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This study is concerned with cooperative spectrum sensing (CSS) mechanisms in three-hop cognitive wireless relay networks (CWRNs). The data transmission from a source to a destination is realised with the aid of two layers of cognitive radio (CR) users which are in the transmission coverage of two primary users. The authors first propose a new CSS scheme for a layer of CR users to improve the spectrum-sensing performance by exploiting both local decisions at the CR users and global decisions at the fusion centre. Particularly, the probabilities of missed detection and false alarm for a practical scenario where all sensing, reporting and backward channels suffer from Rayleigh fading are derived. The derived expressions not only show that the proposed CSS achieves a better sensing performance than the conventional scheme but also characterise the effects of the fading channels on the sensing reliability. Furthermore, the authors propose a CSS scheme for two CR layers in a three-hop CWRN using binary operations to help reduce one phase of sensing for a higher system throughput.