paper

The DoF of the Asymmetric MIMO Interference Channel with Square Direct Link Channel Matrices

arXiv:1410.5107

Abstract

This paper studies the sum Degrees of Freedom (DoF) of $K$ -user {\em asymmetric} MIMO Interference Channel (IC) with square direct link channel matrices, that is, the $u$ -th transmitter and its intended receiver have $M_{u} \in N$ antennas each, where $M_{u}$ need not be the same for all $u \in [1 : K]$ . Starting from a $3$ -user example, it is shown that existing cooperation-based outer bounds are insufficient to characterize the DoF. Moreover, it is shown that two distinct operating regimes exist. With a {\it dominant} user, i.e., a user that has more antennas than the other two users combined, %(say $M_{1} \geq M_{2} + M_{3}$ ), it is DoF optimal to let that user transmit alone on the IC. Otherwise, it is DoF optimal to {\em decompose} and operate the 3-user MIMO IC as an $(M_{1} + M_{2} + M_{3})$ -user SISO IC. This indicates that MIMO operations are useless from a DoF perspective in systems without a dominant user. The main contribution of the paper is the derivation of a novel outer bound for the general $K$ -user case that is tight in the regime where a dominant user is not present; this is done by generalizing the insights from the 3-user example to an arbitrary number of users.

Presented at 52nd Allerton Conference, 2014