Distributed beamforming for cognitive relay networks

Abstract

A distributed beamforming technique for half-duplex underlay cognitive relay networks is developed in this paper. The considered cognitive relay network consists of one pair of primary users, one pair of secondary users and multiple relays. With the help of multiple relays, the secondary transmitter sends data to the secondary receiver. In our approach, the receive quality-of-service at the secondary user is maximized while keeping the total relay transmit power and the interference power below a predefined threshold. The resultant problem is shown to have closed-form solutions with certain conditions or can be formulated as a quasi-convex problem, which can be solved efficiently using bi-section search method and secondorder-cone programming. Simulation results demonstrate that the proposed beamforming method improves the system performance.Index Terms-Distributed beamforming, worst-case, cognitive relay networks INTRODUCTIONCognitive radio has recently attracts much interest as it can improve the utilization of wireless spectrum resources, which enables unlicensed users or secondary users to communicate with each other over licensed bands through three main ways. In the interweave approach, the secondary transmitter exploits the spectrum holes of the licensed users or primary users to transmit data [1], [2]. In the overlay method, the secondary user is allowed to operate simultaneously with the primary user over the same spectrum. The transmission of the primary user is improved by employing sophisticated signal processing and coding techniques [3]. In the underlay way, the secondary user and the primary user share the same spectrum simultaneously while the interference power at the primary user is lower than a predefined threshold or interference temperature [2], [4]. Due to the simplicity of the implementation, we consider the underlay approach in this paper.Cooperative relay networks have also attracted much interest in the literature as they can exploit cooperative spatial diversity of different users in the network [5]- [6]. These advantages are introduced by the scheme that different users in relay networks share their communication resources to help each other in data transmission.Since the transmit power of the secondary user is limited to guarantee the quality-of-service (QoS) of the primary user in the underlay approach, it is an attractive choice to employ cooperative relaying techniques in cognitive networks [7]- [15]. The secondary user can operate as a relaying node for the primary user to improve the QoS of the primary user and thus helps to improve the threshold of the interference temperature [7]. The secondary user can also work as a relaying node for other secondary users to improve the receive QoS