Relay Selection in Wireless Information and Power Transfer Multi-Relay Networks with Energy Causality Muhammad Usman, Zeeshan Samad School of Electronic Information and Electrical Engineering Shanghai Jiao Tong University, Shanghai, China. Email: {usmansaghir, supershan66}@sjtu.edu.cn Abstract—Energy harvesting (EH) and simultaneous wireless information and power transfer (SWIPT) is promising solution for energy constrained network. In this paper, we propose multi-relay selection (MRS) algorithm and adopt relay ordering criterion based on maximum harvested power to improve the performance and life time of battery dependent nodes . We adopt amplify and forward (AF) based relaying strategy for optimum relay selection subset N from available M relays. Simulation and numerical results demonstrated that our proposed scheme is optimum in terms of energy efficiency and system outage probability. I. I NTRODUCTION Energy harvesting (EH) has gained the substantial attention of researcher in academia and industry to provide solution for battery constrained wireless networks where battery replace- ment and recharging is fatal in hazardous environment e.g.; nuclear plants, sensor implanted in human body [1], as well as promising technology to reduce emission of CO 2 and green- house impact on environment [2] which is 0.7% of telecom- munication sector [3]. Simultaneous wireless information and power transfer (SWIPT) with EH enables harvesting ambient radio frequency (RF) signals from the environment to prolong the life time of energy constrained wireless network. Thus energy constrained nodes can scavenge energy and process information simultaneously [4]. The SWIPT-EH scheme for a single input single output (SISO) channel was studied in [5]. Motivated by the benefits of multi antenna techniques, SWIPT for multiple input multiple output (MIMO) and multiple input single output (MISO) broadcast channels was investigated in [6]. The SWIPT schemes for MIMO relay networks were studied in [7]. Performance of EH system with a simple relay selection scheme was analysed in [8], where the selection is based on the current available energy and CSI. In [9], author discussed practical schemes for SWIPT on time switching (TS) and power splitting (PS) under cognitive radio (CR) network consider point to point transmission with or with out CSI. In the context of relaying strategies, transmission policies for enabling wireless EH and information processing at the energy constrained relay is developed in [9],[10]. Relaying strategy is extensively studied due to its ability to overcome mitigating, fading and path loss effect in wireless communi- cation. Relay has capability to extend the coverage area and spatial diversity gain without enlarging transmit power. Recently, bi-directional or two-way relaying strategy has gained much attention in wireless networks to overcome the path loss, fading and transmission limitation. Two-way relay- ing network improve the bandwidth efficiency by allowing users to transmit data simultaneously [11]. The most popu- lar protocols for multi hop relay networks are amplify-and- forward (AF) and the decode-and-forward (DF). AF protocol has less computation complexity because its simply amplify the received signal and forward to destination without decod- ing. Single relay selection (SRS) considering signal to noise ratio (SNR) was proposed in [12], while power allocation for relay ordering was investigated in [13], multi relay selection scheme based on CSI provide better performance but as overhead in terms of cost and complexity, then author proposed a power allocation scheme for relays with out analysing the relay selection criterion. Single relay selection (SRS) and multi relay selection (MRS) schemes were proposed in [14], where trusted relay channel is selected based on CSI to minimize the system outage probability. In [15], DF based relaying scheme to minimize the average sum bit error rate (BER) was proposed where two relays were selected from available relays. Much research efforts has been devoted in two way relay networks,while research on SRS in wireless networks is fre- quently available in the literature. Research on MRS in which more than one relay can be selected for energy constrained network require more attention. By our best knowledge, the research in EH with MRS scheme is very limited, so this motivated us to exploit the multi relay selection by joining the benefits of energy harvesting. This offers solid foundation for our research to combine EH and multi-relay selection inspired by [15], where authors have proposed the spectrum sharing protocol in cognitive two-way relay networks with out considering relay ordering and energy storage. We consider AF relay networks and propose MRS schemes that is different from [16], which is for DF relaying. By comparing the results in [15],[16], our proposed scheme is better in terms of energy efficiency for the energy constrained nodes. In this paper, we investigate energy harvesting and infor- mation transfer in multi-relay based network to increase the life time of battery dependent nodes. The proposed scheme enables an energy harvesting and information transfer under multi-relay M selection from sub set of available N relays. We propose the efficient relay selection algorithm to mini- mize the computation complexity by adopting relay ordering that enhance the system performance. We consider all nodes equipped with single antenna and can not receive and transmit