Fuzzy Logic Power Control in Cognitive Radio Zeljko Tabakovic Croatian Post and Electronic Communications Agency Jurisiceva 13 Zagreb, Croatia zeljko.tabakovic@telekom.hr Sonja Grgic, Mislav Grgic University of Zagreb, Faculty of EE and Comp Department of Wireless Communication Unska 3/XII, Zagreb, Croatia sgrgic@ieee.org; mgrgic@ieee.org Abstract — Opportunistic radio spectrum access has the possibility to improve spectrum utilization needed for next generation mobile networks. The main challenge to opportunistic radio spectrum access lies in finding balance in conflicting goals of satisfying performance requirements while minimizing interference. In this paper we propose new strategy for fuzzy logic transmit power control which enables cognitive secondary user to achieve its required transmission rate and quality, while minimizing interference to the primary users and other concurrent secondary users. Keywords - cognitive radio; opportunistic radio spectrum access; transmit power control; fuzzy logic systems I. INTRODUCTION In recent years, demand for wireless communication services has grown far beyond earlier predictions. Furthermore, in order to satisfy future market demand for mobile and broadband services, we can envisage deployment of next generation mobile networks and services which will need rapid and more flexible access to radio spectrum. Due to policy of exclusive frequency assignment, radio spectrum has become congested and scarce resource. Nevertheless, related surveys have proved that most of the allocated spectrum is underutilized [1, 2]. To deal with increasing conflict of spectrum congestion and spectrum underutilization, cognitive radio technique has been proposed as a flexible method which allows secondary users to utilize already licensed bands opportunistically [3, 4]. Opportunistic radio spectrum access has the possibility to improve spectrum utilization and in perspective allowing next generation mobile networks access to the attractive radio spectrum bands. The main challenge to opportunistic radio spectrum access lies in finding balance in conflicting goals of satisfying performance requirements for secondary user (SU) while minimizing interference to the active primary users (PU) and other secondary users. Secondary user should not degrade performance statistics of licensed primary users. In order to achieve these tasks, secondary user is required to recognize primary users, determine environment characteristics and quickly adapt its system parameters corresponding to the operating environment. Main abilities of cognitive radio (CR) with opportunistic radio spectrum access capabilities are spectrum sensing, dynamic frequency selection and adaptive transmit power control. In recent years, studies on transmit power control (TPC) are progressing in order to investigate different TPC strategies for opportunistic radio spectrum access systems [5-9]. Presented TPC strategies differ depending on settings of primary goals for TPC, presumptions about available input data and on methodology used for transmit power control parameter determination. In [5], opportunistic TPC is presented which enables cognitive user to maximize its transmission rate i.e. power, while guaranteeing PU outage probability. The authors in [6] proposed fuzzy logic TPC scheme which dynamically adjust transmit power relating to SU interference observed at PU, distance between PU and SU and received power difference at the SU base station. In order to avoid interference at PU, exchange of sensing information between PU and SU is required. In [7], authors propose distributed cognitive network access scheme with the aim of providing best QoS with respect of combination of radio link and core network performance. Fuzzy logic decision has been used to choose the most suitable access opportunity even in multi-technology scenarios. A power control approach based on spectrum sensing side information in order to mitigate interference to the PU is presented in [8]. Cognitive radio transmit power is calculated in three step procedure using missing probability of energy detection dependence on distance between PU and SU. In [9], the authors investigate the optimal power control with and without interference temperature constraints based on observed Shannon capacity. The optimal power control in cognitive radio network is modeled as a concave minimization problem [10]. In this paper we propose alternative transmit power control strategy which enables cognitive secondary user to achieve its required transmission rate and quality, while minimizing interference to the primary users and other concurrent secondary users. Proposed TPC ensures that each SU in the network receives and transmits just enough energy to convey necessary information. Spectrum sensing data and regulatory requirements defines maximum acceptable SU transmit power. Depending on the quality of service, SU receiver sets required signal-to-interference-plus-noise ratio (SINR). Comparison of measured and required SINR at SU receiver determines transmit power control ratio and minimum required SU transmit power. SU transmit power is determined by balancing these two requirements of maximum acceptable SU transmit power to satisfy interference constraints and minimum required SU transmit power to satisfy determined level of service. 978-1-4244-4530-1/09/$25.00 ©2009 IEEE