On the Degrees of Freedom in Cognitive Radio Channels

In this work we revisit the multiplexing gain, or degrees of freedom, of the Gaussian cognitive radio channel. Prior work has shown that the cognitive radio channel attains one degree of freedom, indicating that while cognition significantly improves rates over the interference channel at medium SNR, at high SNR the cognitive channel too is interference-limited. In this work we show that by allowing the power of the secondary user to scale infinitesimally faster than the power of the primary user, degrees of freedom arbitrarily close to 2 can be achieved. This is equal to that of an ideal 2 × 2 MIMO channel, and cannot be achieved in a similar manner in the interference channel.