This paper proposes a new framework for spectrum reuse. Existing architectures have centered on secondary users (cognitive radios) that can reliably sense primary users and opportunistically transmit, without directly interacting with the primary system. We argue that a paradigm in which the primary and secondary systems cooperate can result in reduced interference to primary users and more predictable access for secondary users. Because this architecture gives the primary system full control over spectrum sharing, it could be more favorable in the current economic and political environment.

We illustrate a concrete instance of our framework by showing how secondary radios can reuse the entire uplink channel of a cellular network. We also demonstrate a computationally efficient beamforming algorithm, which enables the coexistence of the two systems. The proposed architecture requires only modest changes to the primary infrastructure, and is shown to achieve an interference rejection of up to 20 dB in most practical scenarios.