Alternating Renewal Framework for Estimation in Spectrum Sensing Policy and Proactive Spectrum Handoff Thant Zin Oo Department of Computer Engineering Kyung Hee University Yongin, 446-701 Korea tzoo@khu.ac.kr † Choong Seon Hong, ‡ Sungwon Lee Department of Computer Engineering Kyung Hee University Yongin, 446-701 Korea { † cshong, ‡ drsungwon}@khu.ac.kr Abstract—In wireless communication, radio spectrum is a resource. Dynamic spectrum access or cognitive radio is a viable method to increase spectrum utilization. The unlicensed users access the unused portion of radio spectrum opportunistically. The unlicensed users must vacate the spectrum as soon as the licensed users appear on the channel. Since, the licensed users or Primary Users (PUs) have paid licensing fees for the spectrum, the unlicensed users or Secondary Users (SUs) must not cause interference to PU transmissions. In order that SUs can continue to operate, they must find another spectrum hole on another channel and switch to that channel. These so called, spectrum sensing and spectrum handoff, are the basic foundations of CR technology. We model the random channel as an ON-OFF process and applied renewal theory to build a framework to observe the PU activity. Then from the observations, we estimate the likelihood of PU arrival to perform spectrum handoff. We then propose a spectrum access decision making policy based on the spectrum handoff time. We then numerically evaluate our proposal and make comparisons. Keywords - Cognitive Radio; Dynamic Spectrum Access; Renewal Process. I. INTRODUCTION The exponential growth in the number of wireless devices since leads to dramatic increase in mobile data communication. It, in turn, increases the demand for radio spectrum. At present, the radio spectrum is allocated to licensed users by the government agencies. The static allocation means that some spectrums are under-utilized while others are congested. The uneven demand and supply ratio can be clearly observed in the most popular wireless service, the Wi-Fi or WLAN (IEEE Std. 802.11). The 2.45 gigahertz Industrial, Scientific and Medical ISM) radio band, on which Wi-Fi devices operate, is allocated a total bandwidth of only 100 megahertz. Moreover, Federal Communication Commission (FCC) stated that more than 70% of the spectrum is under-used [1]. Underutilized empty broadcast TV channels known as “white spaces” (TVWS) across major cities were surveyed in [2]. Dynamic Spectrum Access (DSA) has been considered as a viable solution to solving the spectrum scarcity. Cognitive Radio (CR) was introduced and it became a well known DSA system [3] [4]. The basic CR architecture has two types of users; Primary Users (PUs) to whom the channel is licensed by the authorities and unlicensed Secondary Users (SUs). SUs can opportunistically access the channel without any interference to the PUs. This prioritized two-tier access introduces a new randomness to the already challenging wireless communication environment. This dynamic use of spectrum depending on PUs activity, adds new obstacles to make the network protocols adaptive to the varying available spectrum. [5]. Research for CR in TVWS for broadband usage has been conducted for more than a decade and there is an existing standard already; Regional Area Network (RAN) (IEEE 802.22 Standard) [6]. RAN focuses on broadband access network for rural areas. Their architecture is centralized with base stations controlling the air interface of fixed broadband devices. DSA for WLAN is not yet finalized and currently still under development in IEEE 802.11af task group. Accordingly, there are two CR access methods to limit interference to the PU signal. The first one is CR underlay access in which SUs employ power control in PHY layer to mitigate interference to PUs [7] [17]. Another access method is CR overlay access [18] in which the SU network does not use power control. Since there is no power control, SUs cannot coexist in the same channel with the PUs at the same time. So, the SU network must vacate the spectrum as soon as the PU signal appears on the channel. In other words, if a PU appears on a channel, the SUs on that channel must stop transmitting and switch to another channel for further communication. This CR overlay access is compatible the IEEE 802.11 Std. because WLAN radio interface employs Time Division Duplex (TDD), i.e. half-duplex packet mode networks based on Carrier Sense Multiple Access (CSMA). It is also consistent with the asymmetry between upload and download data communication of the Internet. Based on before mentioned facts, CR overlay access is best suited for our proposal. 330 978-1-4673-5742-5/13/$31.00 ©2013 IEEE ICOIN 2013