# Wireless Energy Harvesting with Cooperative Relaying under the Best Relay Selection Scheme
## 1 Introduction  
Cooperative relaying (CR) provides a significant enhancement of energy-efficient transmission techniques by mitigating fading [1]. The CR system sends the signal via orthogonal channels to prevent interference among the source and relay links. Significant focus has been given to multi-hop relaying in industry and academia due to many benefits such as system capacity, improved coverage, battery life, and throughput [2]. The most well-known relaying strategies are amplify-and-forward (AF) and decode-and-forward (DF) [3]. DF decodes, re-modulates, and retransmits the received signal, while AF retransmits and amplifies signals without having to decode. The DF complexity compared to AF relays significantly high due to its full processing capability. The DF protocol also needs a sophisticated media access control layer, while this is not necessary for the AF protocol [4]. On the other hand, a hybrid relaying protocol can be used in certain applications [5; 6]. However, there are several limitations of the AF relays, such as non-linear distortion and noise added by the information receiver. In numerous extreme situations, replacing or recharging the traditional power supply is impossible [7]. For instance, mobile devices implanted in patients to record health data are increasing in number; however, these devices are expensive, and their batteries can be difficult to replace. This issue has increased the demand for wireless battery charging using external sources, which then fuelled the emergence of EH. Some studies have suggested that thermoelectric, wind, and solar power could be utilized as external power sources to charge batteries wirelessly [8]. The conventional energyharvesting (EH) technologies depend on external energy sources like wind, vibration and solar which are not always available and suffers from the time-varying issue and material inefficiency. EH using radio-frequency (RF) is an alternative approach, since RF signals carry energy as well as information without continuous monitoring and maintenance. Wireless networks have developed considerably in terms of energy efficiency (EE). This development is driven by the needs of subscribers, design issues, satisfaction, and recently, the impact on the environment. Various measures have been employed to achieve high signal to noise ratio (SNR) without increasing the transmission power. Advanced measures include power reduction with sleep and active modes [9], having various transfer nodes that are based on payload size, interference reduction that results from poor frequency planning, device selection based on transmission distance, and many other energy-efficient transmission techniques [10]. The above measures have been used in various wireless fields based on environmental conditions and payload applications. One of the advanced techniques relies on exploiting the RF signals that are radiated from the ambient transmitter, known as the simultaneous wireless information and power transfer (SWIPT) network. SWIPT allows effective resource allocation at transceiver designs, which collectively form the potential 5G network technology [11]. Limited battery life in the conventional CR results in a short network lifetime. Traditional EH sources, such as thermal, solar, and wind, are not always available because of their seasonality, low efficiency, and dependency on surrounding nature, which contributes to the decrease of wireless network reliability due to network outages [12]. The use of RF signals for SWIPT has recently become a valuable EH source due to its capability to carry information and energy simultaneously. One of the major performance indicators of the Third Generation Partnership Project (3 GPP) evolution is the maximum value of spectral efficiency (SE). A typical example is an increment in the target downlink SE of 3 GPP from 0.05 to 5 bits/s/Hz due to the evolution of the mobile system from Global System for Mobile (GSM) to Long Term Evolution (LTE).