適用於具指向性天線無線隨意網路之媒介接取控制與路由協定之聯合設計

Abstract

無線隨意網路（wireless ad hoc network）是由多個可任意移動位置之行動主機所組成。為了避免主要由隱藏節點（hidden node）問題所導致的封包衝撞，目前提出的許多媒介接取控制協定採用請求發送/允許發送（RTS/CTS）交換機制以保留媒介給正在傳送或接收的節點。然而當網路負載過重時，許多節點會不必要地被禁止傳輸，而導致網路的整體效能下降。指向性天線由於能將輻射能量集中在某一特定方向，因而顯著地提升了空間再使用的可能性。藉由指向性天線的使用，在鄰近範圍內的二組或以上的節點可同時進行資料傳輸。然而指向性天線的特點卻受到媒介接取控制協定中的媒介保留策略的限制而無法發揮。在本論文中，吾人針對使用指向性天線的媒介接取控制與路由協定提出一整合性的修正。指向性天線具有估測訊號到達方向的能力，因此可利用到達方向的資訊將請求發送/允許發送封包以指向性的方式傳送。如此將僅有在傳輸方向範圍內的節點會被限制傳輸，且這些點並非在所有方向上都受到限制，不僅達成了空間的再使用，也大幅地提升了網路的整體效能。此外，為了能完全發揮指向性天線的特性，吾人提出一能尋找遭受最少干擾路徑之路由策略。利用訊號到達方向可決定與各鄰近節點之相對角度，以及各方向上節點的擁擠程度，如此封包便可經稀疏分佈區域而被送達目的地。最後，相較於使用全向性天線的系統，吾人藉由電腦模擬驗證上述具指向性天線的無線隨意網路系統架構大幅地改進了網路的整體效能。

A wireless ad hoc network is a collection of wireless mobile hosts that are dynamically and arbitrarily located in a certain area. In order to avoid packet collisions which are mainly caused by the hidden node problem, most proposed MAC protocols utilize the RTS/CTS exchanging mechanism to reserve medium for acting nodes. However, in a heavy-load network, lots of nodes are unnecessarily prohibited from transmitting, and the throughput performance goes down accordingly. By focusing energy in an intended direction, directional antennas increase the potential for spatial reuse significantly, and simultaneous data transmissions of two or more pairs of nodes located in each other’s vicinity may be allowed. However, the advantages of directional antennas cannot be exploited under the medium reservation policy in the MAC protocol. In this thesis, we propose an integrated refinement of MAC and routing protocols with the use of directional antennas. Directional antennas have the ability to estimate the direction of arrival (DOA) of an incoming signal. With DOA information available, RTS/CTS packets can be transmitted directionally. Therefore, only those nodes located within the direction of transmission are blocked, and these nodes are not blocked in all directions. Spatial reuse can thus be achieved, and the throughput performance is improved significantly. Furthermore, to fully exploit the advantages of directional antennas, we propose a routing strategy to discover a route that experiences the least interference. The DOA information helps a node to identify the relative directions of its neighbors and the crowd level in all directions. Therefore, packets can be routed to the destination through a sparsely populated area. Finally, we evaluate the performance of the proposed system architecture by computer simulations, and confirm that utilizing directional antennas in ad hoc networks improves the throughput performance greatly over omni-directional communications.