In this dissertation, we propose an accurate analytical model for the IEEE 802.15.4 CSMA/CA MAC protocol. We consider the non-backoff state in modeling the Markov chain for IEEE 802.15.4 CSMA/CA MAC and obtain accurate analysis models on throughput and power consumption. We also investigate the operation of PCA devices under the presence of DRP devices in a WiMedia MAC environment and formulate a mathematical model based on a Markov chain to represent the behavior of PCA devices. We propose a defer operation reflection mechanism, which mathematically supplements a Markov chain-based analysis, to reflect the effect of the existence of DRP periods on the probabilities of successful transmissions, collisions, and idle state of a device. Specifically, this mechanism can overcome the limitation of a Markov chain model, which is caused by the dependency among PCA devices under the presence of DRP devices. Analytical results obtained from our proposed mathematical model agree well with simulation results and show that the saturated throughput of PCA devices decrease as the proportion of DRP periods or the number of DRP period groups increase.
We mathematically analyze the association procedure of IEEE 802.15.4 ZigBee devices and propose an efficient association algorithm which associate ZigBee devices with a piconet coordinator (PNC) with changing CSMA/CA MAC parameters adaptively to the number of unassociated ZigBee devices. The proposed analysis model is based on Markov chain and order statistics concepts and the proposed algorithm is based on a dynamic estimation concept. We considered total association time of ZigBee devices as a performance measure. The maximum number $N_{max}$ of ZigBee devices, with which pre-given association time requirement is satisfied, is also investigated. Numerical results show that analytical results match well with simulation results and the proposed association algorithm shortens total association time compared to the conventional algorithm from \cite{spec:zigbee}. Especially, for the given association time requirement of 1 sec. from \cite{spec:zigbee}, $N_{max}$ is 102 with the proposed algorithm while $N_{max}$ is 47 with the conventional algorithm.
We analyze the effect of interference from heterogeneous communication systems on the performance of CSMA/CA system and mathematically formulate it based on a Markov chain concept. In the proposed analysis, we consider ZigBee as a target CSMA/CA system. Considered two heterogeneous interfering systems are Bluetooth and WLAN systems. For the transmission power, we assume that the transmission power of Bluetooth or WLAN devices is set hundred to thousand times higher than that of ZigBee devices. Bluetooth or WLAN devices may interfere with ZigBee transmission by transmitting the signal during ZigBee transmission, while ZigBee devices do not interfere with Bluetooth or WLAN transmission and defer their transmission during Bluetooth or WLAN transmission. We obtain a new Markov chain model for ZigBee devices in the presence of Bluetooth or WLAN interference. The performance of ZigBee devices: throughput, delay, and energy consumption is derived for varying the input load of Bluetooth or WLAN interference. Numerical results show that the length of ZigBee frames needs to be set considering the characteristics of Bluetooth or WLAN interference in order to maximize the performance of a ZigBee network in the presence of Bluetooth or WLAN interference.
We analyze the performance of carrier sense multiple access/collision avoidance (CSMA/CA) protocols in the presence of carrier sensing errors. Two types of carrier sensing errors, false alarm and miss detection, are considered, and their impact on system performance is analyzed using a new CSMA/CA model based on a Markov chain capturing the sensing error at physical layer. The system throughput and delay as functions of the sensing error probabilities as well as other CSMA/CA parameters are obtained, and their sensitivity with respect to a key physical-layer parameter, sensing threshold, is analyzed for commonly used energy detectors or matched filters. It is shown that the throughput and delay sensitivity depends heavily on the ratio of the contention window size $W$ to the frame length $L$, and the throughput is sensitive to the design of the sensing threshold when the ratio $W/L$ is either small or large. The result provides guidelines about how to operate the CSMA/CA considering imperfect sensing at physical layer.
We propose a new medium access control (MAC) scheme embedding physical channels into multiband carrier sense multiple access / collision avoidance (CSMA/CA) networks to provide strict quality of service (QoS) guarantee to high priority users. In the proposed scheme, two priority classes of users, primary and secondary users, are supported. For primary users physical channels are provided to ensure strict QoS, whereas secondary users are provided with best-effort service using CSMA/CA modified for multiband operation. The performance of the proposed MAC scheme is investigated using a new multiband CSMA/CA Markov chain model capturing the primary user activity and the operation of secondary users in multiple bands. The throughput of secondary users is obtained as a function of the primary user activity and other CSMA/CA parameters. It is shown that the new MAC scheme yields larger throughput than the conventional single-band CSMA/CA when both schemes use the same bandwidth.
본 학위논문에서는 간섭을 완화시키고 에러에 강인한 새로운 CSMA/CA MAC 구조에 대한 연구로 구체적인 연구 내용들은 다음과 같다. 802.15.4 MAC의 성능평가를 위하여 802.15.4 MAC의 clear channel assessment (CCA) 동작을 고려한 discrete time Markov chain (DTMC) 모델을 제안 하였다. 기존 모델은 노드의 동작을 모델링할 때 CCA 상태에 머물 확률을 제대로 고려하지 않았는데 이로 인하여 CCA 동작이 빈번한 상황에서 802.15.4 MAC의 동작을 제대로 모델링하지 못하였다. 제안하는 모델에서는 CCA 상태에 머물 확률을 모델링에 포함시킴으로써 기존 모델보다 더 정확히 802.15.4 MAC의 동작을 모델링할 수 있었다. 센서 네트워크 기술로 주목받고 있는 CSMA/CA 기반의 ZigBee 기술은 CSMA/CA의 임의 접속 특성으로 인하여 노드들 사이의 채널 경쟁이 심해지면 잦은 충돌이 발생하여 이로 인해 성능이 저하될 수 있다. 802.15.4 기반의 ZigBee 센서 네트워크에서의 초기 가입절차 시의 일어나는 현상을 순서통계 (order statistics)를 기반으로 모델링하였고 성능을 분석하였다. 또한, 신속한 가입을 위하여 충돌상황을 인식하여 이에 기반하여 동적으로 접속하는 방안을 제안하여 성능의 개선을 보여주었다. CSMA/CA와 비 CSMA/CA 시스템 간의 간섭현상을 모델링하기 위해 Bluetooth의 간섭 하에서의 ZigBee 망의 동작을 분석하여 이를 모델링하였다. 또한 이질적인 CSMA/CA 시스템 간의 간섭 현상과 을 모델링하기 위해 무선 LAN의 간섭 하에서의 ZigBee 망의 성능 평가를 수행하고 간섭완화 방안을 제시하였다. CSMA/CA 시스템이 불완전한 반송파신호 감지 상황에서의 성능 문제를 다루었고 여러 매개변수들에 대한 감도 (sensitivity) 해석을 수행하였고 게임이론을 이용하여 carrier sensing 문턱값을 사용자 별로 조정하였을 때 Nash equilibrium에 도달하는 것을 보여주었다. 마지막으로, 다중 대역 CSMA/CA 시스템에서 cognitive radio 기술을 적용한 RawPEACH 시스템을 제안하고 CSMA/CA 상황에서 1차와 2차 가입자 간의 상호 간섭을 받지 않는 상황에서 cognitive radio 환경을 구축하고 1차 가입자 용량 개선 방안도 제안하였다.
본 논문에서는 다양한 간섭 하에서의 CSMA/CA 시스템의 성능 분석을 위한 수학모델의 제시, 반송파 감지능력의 불완전성의 영향분석, 이종시스템의 간섭으로 인한 CSMA/CA 시스템의 성능 분석 및 개선방안, CSMA/CA 시스템 환경에서의 cognitive radio의 구현 방안의 제시 등의 연구들이 제시되어 있다.
