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Researches On Medium Access Control Protocol In Broadband Wireless Local Area Networks

Posted on:2015-06-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:F FangFull Text:PDF
GTID:1108330473956044Subject:Communication and Information System
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With the development of the application of Wireless Local Area Network(WLAN), the physical layer techniques and frequency spectrum utilization of WLAN develop rapidly. The MAC protocol of WLAN determines essentially the performance and the implementation of the network system in which all notes share the access channel. The MAC protocol of current release of IEEE 802.11 standards provides a share-medium access by Distributed Coordination Function(DCF) of CSMA/CA protocol. The performance of CSMA/CA decreases as the data rate increases because of applying OFDM to the physical layer. Moreover, CSMA/CA is no more suitable for the scenarios with long-delay and long-coherence time, so the slotted ALOHA is a new choice instead. Slotted ALOHA and its relevant improved protocols have been used extensively for its simplicity in cognitive radio, label recognition of FRID, satellite communication and the new techniques of wireless network. This dissertation treats slotted ALOHA as a MAC protocol to resolve some problems of CSMA/CA in wide-band WLAN. Some novel works had been done on the performance and stability control of slotted ALOHA. The contributions of this dissertation are listed as follows:1. A slotted MAC protocol is proposed for a high-rate WLAN scenario, which is not same as most existing methods that improve DCF throughput in broadband WLAN by frame aggregation. Considering every OFDM symbol can carry more data in a high speed physical channel, we study the effect of channel utilization ratio in term of the number of bits, symbol duration and packet length of OFDM based on Bianchi’s 2-D Markov model. The numerical analysis and simulation results show that the effective channel throughput is not greater than 0.35 if the length of data is not greater than 1000 bytes and physical layer rate exceed 100 Mbps. The result has no concern with RTS/CTS mode, Basic mode and how many the network nodes are. The channel throughput of CSMA/CA is not greater than 0.16 in RTS/CTS mode and also below 25% in Basic mode when physical layer rate exceeds 600 Mbps. Using time division multiple access as MAC layer protocol for broadband wireless local area network, a MAC protocol is proposed which is designed for both centralized dispatching and completely free competition scenarios.2. A MAC scheme is proposed for broadband WLAN with long delay channel. In most current literatures, WLAN is mainly studied in a short-distance wireless access network. In this dissertation, we study WLAN in a long-distance network in which the wireless signal has a long propagation delay. Considering the OFDM symbol duration is very long in thus network with long delay channel that results in a low performance of CSMA/CA, a slotted ALOHA is treated as the MAC protocol in which the OFDM symbol duration is used as the length of time slot. A contention-reservation slotted ALOHA is proposed for centralized network, and a kind of channel access control algorithm of slotted ALOHA is proposed for distributed network. Furthermore, we also analyze the stability of slotted ALOHA.3. An algorithm named p-Persistent Control Algorithm(pPCA) is proposed and the Binary Exponential Back-off Algorithm(BEBA) is used in slotted ALOHA. In most literatures, the control algorithm of slotted ALOHA is investigated under an assumed condition, i.e. the node gets the channel state information immediately after sending data. However, this assumed condition is not true for most factual situations. At first, we analyze the performance of pBCA under traditional assumption. Then, we proposed the pPCA algorithm in which the time slot is used as update unit. The number of nodes is estimated through counting the probability of free slots in a time window. System can adjust the sending probability into 1/n to achieve the maximized steady throughput. Considering the use of BEBA algorithm in CSMA/CA, we apply this algorithm into slotted ALOHA and derive the mathematical expression of steady system throughput based on 2-D Markov model. Finally, we compare these three control algorithms in term of throughput, the time to reach steady maximized throughput and algorithm complexity. The simulation results indicate that these three control algorithms all can achieve a steady system throughput with approximated theoretic maximum. PBCA has better performances in term of the even throughput and the adjusting time than the others in a small scale network. But if the number of nodes is quite large, the even throughput of three algorithms is nearly same, the adjusting time of pPCA is longest and PBCA has the best performance on algorithm complexity.4. A fast-adaptive steady control algorithm for slotted ALOHA is proposed. To resolve the problem of long adjusting time under an abrupt change of the number of nodes in pBCA and pPCA, we make a fast adjustment according to the run-length of channel state. The ratio of actual node number to estimated node number is changed to the area of 0.5-2, and then adjusted by pPCA and pBCA. The simulation results indicate that the fast-adaptive control algorithm is completely suitable for the scenarios with abrupt change of the number of nodes.
Keywords/Search Tags:Wireless Local Area Network, Medium Access Control, Throughput, Fast Adaptive, Stability
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