Energy harvesting in arq-based cooperative communication

ENERGY HARVESTING IN ARQ-BASED COOPERATIVE COMMUNICATION SUMMARY In today's world, the amount of data used in the field of communication, data rates and the number of devices connected to networks on the global scale are rapidly increasing, and the used applications and technologies are developing at the same speed. However, since the battery technology does not develop at the same speed, many users are experiencing charging problems during the day. 2G networks are designed for voice, 3G networks are designed for voice and data, and 4G networks are designed for broadband internet services. To access a technology (5G) beyond 4G, communication resources and processing power will need to be distributed between the networks and the mobile devices. For this, it will be necessary to power even the smallest devices connected to the system to carry out tasks involving a lot of benchmarking and decision-making mechanisms and to keep the applications running without interruption. These next generation networks and standards will need to address many challenges, such as the convergence complexity of communication and benchmarking. Wireless technologies consume much more energy compared to wired technologies such as fiber, copper or DSL. Wireless access networks are clearly the largest and most inefficient consumer of energy in the cloud environment. The ultimate connection between the telecommunication infrastructure and the user is the most energy consuming part within the entire cloud system. While 3G technologies use about 15 times more energy than cable connections, this ratio reaches 23 times in 4G technology. There is not yet an exact data on the size of this energy consumption ratio in 5G. Broadly speaking, the 5G aims to connect everything, everywhere, all the time, through the - Internet of Things (IoT) technology. IoT will enable wearable devices, smartphones, tablets, sensors, etc. to work together with applications and services we use. With 5G, it will be observable that the benchmarking operations are heavily used in the communication systems in each area. In order for trillions of objects, such as wearable devices, to continue without interruption, they must have the required energy for these comparison operations. A number of new solutions are needed for IoT to be successful in balancing the rapidly growing unlimited energy consumption. IoT devices are expected to be low-power devices for this purpose, and the number of IoT devices that Cisco predicts to be 50 billion in 2020, is much higher than the sum of today's smartphones and tablets. If the energy consumed by these devices, networks and data centres is taken into consideration, the energy consumption with IoT will dramatically increase the growth rate of energy consumption. In this context, in IoT studies, energy efficiency has emerged as another challenging issue besides the spectrum activity that is being worked on intensively. The reason for this is the fact that the higher transmission speed and the greater number of connected smart devices consume more energy. Therefore, it is important to understand how to reduce energy consumption and how to use it effectively. Next generation devices must be much faster and smarter than previous generations so that this restriction can be overcome and the 5G networks can show the expected development. For this reason, the nodes / users in the network have to provide more than just serving as an end user, they also have to serve as a relay for other users. In wireless communication, when the source cannot directly transmit sufficient information packets to the destination, a third node in a location between the two nodes can serve as relay. Such a form of communication that enables the relay to transmit information to the destination is called cooperative communication. One of the promising solutions for dealing with battery problems in such systems is to perform Energy Harvesting (EH) at the relay. Radio Signals (RF) can be used to carry energy and information at the same time. In this case, the relay does not need an external energy source while transmitting the source's signal to the destination and can use the harvested energy for his own transmission. If the relay can operate independently from an energy source, the energy harvesting and information transfer processes are fundamentally implemented by two different protocols. One of them is the Time Switching Relaying protocol (TSR) and the other is the Power Splitting Relaying protocol (PSR). On the other hand, one of the envisaged radio access techniques to be used to increase the performance in 5G systems is the Non-Orthogonal Multiple Access (NOMA). The NOMA is based on the transmission of multiple information symbols by power sharing from the same time interval or frequency band. NOMA is expected to provide a better trade-off between user efficiency and system efficiency when compared to the Orthogonal Multiple Access (OMA) technique commonly used in 4G mobile communication systems, by applying the successive interference cancellation (SIC) process at the receiver side. With this technique it is expected that it will be possible to meet the data rate requirements of users in 5G communication. In this thesis, the effects of energy harvesting (EH) on an automatic repeat request (ARQ) – based cooperative communication network comprised of a source, a relay and two destination nodes, are investigated in Rayleigh fading channels. First, a broadcast channel is considered where the source transmits the same data packet to two destination and relay nodes by direct links. If the transmission is successful at both destination nodes, these nodes reply by an acknowledgement (ACK) signal to the source and the relay. Otherwise, there are two alternatives: If the relay has received this data packet successfully and replied by an ACK signal, then it cooperates and transmits the data packet for a second time to two destination nodes. If the relay has failed and replied by a non-acknowledgement (NACK) signal, the source in this case retransmits this packet to the destination nodes by the direct links. Only one retransmission (from the relay or the source) is allowed for each data packet. The energy of the relay is provided by the EH throughout the transmission of only one data packet from the source. In this sense, there is no buffer at the relay. Accordingly, the delay-limited usage of the decode-and-forward (DF) protocol is addressed. Second, the NOMA technique is applied to the same network configuration but the source is now considered as having two different data packets for the two destination nodes. In the same ARQ context, transmission of the superposed data packets from the source and the relay is effectuated by an appropriate power allocation, and the successive interference cancellation (SIC) is performed at the destination node with stronger channel conditions while the other destination node being decoding its own packet directly by treating the other destination node's packet as noise. In the considered two cases, the aim is to increase the reliability of the system by the inclusion of ARQ process and improve the energy efficiency by applying EH at the relay node. Both TSR and PSR protocols are considered for EH. The performances of the considered systems are evaluated in terms of throughput which is related to the outage probabilities of the links in the considered communication network. Analytical throughput expressions are obtained in closed form and the theoretical results are supported by the computer simulation results for each case. Effects of different system parameters as SNR, target data rate, time switching and power splitting factors, energy harvesting efficiency on the throughput are considered. It is concluded from the obtained results that the use of an EH relay for cooperation significantly improves the system throughput compared to the direct transmission from the source and that the PSR protocol for EH outperforms TSR in all cases.