User Cooperation in Heterogeneous Cognitive Radio Networks with Interference Reduction Chunhua Sun and Khaled Ben Letaief, Fellow, IEEE Department of Electronic and Computer Engineering The Hong Kong University of Science and Technology Kowloon, Hong Kong Email: {sunnyhua, eekhaled}@ust.hk Abstract— In cognitive radio systems, secondary users can share the spectrum with the primary user as long as the quality of service (QoS) of the primary system is guaranteed. However, the system throughput of the cognitive system will be limited when the QoS requirement is stringent. Recently cooperative diversity has been proposed as a powerful method that can provide dra- matic gains in wireless environments. In this paper, we investigate the problem of spectrum sharing together with adaptive user cooperation in heterogeneous cognitive relay system. To maximize the throughput of the cognitive system, one best relay will be selected and besides, optimal power allocation is performed between the source and the relay. In addition, beamforming is applied to further reduce the interference and improve the system performance. Simulation results show the improvement of the throughput as opposed to the direct transmission. I. I NTRODUCTION Intense competition for spectrum usage has recently arisen due to the increasing wide deployment of high speed networks for services and low utilization of the licensed spectrum resources which only ranges from 15% to 85% [1]. Aimed at making full use of the underused spectrum resources (white space), the IEEE 802.22 Wireless Region Area Network (WRAN) Group was established to utilize the spectrum be- tween 54 MHz and 862 MHz. As a very promising candidate for WRAN, cognitive radio has been pursued to exploit the existence of spectrum holes [2], [3], so as to more efficiently use the spectrum and without major changes to incumbents. In cognitive radio systems, the unlicensed (cognitive) users can access the licensed (primary) spectrum as long as the QoS of the licensed user is guaranteed [4]. With the Fed- eral Communications Commission’s (FCC) spectrum policy reform, a new metric, called the interference temperature, has been proposed to quantify and manage the interference in a radio environment. However, the coexistence constraint, such as power levels for the cognitive users, should be well predefined to maintain a guarantee of service to the primary users on the same band [5]. Under the scenario when the QoS requirement for the primary system is stringent, the throughput of the cognitive system will be limited due to the low allowed transmitting power level. Recently, user cooperation has been proposed as an ap- proach to form virtual antenna arrays that can exploit user diversity and provide dramatic gains in reliability and capacity increase. To that ends, a wireless relay network, which allows a source assisted by intermediate nodes, offers a significant performance gain advantage [6], [7]. The most popular cooper- ation protocols are amplify-and-forward (AF) where the relay simply retransmits the amplified received signal and decode- and-forward (DF) in which the relay will decode the message, re-encode and retransmit. Motivated by these two promising techniques, i.e., cog- nitive radio and user cooperation, cognitive relay network, has attracted much interest. Such kind of network includes a cognitive network which is defined by a cognitive source node, a cognitive destination node as well as multiple potential cognitive relay nodes and a primary network which consists of a primary transmitter-receiver pair. Related work can be found in [8], in which the cognitive source is assisted by a group of relays only when the primary user is absent. Besides, the impact of spectrum acquisition performance of the cognitive relay nodes on the outage performance of the cognitive system has been investigated. Apart from cooperation among cognitive users in [8], a different cooperative transmission has been investigated in [9], where the cognitive transmitter acts as a transparent relay for the primary link to exploit more free time slots and to increase the stable throughput for the cognitive system, assuming the queuing state information available. In contrast to previous works, here we consider the problem of spectrum sharing in heterogeneous cognitive relay networks together with adaptive relay selection and interference reduc- tion. Coexistence between the primary system and secondary system is allowed under the constraint of interference temper- ature for the primary receiver. The best relay will be selected from the potential cognitive relay group so that the throughput of the cognitive system under the QoS requirement of the primary system is maximized. For simplicity, the AF protocol is employed in this work and direct link from cognitive source to cognitive destination is neglected. In our problem, optimal power allocation between the selected relay and the cognitive source will be considered to further increase the throughput. In addition, beamforming is performed at the cognitive relay and destination to reduce the interference received from the primary transmitter so as to bring further improvement to the system throughput. The rest of this paper is organized as follows. In Section II, the system model is presented and the system input- output relationship is derived. In Section III, the problem is This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2008 proceedings. 978-1-4244-2075-9/08/$25.00 ©2008 IEEE 3193