# Multi-User AF Relay Networks with Power Allocation and Transfer: A Joint Approach
## 1 Introduction  
In today's smart world, wireless communication services are growing in demand exponentially. The Internet-of-Things (IoT) framework has been viewed as an enabler of the smart world. The key requirements to enable IoT services are prolonging the lifetime of operating nodes and extending the coverage range [1, 2]. Since relaying schemes are paramount for improving coverage, capacity, and reliability of wireless networks, the lifetime of wireless networks can be enhanced by deploying intermediate energy harvesting (EH) relay nodes, thus, we don't need to replace the battery sporadically [3, 4, 5]. As a result of the aforementioned advantages, EH cooperative communication can easily become the backbone of modern, green, smart wireless networks. However, the design challenges of EH wireless networks lie in the fact that solar, wind, and thermoelectric energy, which are common environmental energy sources, are intermittent and random in nature. These common energy sources are not always available. Hence, for transmission reliability, these uncertainties become more challenging for designing EH communication systems. In such a scenario, a promising solution has emerged as regards the wireless power transfer (WPT) in an EH relay-assisted network, whereinEH nodes transfer the energy using dedicated radio frequency (RF) signals. RF-enabled WPT has several practical advantages, such as a small receiver form factor, a low production cost, a wide operating range, and efficient energy multi-casting. The joint transfer of information and energy leads to a new paradigm referred to as simultaneous wireless information and power transfer (SWIPT) [6]. With the recent advancements in antenna technologies and RF EH circuits, SWIPT will become a key building block of numerous popular industrial and commercial systems in the future, including upcoming IoT technology consisting of billions of communicating and sensing devices and large-scale wireless sensor networks (WSNs) [7; 8]. Cooperative communication mitigates wireless channel fading and improves the link reliability by utilizing the spatial diversity gains. There have been a lot of recent studies on energy management in EH cooperative networks [9; 10; 11; 12; 13; 14]. The power control scheme was investigated in [9] with the non-causal knowledge of energy state information (ESI) [14]. The authors in [10] extended the work of [9] regarding finite batteries.Fading channels were considered in [11; 12] as well as EH problems. The optimal energy allocation strategy was obtained in [11; 12]. However, the authors in [13; 14; 15] explored the idea of energy cooperation for EH relay networks, wherein [15] a dual-dimensional directional water-filling strategy was investigated to maximize the throughput. The ambient EH has been studied in [16; 17; 18]. The authors in [16] studied power allocation in decode-and-forward (DF) relaying with deterministic EH models. With a finite horizon of transmission blocks, the throughput maximization problem was studied for the cases of delay-constrained traffic or no-delay-constrained traffic. A joint problem of power allocation and time scheduling was investigated in [17] for a dual-hop relay system with an EH source. In [18], power allocation schemes were designed to maximize the throughput of a dual-hop amplify-and-forward (AF) relay system; while the authors in [19; 20] considered multiple source-destination pairs and proposed relay transmission strategies. Dedicated EH from RF signals is naturally appropriate to cooperative networks as it facilitates information relaying. In this direction, the major design concern is to find a suitable time sharing or power splitting ratio to achieve the best trade-off between EH and signal relaying. More recently, SWIPT methods have been investigated in [21; 22; 23; 24]. However, none of these aforementioned works has studied joint power allocation and WPT in multi-user AF-assisted relay networks.