OFDMA 方式 高速 移動 通信 시스템에서 廣帶域 패킷 스케줄링 = Frequency domain packet scheduling for OFDMA-based mobile telecommunication systems

다국어 초록 (Multilingual Abstract)

Next generation mobile communication systems are expected to provide a wide range of multimedia services with variable QoS (Quality of Service). The major issues of the next generation systems are performance enhancement and efficient resource management.
3GPP (3rd Generation Partnership Project) is standardizing LTE (Long Term Evolution), which is a framework for the evolution of the 3GPP radio access technology such as HSDPA/HSUPA (High Speed Downlink/Uplink Packet Access). The target of LTE is to provide significantly higher data rates and the radio transmission is based on OFDMA (Orthogonal Frequency Division Multiple Access) for high spectrum efficiency and robust performance. The OFDMA divides a broadband wireless channel into a number of subcarriers that are transmitted simultaneously. Hence, the OFDMA has a potential for efficient handling of broadband radio resources since subcarriers can be dynamically assigned to different users.
In order to employ the OFDMA to next generation systems, the wireless system requires the channel state information of entire system bandwidth to allocate downlink resources in time varying broadband channel. However, the OFDMA/FDD (Frequency Division Duplex) system needs much signaling overheads for mobile stations to feedback the information to the packet scheduler which is located in the base station. Therefore, an efficient CQI (Channel Quality Information) report mechanism should be considered to reduce the extensive signaling overhead in a fast moving environment. In addition, the performance of a mobile system heavily depends upon packet scheduling and the frequency domain packet scheduling is needed to increase system throughput by exploiting multi-user diversity and frequency-selectivity. The allocation of subcarriers should consider not only throughput performance but also fair allocation among mobile stations. Also, the complexity of scheduling algorithm should be lessened to manage a large number of mobile stations since the packet scheduler takes into account much information, such as wireless channel state, buffer status, and QoS requirement.
This paper proposes the Multi-Level feedback scheme for the efficient report of channel state information. Furthermore, the paper proposes the OPF (OFDMA Proportional Fair) packet scheduling for the sub-optimal resource allocation scheme which guarantees proportional fairness with reduced complexity.
Proposed Multi-Level feedback scheme continuously measures downlink channel state and averages over multiple time slots for reporting. Each channel state per subband is quantized into a discrete value LMCS which designates the possible MCS (Modulation and Coding Scheme) level. Then, the scheme merges the channel state information of adjacent subbands at each reporting time to reduce the size for reporting. The procedure is divided into the frequency domain merging and the Multi-Level mapping. The frequency domain merging compares the channel quality of adjacent subbands and merges it into one MCS level when the difference between adjacent subbands is lower than a certain threshold. The Multi-Level mapping transforms channel state information into indicators which is composed of 2bit per subband. Then, the channel information is reported to the base station for packet scheduling.
Proposed OPF packet scheduling algorithm is based on the conventional Proportional Fair scheduling and the paper extends the scheme for throughput performance and proportional fairness on OFDMA systems. Firstly, the paper redefines the instantaneous channel state into which is the sum of the channel state of selected subbands. Secondly, a weighted variable which represents the delay of incoming data is added for QoS. Then, the algorithm selects mobiles according to the priority function and resource allocation is done by using the ratio of the proportional fairness. In addition, the moving average per user is calculated once per scheduling period to update the average value Rk (t).
For comparison, a system-level simulation approach is used and it assumes an OFDMA/FDD system which has 10 MHz channel bandwidth. The channel model is based on SCME (Spatial Channel Model Extended). The simulation result shows that the proposed Multi-Level feedback scheme reduces signaling overheads up to 73% compared to the full CQI feedback scheme. In addition, the scheme gives the best throughput performance among partial CQI feedback schemes. Also, the proposed OPF packet scheduling follows proportional fair throughput with reduced complexity which is similar to the complexity of the single carrier packet scheduling algorithm. Furthermore, the algorithm increases system performance by efficiently utilizing the frequency diversity.

국문 초록 (Abstract)

본 논문의 구성은 다음과 같다.
제2장은 먼저 이동 통신 시스템에서 통신 매개체로 사용하는 무선 채널의 특성에 대하여 정리한다. 그리고, OFDMA 방식을 사용하는 광대역 이동 통신의 개요를 설명하기 위하여, 3GPP에서 차세대 이동 통신 규격으로 개발 중인 LTE 시스템의 무선 전송 구조 및 무선 자원 구조에 대하여 기술한다. 또한, 광대역 이동 통신 시스템의 무선 채널 상태 개념 및 채널 상태 보고 기술에 대하여 정리하고, 이와 관련된 연구 동향에 대하여 기술한다. 이와 함께, 광대역 이동 통신 시스템에서 필요한 패킷 스케줄링의 기능과 대표적인 패킷 스케줄링 알고리즘의 연구 동향에 대하여 기술한다. 제3장에서는 본 논문에서 제안한 무선 채널 상태 보고 방식 및 광대역 패킷 스케줄링 알고리즘에 대하여 특징과 구조, 그리고 알고리즘의 절차에 대하여 구체적으로 기술한다. 제4장에서는 본 논문에서 제안한 무선 채널 상태 보고 방식 및 패킷 스케줄링 알고리즘의 성능을 시뮬레이션을 통하여 비교한 결과와 장단점을 기술하며, 마지막으로 제5장에서 결론을 정리한다.

목차 (Table of Contents)

• 제1장 서론 1
• 1.1 연구 배경 5
• 1.2 연구 내용 9
• 1.3 논문 구성 13
• 제2장 광대역 이동 통신 기술 14
• 2.1 무선 채널 14
• 2.2 LTE 이동 통신 시스템 22
• 2.3 무선 채널 상태 보고 41
• 2.4 패킷 스케줄링 53
• 제3장 새로운 알고리즘 제안 67
• 3.1 시스템 환경 70
• 3.2 Multi-Level 무선 채널 상태 보고 73
• 3.3 OPF 광대역 패킷 스케줄링 83
• 제4장 성능 비교 90
• 4.1 시험 환경 90
• 4.2 성능 비교 95
• 제5장 결론 109
• 참고문헌 113
• Abstract 122