Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the ine#ciency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) Networks as well as Dynamic Spectrum Access (DSA) and cognitive radio networks. The term xG networks is used throughout the paper. The novel functionalities and current research challenges of the xG networks are explained in detail. More specifically, a brief overview of the cognitive radio technology is provided and the xG network architecture is introduced. Moreover, the xG network functions such as spectrum management, spectrum mobility and spectrum sharing are explained in detail. The influence of these functions on the performance of the upper layer protocols such as routing and transport are investigated and open research issues in these areas are also outlined. Finally, the cross-layer design challenges in xG networks are discussed.

In this paper, we provide a survey of dynamic spectrum access techniques. Various approaches envisioned for dynamic spectrum access are broadly categorized under three models: dynamic exclusive use model, open sharing model, and hierarchical access model. Based on this taxonomy, we provide an overview of the technical challenges and recent advances under each model. Index Terms: Dynamic spectrum access, spectrum property rights, spectrum commons, spectrum underlay, spectrum overlay, opportunistic spectrum access. 1.

Abstract—Existing capacity constrained cellular networks that operate in fixed spectrum bands can be enhanced with capacityon-demand services using the Coordinated Dynamic Spectrum Access (CDSA) model. In this model, a centralized spectrum broker coordinates access to spectrum in a given region and assigns short term spectrum leases to competing wireless service providers and/or end users. In contrast to existing multi-year cellular spectrum licenses that span large regions, a spectrum broker can grant spectrum leases that are for small regions (e.g.: per base station) and valid for short durations (e.g.: tens of minutes). Fast spectrum allocation algorithms are crucial to the design of scalable spectrum brokers that can provide such realtime spectrum access. In this paper, we address this challenge. Specifically, we formulate the spectrum allocation problem as two optimization problems: first with the objective of maximizing the overall demand (Max-Demand) satisfied among the various base stations and the second with the objective of minimizing the overall interference in the network (Min-Interference) when all the demands of the base stations are satisfied. We show that the optimization problems are NP-hard and design efficient algorithms to solve them. Our simulation results on sample network topologies show that our algorithms scale very well for large network sizes. I.

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