Matrix analysis and applied linear algebra
Matrix analysis and applied linear algebra
Fundamentals of wireless communication
Fundamentals of wireless communication
Opportunistic beamforming using dumb antennas
IEEE Transactions on Information Theory
Diversity and multiplexing: a fundamental tradeoff in multiple-antenna channels
IEEE Transactions on Information Theory
Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks
IEEE Transactions on Information Theory
Cognitive radio: brain-empowered wireless communications
IEEE Journal on Selected Areas in Communications
On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming
IEEE Journal on Selected Areas in Communications
Dynamic spectrum access in open spectrum wireless networks
IEEE Journal on Selected Areas in Communications
A simple Cooperative diversity method based on network path selection
IEEE Journal on Selected Areas in Communications
Joint Beamforming and Power Allocation for Multiple Access Channels in Cognitive Radio Networks
IEEE Journal on Selected Areas in Communications
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Cognitive radio is a powerful solution that can significantly improve the utilization of the precious limited radio spectrum. It allows secondary users (SUs) to opportunistically access spectral holes in the licensed spectrum without causing harmful interference to primary users (PUs). However, the secondary communication opportunity becomes extremely poor when primary systems are heavily loaded. In this paper, a distributed beamforming method is proposed to allow concurrent transmissions of PUs and SUs, thereby improving the opportunistic spectrum access. Specifically, a SU source broadcasts a message to a set of cognitive users, which can serve as a set of relays, when PUs are absent. The relays that correctly decode the message will create a distributed beamformer to forward the message to the SU destination without causing any interference irrespective of whether PUs are silent or not. To achieve this, we use the method of orthogonal projection to obtain the beamforming weight vector. In addition, we derive the distribution of the received signal power at the SU destination, based on which the average outage probability of our proposed scheme is analyzed when PUs' occupation changes fast. Theoretical and numerical results reveal that the spatial diversity order of this scheme equals the number of SU relays minus that of primary receivers. Furthermore, numerical results show that the outage probability of this scheme outperforms other schemes that access the spectrum only when PUs are absent.