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Special Graduate Schools
Graduate School of Information and Communication Technology
Department of Information and Communication
3. Theses(Master)
시간 비동기 OFDMA 시스템을 위한 그룹 경쟁 기반 MAI-free 다중접속 기법
- Alternative Title
- Junwoo Jung
- Author(s)
- 정준우
- Alternative Author(s)
- Junwoo Jung
- Advisor
- 임재성
- Department
- 정보통신전문대학원 정보통신공학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2012-02
- Language
- eng
- Alternative Abstract
- 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.
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