Journal of Electrical Systems and Signals, Vol. 1, No. 2, 2013 Cooperative Beamforming in Relay Assisted MIMO-OFDMA Cognitive Radio Systems Mohsen Abbasi-Jannatabad and Hossein Khoshbin Abstract: In this paper, a new algorithm is proposed for cooperative beamforming, power allocation and relay selection in the multiple input multiple output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) CR systems where a pair of SU communicates with each other assisted by some single antenna relay nodes. The objective is to maximize signal to interference plus noise ratio (SINR) of SU subject to guarantee the PU's quality of service and power constraints of SU and relay nodes over all subchannels. The transmitter and receiver beamforming vectors of PU and SU are estimated by the proposed two- step iterative algorithm. Simulation results show that the proposed algorithm is able to increase the spectrum usage efficiency and, further, guarantee the required QOS at the PU. Keywords: Relay assisted MIMO-OFDMA cognitive radio, cooperative beamforming, power allocation, relay selection. 1. Introduction Spectrum usage efficiency can be improved using cognitive radio (CR) technology in which secondary users (SUs) are allowed to access licensed bands that are originally allocated to primary network without causing any harmful interference to primary users (PUs) [1-4]. There are three kinds of CR networks based on secondary access schemes to licensed bands, called interweave, underlay and overlay. The first approach is based on opportunistic communications in which SUs can access the licensed bands only when PUs are absent. This approach is called “interweave” technique. In the second scheme, SUs are allowed to access the licensed bands simultaneously with PUs. Note that in this scheme the interference induced from SUs to PUs must below a specific threshold which is known as ‘interference temperature’. This scheme is called “underlay” technique. In the overlay CR system, SUs regulate the interference induced to PUs by assisting and relaying PU signals to their destinations [5 -8]. Due to adverse effects of environment such as loss, shadowing and fading, the receive signal at SUs may be too weak, thereby decreasing the quality of service (QoS). Manuscript received November 26, 2012; accepted October 20, 2013. The authors are with the Department of Electrical Engineering, Ferdousi University of Mashhad (FUM), Mashhad, Iran. This research has been done at Commonications and Computer Research Center of FUM. The Corresponding author’s email is:mh_abbasi2003@yahoo.com To mitigate these destructive effects of channels, SUs can cooperate with each other and some SUs can act as relay nodes. These relay nodes cooperate with SU transmitters to relay SU signals to their desired destinations. Using relay nodes further enhances the spectrum efficiency by increasing spatial diversity. In recent years, many relay schemes are investigated. In [9], resource allocation in relay OFDM-based CR system is considered based on decode-and-forward (DF) protocol. The dual decomposition technique is adopted to obtain an asymptotically optimal subcarrier pairing, relay selection and power allocation. The optimization problem is based on maximization of sum rate subject to interference limit for PU and power constraints in SU transmitter and relays. In [10], authors investigate power and channel allocation for cooperative relay in a three-node CR network. Three nodes are considered which consists of a source, a relay and a destination. Three end-to-end possible communications are assumed based on three channels consist of direct channel, dual-hop channel and relay channel. The resources are allocated based on maximization of channel capacity under PU’s interference and power constraints in source and relay nodes. In [11], the problem of relay and power allocation for OFDM-based CR systems with single antenna has been considered where the capacity of SU employing relays is maximized subject to total transmission power constraint and interference limit for PU. Due to high computational complexity of the optimization problem, three sub-optimal schemes are presented. The authors in [12] investigate the problem of resource (subcarrier and power) allocation in an OFDMA-based relayed cellular CR network in which a base station services some user equipment via multiple relay stations. The resource allocation problem must decide for each symbol which subcarrier at which relay stations and at what power level would relay. The objective function is maximization of network capacity. Joint relay selection and power allocation are investigated in [13] to maximize system throughput with limited interference to PUs in CR network. The authors develop an optimal approach based on dual method and then propose a suboptimal algorithm to reduce computational complexity. In [14], the authors present an optimal scheme for power allocation and relay selection in CR networks where a pair of cognitive (secondary) transceiver communicates with each other using some two-way relay nodes. The goal of the proposed power allocation and relay selection algorithm is to maximize the achievable rate subject to interference constraint of PU and power constraints of cognitive transceivers and relay node. In [15], a simplified power