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In this paper, we consider a cognitive radio (CR) system in non-ideal fading wireless channels and propose a cooperative spectrum sensing scheme based on linear parallel access channel (PAC),serving as an alternative way to improve the cooperative spectrum sensing performance. We assume that the gains of the observation and transmission channels are all known which is available since we use standard preamble-aided channel estimation techniques to require channel state information. The key feature of the proposed scheme is that the observations are transmitted by amplify and forward transmissions from part of cognitive users (CUs) to a fusion center (FC) while the other CUs keep silent. The optimal weighting coefficients are obtained by maximizing the deflection coefficient under transmit power constraint and the optimal number and set of CUs in cooperation is searched by a sequential forward selection algorithm. Simulation results illustrate that the proposed optimal cooperative spectrum sensing scheme can significantly improve the spectrum sensing performance.