Cooperative spectrum sensing is a technology that allows cooperative sensors to assist cognitive radio (CR) transmitters to intelligently decide their transmission opportunities. Transmissions can only be successful if spectrum is available for both CR transmitters and receivers. Motivated by this observation, we use a Boolean-Poisson model to analyze the geometric property of the geographical region that allows CR transmission to be helped with cooperative sensors. We find that cooperative sensing cannot always be helpful and that the region allowing CR transmission is generally not circular symmetry. We identify the condition that transmission link is bidirectional. We further extend this model into the suboptimal scenario among the secondary users and the corresponding transmission allowable region. We derive the condition for which secondary users cannot reduce their Bayesian risk by using cooperative sensors. We conclude with the guidelines for deploying CRs into the existing network.