IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 13, NO. 11, NOVEMBER 2014 6219 A Cooperative Transmission Scheme for Improving the Secondary Access in Cognitive Radio Networks Wael Jaafar, Student Member, IEEE, Wessam Ajib, Member, IEEE, and David Haccoun, Life Fellow, IEEE Abstract—In this paper, we examine the problem of secondary access blocking in cognitive radio networks when secondary trans- missions cause unacceptably high interference to primary trans- missions. In general, the access of secondary users (SUs) to a licensed spectrum band is only allowed when this access does not alter the performance of primary users that can be defined by the primary QoS requirement. In this paper, we propose a cooperative scheme that allows SUs to increase their access to the spectrum band and access the spectrum even when the primary QoS is not satisfied. Using relay selection and a proper power allocation method, we show that the secondary outage performance can be significantly improved, whereas the primary outage performance is either not altered or slightly improved. Moreover, closed-form expressions of the primary and secondary outage probabilities are derived, and the achieved diversity order is calculated. Finally, analytical and simulation results illustrate the primary outage performance and secondary outage performance of the proposed scheme and show its advantages compared with conventional schemes. Index Terms—Cognitive radio, relaying, decode-and-forward, relay selection, power allocation. I. I NTRODUCTION C OGNITIVE radio (CR) is a key technology for solving the spectrum under-utilization and spectrum congestion issues [1]–[4]. By allowing unlicensed Secondary Users (SUs) to transmit on the licensed spectrum band of the Primary Users (PUs) without disrupting the primary transmissions, the spectrum resources are better exploited. In underlay Cognitive Radio Networks (CRNs), the SUs may transmit at the same time as the PUs as long as the induced interference is below a predefined threshold (such as a Signal-to-Noise-Ratio-SNR- threshold, outage probability threshold, etc.) [4]. User cooperation has also been developed to provide spatial diversity gain and reduce interference due to simultaneous communications [5]–[7]. In [6], several cooperative schemes have been proposed such as fixed relaying, selection relaying and incremental relaying. These schemes provide an increased diversity gain at the expense of a reduced spectral efficiency. This shortcoming can be overcome by selecting only the “best” Manuscript received November 8, 2013; revised April 19, 2014; accepted June 11, 2014. Date of publication June 20, 2014; date of current version November 7, 2014. The associate editor coordinating the review of this paper and approving it for publication was H. Wymeersch. W. Jaafar and D. Haccoun are with the Department of Electrical Engineering, École Polytechnique de Montréal, Montreal QC H3T 1J4, Canada (e-mail: wael.jaafar@polymtl.ca; david.haccoun@polymtl.ca). W. Ajib is with the Department of Computer Science, Université du Québec à Montréal, Montreal QC H2X 3YZ, Canada (e-mail: ajib.wessam@uqam.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TWC.2014.2332161 relay, among a set of relay nodes, to assist the transmission [8]. Hence, the cooperative scheme achieves full diversity while avoiding complex synchronization among the relay nodes. Integrating user cooperation to CRNs has recently attracted attention for enhancing the transmissions performances. The authors in [9] propose assisting the secondary transmissions using a group of co-located CR relay nodes. By “adequately” selecting the relay nodes, the maximal diversity gain can be achieved. In [10], a less-complex scheme has been proposed achieving the maximal diversity gain at high primary SNR and improving the secondary outage probability compared to conventional schemes. In [11], the authors show that by jointly optimizing spectrum sensing and the secondary access, the sec- ondary outage performance can be significantly improved. The same authors investigate an incremental cooperation scheme for secondary transmissions in [12] and derive closed-form expressions of the secondary outage probability. Both schemes are then extended to multiple-relay CRNs and the related diversity-multiplexing tradeoff is obtained. The authors in [13] investigated the cooperative diversity gain in underlay CRNs in terms of outage probability and diversity order. They showed that diversity is lost for a fixed interference power constraint at the primary receiver. However, if the interference power constraint is proportional to the peak transmit power at the secondary transmitters, then full diversity is achieved, which is equal to the number of relay nodes +1 (by accounting the direct transmission). Other works propose cooperative schemes to assist the pri- mary transmissions, using either a cognitive relay node [14], [15] or the secondary transmitter [16]. They showed that cog- nitive relaying is efficient under certain network topologies and nodes locations. It is to be noted that in this type of networks, a limited information exchange between primary and secondary systems is required in order to respect the primary QoS constraint. In [17]–[20], we have proposed cooperative schemes to assist either the primary transmission (as in [14]–[16]) or the secondary one (as in [9]–[13]) or both simultaneously. In [17], the proposed scheme assists simultaneously PUs and SUs whenever the relay node is able to decode both primary and secondary signals. Provided results show that the secondary outage performance improves at the expense of a higher relay transmit power, while the primary QoS is maintained. An extension to the multi-antenna relay with antenna selection is studied in [18]. In [19], assisting the primary or secondary transmission is activated using the incremental relaying tech- nique (acknowledgment-based cooperation as in [12]). Results show that choosing first the best relay to assist the PUs before 1536-1276 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.