無線隨意網路(Wireless Ad Hoc Networks)是由許多具無線裝置之節點所組成的，它不需要透過基地台做溝通就能達到互相通訊的目的。在之前無線隨意網路的研究中，大部分都是使用全方向性天線(Omni-Directional Antennas)，使得頻道(Channel)無法充分使用。在無線隨意網路環境中，使用全方向性天線(Omni-Directional Antennas)的節點在一段通訊時間內只能傳送或接收一個封包，在同一通訊範圍內只能允許一對通訊節點進行通訊活動。
在無線隨意網路中使用智慧型天線(Smart Antennas)，可以提供許多的優點，例如可以增加無線頻道的空間重複使用率(Spatial Reuse)、擴大了涵蓋範圍(Coverage Range)及增加網路容量(Network Capacity)。使用智慧型天線(Smart Antennas)之眾多利益中的其中一點是可以降低非需求性使用者的干擾。在本篇論文中，我們提出一個媒介存取控制協定(MAC Protocol)使用智慧型天線(Smart Antennas)，利用智慧型天線(Smart Antennas)之空間多工存取(SDMA)機制，可以收到超過一個以上節點所傳送的封包。使用空間多工存取(SDMA)機制會基於行動節點之位置資訊，提供無碰撞(Collision-Free)的存取通訊媒介。所以在無線隨意網路中，空間多工存取(SDMA)機制相較於其他多工存取方法，能帶來更多優勢。
在我們所提出的方法中，可以有效地改善隱藏終端點(Hidden Terminal)、暴露終端點(Exposed Terminal)及非對稱增益所產生之隱藏終端點(Hidden Terminal Problem Due to Asymmetry in Gain)的問題。最後，我們將透過模擬實驗來證實我們所提出的方法可以比IEEE 802.11、DMAC與另一個具SDMA機制之MAC通訊協定[17]，能更有效率的改善網路的傳輸量(Throughput)並增加空間頻道的重覆使用率(Spatial Channel Reuse)。

A wireless ad hoc network consists of a set of wireless devices. The wireless devices are capable of communicating with each other without the assistance of base stations. In most of the previous researches on the wireless ad hoc networks, omni-directional antennas are usually used, thus the channel is normally underutilized. The entire node uses omni-directional antennas to allow transmission or receiver only single packet at a communicated period on the wireless ad hoc networks.
In this thesis, we propose an SDMA-based MAC protocol (SMAC) for wireless ad hoc networks with smart antennas based on IEEE 802.11. Smart antennas in wireless ad hoc networks offer many benefits, for example, enhancement of spatial reuse of the wireless channel, extension of the coverage area, and increasement of network capacity. One of the most important benefits of using smart antenna is able to minimize interference from undesired users. The proposed MAC protocol exploits space division multiple access (SDMA) system to receive more than one packet from spatially separated transmitter nodes. Using SDMA technology provides collision-free access to the communication medium based on node’s location information. In wireless ad hoc networks, the SDMA has several advantages over other wireless multiple access schemes.
Our protocol can improve the hidden terminal problem, the exposed terminal problem, and the hidden terminal problem due to asymmetry in gain. Finally, the simulation results show that the proposed protocol can efficiently improve network throughput compared to the IEEE 802.11 and DMAC and a novel MAC protocol for SDMA [17].

Chapter 1 Introduction 1
1.1 Wireless Ad Hoc Networks 1
1.2 Motivation 2
1.3 Design Concept 3
1.4 Thesis Organization 3
Chapter 2 Related Work 4
2.1 A MAC Protocol Using Directional antennas 4
2.2 A MAC Protocol Using Smart antennas 5
2.3 Location Information Problem 9
2.4 Problem with Directional MAC Protocol 9
Chapter 3 Preliminaries 13
3.1 Smart Antennas 13
3.2 Space Division Multiple Access (SDMA) 14
3.3 IEEE 802.11 16
3.3.1 The Hidden Terminal Problem 18
3.3.2 The Exposed Terminal Problem 19
Chapter 4 An SDMA-Based MAC Protocol 20
4.1 Modified RTS/CTS Handshake 20
4.2 Modified CTS/BDTS Frame Formats 24
4.3 Beam Table 25
4.4 Modified Network Allocation Vector (NAV) 27
Chapter 5 Issues of Improvement in Wireless Ad Hoc Networks 29
5.1 Hidden Terminal Problem Due to Directional Transmission 29
5.2 Exposed Terminal Problem 30
5.3 Deafness 30
5.4 Hidden Terminal Problem Due to Asymmetry in Gain 31
Chapter 6 Experimental Results 33
6.1 The Simulation Environment 33
6.2 Simulation Results 34
6.2.1 A Random Topology 34
6.2.2 A Grid Topology 36
Chapter 7 Conclusions and Future Work 43
7.1 Conclusions 43
7.2 Future Work 43
References 45

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