||Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as cognitive radio. Cognitive radios have the ability to listen to the surrounding wireless channel, detect the vacant spectrum bands, and make use of them using a variety of coding schemes. In current cognitive radio protocols, the cognitive device listens to the wireless channel and determines, either in time or frequency, which band of the spectrum is unused. It then adapts its signal to fill this void in the spectrum domain. Therefore, it transmits over a certain time or frequency band only when no authorized and licensed users are not present. In this thesis, we consider a new scenario, in which the cognitive radio behavior is generalized to allow both primary (licensed) and secondary (unlicensed) users to simultaneously transmit over the same time or frequency. Under this scheme, a cognitive radio will listen to the channel and, if sensed idle, could proceed with the traditional cognitive radio protocol in which the signal can be transmitted during the voids. However and in contrast to the conventional approach, if the primary user is detected or miss-detected, the cognitive radio can proceed with simultaneous transmission, if certain conditions are met. To mitigate the harmful interference caused to the primary user when the cognitive one is working at the same area, we will propose different techniques and we will develop them in details through our thesis. Different scenarios will be considered including single user, multiple user, downlink and uplink transmission scheme. In the first part, we will develop a power control approach which intelligently adjusts the transmit power of the cognitive radio while maintaining a quality of service (QoS) for the primary user. In order to limit the interference to the primary user, when one cognitive user is sharing the spectrum with one primary user, the transmit power is controlled by the spectrum sensing side information, the probability of missing which actually includes the implicit location information of the primary user. In the second part of the thesis, we shall investigate multi-user uplink scheduling with QoS provisioning for multiple cognitive users working in the vicinity of the primary user. The proposed scheduling schemes provide a satisfactory tradeoff between maximizing the system capacity, achieving fairness among cognitive users, minimizing the interference to the primary user while satisfying the delay constraints to individual cognitive user. Finally, we explore MIMO technology in the framework of cognitive systems so as to provide efficient algorithms for determining which users should be active under the zero-forcing beamforming (ZFB) criterion. The cognitive users will be selected so that they are semi-orthogonal to one another and to the primary user. These users can be then grouped for simultaneous transmission to enhance the performance and guarantee the QoS requirements of the cognitive system.