시간 비동기 OFDMA 시스템을 위한 그룹 경쟁 기반 MAI-free 다중접속 기법
Group Contention-based MAI-free Multiple Access Schemes for Time-Asynchronous OFDMA Systems
- 주제(키워드) OFDMA , Group Contention , Multiple Access , Multiple Access Interference(MAI)-free , FH-OFDMA
- 발행기관 아주대학교
- 지도교수 임재성
- 발행년도 2012
- 학위수여년월 2012. 2
- 학위명 박사
- 학과 및 전공 정보통신전문대학원 정보통신공학과
- 실제URI http://www.dcollection.net/handler/ajou/000000012096
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Signals transmitted by multiple stations through different multiple subchannels may arrive at a particular station, such as an access point (AP), with different time delays. If the difference in arrival time delays exceeds the cyclic prefix duration, the orthogonality among the subchannels can be broken, which leads to multiple access interference (MAI) among the stations. To solve the MAI problem, we propose a multichannel slotted Aloha scheme based on an MAI-free group for a simple orthogonal frequency division multiple access (OFDMA) wireless network. Each MAI-free group consists of stations whose signals arrive at the AP within the cyclic prefix duration. The proposed scheme outperforms a comparative scheme based on fast retrial algorithm in term of throughput due to a smaller cyclic prefix, lower collision probability, and low block probability. However, the proposed scheme has a higher delay overhead in the low arrival rate region, while the delay of the proposed scheme gets closer to that of the comparative scheme as the arrival rate increases. In addition, the MAI problem can also occur in wireless local area networks (WLANs). In order to solve the MAI problem in WLANs, we propose a group contention-based OFDMA with an adequate cyclic prefix length and support a larger number of STAs. The STAs within the cyclic prefix duration that are synchronized to an AP, are included in the same contending group and compete with other STAs of the group in transmitting packets. The proposed scheme allows for the adoption of an adequate cyclic prefix length without MAI, and is able to reduce the cyclic prefix overhead. In addition, since no packet errors are induced by MAI in any of the STAs, the STAs can transmit simultaneously without redundant short interframe spaces (SIFSs) and other control packets. Through analysis and simulation, we show that for a large number of STAs, the proposed scheme achieves higher throughput than 802.11 protocols and a conventional CSMA combined with OFDMA. Furthermore, we have an invaluable study of the low probability of intercept (LPI) in the OFDMA systems. Especially, we focus on the frequency hopping based OFDMA (FH-OFDMA) in this thesis. In FH-OFDMA systems, the transmitted symbols on all subcarriers are demodulated by fast Fourier transform (FFT) at once at the receiver. Thus, these symbols can be easily intercepted by an adversary when the number of simultaneous transmitting users is small. In this thesis, we design a chaotic standard map (CSM) and cat map based two dimensional time/frequency hopping patterns for LPI. Through the proposed scheme, more than two symbols of the same user can be shown in the same symbol period, or none. Moreover, the order of these symbols is mixed. It makes the radiometer difficult in intercepting these symbols, and thus, it achieves a low probability of intercept. We also show that the proposed schemes can satisfy requirements of a desired FH pattern and achieve the maximum frequency diversity. Due to this property, the two proposed time/frequency hopping OFDMA schemes can achieve anti-jamming capabilities against partial band jamming and random pulse noise jamming attacks.
more목차
CHAPTER 1 INTRODUCTION 1
1.1 Background 1
1.2 Original contributions 2
1.3 Outlines of thesis 4
CHAPTER 2 OFDMA and FH-OFDMA Systems 5
2.1 OFDM Basics 5
2.2 OFDM Transmitter and Receiver 9
2.3 Background of OFDMA Systems 12
2.4 Background of FH-OFDMA Systems 14
CHAPTER 3 Group Contention based MAI-free Multiple Access Schemes 16
3.1 MAI Problem in OFDMA Systems 16
3.2 Related Works of Group Contention Multiple Access 20
3.3 MAI-free Group Assignment Procedure 23
3.4 Group Contention based Multichannel Slotted Aloha 28
3.4.1 System model 28
3.4.2 Group contention-based multichannel slotted Aloha scheme 29
3.4.3 Analytical model 33
3.4.4 Simulation and numerical results 41
3.5 Group Contention based CSMA/CA MAC Protocol 54
3.5.1 System model 54
3.5.2 Group contention-based CSMA/CA MAC protocol scheme 55
3.5.3 Analytical model 59
3.5.4 Simulation and numerical results 66
3.5.5 Discussions 77
CHAPTER 4 Time/Frequency Hopping (TFH)-OFDMA for LPI Systems 83
4.1 Related Works of TFH-OFDMA 83
4.2 Frequency Hopping Pattern Problem in OFDMA Systems 84
4.3 Time/Frequency Hopping OFDMA Schemes 86
4.3.1 System model 86
4.3.2 Chaotic standard map and cat map based TFH-OFDMA 89
4.3.3 Analysis of probability of intercept 94
4.3.4 Simulation and numerical results 95
CHAPTER 5 Conclusions 102
Bibliography 105
