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The increasing demand for high data rate services such as streaming video and file sharing motivates the need to develop schemes that can more efficiently exploit the limited availability of the electromagnetic radio spectrum. It has been previously noted that in conventional fixed frequency allocation schemes a large portion of the assigned spectrum is used in a sporadic manner, with the usage of the licensed frequency bands ranging from 
to 
(Akyildiz et al., 2006). As a consequence, emerging wireless devices are being designed to exploit the temporal vacancies in licensed frequency bands (Yucek et al., 2009).
Cognitive radio (CR) is a low-cost, highly flexible alternative to the classic single frequency wireless devices that offers a novel approach for improving the utilization of the precious natural frequency spectrum (Mitola, 1999; Devroye et al., 2006; Haykin, 2005). A completely adaptable physical layer is offered by CR devices where the radio features are altered autonomously according to the conditions of the wireless channel and to the user’s requirement by sensing the spectrum. CR devices are different from traditional radio devices due to their ability to equip users with cognitive capability and reconfigurability (Gandetto et al., 2005; Wang et al., 2011).
In a CR system, the new (overlay) user has a lower priority in using the spectrum licensed to the legitimate (legacy) user. The CR is required to sense the spectrum properly in order to ensure that there is minimal interference to the existing system while utilizing the available bandwidth. Various detection techniques can be used to sense the presence of an existing user. However, the performance of most of these techniques degrades with channel shadowing or fading (Yucek et al., 2009); thus, using a paradigm which allows the overlay system to operate simultaneously with the legacy user, rather than an approach that needs continuous spectrum sensing to determine when the channel is vacant, unveils more efficient usage of the shared spectrum. However, the introduction of the overlay system to the legacy system leads to mutual interference between the systems, as shown in Figure 1. Additionally, the legacy system already exists with a fixed infrastructure that cannot be altered. This imposes a constraint on the design of the overlay system.
Figure 1. Interference between overlay/legacy systems
In this paper, we employ an MSE (mean-squared error) based criterion to design an overlay system for cognitive radio application. This CR system allows the legacy and the overlay users to simultaneously share the spectrum under the constraint that the overlay user does not cause significant degradation in the performance of the legacy system while maintaining acceptable performance for its own link.