Downlink channel access performance of NR-U: Impact of numerology and mini-slots on coexistence with Wi-Fi in the 5 GHz band

Coexistence between cellular systems and Wi-Fi gained the attention of the research community when LTE License Assisted Access (LAA) entered the unlicensed band. The recent introduction of NR-U as part of 5G introduces new coexistence opportunities because it implements scalable numerology (flexible subcarrier spacing and OFDM symbol lengths), and non-slot based scheduling (mini-slots), which considerably impact channel access. This paper analyzes the impact of NR-U settings on its coexistence with Wi-Fi networks and compares it with LAA operation using simulations and experiments. First, we propose a downlink channel access simulation model, which addresses the problem of the dependency and non-uniformity of transmission attempts of different nodes, as a result of the synchronization mechanism introduced by NR-U. Second, we validate the accuracy of the proposed model using FPGA-based LAA, NR-U, and Wi-Fi prototypes with over-the-air transmissions. Additionally, we show that replacing LAA with NR-U would not only allow to overcome the problem of bandwidth wastage caused by reservation signals but also, in some cases, to preserve fairness in channel access for both scheduled and random-access systems. Finally, we conclude that fair coexistence of the aforementioned systems in unlicensed bands is not guaranteed in general, and novel mechanisms are necessary for improving the sharing of resources between scheduled and contention-based technologies

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

비면허대역 셀룰라 통신의 성능 분석 및 성능 향상 기법 연구

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 전기·정보공학부, 2021. 2. 박세웅.3GPP는 LAA (licensed-assisted access)라고하는 5GHz 비면허 대역 LTE를 개발했습니다. LAA는 충돌 방지 기능을 사용하기 위해 Wi-Fi의 CSMA / CA (Carrier Sense Multiple Access with Collision avoidance)와 유사한 LBT (Listen Before Talk) 작업을 채택하여 각 LAA 다운 링크 버스트의 프레임 구조 오버 헤드는 각각의 종료 시간에 따라 달라집니다. 이전 LBT 작업. 이 논문에서는 비면허 대역 셀룰러 통신을 분석하기위한 수치 모델을 제안한다. 다음으로, 비면허 대역 셀룰러 통신의 다음 두 가지 향상된 기능을 고려합니다. 대역 독립형 셀룰러 통신. 기존 WiFi 분석 모델로는 LAA의 성능을 평가할 수 없다는 점을 감안하여 본 서신에서는 여러 경합 진화 된 NodeB로 구성된 LAA 네트워크의 성능을 분석하기위한 새로운 Markov 체인 기반 분석 모델을 제안합니다. LAA 프레임 구조 오버 헤드의 변형. LTE-LAA는 LTE에서 상속 된 속도 적응 알고리즘을 위해 적응 변조 및 코딩 (AMC) 을 채택합니다. AMC는 진화 된 nodeB (eNB)가 현재 전송의 채널 품질 표시기 피드 백을 사용하여 다음 전송을위한 변조 및 코딩 방식 (MCS)을 선택하도록 돕습니다. 라이선스 대역에서 동작하는 기존 LTE의 경우 노드 경합 문제가 없으며 AMC 성능 에 대한 연구가 잘 진행되고 있습니다. 그러나 비면허 대역에서 동작하는 LTE-LAA 의 경우 충돌 문제로 인해 AMC 성능이 제대로 처리되지 않았습니다. 이 편지에서는 AMC 운영을 고려한 현실적인 채널 모델에서 LTELAA 성능을 분석하기위한 새로 운 Markov 체인 기반 분석 모델을 제안합니다. 무선 네트워크 분석에 널리 사용되는 Rayleigh 페이딩 채널 모델을 채택하고 분석 결과를 ns-3 시뮬레이터에서 얻은 결과 와 비교합니다. 비교 결과는 평균 정확도가 99.5%로 분석 모델의 정확도를 보여줍니다. 높은 데이터 속도에 대한 요구 사항으로 인해 3GPP는 LTE-LAA를위한 다중 반송파 운영을 제공했습니다. 그러나 다중 반송파 동작은 OOBE에 취약하고 제한된 전송 전력을 사용하여 비효율적 인 채널 사용을 초래합니다. 본 논문은 채널 효율을 높이기위한 새로운 다중 반송파 접근 방식을 제안한다. 우리가 제안한 방식은 전송 버스트를 여러 개로 분할하고 전송 전력 제한을 충족하면서 짧은 서브 프레임 전송 을 사용합니다. 또한 채널 상태를 정확하게 판단하여 OOBE 문제를 극복 할 수있는 에너지 감지 알고리즘을 제안합니다. 소프트웨어 정의 라디오를 사용하는 프로토 타입은 99% 이상의 정확도로 채널 상태를 결정하는 에너지 감지 알고리즘의 실행 가능성과 성능을 보여줍니다. ns-3 시뮬레이션을 통해 제안 된 다중 반송파 액세스 방식이 기존 LBT 유형 A 및 유형 B에 비해 사용자인지 처리량에서 각각 최대 59% 및 21.5%의 성능 향상을 달성 함을 확인했습니다. 레거시 LAA에는 배포 문제가 있기 때문에 3GPP와 MulteFire 얼라이언스는 비면허 대역 독립형 셀룰러 통신 시스 템을 제안했습니다. 그러나, 종래의 비면허 대역 독립형 셀룰러 통신 시스템은 상향 링크 제어 메시지의 전송 확률이 낮다. 이 논문은 Wi-Fi 블록 ACK 프레임에 업 링크 제어 메시지를 넣는 W ARQ : Wi-Fi 지원 HARQ를 제안합니다. 또한 W-ARQ의 처 리 성능을 향상시키기 위해 병렬 HARQ 및 클러스터링 된 Minstrel을 제안합니다. 우리가 제안한 알고리즘은 기존 MulteFire가 거의 제로 처리량 성능을 보이는 경우 높은 처리량 성능을 보여줍니다. 요약하면 비면허 대역 셀룰러 통신의 성능을 분석 합니다. 제안 된 모델을 사용함으로써 우리는 레거시 다중 반송파 동작을 주장하며 비면허 셀룰러 통신의 HARQ는 효율적이지 않다. 이러한 이유로, 우리는 최첨단 기 술에 비해 UPT 및 처리량과 같은 네트워크 성능 향상을 달성하는 OOBE 인식 추가 액세스 및 W-ARQ를 제안합니다.3GPP has developed 5 GHz unlicensed band LTE, referred to as licensed-assisted access (LAA). LAA adopts listen before talk (LBT) operation, resembling Wi-Fis carrier sense multiple access with collision avoidance (CSMA/CA), to enable collision avoidance capability, while the frame structure overhead of each LAA downlink burst varies with the ending time of each preceding LBT operation. In this dissertation, we propose numerical model to analyze unlicensed band cellular communication. Next, we consider the following two enhancements of unlicensed band cellular communication: (i) out-of-band emission (OOBE) aware additional carrier access, and (ii) Wi-Fi assisted hybrid automatic repeat request (H-ARQ) for unlicensed-band stand-alone cellular communication. Given that, existing analytic models of Wi-Fi cannot be used to evaluate the performance of LAA, in this letter, we propose a novel Markov chain-based analytic model to analyze the performance of LAA network composed of multiple contending evolved NodeBs by considering the variation of the LAA frame structure overhead. LTE-LAA adopts adaptive modulation and coding (AMC) for the rate adaptation algorithm inherited from LTE. AMC helps the evolved nodeB (eNB) to select a modulation and coding scheme (MCS) for the next transmission using the channel quality indicator feedback of the current transmission. For the conventional LTE operating in the licensed band, there is no node contention problem and AMC performance has been well studied. However, in the case of LTE-LAA operating in the unlicensed band, AMC performance has not been properly addressed due to the collision problem. In this letter, we propose a novel Markov chain-based analysis model for analyzing LTELAA performance under a realistic channel model considering AMC operation. We adopt Rayleigh fading channel model widely used in wireless network analysis, and compare our analysis results with the results obtained from ns-3 simulator. Comparison results show an average accuracy of 99.5%, which demonstrates the accuracy of our analysis model. Due to the requirement for a high data rate, the 3GPP has provided multi-carrier operation for LTE-LAA. However, multi-carrier operation is susceptible to OOBE and uses limited transmission power, resulting in inefficient channel usage. This paper proposes a novel multi-carrier access scheme to enhance channel efficiency. Our proposed scheme divides a transmission burst into multiple ones and uses short subframe transmission while meeting the transmission power limitation. In addition, we propose an energy detection algorithm to overcome the OOBE problem by deciding the channel status accurately. Our prototype using software-defined radio shows the feasibility and performance of the energy detection algorithm that determines the channel status with over 99% accuracy. Through ns-3 simulation, we confirm that the proposed multi-carrier access scheme achieves up to 59% and 21.5% performance gain in userperceived throughput compared with the conventional LBT type A and type B, respectively. Since the legacy LAA has deployment problem, 3GPP and MulteFire alliance proposed unlicensed band stand-alone cellular communication system. However, conventional unlicensed band stand-alone cellular communication system has low transmission probability of uplink control messages. This disertation proposes W-ARQ: Wi-Fi assisted HARQ which put uplink control messages into Wi-Fi block ACK frame. In addition we propose parallel HARQ and clustered Minstrel to enhance throughput performance of W-ARQ. Our proposed algorithm shows high throughput performance where conventional MulteFire shows almost zero throughput performance. In summary, we analyze the performance of unlicensed-band cellular communication. By using the proposed model, we insist the legacy multi-carrier operation and HARQ of unlicensed cellular communication is not efficient. By this reason, we propose OOBE aware additional access and W-ARQ which achievee enhancements of network performance such as UPT and throughput compared with state-of-the-art techniques.Abstract i Contents iv List of Tables vii List of Figures viii 1 Introduction 1 1.1 Unlicensed Band Communication System . . . . . . . . . . . . . . . 1 1.2 Overview of Existing Approaches . . . . . . . . . . . . . . . . . . . 2 1.2.1 License-assisted access . . . . . . . . . . . . . . . . . . . . . 2 1.2.2 Further LAA . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.3 Non-3GPP Unlicensed Band Cellular Communication . . . . 6 1.3 Main Contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.1 Performance Analysis of LTE-LAA . . . . . . . . . . . . . . 6 1.3.2 Out-of-Band Emission Aware Additional Carrier Access for LTE-LAA Network . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.3 W-ARQ: Wi-Fi Assisted HARQ for Unlicensed Band StandAlone Cellular Communication System . . . . . . . . . . . . 8 1.4 Organization of the Dissertation . . . . . . . . . . . . . . . . . . . . 8 2 Performance Analysis of LTE-LAA network 10 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3 Proposed Markov-Chain Model . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 Markov Property . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.2 Markov Chain Model for EPS Type Variation . . . . . . . . . 16 2.3.3 LAA Network Throughput Estimation . . . . . . . . . . . . . 18 2.4 Model Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3 Out-of-Band Emission Aware Additional Carrier Access for LTE-LAA Network 35 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2 Related work and Background . . . . . . . . . . . . . . . . . . . . . 37 3.2.1 Related work . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.2.2 Listen Before Talk . . . . . . . . . . . . . . . . . . . . . . . 38 3.2.3 Out-of-Band Emission . . . . . . . . . . . . . . . . . . . . . 39 3.3 Multi-carrier Operation of LTE-LAA . . . . . . . . . . . . . . . . . . 39 3.4 Carrier Sensing considering Out-of-Band Emission . . . . . . . . . . 47 3.4.1 Energy Detection Algorithm . . . . . . . . . . . . . . . . . . 49 3.4.2 Nominal Band Energy Detection . . . . . . . . . . . . . . . . 50 3.4.3 OOBE-Free Region Energy Detection . . . . . . . . . . . . . 51 3.5 Additional Carrier Access Scheme . . . . . . . . . . . . . . . . . . . 52 3.5.1 Basic Operation . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.5.2 Transmission Power Limitation . . . . . . . . . . . . . . . . 53 3.5.3 Dividing Transmission Burst . . . . . . . . . . . . . . . . . . 54 3.5.4 Short Subframe Decision . . . . . . . . . . . . . . . . . . . . 54 3.6 Performance Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.6.1 Performance of Energy Detection considering OOBE . . . . . 57 3.6.2 Simulation Environments . . . . . . . . . . . . . . . . . . . . 57 3.6.3 Performance of Proposed Carrier Access Scheme . . . . . . . 58 3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4 W-ARQ: Wi-Fi Assisted HARQ for Unlicensed Band Stand-Alone Cellular Communication System 66 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.3 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.4 W-ARQ: Wi-Fi assisted HARQ for Unlicensed Band Stand-Alone Cellular Communication System . . . . . . . . . . . . . . . . . . . . . . 69 4.4.1 Parallel HARQ . . . . . . . . . . . . . . . . . . . . . . . . . 71 4.4.2 Clustered Minstrel . . . . . . . . . . . . . . . . . . . . . . . 72 4.5 Performance Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5 Concluding Remarks 80 5.1 Research Contributions . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Abstract (In Korean) 90 감사의 글 93Docto

Energy and throughput efficient strategies for heterogeneous future communication networks

As a result of the proliferation of wireless-enabled user equipment and data-hungry applications, mobile data traffic has exponentially increased in recent years.This in-crease has not only forced mobile networks to compete on the scarce wireless spectrum but also to intensify their power consumption to serve an ever-increasing number of user devices. The Heterogeneous Network (HetNet) concept, where mixed types of low-power base stations coexist with large macro base stations, has emerged as a potential solution to address power consumption and spectrum scarcity challenges. However, as a consequence of their inflexible, constrained, and hardware-based configurations, HetNets have major limitations in adapting to fluctuating traffic patterns. Moreover, for large mobile networks, the number of low-power base stations (BSs) may increase dramatically leading to sever power consumption. This can easily overwhelm the benefits of the HetNet concept. This thesis exploits the adaptive nature of Software-defined Radio (SDR) technology to design novel and optimal communication strategies. These strategies have been designed to leverage the spectrum-based cell zooming technique, the long-term evolution licensed assisted access (LTE-LAA) concept, and green energy, in order to introduce a novel communication framework that endeavors to minimize overall network on-grid power consumption and to maximize aggregated throughput, which brings significant benefits for both network operators and their customers. The proposed strategies take into consideration user data demands, BS loads, BS power consumption, and available spectrum to model the research questions as optimization problems. In addition, this thesis leverages the opportunistic nature of the cognitive radio (CR) technique and the adaptive nature of the SDR to introduce a CR-based communication strategy. This proposed CR-based strategy alleviates the power consumption of the CR technique and enhances its security measures according to the confidentiality level of the data being sent. Furthermore, the introduced strategy takes into account user-related factors, such as user battery levels and user data types, and network-related factors, such as the number of unutilized bands and vulnerability level, and then models the research question as a constrained optimization problem. Considering the time complexity of the optimum solutions for the above-mentioned strategies, heuristic solutions were proposed and examined against existing solutions. The obtained results show that the proposed strategies can save energy consumption up to 18%, increase user throughput up to 23%, and achieve better spectrum utilization. Therefore, the proposed strategies offer substantial benefits for both network operators and users