5G URLLC를 위한 저지연 통신 프로토콜

학위논문(박사)--서울대학교 대학원 :공과대학 전기·컴퓨터공학부,2020. 2. 심병효.2020년 IMT 비전에 따르면 5 세대 (5G) 이동 통신 서비스는 eMBB (Enhanced Mobile Broadband), mMTC (Massive Machine Type Communication) 및 URLLC (Ultra Reliability and Low Latency Communication)의 세 가지 서비스로 분류된다. 낮은 지연 시간과 높은 신뢰도를 동시에 보장하는 것은 실시간 서비스 및 응용 프로그램의 상용화를 위하여 필요한 핵심 기술이고, 3 개의 5G 서비스 중 URLLC는 가장 어려운 시나리오로 여겨지고 있다. 본 학위 논문에서는 URLLC 서비스를 지원하기 위해 다음과 같은 3가지 저지연 통신 프로토콜을 제안한다: (i) 2-way 핸드쉐이크 기반 랜덤 액세스, (ii) Fast Grant Multiple Access 및 (iii) UE가 시작하는 핸드 오버 방식. 첫째, 5G에서 목표로 하는 성능 지표는 데이터 전송률의 증가뿐만 아니라 지연 시간을 감소시키는 것도 포함하고 있다. 현재 LTE-Advanced 시스템은 랜덤 액세스 및 상향 링크 전송 절차에서 4개의 메시지 교환을 필요로하고, 이는 높은 지연 시간을 야기한다. 본 논문에서는 이러한 지연 시간을 효과적으로 줄이기 위하여 2-way 랜덤 액세스 방식을 제안한다. 제안한 2-way 랜덤 액세스 기술은 프리앰블의 수를 증가시킴으로써 해당 절차를 완료하는데 단 2개의 메시지 만 필요하다. 우리는 이러한 프리앰블을 생성하고 활용하는 방법을 연구했고, 다양한 시뮬레이션을 통하여 제안한 랜덤 액세스 방식이 기존 기술과 비교하여 지연 시간을 최대 43% 줄이는 것 을 확인했다. 또한 제안한 랜덤 액세스는 계산 복잡도가 약간 증가하지만, 네트워크 로드는 기존 기술에 비해 절반 이상 감소한다. 둘째,원격 동작,자율 주행,몰입형 가상 현실 등과 같은 다양한 미션 크리티컬 어플리케이션이 등장하고 있다. 다양한 URLLC 트래픽은 다양한 지연 시간 및 신뢰도 수준을 요구 사항으로 가지고 있고, 이와 함께 필요한 데이터 크기 및 패킷의 발생율 등의 측면에서 다양한 특성을 가지고 있다. 미션 크리티컬 애플리케이션의 다양한 요구 사항을 지원하기 위해 상향 링크 전송에 중점을 둔 FGMA(Fast Grant Multiple Access)를 제안했다. FGMA는 승인 제어 알고리즘, 동적 프리앰블 구조, 상향 링크 스케줄링 및 적응적 대역폭 조절의 네 가지 부분으로 구성된다. FGMA에서는 지연 시간을 최소화 하는 방향으로 자원 할당을 한다. 이 방법을 활용하면 적응적 대역폭 조절 알고리즘을 통해 지연 시간 요구 사항이 다른 트래픽의 불균형을 완화 시킬 수 있다. 또한 승인 제어 알고리즘을 통해 FGMA 시스템에 이미 승인된 모든 UE들에 대한 요구 사항을 항상 보장한다. FGMA는 시간에 따라 변하는 환경에서도 UE의 QoS 요구 사항을 효율적으로 보장한다는 것을 확인 할 수 있다. 마지막으로, 소형 셀은 셀룰러 서비스 범위를 개선하고 시스템 용량을 향상 시 키고, 많은 수의 무선 단말을 지원하는 핵심 기술로 떠오르고 있다. 하지만 셀의 서비스 범위의 감소는 빈번한 핸드오버를 유도하기 때문에, 효과적인 핸드오버 방식이URLLC 애플리케이션을 지원하기 위해서 필요하다. 따라서, URLLC서비스를 요구하는 이동성이 있는 UE를 서비스하기 위해 적응적 핸드오버 파라미터를 선택 및 단말의 동작을 미리 준비해 놓는 방식을 적용한 단말이 시작하는 핸드오버 방식을 제안한다. 시뮬레이션 결과는 제안한 핸드오버가 수율을 향상시킴과 동시에 저지연을 달성하는 것을 확인 할 수 있다. 본 논몬을 간략히 요약하면 지연 시간의 종류를 랜덤 액세스 지연 시간, 상향 링크 데이터 전송 지연 시간 및 핸드오버 지연 시간과 같이 3가지로 구분하였다. 3가지 종류의 지연 시간에 대해서 각각 저지연을 달성 할 수 있는 프로토콜과 알고리즘을 제안하였다.According to IMT vision for 2020, the fifth generation (5G) wireless services are classified into three categories, namely, Enhanced Mobile Broadband (eMBB), Massive Machine Type Communication (mMTC), and Ultra Reliable and Low Latency Communication (URLLC). Among three 5G service categories, URLLC is considered as the most challenging scenario. Thus, ensuring the latency and reliability is a key to the success of real-time services and applications. In this dissertation, we propose the following three latency reduction protocols to support the URLLC services: (i)2-way handshake-based random access, (ii) Fast grant multiple access, and (iii) UE-initiated handover scheme. First, the performance target includes not only increasing data rate, but also reducing latency in 5G cellular networks. The current LTE-Advanced systems require four message exchanges in the random access and uplink transmission procedure, thus inducing high latency. We propose a 2-way random access scheme which effectively reduces the latency. The proposed 2-way random access requires only two messages to complete the procedure at the cost of increased number of preambles. We study how to generate such preambles and how to utilize them. According to extensive simulation results, the proposed random access scheme significantly outperforms conventional schemes by reducing latency by up to 43%. We also demonstrate that computational complexity slightly increases in the proposed scheme, while network load is reduced more than a half compared to the conventional schemes. Second, various mission-critical applications are emerging such as teleoperation, autonomous driving, immersive virtual reality, and so on. A variety of URLLC traffic has various characteristics in terms of required data sizes and arrival rates with a variety of requirements of latency and reliability. To support the various requirements of the mission-critical applications, We propose a fast grant multiple access (FGMA) focusing on the uplink transmission. FGMA consists of four important parts, namely, admission control, dynamic preamble structure, the uplink scheduling, and bandwidth adaptation. The latency minimization scheduling policy is adopted in FGMA. Taking advantage of this method, the bandwidth adaptation algorithm makes even for the imbalanced arrival of the traffic requiring different latency requirements. With the proposed admission control, FGMA guarantee the requirements to all admitted UEs in the systems. We observe that the proposed FGMA efficiently guarantee the QoS requirements of the UEs even with the dynamic time-varying environment. Finally, small cells are considered a promising solution for improving cellular coverage, enhancing system capacity and supporting the massive number of things. Reduction of the cell coverage induced the frequent handover, so that the effective handover scheme is of importance in the presence of the URLLC applications. Thus, we propose a UE-initiated handover to deal with the mobile UEs requiring URLLC services taking into account the adaptive handover parameter selection and the logic of preparing in advance. The simulation results show that the proposed handover enhances the throughput performance as well as achieving low latency. In summary, we identify interesting problem in terms of latency. We classify three latency, random access latency, data transmission latency, and handover latency. With compelling protocols and algorithms, we resolve the above three problems.1 Introduction 1 1.1 Motivation 1 1.2 Main Contributions 2 1.2.1 Low Latency Random Access for Small Cell Toward Future Cellular Networks 2 1.2.2 Fast Grant Multiple Access in Large-Scale Antenna Systems for URLLC Services 3 1.2.3 UE-initiated Handover for Low Latency Communications 4 1.3 Organization of the Dissertation 4 2 Low Latency Random Access for Small Cell Toward Future Cellular Networks 6 2.1 Introduction 6 2.2 Related Work 9 2.3 Random Access and Uplink Transmission Procedure in LTE-A 11 2.3.1 Random Access in LTE-A 12 2.3.2 Uplink Transmission Procedure 14 2.3.3 Latency Issue in LTE-A 15 2.4 Proposed Random Access 16 2.4.1 Key Idea . 17 2.4.2 Proposed Preamble and Categorization 18 2.5 Preamble Sequence Analysis 23 2.5.1 Preamble Sequence Generation in LTE-A 23 2.5.2 Proposed Preamble Sequence Generation 25 2.5.3 Proposed Preamble Detection 26 2.6 Performance Evaluation 31 2.6.1 Network Latency 32 2.6.2 One-way Latency 33 2.6.3 Network Load 36 2.6.4 Computational Complexity 37 2.7 Conclusion 39 3 Fast Grant Multiple Access in Large-Scale Antenna Systems for URLLC Services 40 3.1 Introduction 40 3.2 Related Work 43 3.3 System Model 44 3.3.1 QoS Information and Service Category 45 3.3.2 Channel Structure 47 3.3.3 Frame Structure 48 3.4 Fast Grant Multiple Access 49 3.4.1 The Uplink Scheduling Policy 51 3.4.2 Dynamic Preamble Structure 53 3.4.3 Admission Control 54 3.4.4 Bandwidth Adaptation 55 3.5 Performance Evaluation 57 3.5.1 Impact of admission control 59 3.5.2 Impact of bandwidth adaptation 61 3.5.3 FGMA performance 62 3.6 Conclusion 64 4 UE-initiated Handover for Low Latency Communications 67 4.1 Introduction 67 4.2 Background and Motivation 69 4.2.1 Handover Decision Principle 69 4.2.2 Handover Procedure 70 4.2.3 Summary of the latency issues 72 4.3 UE-initiated Handover 73 4.3.1 The proposed handover design principles 73 4.3.2 The proposed handover procedure 75 4.4 Performance Evaluation 77 4.4.1 Low mobility environment 77 4.4.2 Low mobility environment 78 4.4.3 High mobility environment 80 4.5 Conclusion 82 5 ConcludingRemarks 84 5.1 Research Contributions 84 Abstract (InKorean) 92Docto

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