On the Maximum Sum-rate Capacity of Cognitive Multiple Access Channel
  We consider the communication scenario where multiple cognitive users wish to communicate to the same receiver, in the presence of primary transmission. The cognitive transmitters are assumed to have the side information about the primary transmission. The capacity region of cognitive users is formulated under the constraint that the capacity of primary transmission is not changed as if no cognitive users exist. Moreover, the maximum sum-rate point of the capacity region is characterized, by optimally allocating the power of each cognitive user to transmit its own information. 
Peng Cheng, Guanding Yu, Zhaoyang Zhang, Hsiao-Hwa Chen and Peiliang   Qiu