Interference suppression in wireless ad hoc networks
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Wireless ad hoc networks are infrastructure-free self-organizing networks formed by cooperating nodes. They are highly desirable for various emerging applications and to extend the range and capacity of infrastructure-based wireless networks. Scheduling algorithms in ad hoc networks allow nodes to share the wireless channel so that concurrent transmissions can be decoded successfully. On one hand, scheduling needs to be eﬃcient to maximize the spatial reuse. But on the other hand the scheduling algorithm needs to be easily implementable with little, if any, coordination between nodes in the network. The goal of this dissertation is to propose and evaluate a simple scheduling technique that suppresses transmissions by nodes around the desired receiver in order to achieve successful communication. This minimum separation, the guard zone, has important implications on the network performance and impacts the MAC design. In particular, using stochastic geometry, a near-optimal guard zone for spread spectrum ad hoc networks is derived – narrow-band transmission (spreading gain of unity) is a special case. In ad hoc networks employing a Direct-Sequence Code Division Multiple Access (DS-CDMA), the guard zone can easily be realized in a distributed manner, and oﬀers a 2−100 fold increase in capacity as compared to an ALOHA network; the capacity increase depending primarily on the required outage probability, as higher required QoS increasingly rewards scheduling. By implementing guard zone-based scheduling, the attained performance is about 70− 80% of a well-known near-optimal (and practically infeasible) centralized scheme. One major advantage of DS-CDMA is its ability to reduce the required guard zone size compared to a narrow-band system. A guard zone smaller than transmission range ensures that nodes that can potentially cause an outage are within the decoding range of a receiver. This lowers the complexity of scheduling algorithms as smaller area, which lies with in the transmission range of the receiver, needs to be managed by the MAC protocol. The dissertation considers primarily a physical and MAC layer view of the network to investigate and deﬁne what is optimal at the physical/MAC layer.