Interactive Interference Alignment

Abstract: We study interference channels (IFC) where interaction among sources and destinations is enabled, e.g., both sources and destinations can talk to each other using full-duplex radios. The interaction can come in two ways: 1) {\em In-band interaction:} sources and destinations can transmit and listen in the same channel simultaneously, enabling interaction. 2) {\em out-of-band interaction:} destinations talk back to the sources on an out-of-band channel, possible from white-space channels. The flexibility afforded by interaction among sources and destinations allows for the derivation of interference alignment (IA) strategies that have desirable "engineering properties": insensitivity to the rationality or irrationality of channel parameters, small block lengths and finite SNR operations. We show that for several classes of interference channels the interactive interference alignment scheme can achieve the optimal degrees of freedom. In particular, we show the {\em first simple scheme} (having finite block length, for channels having no diversity) for $K=3,4$ that can achieve the optimal degrees of freedom of $\frac{K}{2}$ even after accounting for the cost of interaction. We also give simulation results on the finite SNR performance of interactive alignment under some settings. On the technical side, we show using a Gr\"{o}bner basis argument that in a general network potentially utilizing cooperation and feedback, the optimal degrees of freedom under linear schemes of a fixed block length is the same for channel coefficients with probability 1. Furthermore, a numerical method to estimate this value is also presented. These tools have potentially wider utility in studying other wireless networks as well.