doms

Papers

Micropower energy harvesting

Micropower energy harvesting

Capacitive Power Management Circuit for Micropower Thermoelectric Generators With a 1.4 $\mu$ A Controller

Integrated capacitive power-management circuit for thermal harvesters with output power 10 to 1000µW

Capacitive Power-Management Circuit for Micropower Thermoelectric Generators with a 2.1μW Controller