The frequency spectrum is an essential resource for wireless communication. Special sections of the spectrum are used for military purposes, governments sell some frequency bands to broadcasting and mobile communications companies for commercial use, others such as ISM (Industrial, Science and Medical) bands are available for the public free of charge. As the spectrum becomes overcrowded, there seem to be two possible solutions: pushing the frequency limits higher to frequencies of 60 GHz and above, or reaggregating the densely used licensed frequency bands. The new Cognitive Radio (CR) approach comes with the feasible solution to spectrum scarcity. Secondary utilization of a licensed spectrum band can enhance the spectrum usage and introduce a reliable solution to its dearth. In such a cognitive radio network, secondary users can access the spectrum under the constraint that a minimum quality of service is guaranteed for the licensed primary users. In this thesis, we focus on spectrum sharing techniques in cognitive radio network where there is a number of secondary users sharing unoccupied spectrum holes. More specifically, we introduce two collaborative cognitive radio networks in which the secondary user cooperate with the primary user to deliver the data of the primary user.