Spatial Degrees of Freedom of the Multicell MIMO Multiple Access Channel
  We consider a homogeneous multiple cellular scenario with multiple users per cell, i.e., $K\geq 1$ where $K$ denotes the number of users in a cell. In this scenario, a degrees of freedom outer bound as well as an achievable scheme that attains the degrees of freedom outer bound of the multicell multiple access channel (MAC) with constant channel coefficients are investigated. The users have $M$ antennas, and the base stations are equipped with $N$ antennas. The found outer bound is general in that it characterizes a degrees of freedom upper bound for $K\geq 1$ and $L>1$ where $L$ denotes the number of cells. The achievability of the degrees of freedom outer bound is studied for two cell case (i.e., L=2). The achievable schemes that attains the degrees of freedom outer bound for L=2 are based on two approaches. The first scheme is a simple zero forcing with $M=K\beta+\beta$ and $N=K\beta$, and the second approach is null space interference alignment with $M=K\beta$ and $N=K\beta+\beta$ where $\beta>0$ is a positive integer. 
Taejoon Kim, David J. Love, Bruno Clerckx, and Duckdong Hwang
cs.IT math.IT