Exact Outage Probability of Dual-hop Cooperative Cognitive Networks with Relay Selection Methods, Hardware Impairment and MRC Receiver Pham Thi Dan Ngoc [1],[2] , Tran Trung Duy [2] , Vo Nguyen Quoc Bao [2] and Khuong Ho-Van [1] [1] HoChiMinh City University of Technology, Vietnam [2] Posts and Telecommunications Institute of Technology, Vietnam Email: {ngocptd, trantrungduy, baovnq}@ptithcm.edu.vn Email: khuong.hovan@yahoo.ca Abstract—In this paper, we evaluate outage probability of dual- hop cooperative cognitive networks under impact of hardware impairment. In particular, we propose two relay selection meth- ods to compensate the performance loss due to the imperfect hardware and the interference constraint at primary network. In the first proposed method, named PRO, the best secondary relay is selected by relying on the channel state information (CSI) of the links between the secondary source and the secondary relays. In the second one, named RE, one of active relays which provides the highest channel gain to the secondary destination is chosen for the cooperation. At the secondary destination, the maximal ratio combining (MRC) technique is used to combine the data received from the source and the selected relay. For performance evaluation and comparison, we drive exact closed- form expressions of outage probability over Rayleigh fading channel, which are verified by Monte Carlo simulations. Index Terms—Hardware impairment, underlay cognitive ra- dio, relay selection, outage probability, maximal ratio combining. I. I NTRODUCTION To solve the spectrum scarcity problem, Mitola [1] intro- duced a new method, called cognitive radio (CR), in which the primary users (PUs) can share the licensed bands to secondary users (SUs). One of efficient methods in CR is underlay protocol [2]–[4], in which PUs and SUs can operate on the frequency bands simultaneously, provided that the SUs’ opera- tions must satisfy the co-channel interference constraint given by PUs. To enhance the performance of the secondary network under the limited transmit power, cooperative communication [5] was widely used in underlay CR. In [6]–[8], pro-active decode-and-forward (DF) relay selection (RS) methods were proposed and analyzed, where the best relay is selected by using channel state information (CSI) of the source-relay links. In [9]–[11], the authors studied outage performance of the re- active DF RS protocol, in which the successful relay which is ready for the cooperation and provides highest channel gain to the destination is considered as the best candidate. For full RS strategies, cooperative cognitive protocols in one-way and two- way networks were investigated in [12] and [13], respectively. However, almost of the available performance analyzes of cooperative cognitive protocols are based on the underlying assumption hardware of the secondary transceivers is perfect. In practice, the transceiver hardware is suffered from impair- ment due to amplifier-amplitude non-linearity, I/Q imbalance and phase noise [14], [15]. This motivates us to evaluate the performances of underlay CR networks under the impact of the imperfect hardware. To the best of our knowledge, there have been several published works researching on impact of hard- ware impairment in CR. In [16], the authors considered the outage performance of a dual-hop underlay CR scenario with a single-relay. The authors in [17] proposed various MIMO relaying schemes under the joint impact of the hardware noise and the interference constraint. In this paper, dual-hop underlay relaying protocols with re- lay selection methods are proposed and analyzed. In particular, in the first proposed protocol, named PRO, the best secondary relay is selected based on the quality of channel of the source- relay links. In the second proposal, named RE, the secondary relay which successfully decodes the source data and has the best channel quality to the secondary destination is chosen. Moreover, to improve the reliability of the decoding process at the secondary destination, maximal ratio combining (MRC) technique is used. For performance evaluation and comparison, we derive exact closed-form expressions of outage probability for the proposed protocols over Rayleigh fading channel. Then, Monte Carlo simulations are performed to verify our derivations. The results present that the PRO and RE protocols can provide higher performance than the conventional direct transmission protocol (DT), while the RE always outperforms the PRO. The rest of this paper is organized as follows. We present the system model of the proposed protocols in Section II. In Section III, the exact outage probability of the RE and PRO methods are evaluated. The simulation results are shown in section IV and Section V concludes this paper. II. SYSTEM MODEL In Fig. 1, we present the system model of the proposed protocols. In this figure, the primary network consists of a PU while secondary relay network includes one source node (S), one destination (D) and M relay nodes (R). The secondary source communicates with the secondary destination via help 978-1-4673-6547-5/15/$31.00 2015 IEEE