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Cognitive radio is a technology with tremendous potential for more efficient utilisation of the radio spectrum. Based on the available network side information to the secondary user, three transmission models are proposed in the literature: the `underlay` method, the `overlay` method and the `hybrid` method. The authors study here the capacity and power allocation for the hybrid method. In particular, the authors investigate the ergodic capacity of the secondary user under joint average transmission power and average interference to the primary user receiver constraints when no channel state information is available to the secondary user except its probability density function. An algorithm based on the barrier method is proposed to achieve this. Finally, simulation results are presented and analysed to validate the authors` theoretical results.