Browsing by Author "Kozat, Ulas C."
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Item Dynamic Code Assignment and Spreading Gain Adaptation in Synchronous CDMA Wireless Networks(2002) Koutsopoulos, Iordanis; Kozat, Ulas C.; Tassiulas, Leandros; Tassiulas, Leandros; ISR; CSHCNDS-CDMA has been recognized as the major candidate for providing high and variable data rates in third generation wireless networks, where multi-code structures and different spreading gains will be employed. In this paper, we address the problem of assignment of variable spreading gain deterministic codes to a set of users, with the objective to maximize down-link system throughput. We propose an algorithm to allocate codes to users with different minimum rate requirements, based on code cross-correlation properties and spreading gains. Our algorithm first constructs an admissible set of codes by using criteria which are based on induced interference to the system and code rates. These codes are then appropriately assigned to users, so that user rate requirements are satisfied. Comparative numerical results for different performance measures of these criteria are also provided.Item A Framework for Cross-layer Design of Energy-efficient Communication with QoS Provisioning in Multi-hop Wireless Networks(2004) Kozat, Ulas C.; Koutsopoulos, Iordanis; Tassiulas, Leandros; Tassiulas, Leandros; ISR; CSHCNEfficient use of energy while providing an adequate level of connection to individual sessions is of paramount importance in multi-hop wireless networks. Energy efficiency and connection quality depend on mechanisms that span several communication layers due to the existing co-channel interference among competing flows that must reuse the limited radio spectrum. Although independent consideration of these layers simplifies the system design, it is often insufficient for wireless networks when the overall system performance is examined carefully. The multi-hop wireless extensions and the need for routing users' sessions from source to the destination only intensify this point of view. In this work, we present a framework for cross-layer design towards energy-efficient communication. Our approach is characterized by a synergy between the physical and the medium access control (MAC) layers with a view towards inclusion of higher layers as well. More specifically, we address the joint problem of power control and scheduling with the objective of minimizing the total transmit power subject to the end-to-end quality of service (QoS) guarantees for sessions in terms of their bandwidth and bit error rate guarantees. Bearing to the NP-hardness of this combinatorial optimization problem, we propose our heuristic solutions that follow greedy approaches.Item Network Layer Support for Service Discovery in Mobile Ad Hoc Networks(2003) Kozat, Ulas C.; Tassiulas, Leandros; ISRService discovery is an integral part of the ad hoc networking toachieve stand-alone and self-configurable communication networks. Inthis paper, we discuss possible service discovery architectures alongwith the required network support for their implementation, and wepropose a distributed service discovery architecture which relies on avirtual backbone for locating and registering available serviceswithin a dynamic network topology. Our proposal consists of twoindependent components: (i) formation of a virtual backbone and (ii)distribution of service registrations, requests, and replies. Thefirst component creates a mesh structure from a subset of a givennetwork graph that includes the nodes acting as service brokers and asubset of paths (which we refer as virtual links) connectingthem. Service broker nodes (SBNs) constitute a dominating set, i.e.all the nodes in the network are either in this set or only one-hopaway from at least one member of the set. The second componentestablishes sub-trees rooted at service requesting nodes andregistering servers for efficient dissemination of the servicediscovery probing messages. Extensive simulation results are providedfor comparison of performance measures ,i.e. latency, success rate,and control message overhead, when different architectures and networksupport mechanisms are utilized in service discovery.Item Throughput Capacity of Random Ad Hoc Networks with Infrastructure Support(2003) Kozat, Ulas C.; Tassiulas, Leandros; ISRIn this paper, we consider the transport capacity of ad hoc networkswith a random flat topology under the present support of an infinitecapacity infrastructure network. Such a network architecture allowsad hoc nodes to reach each other by purely using ad hoc nodes asrelays. In addition, ad hoc nodes can also utilize the existinginfrastructure fully or partially by reaching any access point (orgateway) of the infrastructure network in a single or multi-hopfashion.Using the same tools as in cite{gupta-capacity2000}, weshowed that the per source node capacity of $Theta(W/log(N))$ can beachieved in a random network scenario with the assumptions that thenumber of ad hoc nodes per access points is bounded above and that $N$ad hoc nodes excluding the access points, each capable of transmittingat $W$ bits/sec using a fixed transmission range, constitute aconnected graph.
This is a significant improvement over the capacityof random ad hoc networks with no infrastructure support which isfound as $Theta(W/sqrt{N log(N)})$ in cite{gupta-capacity2000}.Although better capacity figures are obtained by complex networkcoding or exploiting mobility in the network, infrastructure approachprovides a simpler mechanism that has more practical aspects.
We alsoshow that even when less stringent requirements are imposed ontopology connectivity, a per source node capacity figure that isarbitrarily close to $Theta(1)$ can not be obtained. Neverthelessunder these weak conditions, we can further improve per nodethroughput significantly.