Wireless devices operations have always been constrained by the limited battery power, meaning that frequent disruptions to replace or recharge the battery were needed. Advances in RF enabled wireless energy transfer technology, giving birth to wireless powered communications, eliminating the need for manual replacement of batteries and thus improving the performance of conventional battery-powered communications systems by obtaining higher throughput and longer device lifetime. In this report, an overview of the key structures and technologies is provided, along with highlights on the design challenges and solutions to build efficient WPCN.
The design of new wireless devices and new protocols has always been constrained by the limited amount of power that can be supplied to mobile devices by their batteries. Frequent battery change or recharging is often infeasible either for its cost, due to the large number of mobile devices, or for the critical applications of sensors, e.g. medical devices implanted. Thus the question arises: is there a way these elements can be powered without needing to be connected to the electric network?
In the 1890s, Nikola Tesla envisioned power supply without the necessity of cables by using radio-frequency waves and particular resonant coils divided between a transmitter and a receiver. The transmitter end is energised and produces a time-varying electromagnetic field, which in turn induces an electric current in the receiver. This technology, which is still under research, has numerous advantages, like an extended coverage, low capital expenditure (CapEx) and small form factor of the receiver. Probably, the most important application of wireless energy transfer (WET) state-of-art technology is represented by wireless powered communications (WPC), where wireless devices (WDs) harvest up to dozens of microwatts power from received radio frequency signals in order to send data over the air to a base station distant few tens of meters 1.
2 Working principles of WET
Like well-known wireless information transfer, WET is also based on the electromagnetic field and its theoretical grounds can be found in Faraday’s law of induction that predicts how this physical field will interact with an electric circuit to produce an electromotive force (EMF) – a phenomenon called electromagnetic induction. While in information transfer, the signal-to-noise ratio is the key factor to consider, in energy transfer technique it is necessary to evaluate the receive-to-transmit ratio, and usually high values of this parameter are more difficult to achieve rather than those of SNR over long distances.
In general, to achieve power transfer, Direct Current (DC) is supplied by an unlimited power source, often the national electric network, upconverted to Alternating Current (AC) by specific circuitry elements in the transmitter and used to energise specific elements in the transmitter. Depending on whether radiative electromagnetic field is used or not, wireless transfer methods are classified into two categories: near-field and far-field.
2.1 Near-field Transfer
This first scheme, already deployed in wireless charging of smartphones and electric brushes, makes use of two separate coils, spaced of the coils’ dimensions. During the charging operations, alternating current is supplied to the copper coil in the transmitter to generate a magnetic field, which in turn, as Faraday’s law of induction states, would generate electric current in the receiver coil.
In a perfectly designed Tesla coil, when the receiver coil reaches its maximum charge, the whole process should start over again and the device should become self-sustaining, a phenomenon referred to as resonance. In practice, however, this does not happen as the heated air in the gap pulls some of the electricity away from the secondary coil, and eventually, the Tesla coil will run out of energy. This is why the transmitter end must be connected to an outside power supply. Furthermore, the coupling efficiency between the two coils drops dramatically when either the distance between the coils increases or the dimensions of one of them is reduced 2.
2.2 Far-field Transfer
When the distance between transmitter and receiver increases, it is necessary to use electromagnetic resonance supported by radio-frequency power transmission. Far-field transfer is based on the radiative property of electromagnetic waves within different wave bands. Electric energy is transferred as a strong beam of radio waves or microwaves by a dish-like antenna, travels through the atmosphere and then received by another antenna which transfers it back to AC electric current.
As for electromagnetic waves, frequency, wavelength and the speed of light are related by the formula , where is the wavelength and is the frequency, the longer the wavelength is, the bigger the antennas must be in order to achieve acceptable directionality, that is, high gain. Since the speed of light in the air is about , the wavelength of radiowaves and microwaves originally used ( ) was about one meter, which required an antenna with a diameter of some meters. Obviously, as we can’t equip a mobile handset with an antenna with such dimensions, we have to use electromagnetic waves with shorter wavelength, that is, higher frequency.
3 Basic Models of Wireless Powered Communications
Nowadays, two types of wireless powered communications networks to transfer power are under research: the first type makes use of separate energy and information nodes, i.e. there have to be nodes to supply energy wirelessly and nodes to send data signals. In the second type, power beacons and data nodes are co-located, reducing the CapEx for the operators by reducing the deployment costs and sharing their communication and signal processing modules 1. In this case, the pair of energy node and access point is referred to as Hybrid Access Point (HAP). However, such a scheme introduces an important challenge, commonly known as doubly-near-far problem. In conventional wireless systems, far away users achieve lower data rates than those near the access point. This issue is even more critical in WPCN. In fact, due to signal attenuation, a user far away from the associated HAP harvests little energy in the downlink but uses more to transmit data to the access point on the uplink channel and eventually, consumes more energy to transmit data than a user closer to the access point 3.