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Cognitive Radio networks are emerging as a new paradigm of communication and channels sharing in multimedia and wireless networks. In this paper, we consider multimedia traffic transmission over shared Cognitive Radio (CR) networks using fountain codes under different subchannel selection policies in a fading environment. We assume that the primary traffic arrival follows a Poisson process. To share this CR infrastructure, we propose to make use of the Time Division Multiple Access (TDMA) method, each secondary user (SU) is assigned one time slot where he transmits with a certain probability, the proposed model permits to this SU to transmits also in the remaining slots if he has data to transmit and has missed his own slot. Therefore, packets are not only lost by reason of Primary traffic interruptions, but also we consider losses caused by collisions between several SUs due to our specific time slot allocation. We evaluate the influence of the proposed model parameter settings on the secondary traffic transmission performance in view of the Spectral Efficiency. Numerical simulations show that the system still exhibit good secondary traffic efficiency despite of the presence of primary interferences and secondary collisions.