Characterizing degrees of freedom through additive combinatorics

Abstract: We establish a formal connection between the problem of characterizing degrees of freedom (DoF) in constant single-antenna interference channels (ICs), with general channel matrix, and the field of additive combinatorics. The theory we develop is based on a recent breakthrough result by Hochman in fractal geometry. Our first main contribution is an explicit condition on the channel matrix to admit full, i.e., $K/2$ DoF; this condition is satisfied for almost all channel matrices. We also provide a construction of corresponding DoF-optimal input distributions. The second main result is a new DoF-formula exclusively in terms of Shannon entropies. This formula is more amenable to both analytical statements and numerical evaluations than the DoF-formula by Wu et al., which is in terms of R\'enyi information dimension. We then use the new DoF-formula to shed light on the hardness of finding the exact number of DoF in ICs with rational channel coefficients, and to improve the best known bounds on the DoF of a well-studied channel matrix.