Index Coding Capacity: How Far Can One Go With Only Shannon Inequalities?

An interference alignment perspective is used to identify the simplest instances (minimum possible number of edges in the alignment graph, not more than 2 interfering messages at any destination) of index coding problems where non-Shannon information inequalities are necessary for capacity characterization. In particular, this includes the first known example of a multiple unicast (one destination per message) index coding problem where non-Shannon information inequalities are shown to be necessary. The simplest multiple unicast example has 7 edges in the alignment graph and 11 messages. The simplest multiple groupcast (multiple destinations per message) example has 6 edges in the alignment graph, 6 messages, and 10 receivers. For both the simplest multiple unicast and multiple groupcast instances, the best outer bound based on only Shannon inequalities is 2/5, which is tightened to 11/28 by the use of the Zhang-Yeung non-Shannon type information inequality, and the linear capacity is shown to be 5/13 using the Ingleton inequality. Conversely, identifying the minimal challenging aspects of the index coding problem allows an expansion of the class of solved index coding problems up to (but not including) these instances.