Achieving Full Sum DoF in the SISO Interference Channel With Feedback Delay

Channel state information at the transmitter (CSIT) is of utmost importance to manage interference in multi-user wireless networks, in which transmission rates at a high signal-to-noise ratio are characterized by (sum) degrees of freedom (DoF). Ingenious techniques have been proposed to deal with delayed CSIT, notably a scheme by Maddah-Ali and Tse, providing significant DoF with outdated CSIT. However, with most techniques, any delay in the CSIT feedback still results in a DoF loss. A notable exception is the work by Lee and Heath, allowing one to preserve the optimal DoF in the underdetermined MISO BC and IC, but for feedback delays $T_{fb}$ decreasing with the number of users $K$, up to a fraction $1/K$ of the channel coherence time $T_{c}$. In this work, an ergodic interference alignment scheme that preserves the $K/2$ sum DoF of the $K$-user IC is proposed for $T_{fb}\leq T_{c}/2$. It is also proven that $T_{fb}=T_{c}/2$ is the longest for which the optimal $K/2$ DoF can still be obtained for all $K$.