Aligned Image Sets under Channel Uncertainty: Settling a Conjecture by Lapidoth, Shamai and Wigger on the Collapse of Degrees of Freedom under Finite Precision CSIT

Abstract: A conjecture made by Lapidoth, Shamai and Wigger at Allerton 2005 (also an open problem presented at ITA 2006) states that the DoF of a 2 user broadcast channel, where the transmitter is equipped with 2 antennas and each user is equipped with 1 antenna, must collapse under finite precision CSIT. In this work we prove that the conjecture is true in all non-degenerate settings (e.g., where the probability density function of unknown channel coefficients exists and is bounded). The DoF collapse even when perfect channel knowledge for one user is available to the transmitter. This also settles a related recent conjecture by Tandon et al. The key to our proof is a bound on the number of codewords that can cast the same image (within noise distortion) at the undesired receiver whose channel is subject to finite precision CSIT, while remaining resolvable at the desired receiver whose channel is precisely known by the transmitter. We are also able to generalize the result along two directions. First, if the peak of the probability density function is allowed to scale as O(P^\lpha/2)), representing the concentration of probability density (improving CSIT) due to, e.g., quantized feedback at rate (\alpha/2)\log(P), then the DoF are bounded above by 1+\alpha, which is also achievable under quantized feedback. Second, we generalize the result to the K user broadcast channel with K antennas at the transmitter and a single antenna at each receiver. Here also the DoF collapse under non-degenerate channel uncertainty. The result directly implies a collapse of DoF to unity under non-degenerate channel uncertainty for the general K-user interference and MxN user X networks as well.