Decision-Theoretic Distributed Channel Selection for Opportunistic Spectrum Access: Strategies, Challenges and Solutions

Opportunistic spectrum access (OSA) has been regarded as the most promising approach to solve the paradox between spectrum scarcity and waste. Intelligent decision making is key to OSA and differentiates it from previous wireless technologies. In this article, a survey of decision-theoretic solutions for channel selection and access strategies for OSA system is presented. We analyze the challenges facing OSA systems globally, which mainly include interactions among multiple users, dynamic spectrum opportunity, tradeoff between sequential sensing cost and expected reward, and tradeoff between exploitation and exploration in the absence of prior statistical information. We provide comprehensive review and comparison of each kind of existing decision-theoretic solution, i.e., game models, Markovian decision process, optimal stopping problem and multi-armed bandit problem. We analyze their strengths and limitations and outline further research for both technical contents and methodologies. In particular, these solutions are critically analyzed in terms of information, cost and convergence speed, which are key concerns for practical implementation. Moreover, it is noted that each kind of existing decision-theoretic solution mainly addresses one aspect of the challenges, which implies that two or more kinds of decision-theoretic solutions should be incorporated to address more challenges simultaneously.

I. Introduction

The explosive increase in wireless service demand has made spectrum scarcity a serious problem facing today's wireless communication systems. Historically, most of the spectrum less than 6 GHz has been almost completely assigned to different services, which were called the licensed spectrum owners. However, the reports released by FCC and the practical measurements carried out by researchers show that the allocated spectrum is largely under-utilized in time and space, which is referred to as spectrum waste. To address the paradox between spectrum scarcity and waste, lots of attentions have been given to the following two aspects of effort: (i) explore and utilize unknown spectrum, e.g., non-line-of-sight ultraviolet communications [1], visible light wireless communications [2] and Terahertz communications [3], and (ii) seek and opportunistically utilize the “spectrum hole” that is not utilized by the licensed owners in the current spectrum, which is referred to as opportunistic spectrum access (OSA) technology. In comparison, OSA technology operates in the current spectrum and hence is compatible with existing technologies, which has made it a hot research topic in the last decade. In this article, we focus on opportunistic spectrum access technologies.