Improvements of link performance and capacity in DSA systems

Abstract : Recently, several studies initiated mainly by the Federal Communications Commission (FCC), which is charged with regulating communications by radio, television, wire, satellite and cable in US, have shown that the frequency spectrum is inefficiently exploited: some bands are highly crowded at some day hours or in certain dense urban areas while others remain poorly utilized. These problems, together with the rapid evolution of Cognitive Radio (CR) technology have pointed to the implementation of Dynamic Spectrum Access (DSA) in next generation wireless networks. The key promise of these systems is the possibility of highly flexible and efficient management and reuse of spectrum across all its dimensions. DSA systems use innovative spectrum management techniques, which allow different systems to share the same frequency band to utilize the radio spectrum in an efficient way. CR technology enables the development of intelligent and adaptive wireless communication systems that are able to work in an environment aware manner. DSA networks using CR technology are expected to provide significant throughput improvement and coverage extension for next generation wireless systems. During our research work, we propose different approaches to solve the dynamic spectrum access/allocation problem for future CR systems and we present some of the key research challenges associated with this new paradigm. For that purpose, we study Adhoc as well as cellular orientations allowing dynamic access to spectrum. Moreover, we investigate different network architectures for DSA, ranging from fully autonomous and distributed to fully centralized architectures in which dynamic access to spectrum is centrally managed. In addition, we also study two different techniques for spectrum sharing: Overlay and Underlay. Even if our proposals differ in the network orientation, in the network architecture and in the spectrum access technique for spectrum sharing, all of them have as goal the improvement of the link performance and the capacity of secondary networks while granting the activity of primary users (PUs). In this thesis, we firstly develop an extensive analysis of existing multi-channel MAC protocols for Ad-hoc networks. These protocols were proposed to increase network throughput, to improve spectrum utilization and to reduce interference caused by secondary use of the spectrum in an opportunistic (i.e. Overlay) manner. In our analysis we make a comparison of the key features of each protocol according to the number of transceivers, the need for synchronization, the need for a common control channel and the different ways to make rendezvous. By pinpointing the advantages, disadvantages and hardware requirements of each protocol, we facilitate the accurately selection of the appropriate solution to be implemented in future distributed DSA networks. Nevertheless, to obtain the necessary parameters for spectrum access in distributed Ad-hoc networks, a mobile station has to scan the entire spectrum looking for occupancy information. This scanning process may require a lot of time and can greatly impact the battery consumption in mobile devices. To overcome this problem, in this thesis we propose the use of a Cognitive Beacon Channel (CBC). This cognitive control channel helps to improve spectrum awareness by conveying signalization to mobile users in a multi-radio access technology environment and allows DSA using a centralized or a coordinated architecture. The main advantage of our proposal is the fact that our CBC re-use existing 3GPP technologies, proved to be efficient and accepted worldwide. The Interference Temperature model used for spectrum sharing is our last research axis. This approach is an Underlay technique in which secondary users (SUs) attempt to coexist with PUs instead of avoiding primary networks. Using the Poisson point process, we develop a new model to evaluate the achieved capacity of a secondary network, the interference caused to a primary network and the allowed transmission power of SUs to guarantee that activity of the PUs won't be affected by their transmissions. Afterwards, by the use of Concentration Inequalities, we determine an upper bound on the outage probability of the primary network when the SUs transmit following the Ideal and the Generalized Interference Temperature models. The results obtained from our research work indicate that efficient DSA is feasible and can be done with the current wireless technologies in the market. We also confirmed that reliable communication between SUs, preserving undisturbed at all time the activity of primary networks is possible following different DSA techniques and different DSA architectures for spectrum sharing. Thus, from our results we can state that by implementing the proposed techniques for the secondary use of the spectrum, we can improve the link performance and the capacity in future DSA systems.
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Submitted on : Tuesday, May 22, 2012 - 1:36:13 AM
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Rodrigo Soule de Castro. Improvements of link performance and capacity in DSA systems. Networking and Internet Architecture [cs.NI]. Télécom ParisTech, 2011. English. ⟨NNT : 2011ENST0054⟩. ⟨pastel-00699924⟩

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