1 A Simple Relay Selection Algorithm in Multi User OFDM Based Cooperative CR Networks Ahmad Farjami * Vahid Tabataba Vakili Shahriyar Shirvani Moghadam St_a_farjami@azad.ac.ir Vakili@iust.ac.ir sh_shirvani@srttu.edu * Islamic Azad University (IUT) Tehran south branch ** Iran University of Science and Technology *** Shahid Rajaee Teacher Training University Abstract – this paper presents a simple relay selection algorithm in multiuser orthogonal frequency-division multiplexing (MU-OFDM) cooperative Cognitive Radio (CR) networks to enhance normalized sum capacity of secondary users (SUs). After simplifying three node relay network to its equivalent two node network with equivalent gain on each subcarrier, we used a simple method for relay selection by considering interference to primary users, and a simple relay selection algorithm proposed. This simple relay selection algorithm selects the relay-node for each source- node by a fixed power allocation on all subcarriers (SCs) in independent phase before joint subcarrier and power allocation algorithm perform. Simulation results demonstrated that this relay selection algorithm can enhance normalized sum capacity of SUs significantly compared to random relay selection algorithm. Keywords- cognitive radio (CR), cooperative communication, multiuser orthogonal frequency-division multiplexing (MU-OFDM), peak power constrain, relay selection, resource allocation. I. INTRODUCTION In recent years with rapid growth of wireless devices and spectrum needs, efficient use of limited spectrum sources becomes more important. As traditional static spectrum allocation mechanism is inefficient, Cognitive Radio (CR) has introduced as a promising solution to the problem of spectrum scarcity [1], [2]. In a CR system when licensed primary user (PU) is ideal, an unlicensed secondary user (SU) is allowed to access the PU spectrum opportunistically or to share the spectrum simultaneously subject to interference limitation [3], [2]. In recent years combination of OFDM and CR becomes an important technology for next-generation (XG) wireless systems [4], [5], as or thogonal frequency- division multiplexing (OFDM) technique can effectively combat intersymbol interference and enhance transmission efficiency [6]. One of the probable multiple access techniques for XG wireless broadband networks is Multiuser Orthogonal Frequency Division Multiplexing (MU-OFDM) [7], [8]. In MU-OFDM networks, several users can transmits their signals simultaneously over different subcarriers on each OFDM symbol. MU-OFDM technique advantages compared to other multiple access schemes, like finer granularity, better link budget, simpler receiver design (eliminating multiuser detection), and better multiuser diversity [ 9 ] caused this technique has been adopted as the IEEE 802.22 standard for CR wireless regional area networks [10] . On the other hand, to reduce multipath fading and consequently enhance the channel capacity, cooperative diversity technique has been considered. In this technique instead of using extra antennas in each transmitter (which is impractical in some devices), the spatial diversity is implemented by sharing other users' antennas for a source node. As the multiuser diversity gained by dynamic resource allocation is limited because of variation of channel quality among users, cooperative diversity is considered as a way to mitigate this issue. A way to form cooperative communication is to let some users to act as relay for other ones. Thus the cognitive relay networks are of great interests in recent years [11], [12], [13]. Resource allocation algorithms in MU-OFDM CR network have great importance in this network [14], [13]. depending on the current channel state information (CSI), a dynamic resource allocation algorithm for cooperative MU-OFDM CR system, allocates the available limited resources (relays, subcarriers and power) to users who can best exploit them in order to best quality of service could be achieved at the lowest cost. In this paper we consider resource allocation problem for centralized cooperative MU-OFDM CR uplink system, in which some SUs act as relay for other ones in three-node fashion. The relay uses decode-and-forward (DF) cooperative strategy to transmit source-node signal to the destination and destination combines the signals from the source-node and relay-node by maximal ratio combining (MRC). The joint optimization of relay selection, power and subcarrier allocation is a mixed binary integer programming problem which is NP-hard. Thus, resource allocation procedure can be divided into two phases. In the first phase, with the help of proposed algorithm and according to CSI the relay-source pair is identified with fix power allocation. Then the three-node network is simplified to its equivalent two node network for each cooperative link with corresponding channel gain [13]. After relay selection and node simplification in second phase, this equivalent model can be applied to a cooperative MU-OFDM CR system such that the traditional joint subcarrier and power allocation algorithms for uncooperative MU-OFDM CR system could be used [15], [14]. In this paper, after relay selection, the joint subcarrier and power allocation algorithm proposed in [14] is used in order to