Thesis

Buffer-Aided Cooperative Communication in Wireless Networks

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Abstract

In this thesis, a suite of schemes is presented to enhance the performance of cooperative communication networks. In particular, techniques to improve the outage probability, end-to-end delay and throughput performances are presented. Firstly, a buffer-aided cooperative communication is studied and analyzed for packet selection and relay selection. A three-node network is considered in the beginning and the phenomenon of packet diversity is taken into consideration to overcome bad channel conditions of the source to relay (SR) and relay to destination (RD) links. The closed-form expressions for the computation of the outage probability along with the delay, throughput and diversity gain are derived. Then, packet selection is studied along with relay selection for buffer-aided amplify and forward (AF) cooperative relaying networks. The proposed protocol is analyzed for both symmetric and asymmetric channel conditions and buffer size using multiple antennas at relays and compared against the existing buffer-aided schemes. Markov chain (MC) is used to derive the closed-form expressions for outage probability, diversity gain, delay and throughput. Next, the performance of SNR based hybrid decode-amplify-forward relaying protocol is observed. When SR link is the strongest, data is transmitted to the selected relay and checked against the predefined threshold at the relay. If it is greater than the threshold, data is decoded and stored in the corresponding buffer. Otherwise, it is amplified and stored in the respective buffer. When RD link is the strongest, data is transmitted to the destination. MC based theoretical framework is used to derive an expression for the outage probability, the average end-to-end delay and throughput. Then, relay selection schemes considering the instantaneous link quality along with buffer status in the relay selection are proposed. A scheme is proposed that simultaneously considers buffer status and link quality. Then, we discuss multiple links with equal weights using a general relay selection factor. It includes the weight of the link as the first metric and the link quality, or priority, as the second metric for different cases of the same weight. The proposed scheme is evaluated for symmetric and asymmetric channel conditions. Moreover, we propose a specific parameter, termed as the bu�er-limit, which controls the selection of SR or RD links and also have its impact on the average delay and throughput. In this scheme, the outage probability is traded with the average end-to-end queuing delay or the average throughput by adjusting the values of the buffer-limit. The MC based framework is employed to derive the closed-form expressions for the outage probability, average end-to-end queuing delay and the average throughput. The suggested schemes are compared to the existing bufferaided relay selection schemes. Lastly, we consider the energy constraint cooperative network and propose a generalized approach to study the performance of energy harvesting relaying schemes. The unified modeling of generalized energy harvesting relaying (GEHR) scheme covers the non-energy harvesting schemes and the well-known energy harvesting schemes, i.e., time switching based relaying (TSR) and power splitting based relaying (PSR). Moreover, the scheme also caters the hybrid of both TSR and PSR schemes. The closed-form expressions for the outage probability and ergodic capacity and average throughput are formulated for non-mixed Rayleigh fading and mixed Rayleigh-Rician fading channels. Each case is analyzed for AF and decode and forward relaying models. Comprehensive Monte-Carlo simulations confirm all theoretical results.

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A decode-and-forward system with an energy-harvesting relay is analyzed for the case when an arbitrary number of independent interference signals affect the communication at both the relay and the destination nodes. The scenario in which the relay harvests energy from both the source and interference signals using a time switching scheme is analyzed. The analysis is performed for the interference-limited Nakagami-m fading environment, assuming a realistic nonlinearity for the electronic devices. The closed-form outage probability expression for the system with a nonlinear energy harvester is derived. An asymptotic expression valid for the case of a simpler linear harvesting model is also provided. The derived analytical results are corroborated by an independent simulation model. The impacts of the saturation threshold power, the energy-harvesting ratio, and the number and power of the interference signals on the system performance are analyzed.
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Energy efficiency will play a crucial role in future communication systems and has become a main design target for all 5G radio access networks. The high operational costs and impossibility of replacing or recharging wireless device batteries in multiple scenarios, such as wireless medical sensors inside the human body, call for a new technology by which wireless devices can harvest energy from the environment via capturing ambient RF signals. SWIPT has emerged as a powerful means to address this issue. In this article, we survey the current architectures and enabling technologies for SWIPT and identify technical challenges to implement SWIPT. Following an overview of enabling technologies for SWIPT and SWIPT-assisted wireless systems, we showcase a novel SWIPT-supported power allocation mechanism for D2D communications to illustrate the importance of the application of SWIPT. As an ending note, we point out some future research directions to encourage and motivate more research efforts on SWIPT.
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The outage probability of a decode-and-forward relaying system based on simultaneous wireless information and power transfer (SWIPT) in the presence of Nakagami fading is investigated. The relaying model considers both the source-destination direct link the the source-relay-destination link. The power splitter at the relaying device provides energy to the relay by splitting the received signal power into energy harvesting and information transfer parts. The derived outage expression is verified using simulation results. The results show an impressive amount of percentage decrease in outage probability for Nakagami fading with different values of shape parameter in comparison to Rayleigh fading.
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In this letter, we discuss multiple links with equal weights, in buffer size based relay selection schemes in cooperative wireless networks. A general relay selection factor is defined, which includes the weight of the link as the first metric and the link quality, or priority, as the second metric for different cases of the same weight. The Markov chain based theoretical framework is employed to evaluate the outage probability, delay and throughput of the system. The proposed scheme is evaluated for symmetric and asymmetric channel conditions. The link quality based second selection metric achieves lower outage probability, while the link priority based selection shows significant improvements in terms of delay and throughput. Theoretical results are validated through extensive Monte carlo simulations.
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In this paper, we propose a relay selection scheme for cooperative buffer-aided relay networks with specific information rates and delay bounds. The proposed scheme differs from most of the well known schemes in the literature that assume the source is saturated with data and/or without delay constraints. The proposed scheme is designed to encompass the delay-sensitive applications that are subject to delay limits and have certain rates. The proposed scheme exploits the channel state information (CSI), buffer state information (BSI) and delay state information (DSI) to minimize the outage probability and achieve higher throughput. To achieve that, it uses these information to compromise between the selections of relays for reception and transmission. The proposed scheme is analyzed in the independent and identically distributed (i.i.d.) and independent and non-identically distributed (i.n.d.) Rayleigh fading channels. The analysis is in terms of outage probability, packet dropping probability of the source node, packet dropping probability of the entire system and throughput. For systems with high delay constraints, simulation results show that the proposed scheme offers lower packet dropping probability and higher throughput as compared to the renown relay selection schemes.
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In this paper, we propose a novel buffer-state-based amplify-and-forward relaying protocol for dual-hop cooperative networks. The proposed protocol is designed to amalgamate the following three concepts: the exploitation of multiple source-to-relay broadcast channels, the introduction of thresholding at relay and destination nodes, and buffer-state-based relay selection. Our numerical results demonstrate that the proposed protocol is capable of attaining a lower end-to-end delay and a lower outage probability than previous amplify-and-forward relay selection schemes while simultaneously imposing no additional costs.
Conference Paper
Relay selection in cooperative communication is an efficient approach to mitigate the spectral efficiency loss faced in cooperative diversity systems. In this paper, we propose a relay selection scheme for buffer-aided cooperative systems. It simultaneously considers the instantaneous link quality and the buffer status of relay nodes in the relay selection decision. The normalized and weighted sum of these parameters results into the overall score of each link, and a link with maximum score is selected. First, the concept of equivalent outage of a link corresponding to a buffer-aided relay is explained, and then Markov chain (MC) modeling is used for the evaluation of states of buffer. We provide the specific examples with fixed values of number of relays and buffer size. The system achieves the full diversity gain of 2K for the smaller buffer sizes.