A novel joint transmit beamforming and receive time switching strategy for MISO SWIPT system

Ma et al. EURASIP Journal on Wireless Communications and Networking (2018) 2018:172 https://doi.org/10.1186/s13638-018-1189-z RESEARCH Open Access A novel joint transmit beamforming and receive time switching strategy for MISO SWIPT system Lin Ma* , Yun Wang, Yubin Xu, Shiqi Wang and Teklu Merhawit Berhane Abstract In multi-antenna simultaneous wireless information and power transfer (SWIPT) system, beamforming strategy has been widely analyzed due to the increasing signal strength. It can not only increase the signal strength in the direction of the antenna array but also reduce the interference strength, which is a good option for SWIPT system to achieve directional transmission of information and energy. However, the traditional beamforming strategy only uses single beamforming vector, and it does not consider the differences between information and energy in SWIPT system. Actually, interference can also be collected as energy. Based on the traditional beamforming strategy, the resources in SWIPT system are not properly utilized. Therefore, this paper proposes a joint beamforming strategy in multi-input and single-output (MISO) SWIPT system. We extend the traditional single beamforming vector into two beamforming vectors to realize independent control of information and energy based on time switching (TS) receiving mode. In information receiving phase, information beamforming vector is used to carry specific user’s information for information alignment. Since there is an orthogonal relationship between information beamforming vector and channel gain vector, we can achieve to eliminate interference and realize error-free information transmission. In energy receiving phase, energy beamforming vector is used to carry user’s energy. Energy beamforming vector and channel gain vector do not require the orthogonality so that the interference can also be collected as energy. In this paper, we model it as a transmission power optimization (TPO) problem, which is a complex non-convex problem. We firstly transform it into a convex problem, and then, it can be solved using CVX toolbox. Simulation results show that the proposed strategy could increase the energy collection at the same transmission power and also decrease the transmission power at the same energy collection. Keywords: MISO, SWIPT, Joint beamforming, Time switching, Convex optimization 1 Introduction With the development of Internet of Things and communication, low-power sensors are becoming ubiquitous, which increasingly leads to convenient electric charging. In our daily life, a power line is the most popular way to transmit power for charging the battery [1]. But in some cases, there are obvious limitations for cable charging, such as sensors in the wall and furniture for a smart home or in the body for medical implantation [2, 3]. Thus, the wireless charging is a prominent solution to a wide range of applications. Wireless charging is a charging method *Correspondence: Communication Research Center, Harbin Institute of Technology, 150001 Harbin, People’s Republic of China in which the device and the charger do not have to be connected to each other by the cable but by the wireless medium as the carrier for energy transmission. It is a new idea for a terminal to obtain energy. In a traditional wireless communication system, the radio frequency (RF) signal plays a role of information transmitter; nevertheless, the RF signal could carry not only information but also energy [4–6]. Therefore a new wireless communication system that uses the RF signal to transmit information as well as energy has emerged, which was called simultaneous wireless information and power transfer (SWIPT) system proposed by Varshney in 2008 [7]. In a SWIPT system, there are two kinds of receivers, information receiver (IR) and energy receiver (ER) for information reception and energy collection, respectively. © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ma et al. EURASIP Journal on Wireless Communications and Networking (2018) 2018:172 It outperforms the traditional communication system by wireless charging to prolong the system service lifetime. Based on a special hardware circuit design, ER has the ability to convert the received RF signal to energy and store it in the battery to charge IR for reception of the information [8, 9]. In SWIPT system, the transmission power is a key performance parameter. It provides an evaluation criterion on the resource consumption. However, due to the power dissipation in the free space and low efficiency of energy conversion, the energy collection efficiency is usually not high. Currently, scholars throughout the world have paid much attention to decrease the transmission power by focusing on the transmitter design, receiver design, and transmitter and receiver joint design [10]. Transmitter design involves beamforming, user selection, and power control [11–13]. Receiver design studies the RF signal receiving modes, such as power splitting (PS) mode and time switching (TS) mode [14, 15]. Based on the above studies, joint designs of transmitter and receiver with TS or PS scheme [16, 17] were proposed. Especially, in the SWIPT studies, beamforming strategy has widely attracted attention because it well combines the antenna technology and digital signal processing technology for directional information transmission. In the traditional wireless communication system, beamforming strategy could increase the signal strength in the direction of the antenna array and suppress the interference between users. When beamforming strategy is introduced to the SWIPT system, it just makes use of the traditional beamforming vector to achieve the information and energy alignment for directional information and energy transmission. As a result, the transmission power could be reduced by the beamforming strategy. In order to achieve good transmission power performance, beamforming strategies based on different system models, receiving modes, and optimization goals were proposed and analyzed. The research of beamforming in SWIPT system based on different system models includes multiple antenna technology, IR and ER position relation, and single-cell or multi-cell scenario. For multiple antenna technologies, multi-input and single-output (MISO) and multiple-input and multiple-output (MIMO) are usually the hot topics. When the number of the base station antenna is more than the number of users, interference could be suppressed by the beamforming strategy. In [18, 19], MISO technology was well studied in SWIPT system. In this case, user term (...truncated)