비면허대역 셀룰라 통신을 위한 성능 향상 기법

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2021.8. 박세웅.The 3rd generation partnership project (3GPP) has standardized long-term evolution (LTE) licensed-assisted access (LTE-LAA) that uses a wide unlicensed band as an alternative solution to the insufficient bandwidth problem of the existing LTE. 3GPP cellular communications in unlicensed spectrum allow transmission only after completing listen-before-talk (LBT) operation. For downlink, the LBT operation helps cellular traffic to coexist well with Wi-Fi traffic. However, cellular uplink transmission is attempted only at the time specifically determined by the base station after having a successful LBT and the user equipment (UE) may suffer transmission failure and delayed transmission due to Wi-Fi interference. As a result, cellular uplink traffic does not coexist well with Wi-Fi traffic. NR-U suffers from the collision issue because its channel access mechanism is similar to that of Wi-Fi. Wi-Fi solves the collision problem through the request-to-send/clear-to-send (RTS/CTS) mechanism. However, NR-U has no way of solving the collision problem. As a result, NR-U suffers severe performance degradation due to collisions as the number of contending nodes increases. In this dissertation, we consider the following two enhancements to cellular communication in the unlicensed spectrum: (i) Uplink channel access enhancement for solving poor uplink performance and (ii) collision minimization for efficient channel utilization. First, we mathematically analyze the problem of unfairness between cellular and Wi-Fi for uplink channel access. To address the coexistence problem in unlicensed spectrum, we propose a standard-compliant approach, termed UpChance, which allows the UE to use a minimum length of uplink reservation signal (RS) and the base station to determine the optimal timing for the UE's uplink transmission. Through ns-3 simulation, we verify that UpChance improves the performance of fairness and random access completion time by up to 88% and 99%, respectively. Second, we propose to extend an RS duration and use a split RS for reservation in NR-U that consists of front RS and rear RS and design a new collision minimization scheme, termed R-SplitC, that contains two components: new split RS operation and contention window size (CWS) control. New split RS operation helps to minimize collisions in NR-U transmissions, and CWS control works to protect the performance of other communication technologies such as Wi-Fi. We mathematically analyze and evaluate the performance of our scheme and confirm that R-SplitC improves network throughput by up to 100.6% compared to the baseline RS scheme without degrading Wi-Fi performance. In summary, we propose standard-compliant uplink channel access enhancement scheme and collision minimization scheme for cellular communication in unlicensed spectrum. Through this research, we achieve enhancements of network performance such as throughput and fairness.3세대 파트너십 프로젝트는 기존 LTE의 부족한 대역폭 문제에 대한 대안으로 넓은 비면허 대역을 사용하는 라이선스 지원 접속을 표준화하고 있다. 비면허 대역에서 3GPP 셀룰러 통신은 LBT 동작을 완료한 후에만 전송을 허용한다. 다운링크의 경우 LBT 작업을 통해 셀룰러 트래픽이 와이파이 트래픽과 잘 공존할 수 있습니다. 그러나, 셀룰러 업링크 전송은 LBT 성공 후 기지국에 의해 특별히 결정된 시간에만 시도되며, 사용자 장비는 와이파이의 간섭으로 인해 전송 실패와 전송 지연을 겪을 확률이 높다. 따라서 셀룰러 업링크 트래픽이 와이파이 트래픽과 잘 공존하지 못한다. 라이선스 지원 접속 기술은 또한 채널 액세스 메커니즘이 와이파이의 채널 액세스 메커니즘과 유사하기 때문에 동시 전송으로 충돌 문제를 겪고 있다. 와이파이는 RTS/CTS 메커니즘을 통해 충돌 문제를 해결한다. 그러나 현재 라이선스 지원 접속 기술은 충돌 문제를 해결할 방법이 존재하지 않는다. 따라서 라이선스 지원 접속 기술은 경합 노드 수가 증가함에 따라 충돌로 인해 심각한 성능 저하를 겪는다. 본 논문에서는 비면허 대역에서 셀룰러 통신에 대한 다음과 같은 두 가지 개선을 고려한다. (i) 업링크 성능 저하를 해결하기 위한 업링크 채널 액세스 향상 및 (ii) 효율적인 채널 활용을 위한 충돌 최소화. 첫째, 업링크 채널 액세스를 위한 셀룰러와 와이파이 사이의 불공정성 문제를 수학적으로 분석한다. 비면허 대역에서의 공존 문제를 해결하기 위해, 우리는 단말이 최소 길이의 업링크 예약 신호를 사용하고 기지국이 단말의 업링크 전송에 대한 최적의 타이밍을 결정할 수 있는 UpChance라는 표준을 만족하는 상향 링크 채널 접근 방식을 제안한다. ns-3 시뮬레이션을 통해 UpChance가 공정성과 랜덤 액세스 완료 시간을 각각 최대 88%, 99% 향상시키는 것을 검증한다. 둘째, 우리는 전방 예약신호와 후방 예약신호로 구성된 분할 예약 신호를 사용하고 경합 창 크기를 추가적으로 제어하는 R-SplitC라는 새로운 충돌 최소화 체계를 제안한다. 새로운 분할 예약 신호는 라이선스 지원 접속 기술의 전송간의 충돌을 최소화하는 데 도움을 주며, 경합 창 크기 제어는 와이파이와 같은 다른 통신 기술의 성능을 보호한다. 우리는 우리 체계의 성능을 수학적으로 분석하고 평가하여 R-SplitC가 와이파이 성능을 저하시키지 않고 기존의 예약 신호 체계에 비해 네트워크 처리량을 최대 100.6% 향상시키는 것을 확인한다. 요약하면, 우리는 비면허 대역에서 셀룰러 통신을 위한 업링크 채널 액세스 향상 기법 및 충돌 최소화 기법을 제안한다. 본 연구를 통해, 우리는 최첨단 기술에 비해 처리량 및 공정성과 같은 네트워크 성능의 향상을 달성한다.1 Introduction 1 1.1 Motivation 1 1.2 Main Contributions 2 1.2.1 Uplink Channel Access Enhancement for Cellular Communication in Unlicensed Spectrum 2 1.2.2 R-SplitC: Collision Minimization for Cellular Communication in Unlicensed Spectrum 3 1.3 Organization of the Dissertation 4 2 Uplink Channel Access Enhancement for Cellular Communication in Unlicensed Spectrum 5 2.1 Introduction 5 2.2 Related Work and Preliminaries 7 2.2.1 Related Work 7 2.2.2 Preliminaries 8 2.3 Mathematical Analysis for Unfairness between Uplink Cellular and Wi-Fi 10 2.3.1 PRACH scenario 10 2.3.2 UL data scenario 13 2.4 Proposed Scheme 17 2.4.1 UE Operation 18 2.4.2 eNB Operation 19 2.5 Performance Evaluation 24 2.5.1 Simulation Environments 24 2.5.2 UL data transmission 25 2.5.3 Random access 27 2.6 Summary 29 3 R-SplitC: Collision Minimization for Cellular Communication in Unlicensed Spectrum 37 3.1 Introduction 37 3.2 Related Work and Preliminaries 39 3.2.1 Related Work 39 3.2.2 NR-U 40 3.2.3 listen-before-talk (LBT) 41 3.2.4 reservation signal and mini-slot 41 3.2.5 Wi-Fi 42 3.3 Proposed Scheme 44 3.3.1 New RS structure 46 3.3.2 CWS control 48 3.4 Performance Analysis 49 3.4.1 Throughput Analysis for R-Split 49 3.4.2 Throughput Analysis for R-SplitC 55 3.5 Performance Evaluation 57 3.5.1 Performance Evaluation for an NR-U only Network 58 3.5.2 Performance Evaluation for an NR-U/Wi-Fi Network 61 3.6 Summary 65 4 Concluding Remarks 67 4.1 Research Contributions 67 4.2Future Work 68 Abstract (In Korean) 75 감사의글 78박

D2D-Based Grouped Random Access to Mitigate Mobile Access Congestion in 5G Sensor Networks

The Fifth Generation (5G) wireless service of sensor networks involves significant challenges when dealing with the coordination of ever-increasing number of devices accessing shared resources. This has drawn major interest from the research community as many existing works focus on the radio access network congestion control to efficiently manage resources in the context of device-to-device (D2D) interaction in huge sensor networks. In this context, this paper pioneers a study on the impact of D2D link reliability in group-assisted random access protocols, by shedding the light on beneficial performance and potential limitations of approaches of this kind against tunable parameters such as group size, number of sensors and reliability of D2D links. Additionally, we leverage on the association with a Geolocation Database (GDB) capability to assist the grouping decisions by drawing parallels with recent regulatory-driven initiatives around GDBs and arguing benefits of the suggested proposal. Finally, the proposed method is approved to significantly reduce the delay over random access channels, by means of an exhaustive simulation campaign.Comment: First submission to IEEE Communications Magazine on Oct.28.2017. Accepted on Aug.18.2019. This is the camera-ready versio

Massive Non-Orthogonal Multiple Access for Cellular IoT: Potentials and Limitations

The Internet of Things (IoT) promises ubiquitous connectivity of everything everywhere, which represents the biggest technology trend in the years to come. It is expected that by 2020 over 25 billion devices will be connected to cellular networks; far beyond the number of devices in current wireless networks. Machine-to-Machine (M2M) communications aims at providing the communication infrastructure for enabling IoT by facilitating the billions of multi-role devices to communicate with each other and with the underlying data transport infrastructure without, or with little, human intervention. Providing this infrastructure will require a dramatic shift from the current protocols mostly designed for human-to-human (H2H) applications. This article reviews recent 3GPP solutions for enabling massive cellular IoT and investigates the random access strategies for M2M communications, which shows that cellular networks must evolve to handle the new ways in which devices will connect and communicate with the system. A massive non-orthogonal multiple access (NOMA) technique is then presented as a promising solution to support a massive number of IoT devices in cellular networks, where we also identify its practical challenges and future research directions.Comment: To appear in IEEE Communications Magazin

Enabling Technologies for Ultra-Reliable and Low Latency Communications: From PHY and MAC Layer Perspectives

© 1998-2012 IEEE. Future 5th generation networks are expected to enable three key services-enhanced mobile broadband, massive machine type communications and ultra-reliable and low latency communications (URLLC). As per the 3rd generation partnership project URLLC requirements, it is expected that the reliability of one transmission of a 32 byte packet will be at least 99.999% and the latency will be at most 1 ms. This unprecedented level of reliability and latency will yield various new applications, such as smart grids, industrial automation and intelligent transport systems. In this survey we present potential future URLLC applications, and summarize the corresponding reliability and latency requirements. We provide a comprehensive discussion on physical (PHY) and medium access control (MAC) layer techniques that enable URLLC, addressing both licensed and unlicensed bands. This paper evaluates the relevant PHY and MAC techniques for their ability to improve the reliability and reduce the latency. We identify that enabling long-term evolution to coexist in the unlicensed spectrum is also a potential enabler of URLLC in the unlicensed band, and provide numerical evaluations. Lastly, this paper discusses the potential future research directions and challenges in achieving the URLLC requirements

Uplink Transmission Schemes for 5G NR Unlicensed : Design Principles and Achievable Performance

In this paper, we address and analyze different uplink (UL) transmission schemes for the 5G New Radio (NR) deployments at unlicensed spectrum. Specific emphasis is on the new NR-unlicensed (NR-U) wideband physical random access channel (PRACH) under short preamble formats as well as on the base-station receiver sensitivity requirements for the fixed reference channels. In general, in order to comply with ETSI regulations, the UL waveform resource allocation has been revised in NR-U. On one hand, the bandwidth (BW) of the PRACH sequences has been increased according to numerology, which based on the performance analysis presented in this paper provides a substantially improved detection performance while fulfilling the coverage requirements of Re1-15 preambles. On the other hand, the physical uplink shared channel (PUSCH) resource allocation is based on block interlace frequency division multiple access (B-IDFMA). This design is characterized by the number of interlaces allocated within the transmission BW and it should be properly defined for the multiple physical layer numerologies supported in NR systems. The NR-U PUSCH performance results provided in this paper show that the B-IFDMA design yields the best performance for the case of one interlace allocation compared to contiguous resource allocation for all numerologies. Additionally, it is shown that the non-B-IFDMA PUSCH design outperforms the NR-U design for larger number of allocated interlaces. There is thus a trade-off between the frequency diversity gain achieved by the sparse PRB distribution in NR-U and the corresponding channel estimation challenges impacting the demodulation performance.acceptedVersionPeer reviewe

Towards Massive Machine Type Communications in Ultra-Dense Cellular IoT Networks: Current Issues and Machine Learning-Assisted Solutions

The ever-increasing number of resource-constrained Machine-Type Communication (MTC) devices is leading to the critical challenge of fulfilling diverse communication requirements in dynamic and ultra-dense wireless environments. Among different application scenarios that the upcoming 5G and beyond cellular networks are expected to support, such as eMBB, mMTC and URLLC, mMTC brings the unique technical challenge of supporting a huge number of MTC devices, which is the main focus of this paper. The related challenges include QoS provisioning, handling highly dynamic and sporadic MTC traffic, huge signalling overhead and Radio Access Network (RAN) congestion. In this regard, this paper aims to identify and analyze the involved technical issues, to review recent advances, to highlight potential solutions and to propose new research directions. First, starting with an overview of mMTC features and QoS provisioning issues, we present the key enablers for mMTC in cellular networks. Along with the highlights on the inefficiency of the legacy Random Access (RA) procedure in the mMTC scenario, we then present the key features and channel access mechanisms in the emerging cellular IoT standards, namely, LTE-M and NB-IoT. Subsequently, we present a framework for the performance analysis of transmission scheduling with the QoS support along with the issues involved in short data packet transmission. Next, we provide a detailed overview of the existing and emerging solutions towards addressing RAN congestion problem, and then identify potential advantages, challenges and use cases for the applications of emerging Machine Learning (ML) techniques in ultra-dense cellular networks. Out of several ML techniques, we focus on the application of low-complexity Q-learning approach in the mMTC scenarios. Finally, we discuss some open research challenges and promising future research directions.Comment: 37 pages, 8 figures, 7 tables, submitted for a possible future publication in IEEE Communications Surveys and Tutorial

Models of Control Channels in the LTE System

Dizertační práce se zabývá zpracováním signálu fyzických řídicích kanálů systému LTE a vyšetřováním bitové chybovosti při přenosu řídicí informace z vysílače do přijímače v závislosti na podmínkách příjmu. Práce je rozdělena do dvou hlavních částí. První část práce je zaměřena na simulaci přenosu řídicí informace LTE v základním pásmu. Jsou zde prezentovány vytvořené simulátory řídicích kanálů ve směru uplink i downlink. Simulace jsou provedeny pro všechny druhy nastavení systému a základní modely přenosového prostředí. Jsou zde popsány výsledky vlivu použití MIMO technologií na kvalitu příjmu řídicí informace především v únikových kanálech. Druhá část práce je zaměřena na možnost nasazení systému LTE ve sdíleném pásmu ISM (2.4 GHz). Jsou zde představeny základní koncepce použití, na jejichž základě je vytvořen scénář simulací. Kapitola dále popisuje tvorbu simulátoru koexistence LTE a systému Wi-Fi v přeneseném pásmu ISM 2.4GHz. Jsou zde uvedeny výsledky simulací koexistence LTE a rušivého systému Wi-Fi provedených dle vytvořeného scénáře. Výsledky simulací koexistence LTE a Wi-Fi jsou ověřeny měřením v laboratorních podmínkách. Toto porovnání je důležité z hlediska optimalizace simulátoru koexistence. Dle výsledků obou typů simulací a měření jsou stanovena provozní doporučení, která mají přispět k bezpečnému a spolehlivému vysílání a příjmu řídicích informací LTE i při nepříznivých podmínkách příjmu.The doctoral thesis is focused on a signal processing in the LTE physical control channels and performance analysis of control information transmission according to receiving conditions. The thesis is divided into two parts. The first part deals with simulation of the transmission of control information in baseband. The created simulators for uplink and downlink are presented. The simulations are performed for all possible system settings and various channel models. The MIMO influence on a quality of control information reception under fading channels is also presented. The second part of the thesis is focused on LTE utilization in shared channel ISM (2.4 GHz). The basic LTE application concept for ISM band is presented. This concept is fundamental to created simulation scenario. The chapter also presents the LTE and Wi-Fi coexistence simulator in 2.4 GHz ISM passband. The coexistence simulation are presented according to simulation scenario and the results are shown. The simulated coexistence analysis results are verified in laboratory environment. The comparison of the simulated and the measured coexistence analysis results is crucial for further optimization of the coexistence simulator. Recommendations for optimal and reliable operation of LTE are specified according to the simulated and the measured results. Recommendations should be useful to the reliable transmission of LTE control information in bad receiving conditions.