Poor Application of Smart Antenna Technologies in Simultaneous Wireless Information and Power Transfer

In wireless power transfer, a concept originally conceived by Nikola Tesla in the 1890s, energy is transmitted from a power source to a destination over the wireless medium. The use of wireless power transfer can avoid the costly process of planning and installing power cables in buildings and infrastructure. One of the challenges for implementing wireless power transfer is its low energy transfer efficiency, as only a small fraction of the emitted energy can be harvested at the receiver due to severe path loss and the low efficiency of radio frequency (RF)–direct current (DC) conversion. In addition, early electronic devices, such as first generation mobile phones, were bulky and suffered from high power consumption. For the aforementioned reasons, wireless power transfer had not received much attention until recently, although Tesla had already provided a successful demonstration to light electric lamps wirelessly in 1891. In recent years, a significant amount of research effort has been dedicated to reviving the old ambition of wireless power transfer, which is motivated by the following two reasons [1, 2]. The first reason is the tremendous success of wireless sensor networks (WSNs) which have been widely applied for intelligent transportation, environmental monitoring, etc. However, WSNs are energy constrained, as each sensor has to be equipped with a battery that has a limited lifetime in most practical cases. It is often costly to replace these batteries and the application of conventional energy harvesting (EH) technologies relying on natural energy sources is problematic due to their intermittent nature. Wireless power transfer can be used as a promising alternative to increase the lifetime of WSNs. The second reason is the now widespread use of low-power devices that can be charged wirelessly. For example, Intel has demonstrated the wireless charging of a temperature and humidity meter as well as a liquid-crystal display using the signals of a TV station 4 km away [4]. This article considers the combination of wireless power transfer and information transmission, a recently developed technique termed simultaneous wireless information and power transfer (SWIPT), in which information carrying signals are also used for energy extraction. Efficient SWIPT requires some fundamental changes in the design of wireless communication networks. For example, the conventional criteria for evaluating the performance of a wireless system are the information transfer rates and the reception reliability. However, if some users in the system perform EH by using RF signals, the trade-off between the achievable information rates and the amount of harvested energy becomes an important figure of merit [1]. In this context, an ideal receiver, which has the capability to perform information decoding (ID) and EH simultaneously, was considered in [1]. In [2] a more practical receiver architecture was proposed, in which the receiver has two circuits to perform ID and EH separately. This article focuses on the application of smart antenna technologies in SWIPT systems, namely multiple-input multiple-output (MIMO) ABSTRACT