Opportunistic Spectrum Access in Self-Similar Primary Traffic
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Research Article Opportunistic Spectrum Access in Self-Similar Primary Traffic Xiangyang Xiao, Keqin Liu, and Qing Zhao Department of Electrical and Computer Engineering, University of California, Davis, CA 95616, USA Correspondence should be addressed to Qing Zhao, [email protected] Received 16 February 2009; Revised 17 June 2009; Accepted 14 July 2009 Recommended by Ananthram Swami We take a stochastic optimization approach to opportunity tracking and access in self-similar primary traffic. Based on a multiple time-scale hierarchical Markovian model, we formulate opportunity tracking and access in self-similar primary traffic as a Partially Observable Markov Decision Process. We show that for independent and stochastically identical channels under certain conditions, the myopic sensing policy has a simple round-robin structure that obviates the need to know the channel parameters; thus it is robust to channel model mismatch and variations. Furthermore, the myopic policy achieves comparable performance as the optimal policy that requires exponential complexity and assumes full knowledge of the channel model. Copyright © 2009 Xiangyang Xiao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction 1.1. Opportunistic Spectrum Access. The “spectrum paradox” is by now widely recognized. On the one hand, the projected spectrum need for wireless devices and services continues to grow, and virtually all usable radio frequencies have already been allocated. Such an imbalance in supply and demand threatens one of the most explosive economic and technological growths in the past decades. On the other hand, extensive measurements conducted in the recent years reveal that much of the prized spectrum lies unused at any given time and location [1]. For example, in a recent measurement study of wireless LAN traffic [2], a typical active FTP session has about 75% idle time, and voice-overIP applications such as Skype have up to 90% idle time. These measurements of actual spectrum usage highlight the drawbacks of the current static spectrum allotment policy that is at the root of this spectrum paradox. They also form the key rationale for Opportunistic Spectrum Access (OSA) envisioned by the DARPA XG program and currently being considered by the FCC [3]. The idea of OSA is to exploit instantaneous spectrum opportunities by opening the licensed spectrum to secondary users. This would allow secondary users to identify available spectrum resources and communicate nonintrusively by limiting interference to primary users. Even for unlicensed bands, OSA may be of considerable value for spectrum efficiency by adopting a
hierarchical pricing structure to support both subscribers and opportunistic users. 1.2. Opportunistic Spectrum Access in Self-Similar Primary Traffic. Since the seminal work of Leland et al. [4], extensive studies have shown that self-similarity ma
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