On feasibility of linear interference alignment with symbol extension for two-cell MIMO interfering multiple access channel

In this paper, we study the non-integer degrees of freedoms (DoF) of the time-invariant multiple-input multiple-output (MIMO) interfering multiple access channel (IMAC) in the simple setting of two cells, K users per cell, and M antennas at all nodes. We apply the idea of channel diversity developed by Li, Jafarkhani, and Jafar to this setting to derive the exact characterization of the maximum achievable sum DoF under the constraint of using linear interference alignment (IA) scheme with symbol extension. We show that the sum DoF characterization 2MK/(1 + K) provided by the linear IA scheme only holds for K ≤ M². Both the lower and upper DoF bounds are constructed by exploring the block-diagonal structure of the symbol-extended channel matrices, where all blocks are the same.