On Performance Optimization of MIMO SDR MANETs: From MAC to PHY Layer.

摘要

Mobile ad hoc networks (MANETs) consist of wireless mobile nodes which form a multi-hop wireless network without the support of centralized administration. Due to the multi-hop nature of MANETs, the hidden terminal problem makes dominant impacts on the performance of MANETs in terms of throughput and delay. Although many collision avoidance MAC protocols have been proposed to combat the hidden terminal problem, their performance is a function of network contention level and the capabilities of the underlying network nodes. In this thesis, we propose a multi-layer solution to this problem from the perspective of both MAC and PHY layer. We design and implement a hybrid MAC protocol (LA-MAC) for MANETs formed by a collection of Multiple-Input Multiple-Output (MIMO) equipped Software Defined Radio (SDR) nodes. By adaptively switching its mode of operation between CSMA and TDMA, LA-MAC achieves high channel utilization and a reasonable delay profile under both high and low contention levels identified by the measured collision rate of the MAC frames. As a cross-layer protocol, LA-MAC relies on the information received from the physical layer to distinguish between corrupted frames due to actual collisions as opposed to those corrupted by wireless link effects. The analytical and experimental studies show the improvements achieved by LA-MAC in comparison with other alternatives. To further exploit the potential of MIMO-capable nodes, in PHY layer, we utilize interference cancellation techniques to mitigate the effect of interference when inevitable collisions occur. The experimental results demonstrate that the performance can be further improved if interference mitigation is enabled in the nodes. Additionally, as an extension of our previously mentioned work, we report on a simulation-emulation study of dynamically monitoring the connectivity of MANET nodes. We compare the link strength using the Frame Error Rate (FER) reported by LA-MAC against a predefined threshold value to determine whether a link is formed between an arbitrary pair of MANET nodes. By interfacing the Google Maps' API with NS-3, we form connectivity graphs of MANET nodes using simulated GPS positioning data in Google Maps and the measured link strength values.