无线自组织网（Ad Hoc网络）作为一种无中心、自组织的无线通信网络，由于其高效的移动性，支持动态变换的网络拓扑结构和多跳转发技术，以及无中心节点的鲁棒性和抗毁性，使其广泛运用于军事战术环境，事故突发现场等紧急环境和民用无线数据通信领域。但是由于无线信道的传输特性和节点位置的不确定性，造成网络拓扑结构始终处于不稳定状态，传统的路由协议根本无法适应自组织网的这些特点。现有的Ad hoc网络路由协议也具有各自的局限性。因此如何对现有的路由协议进行改进，提高网络的健壮性，从而能在各种恶劣条件下保证网络的稳定性和可靠性，提供较高的服务质量是本文研究的目的。本文首先介绍了当前Ad Hoc网络的国内外研究现状和研究热点，然后按照路由发现和维护机制对现有的Ad Hoc网络的路由协议进行了分类，介绍了几种典型的路由协议，尤其是最常用最稳定的按需距离向量路由协议AODV。接下来，针对AODV在节点高速运动，网络拓扑变换频繁和网络负载较重的网络中性能下降严重的缺点，提出了一种基于风险避免机制的多路径路由协议RA-AOMDV。该协议在多路径按需距离向量路由协议AOMDV的基础上，通过对节点电量，信号接收功率，节点密度和路由重叠进行定时预测和基于风险等级的处理机制，避免了因各种原因造成的节点间链路断裂，路由失效的情况发生。最后在NS2平台上对RA-AOMDV和其它协议进行仿真，详细对比了其各项性能。实验结果表明：RA-AOMDV在网络负载较重，拓扑变换频繁的环境中，在路由负载得到了有效控制的同时，平均端到端延时，数据传输率和吞吐量均得到了显著提高。

Wireless Ad Hoc Network is a kind of self-organized communication network without any centers. Because of its unique characteristics, such as highly efficient mobility, supporting of dynamic mapping topology structure, multi-hop retransmission, robustness and the ability of anti-destroying, it is widely applied in the fields of civil wireless data transmission and emergency circumstances such as military strategy environment and accident environment.Because of the transmission characteristic of wireless channel and uncertainty of the node position, the network topology structure is in an unstable state. Traditional routing protocols have not fit for the mobile ad hoc network any longer. Existing routing protocols for Ad Hoc network have their limitations respectively. So this paper aims at how to improving the existing routing protocols to enhancing the robustness of the network, so as to ensure the stability and reliability of the network and provide better quality of service under bad circumstances.In this paper, both abroad and domestic research situations and interests on the Ad Hoc network are introduced firstly. Then the routing protocols of the Ad Hoc network are sorted by their routing discovery and maintenance mechanisms, some classic routing protocols are introduced, especially the most common and stable protocol AODV. Next, since the performance of AODV decreases sharply in the frequent topology -changing and heavy load network, a new Risk-Avoidance Ad Hoc Multipath On-Demand Distance Vector(RA-AOMDV) routing protocol is proposed. Through the prediction of the energy of the node, the receiving power of signal, the nodes density and the route overlap, it handles the risk of link break according to the alarm grades, so the route failure could be avoided. In the end, the RA-AOMDV and other protocols are simulated on the NS2 platform and the performances of them are evaluated in detail. The results show that RA-AOMDV improves obviously in the average end-to-end delay, the data delivery fraction and the throughput, while the routing load is effectively controlled.