Energy-Efficient Two-Way Relaying with Multiple Antennas Thanaphat Srivantana College of Information and Communication Technology Rangsit University Pathumthani, Thailand Kiattisak Maichalernnukul College of Information and Communication Technology Rangsit University Pathumthani, Thailand Abstract—In this paper, we propose various kinds of energy- efficient two-way multi-antenna relaying with simultaneous wireless information and power transfer (SWIPT) and investigate their performance. Specifically, we first consider a two-way relay network where a pair of single-antenna end nodes communicate with each other through a multi-antenna relay node that is energy constrained. This relay node harvests energy from the two end nodes and use the harvested energy for forwarding their information. Three relaying schemes which support the considered network then build on the power splitting-based relaying protocol. The average bit error rates of these schemes are evaluated and compared by computer simulations considering several network parameters, including the number of relay antennas and the power splitting ratio. Such evaluation and comparison provide useful insights into the performance of SWIPT-based two-way multi-antenna relaying. Keywords—bit error rate; simultaneous wireless information and power transfer; two-way multi-antenna relaying. I. INTRODUCTION Recently, simultaneous wireless information and power transfer (SWIPT) has gained great interest due to its capability to deal with the energy scarcity in energy-constrained wireless networks [1-6]. In the seminal work [1], the fundamental trade-off between information and power transfer in different point-to-point wireless channels was studied. On the other hand, a pair of practical receiver designs for SWIPT, namely power splitting (PS) and time switching (TS), were firstly presented in [2]. Specifically, the PS-based receiver spits the received radio-frequency signal into two streams of different power for harvesting energy and decoding information, whereas the TS- based receiver switches over time between those two operations. The SWIPT has been adopted later in more complicated communication scenarios, including the broadband wireless system [3], the cellular network [4], the interference channel [5], and the relay channel [6]. This paper focuses on the last scenario. Many works in the literature have been devoted to two-way multi- antenna relaying (without SWIPT) as this approach can not only extend communication range but also improve spectral efficiency. In a basic two-way multi-antenna relay network (see Fig. 1), an intermediate relay node equipped with multiple antennas is used to assist two end nodes in exchanging their information. Nevertheless, application of SWIPT to this kind of network is still in its infancy [7- 9]. In [7], the SWIPT-based beamforming design for a multi-antenna relay was considered to maximize the sum rate of its two-way relay network. In [8], the authors presented a three-phase two-way relay network where an energy-constrained multi-antenna relay node harvests energy from a pair of single-antenna source nodes, and presented an optimal power allocation solution. In [9], an optimal joint source and relay beamforming scheme for two-way multi-antenna relay networks with SWIPT was proposed based on the principle of singular value decomposition. Depending on the nature and complexity of relays, relaying schemes can be classified into two main categories: non-regenerative relaying and regenerative relaying. Non-regenerative relaying generally implies that the relays only amplify their received signals before retransmitting them. Then, it is often referred to as amplify- and-forward (AF) relaying in the literature. Note that all existing works on two-way multi-antenna relaying with SWIPT, i.e., [7-9], are non-regenerative. On the other hand, regenerative relaying requires the relays to change the waveforms and/or the data contents by performing some processing in the digital domain. An example is the decode-and-forward (DF) relay, which receives the data from its immediate predecessor, decodes, re-encodes, and finally retransmits it. To the best of our knowledge, regenerative relaying has not yet been considered in the SWIPT-based two-way multi-antenna relay networks. In this paper, we consider a two-way relay network in which a pair of single-antenna end nodes communicate with each other through a multi-antenna relay node that is energy constrained. This relay node harvests energy from the two end nodes and use the harvested energy for forwarding their information. Based on a half- duplex relaying protocol, called power splitting-based relaying (PSR) [6], for separate energy harvesting and information processing at the relay node, three multiple-antenna relaying schemes, namely PS-AF, PS-DF, and PS-DF with space-time coding (PS-DF-STC), are designed for the considered network. In the DF-oriented design, network coding (NC) [10] is applied to the end nodes’ information that is decoded at the relay node. Moreover, by having multiple antennas at the relay node, STC [11] is used in the PS-DF-STC scheme with the aim of achieving a better end-to-end decoding performance. Unlike the aforementioned works [7-9] which are devoted to analyzing the relevant sum-rate performance, this paper will investigate the average bit error rates (BERs) of the proposed relaying schemes as a function of the number of relay antennas and power splitting ratio. II. SYSTEM MODEL Fig. 1. System model. Consider a two-way relay network as shown in Fig. 1, where end nodes 1 T and 2 T , each of which is equipped with one antenna, exchange information through an energy-constrained intermediate The Second International Workshop on Sustainability and Green Technologies, March 6-8, 2017, Danang, Vietnam