높은 효율의 무선랜을 위한 MAC/PHY 기법

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 8. 최성현.Along with the steady increase in mobile data traffic, wireless local area network (WLAN) technology has been developed to support heavy traffic for various mobile devices. The-state-of-art IEEE 802.11 specifications such as 802.11n and 802.11ac have focused on improving physical layer (PHY) rate by enabling multiple stream transmission via multiple-input multiple-output (MIMO) technology, wide bandwidth transmission via channel bonding, and high order modulation via 256-QAM and short guard interval. While the emerging technologies greatly increase PHY rate over 1~Gb/s, the achievable throughput is much limited due to the low reliability with high PHY rates and medium sharing among the nodes operating on the same channel. In this dissertation, we tackle three different strategies to enhance the achievable throughput in IEEE 802.11 WLANs. Firstly, we study a cost-effective approach, namely antenna subset selection, to enhance reliability even for the high PHY rates. There are practical challenges to employ antenna subset selection in WLANs such as the lack of channel state information at the transmitter and multiple retry chain utilization. Only few researches have addressed those practical challenges, which result in a limited employment of antenna subset selection in WLANs. We propose a practical antenna subset selection system considering those practical challenges, and evaluate the performance of the proposed system via prototype implementation and extensive experiments. Secondly, we focus on the clear channel assessment (CCA) of IEEE 802.11 WLAN which is too conservative to exploit spatial reuse. The problem is arise due to a limitation of the current CCA mechanism. Only the received signal strength (RSS) of an ongoing transmission is used to determine the status of the medium, i.e., busy or idle. We propose a novel CCA mechanism which utilizes the information delivered in PHY header of the ongoing transmission so that it can stimulate concurrent transmissions for better spatial reuse. Through simulations, we evaluate our proposed approach and demonstrate throughput gain in various scenarios. Lastly, we investigate transmit power and data rate control method to further exploit spatial reuse. Along with our proposed CCA mechanism, more concurrent transmissions become feasible by adapting transmit power and data rate depending on the ongoing transmission. Accordingly, we propose a joint transmit power and data rate control algorithm which operates dynamically depending on the existence of ongoing transmission. We evaluate our proposed algorithm under various scenarios through extensive simulations. In summary, we propose three different methodologies for high efficiency WLANs, one for reliability enhancement and the other two for better spatial reuse. The operation and performance gain of each methodology are verified by testbed experiments or network level simulations.Chapter 1 Introduction 1 1.1 Motivation 1 1.2 Overview of Existing Approaches 4 1.2.1 Practical Antenna Selection for WLAN AP 4 1.2.2 Protective Clear Channel Assessment 5 1.2.3 Dynamic Transmit Power and Data Rate Control 5 1.3 Main Contributions 7 1.3.1 Reliability Enhancement 7 1.3.2 Spatial Reuse Exploitation 9 1.4 Organization of the Dissertation 9 Chapter 2 Practical Antenna Selection for WLAN AP 11 2.1 Introduction 11 2.2 System Description 14 2.2.1 Characteristics of O-the-Shelf Devices 14 2.2.2 Antenna Selection for WLAN AP 18 2.3 Measurement Studies 22 2.3.1 Throughput with Dierent Antenna Combinations 22 2.3.2 DL/UL Link Analysis 26 2.4 Proposed Antenna Selection Algorithm 28 2.4.1 Transmit Antenna Selection 28 2.4.2 Default Antenna Selection 36 2.5 Performance Evaluation 38 2.5.1 Performance of Proposed Transmit Antenna Selection 38 2.5.2 Performance of Proposed Default Antenna Selection 45 2.6 Summary 47 Chapter 3 Protective Clear Channel Assessment 48 3.1 Introduction 48 3.2 Background and Motivation 50 3.2.1 Ideal Operation of CCA 50 3.2.2 IEEE 802.11 Frame Format and CCA Method 52 3.2.3 Physical Layer Header Utilization for CCA 54 3.3 Protective Clear Channel Assessment 55 3.3.1 Signal Quality Table 56 3.3.2 Feasibility Check of Spatial Reuse 57 3.3.3 Consideration of Link Asymmetry 58 3.4 Performance Evaluation 60 3.5 Summary 64 Chapter 4 Dynamic Transmit Power and Data Rate Control 66 4.1 Introduction 66 4.2 Transmit Power and Rate Control for Spatial Reuse 68 4.2.1 RTS-CTS Based 69 4.2.2 Feedback Based 70 4.2.3 Limitations of existing approach 71 4.3 Dynamic Transmit Power and Rate Control 72 4.3.1 Information Gathering 73 4.3.2 Dynamic TPC & RA 76 4.3.3 Normal TPC & RA 77 4.3.4 DCF Throughput Analysis 78 4.3.5 Multi-cell Consideration 85 4.4 Performance Evaluation 86 4.5 Summary 91 Chapter 5 Conclusion 93 5.1 Research Contributions 93 5.2 Future Research Directions 95 Bibliography 97 초록 102Docto

Enabling Millimeter Wave Communication for 5G Cellular Networks: MAC-layer Perspective

Data traffic among mobile devices increases dramatically with emerging high-speed multimedia applications such as uncompressed video streaming. Many new applications beyond personal communications involve tens or even hundreds of billions wireless devices, such as wireless watch, e-health sensors, and wireless glass. The number of wireless devices and the data rates will continue to grow exponentially. Quantitative evidences forecast that total data rate by 2020 will be 1000 times of current 4G data rate. Next generation wireless networks need fundamental changes to satisfy the overwhelming capacity demands. Millimeter wave (mmWave) communication with huge available bandwidth is a very promising solution for next generation wireless networks to overcome the global bandwidth shortage at saturated microwave spectrum. The large available bandwidth can be directly translated into high capacity. mmWave communication has several propagation characteristics including strong pathloss, atmospheric and rain absorption, low diffraction around obstacles and penetration through objects. These propagation characteristics create challenges for next generation wireless networks to support various kinds of emerging applications with different QoS requirements. Our research focuses on how to effectively and efficiently exploit the large available mmWave bandwidth to achieve high capacity demand while overcoming these challenges on QoS provisioning for various kinds of applications. This thesis focuses on MAC protocol design and analysis for mmWave communication to provide required capacity and QoS to support various kinds of applications in next generation wireless networks. Specifically, from the transmitter/receiver perspective, multi-user beamforming based on codebook is conducted to determine best transmission/reception beams to increase network capacity considering the mutual interferences among concurrent links. From the channel perspective, both interfering and non-interfering concurrent links are scheduled to operate simultaneously to exploit spatial reuse and improve network capacity. Link outage problem resulting from the limited diffraction capability and low penetration capability of mmWave band is addressed for quality provisioning by enabling multi-hop transmission to replace the link in outage (for low-mobility scenarios) and buffer design with dynamic bandwidth allocation among all the users in the whole coverage area (for high-mobility scenarios). From the system perspective, system structure, network architecture, and candidate MAC are investigated and novel backoff mechanism for CSMA/CA is proposed to give more transmission opportunity to faraway nodes than nearby nodes in order to achieve better fairness and higher network capacity. In this thesis, we formulate each problem mentioned above as an optimization problem with the proposed algorithms to solve it. Extensive analytical and simulation results are provided to demonstrate the performance of the proposed algorithms in several aspects, such as network capacity, energy efficiency, link connectivity and so on

Mitigating the Impact of Physical Layer Capture and ACK Interference in Wireless 802.11 Networks

Ph.DDOCTOR OF PHILOSOPH

Mobile Ad hoc Networking: Imperatives and Challenges

Mobile ad hoc networks (MANETs) represent complex distributed systems that comprise wireless mobile nodes that can freely and dynamically self-organize into arbitrary and temporary, "ad-hoc" network topologies, allowing people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure, e.g., disaster recovery environments. Ad hoc networking concept is not a new one, having been around in various forms for over 20 years. Traditionally, tactical networks have been the only communication networking application that followed the ad hoc paradigm. Recently, the introduction of new technologies such as the Bluetooth, IEEE 802.11 and Hyperlan are helping enable eventual commercial MANET deployments outside the military domain. These recent evolutions have been generating a renewed and growing interest in the research and development of MANET. This paper attempts to provide a comprehensive overview of this dynamic field. It first explains the important role that mobile ad hoc networks play in the evolution of future wireless technologies. Then, it reviews the latest research activities in these areas, including a summary of MANET\u27s characteristics, capabilities, applications, and design constraints. The paper concludes by presenting a set of challenges and problems requiring further research in the future

MAC/PHY Co-Design of CSMA Wireless Networks Using Software Radios.

In the past decade, CSMA-based protocols have spawned numerous network standards (e.g., the WiFi family), and played a key role in improving the ubiquity of wireless networks. However, the rapid evolution of CSMA brings unprecedented challenges, especially the coexistence of different network architectures and communications devices. Meanwhile, many intrinsic limitations of CSMA have been the main obstacle to the performance of its derivatives, such as ZigBee, WiFi, and mesh networks. Most of these problems are observed to root in the abstract interface of the CSMA MAC and PHY layers --- the MAC simply abstracts the advancement of PHY technologies as a change of data rate. Hence, the benefits of new PHY technologies are either not fully exploited, or they even may harm the performance of existing network protocols due to poor interoperability. In this dissertation, we show that a joint design of the MAC/PHY layers can achieve a substantially higher level of capacity, interoperability and energy efficiency than the weakly coupled MAC/PHY design in the current CSMA wireless networks. In the proposed MAC/PHY co-design, the PHY layer exposes more states and capabilities to the MAC, and the MAC performs intelligent adaptation to and control over the PHY layer. We leverage the reconfigurability of software radios to design smart signal processing algorithms that meet the challenge of making PHY capabilities usable by the MAC layer. With the approach of MAC/PHY co-design, we have revisited the primitive operations of CSMA (collision avoidance, carrier signaling, carrier sensing, spectrum access and transmitter cooperation), and overcome its limitations in relay and broadcast applications, coexistence of heterogeneous networks, energy efficiency, coexistence of different spectrum widths, and scalability for MIMO networks. We have validated the feasibility and performance of our design using extensive analysis, simulation and testbed implementation.PHDComputer Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/95944/1/xyzhang_1.pd

Cooperative Communications inWireless Local Area Networks: MAC Protocol Design and Multi-layer Solutions

This dissertation addresses cooperative communications and proposes multi-layer solu- tions for wireless local area networks, focusing on cooperative MAC design. The coop- erative MAC design starts from CSMA/CA based wireless networks. Three key issues of cooperation from the MAC layer are dealt with: i.e., when to cooperate (opportunistic cooperation), whom to cooperate with (relay selection), and how to protect cooperative transmissions (message procedure design). In addition, a cooperative MAC protocol that addresses these three issues is proposed. The relay selection scheme is further optimized in a clustered network to solve the problem of high collision probability in a dense network. The performance of the proposed schemes is evaluated in terms of through- put, packet delivery rate and energy efficiency. Furthermore, the proposed protocol is verified through formal model checking using SPIN. Moreover, a cooperative code allo- cation scheme is proposed targeting at a clustered network where multiple relay nodes can transmit simultaneously. The cooperative communication design is then extended to the routing layer through cross layer routing metrics. Another part of the work aims at enabling concurrent transmissions using cooperative carrier sensing to improve the per- formance in a WLAN network with multiple access points sharing the same channel