무선 전력 통신 네트워크에서 합통신량 최대화 기반 자원 할당 기법

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2020. 8. 이정우.With the explosive growth of smart devices equipped with wireless communication, there have been numerous challenges to untangle for supporting user demands in the next generation of communication networks such as Internet of Things networks. One of prime concerns is to overcome the finite lifespan of networks due to the limited battery capacity. Wireless power transfer (WPT) has been considered as a promising solution for providing self-sustainability to energy-constrained networks. WPT enables users to charge their batteries by collecting energy from a radio-frequency signal transmitted by a dedicated energy source. As a framework to the design of wireless networks with WPT, a wireless powered communication network (WPCN) consisting of a hybrid access-point (H-AP) and multiple users has emerged. A H-AP serves users in a WPCN as a base station as well as delivers energy to users as a dedicated energy source. In a WPCN, users charge their batteries by WPT via downlink, and use the energy for uplink transmission. Due to the scarcity of resources, an efficient design is crucial to exploit the system. To support this, I explore system design and resource allocation for WPCNs, especially in the perspective of throughput performance. In addition, I aim to mitigate severe rate disparity which originates from the doubly near-far problem, an inherent characteristic of a WPCN. To begin with, I discuss a cooperative WPCN, in which a user with good channel condition relays information of a user with bad channel condition to enhance user fairness. The sum-throughput is maximized in the considered network subject to a set of quality of service (QoS) requirements. By analyzing the optimal solution, the conditions under which the WPCN benefits from the cooperation are characterized. Based on the new findings, I propose a novel resource allocation algorithm for sum-throughput maximization, which is helpful to practical use of user cooperation. Secondly, I discuss a multi-antenna WPCN where non-orthogonal multiple access (NOMA) transmission is employed in the uplink. To address issues regarding adopting NOMA, user clustering exploiting the multi-antenna system is further applied so that the number of users in a single NOMA transmission is reduced. To deal with the difficulty of jointly optimizing cluster-specific beamforming and time/energy resources for sum-throughput maximization, the beamforming is determined first, and then the resources are optimized for given beamforming. A novel algorithm for cluster-specific beamforming design followed by the sum-throughput maximization algorithm is proposed. Lastly, I consider a WPCN assisted by intelligent reflecting surface (IRS) which has recently received significant attention for its potential of enhancing wireless communication experience. By employing an IRS in a WPCN,users harvest extra energy, and the signal strength of each user can be elevated. For the considered system model, beamforming at the IRS and resources are optimized to maximize sum-throughput. In particular, both NOMA and orthogonal multiple access are considered for uplink transmission, and the performance comparison between the two multiple access schemes are presented.무선 통신이 탑재된 스마트 기기의 폭발적인 성장으로, 사물 인터넷 네트워크와 같은 차세대 통신 네트워크에서 요구하는 성능을 충족하기 위하여 해결해야 할 여러 문제가 발생하였다. 주요 문제 중 하나는 기기의 한정된 배터리 용량으로 네트워크가 제한된 시간 동안에만 동작할 수 있는 것을 극복하는 것이다. 무선 전력 전송은 이와 같이 에너지가 제한된 네트워크에 자기 지속성을 부여할 수 있는 해결 방법으로 고려되고 있다. 사용자들은 무선 전력 전송을 통하여 전용 에너지원에 의해 전송되는 무선 주파수 신호로부터 에너지를 수집하고 배터리를 충전시킬 수 있다. 무선 전력 전송이 가능한 무선 네트워크를 설계하기 위한 체계로서 무선 전력 통신 네트워크(Wireless powered communication network, WPCN)가 제안되었다. WPCN은 기지국과 전용 에너지원의 역할을 같이 하는 hybrid access-point (H-AP)와 여러 사용자로 구성된다. WPCN에서 사용자들은 하향링크를 통하여 무선 전력 전송으로 배터리를 충전시키고, 상향링크를 통하여 해당 에너지로 정보를 전송한다. 이 때, 자원이 부족하므로 WPCN의 시스템을 활용하기 위해서는 효율적인 설계가 필수적이다. 이를 위하여, 본 논문에서는 WPCN을 위한 시스템 설계와 자원 할당에 대하여, 특히 통신량 관점에서 탐구하고자 한다. 또한, WPCN의 특징인 이중 근거리 문제에서 비롯된 높은 사용자간 전송 속도 격차를 완화하고자 한다. 우선, 협력 무선 전력 통신 네트워크에 대해 논의한다. 해당 네트워크에서는 채널 상태가 좋은 사용자가 그렇지 않은 사용자의 정보를 중계하여 사용자 공정성을 향상시킨다. 고려하는 시스템 모델에서 합통신량을 최대화하는 데 각 사용자의 서비스 품질(Quality of service, QoS)을 보장하도록 한다. 위 문제의 최적해를 분석하여, WPCN이 사용자 협력 기법을 통하여 이득을 얻는 조건을 밝히고, 이를 기반으로 사용자 협력 기법을 실용적으로 활용할 수 있는 합통신량 최대화를 위한 새로운 자원 할당 알고리즘을 제안한다. 다음으로, 상향링크에서 비직교 다중 접속(Non-orthogonal multiple access, NOMA)이 적용된 다중 안테나 WPCN에 대하여 논의한다. NOMA를 활용하는 것과 관련된 여러 문제를 해결하기 위하여, 다중 안테나 시스템을 이용한 사용자 클러스터링 기법이 추가로 적용되고, 이에 단일 NOMA 전송의 사용자 수가 감소한다. 합통신량 최대화를 위하여 클러스터별 빔형성과 시간 및 에너지 자원을 공동으로 최적화하는 것이 어렵기 때문에, 먼저 빔형성을 설계한 다음, 해당 빔형성이 적용된 네트워크에 대하여 자원을 최적화한다. 이에, 클러스터별 빔형성 설계와 합통신량 최대화를 위한 새로운 알고리즘을 제안한다. 마지막으로, 무선 통신의 성능을 향상시킬 후보 기술 중 하나인 지능형 반사 표면(Intelligent reflecting surface, IRS)이 도입된 WPCN을 고려한다. IRS를 도입함으로써, 사용자들은 추가로 에너지를 얻을 수 있으며 신호 세기를 높일 수 있다. 고려된 시스템 모델의 합통신량을 최대화하도록 IRS의 빔형성과 자원을 최적화한다. 특히, 상향링크를 위하여 NOMA와 직교 다중 접속이 고려되고, 두 다중 접속 기법간의 성능 비교가 이루어진다.1 Introduction 1 1.1 Related Work 3 1.1.1 Wireless Powered Communication Networks 3 1.1.2 A NOMA-Based WPCN 4 1.2 Contributions and Organization 6 1.3 Notation 8 2 Wireless Powered Communication Networks with User Cooperation 9 2.1 Introduction 10 2.2 System model 14 2.3 Problem Formulation 18 2.4 Optimal Solution of QoS Constrained Sum-Throughput Maximization 21 2.4.1 Case (I): Positive z1, z21 and z22 25 2.4.2 Case (II): Positive z1 and z22, and undefinable z21 35 2.5 QoS Constrained Sum-Throughput Maximization Algorithm 40 2.5.1 Proposed Algorithm 41 2.5.2 Computational Complexity Comparison 42 2.6 Sum-Throughput Maximization with Processing Cost 46 2.7 Simulation Results 48 2.8 Conclusion 53 3 NOMA-Based Wireless Powered Communication Networks with User Clustering 56 3.1 Introduction 57 3.1.1 Throughput Maximization in WPCN 58 3.1.2 User Clustering in NOMA 59 3.1.3 Motivation and Contribution 60 3.2 System model 62 3.3 Optimal Beamforming And Resource Allocation 66 3.3.1 Beamforming Design 67 3.3.2 Sum-Throughput Maximization 69 3.3.3 TDMA-based WPCN with Cluster-specific Beamforming 76 3.4 Simulation Results 79 3.5 Conclusion 87 4 IRS-Assisted Wireless Powered Communication Networks: Comparison of NOMA and OMA 89 4.1 Introduction 90 4.2 System Model 91 4.2.1 NOMA-based WPCN 92 4.2.2 OMA-based WPCN 94 4.3 Sum-Throughput Maximization 94 4.3.1 NOMA-based WPCN with throughput constraints 95 4.3.2 OMA-based WPCN with throughput constraints 98 4.4 Simulation Results 99 4.5 Conclusion 102 5 Conclusion 106 5.1 Summary 106 5.2 Future directions 107 Abstract (In Korean) 117 감사의 글 119Docto

Extending Wireless Powered Communication Networks for Future Internet of Things

Energy limitation has always been a major concern for long-term operation of wireless networks. With today's exponential growth of wireless technologies and the rapid movement towards the so-called Internet of Things (IoT), the need for a reliable energy supply is more tangible than ever. Recently, energy harvesting has gained considerable attention in research communities as a sustainable solution for prolonging the lifetime of wireless networks. Beside conventional energy harvesting sources such as solar, wind, vibration, etc. harvesting energy from radio frequency (RF) signals has drawn significant research interest in recent years as a promising way to overcome the energy bottleneck. Lately, the integration of RF energy transfer with wireless communication networks has led to the emergence of an interesting research area, namely, wireless powered communication network (WPCN), where network users are powered by a hybrid access point (HAP) which transfers wireless energy to the users in addition to serving the functionalities of a conventional access point. The primary aim of this thesis is to extend the baseline model of WPCN to a dual-hop WPCN (DH-WPCN) in which a number of energy-limited relays are in charge of assisting the information exchange between energy-stable users and the HAP. Unlike most of the existing research in this area which has merely focused on designing methods and protocols for uplink communication, we study both uplink and downlink information transmission in the DH-WPCN. We investigate sum-throughput maximization problems in both directions and propose algorithms for optimizing the values of the related parameters. We also tackle the doubly near-far problem which occurs due to unequal distance of the relays from the HAP by proposing a fairness enhancement algorithm which guarantees throughput fairness among all users

Wireless Information and Power Transfer in Communication Networks: Performance Analysis and Optimal Resource Allocation

Energy harvesting is considered as a prominent solution to supply the energy demand for low-power consuming devices and sensor nodes. This approach relinquishes the requirements of wired connections and regular battery replacements. This thesis analyzes the performance of energy harvesting communication networks under various operation protocols and multiple access schemes. Furthermore, since the radio frequency signal has energy, in addition to conveying information, it is also possible to power energy harvesting component while establishing data connectivity with information-decoding component. This leads to the concept of simultaneous wireless information and power transfer. The central goal of this thesis is to conduct a performance analysis in terms of throughput and energy efficiency, and determine optimal resource allocation strategies for wireless information and power transfer. In the first part of the thesis, simultaneous transfer of information and power through wireless links to energy harvesting and information decoding components is studied considering finite alphabet inputs. The concept of non-uniform probability distribution is introduced for an arbitrary input, and mathematical formulations that relate probability distribution to the required harvested energy level are provided. In addition, impact of statistical quality of service (QoS) constraints on the overall performance is studied, and power control algorithms are provided. Next, power allocation strategies that maximize the system energy efficiency subject to peak power constraints are determined for fading multiple access channels. The impact of channel characteristics, circuit power consumption and peak power level on the node selection, i.e., activation of user equipment, and the corresponding optimal transmit power level are addressed. Initially, wireless information transfer only is considered and subsequently wireless power transfer is taken into account. Assuming energy harvesting components, two scenarios are addressed based on the receiver architecture, i.e, having separated antenna or common antenna for the information decoding and energy harvesting components. In both cases, optimal SWIPT power control policies are identified, and impact of the required harvested energy is analyzed. The second line of research in this thesis focuses on wireless-powered communication devices that operate based on harvest-then-transmit protocol. Optimal time allocation for the downlink and uplink operation interval are identified formulating throughput maximization and energy-efficiency maximization problems. In addition, the performance gain among various types of downlink-uplink operation protocols is analyzed taking into account statistical QoS constraints. Furthermore, the performance analysis of energy harvesting user equipment is extended to full-duplex wireless information and power transfer as well as cellular networks. In full-duplex operation, optimal power control policies are identified, and the significance of introducing non-zero mean component on the information-bearing signal is analyzed. Meanwhile, SINR coverage probabilities, average throughput and energy efficiency are explicitly characterized for wireless-powered cellular networks, and the impact of downlink SWIPT and uplink mmWave schemes are addressed. In the final part of the thesis, energy efficiency is considered as the performance metric, and time allocation strategies that maximize energy efficiency for wireless powered communication networks with non-orthogonal multiple access scheme are determined. Low complex algorithms are proposed based on Dinkelbach’s method. In addition, the impact of statistical QoS constraints imposed as limitations on the buffer violation probabilities is addressed

Secrecy Throughput Maximization for Full-Duplex Wireless Powered IoT Networks under Fairness Constraints

In this paper, we study the secrecy throughput of a full-duplex wireless powered communication network (WPCN) for internet of things (IoT). The WPCN consists of a full-duplex multi-antenna base station (BS) and a number of sensor nodes. The BS transmits energy all the time, and each node harvests energy prior to its transmission time slot. The nodes sequentially transmit their confidential information to the BS, and the other nodes are considered as potential eavesdroppers. We first formulate the sum secrecy throughput optimization problem of all the nodes. The optimization variables are the duration of the time slots and the BS beamforming vectors in different time slots. The problem is shown to be non-convex. To tackle the problem, we propose a suboptimal two stage approach, referred to as sum secrecy throughput maximization (SSTM). In the first stage, the BS focuses its beamforming to blind the potential eavesdroppers (other nodes) during information transmission time slots. Then, the optimal beamforming vector in the initial non-information transmission time slot and the optimal time slots are derived. We then consider fairness among the nodes and propose max-min fair (MMF) and proportional fair (PLF) algorithms. The MMF algorithm maximizes the minimum secrecy throughput of the nodes, while the PLF tries to achieve a good trade-off between the sum secrecy throughput and fairness among the nodes. Through numerical simulations, we first demonstrate the superior performance of the SSTM to uniform time slotting and beamforming in different settings. Then, we show the effectiveness of the proposed fair algorithms