Partially Connected Interference Networks with No CSIT: Symmetric Degrees of Freedom and Multicast Across Alignment Blocks

Abstract

We explore the symmetric degrees of freedom (DoF) of partially connected interference networks for the case in which transmitters have no knowledge of their channel states (channel coefficients and coherence times) but are aware of the network topology. In particular, we consider interference networks which have no internal conflicts in their alignment graph. This class of networks can be used to model various interference scenarios in modern wireless systems. A sufficient condition is established for which half DoF can be achieved per message, which coincide with the best known outer bound, thereby establishing the optimal symmetric DoF. For networks in this class that do not satisfy the sufficient condition, a systematic achievable scheme, termed multicast across alignment blocks, is derived and an analytical expression is obtained for the achieved symmetric DoF. The new scheme is based on our insight that an interference alignment-based scheme may not utilize all available signalling dimensions, and the unused signalling dimensions can be further exploited through multicast transmission.