Ad hoc networking with directional antennas: a complete system solution
2004 IEEE Wireless Communications and Networking Conference (IEEE Cat. No.04TH8733)
Directional antennas offer tremendous potential for improving the performance of ad hoc networks. Harnessing this potential, however, requires new mechanisms at the medium access and network layers for intelligently and adaptively exploiting the antenna system. While recent years have seen a surge of research into such mechanisms, the problem of developing a complete ad hoc networking system, including the unique challenge of real-life prototype development and experimentation has not been
... ssed. In this paper, we present UDAAN ("Utilizing Directional Antennas for Ad Hoc Networking"). UDAAN is an interacting suite of modular network-and MAC-layer mechanisms for adaptive control of steered or switched antenna systems in an ad hoc network. UDAAN consists of several new mechanisms -a directional power-controlled MAC, neighbor discovery with beamforming, link characterization for directional antennas, proactive routing and forwarding -all working cohesively to provide the first complete systems solution. We also describe the development of a real-life ad hoc network testbed using UDAAN with switched directional antennas, and we discuss the lessons learned during field trials. High fidelity simulation results, using the same networking code as in the prototype, are also presented both for a specific scenario and using random mobility models. For the range of parameters studied, our results show that UDAAN can produce up to a factor-of-10 improvement in throughput over omni-directional communications. Introduction Over the past few years, research into ad hoc networks has yielded considerable advances, notably in the areas of new routing and medium access techniques. Yet, significant shortcomings of ad hoc networks remain, especially when compared with wireline networks. Ad hoc networks suffer from fundamental capacity limitations , connectivity/survivability problems, and are prone to jamming and eavesdropping. Attempts to surmount these problems at the medium access, network and transport layers by way of innovative and often ingenious protocols have yielded only incremental success. In this paper, we consider a physical layer enhancement -directional communicationsand its exploitation by the various components of an ad hoc networking system to address the above problems and provide a significant improvement in network capacity. Directional antennas have a number of advantages over omni-directional antennas in ad hoc networking. By focusing energy only in the intended direction, directional antennas can increase the potential for spatial reuse and can provide longer transmission and reception ranges for the same amount of power. Increased spatial reuse and longer range translates into higher ad hoc network capacity (more simultaneous transmissions and fewer hops), and longer range also provides connectivity. Further, since the spatial signature of the energy is reduced to a smaller area, chances of eavesdropping are reduced, and with "smart" antennas, the steering of nulls can allow the suppression of unnecessary interference (such as jammers) at the receiver. Replacing an omni-directional antenna by a directional one in an ad hoc network is not by itself sufficient to exploit the offered potential. The antenna system needs to be appropriately controlled by the each layer of the ad hoc networking protocol stack. Such control includes pointing in the right direction at the right time for transmitting and receiving, controlling the transmit power in accordance with the antenna gains, etc. Further, mechanisms that were designed with omnidirectional communications in mind -for example, medium access, neighbor discovery and routing -have to be redesigned for directional antennas. Finally, modifications to such network mechanisms can interact with each other -for instance, medium access control may require knowledge of the antenna beam to use for a particular neighbor discovered by the neighbor discovery mechanism.