A radio resource allocation framework is proposed for underlay spectrum sharing. The ergodic capacity maximization problem in orthogonal frequency division multiple access-based network on point to multi point transmission is formulated and solved. Heterogeneous traffic is also considered in which two types of traffic is assumed: streaming traffic which has strict delay requirements, and elastic traffic with flexible delay requirements. Considering the effect of channel state information (CSI) imperfection in the evaluation of the secondary users' expected rate, we further assume that the estimated CSI between the secondary users and secondary base station (secondary channel) is not perfect. Moreover three different cases are considered depending on the availability of the CSI between the secondary base station and the primary receivers (interference channel). Using simulations, we evaluate the impact of streaming traffic and imperfect CSI on the sum capacity of the secondary elastic users for different values of parameter systems.