Various MAC protocols have been suggested for exploiting the possible benefits provided by directional antennas, but most of the solutions suggested are either based on the concept of sending (MAC level) control packets (such as RTS and CTS) in all directions, or based on the use of some omnidirectional control packet transmissions. We believe that these approaches fail short of exploiting the capability of directional transmission/reception fully and are incapable of handling all the problems resulting from the use of directional antennas in multi-hop wireless networks. Hence we first propose a set of requirements that should be met by a good MAC protocol that uses directional antennas. Then we propose a directional busy signal multiple access (DBSMA) protocol that meets these requirements. In DBSMA, all the transmissions, receptions, and idle listening are directional. The need to listen in many directions when in an idle state is achieved by constantly changing the listen direction to cover the whole space. We also propose a novel directional back-off mechanism in which every node maintains independent back-off windows for each direction and show how it yields better performance. We show that DBSMA is well suited for ad-hoc multi-hop networks with directional antennas

The problem of connectivity in multi-hop wireless networks has been extensively studied in the literature and general results for node density requirements have been obtained (P. Gupta and P.R. Kumar, Proc. 37th IEEE Conf. of Decision and Control, 1998). These results have been obtained based on the implicit assumption of a generic routing protocol, capable of exhaustively searching all possible routes, between all pairs of nodes. Hence these results would be too optimistic in several practical cases, where the routing protocols are not generic but optimized for specific applications. In this paper, we provide a framework for defining the appropriate notion of connectivity that reflects the underlying network architecture and protocols. Based on this framework, we define and analyze connectivity requirements for two network architectures proposed in the literature, namely, GAF (geographic adaptive fidelity) with Manhattan routing (Y. Xu et al., Proc. Int. Conf. on Mobile Comp. and Netw., 2001), proposed for ad-hoc networks and AIMRP (address-light integrated MAC and routing protocol) which employs tier-based routing in sensor networks (S.S. Kulkarni et al., submitted for publication in IEEE Trans. on Networking, Dec. 2003). By comparing the critical node density requirements for connectivity, obtained through our framework, with the results in Gupta and Kumar, we show that the earlier results are too optimistic and hence it is important to consider the underlying routing protocol to dimension the density of nodes appropriately.

We consider a scheduling problem for packet based wireless systems with time-varying channel conditions. Designing scheduling mechanisms that take advantage of time-varying channel conditions, which are different for different users, is necessary to improve the wireless system performance. Such scheduling mechanisms are called opportunistic. In this paper we formulate an opportunistic scheduling problem with short term processor sharing fairness constraints as an optimization problem where short term refers to the time window on which the fairness is guaranteed. In its most general form, this problem cannot be solved analytically. We first solve the above optimization problem for three special cases. We consider the scheduling problem with long term fairness constraints; then we consider the scheduling problem for the shortest possible window under two sets of assumptions namely, one in which users have identically distributed channel conditions and another in which users have independent channel conditions. Observing the form of the corresponding optimal policies, we define a heuristic policy for our original opportunistic scheduling problem with short term fairness constraints. We show via simulation that our heuristic policy attains a good trade-off by guaranteeing short term fairness while achieving high average system throughput. We also illustrate that the optimal opportunistic scheduling policy with long term fairness constraint is in fact unfair in practical scenarios.

We consider a scheduling problem for packet based systems with time-varying channel conditions. Designing scheduling mechanisms that take advantage of time-varying channel conditions, which are different for different users, is necessary to improve system performance; however this has to be done in a way that provides some level of fairness among the users. Such scheduling mechanisms are termed opportunistic. We generalize the opportunistic scheduling mechanisms in the literature on two fronts. First, we formulate and solve an opportunistic scheduling problem with multiple general long term QoS constraints and a general system objective function. We call the solution of this opportunistic scheduling problem a generalized water filling solution. Then, we generalize this problem to include multiple interface systems in which several users can be served simultaneously. Apart from the long term QoS constraints specified by each user, multiple interface systems are constrained with other physical limitations imposed by the system. Our main contribution is to show that the structure of the optimal opportunistic scheduling policy is carried over to the problem with general constraints and multiple interfaces. We also study the stability of the multiple interface systems and propose a throughput optimal scheduling rule for such systems.