On the Spatial Degrees of Freedom of Multicell and Multiuser MIMO Channels

TL;DR

This paper investigates the maximum achievable degrees of freedom in multicell MIMO channels with multiple users, providing bounds, schemes, and conditions for optimal spatial multiplexing in both uplink and downlink scenarios.

Contribution

It formulates the outer bounds for degrees of freedom in multicell MIMO MACs and proposes simple schemes that attain these bounds under certain conditions.

Findings

Outer bounds for degrees of freedom are established.

Simple linear schemes can achieve the outer bounds in specific cases.

The results are applicable to both uplink and downlink channels.

Abstract

We study the converse and achievability for the degrees of freedom of the multicellular multiple-input multiple-output (MIMO) multiple access channel (MAC) with constant channel coefficients. We assume L>1 homogeneous cells with K>0 users per cell where the users have M antennas and the base stations are equipped with N antennas. The degrees of freedom outer bound for this L-cell and K-user MIMO MAC is formulated. The characterized outer bound uses insight from a limit on the total degrees of freedom for the L-cell heterogeneous MIMO network. We also show through an example that a scheme selecting a transmitter and performing partial message sharing outperforms a multiple distributed transmission strategy in terms of the total degrees of freedom. Simple linear schemes attaining the outer bound (i.e., those achieving the optimal degrees of freedom) are explores for a few cases. The conditions for the required spatial dimensions attaining the optimal degrees of freedom are characterized in terms of K, L, and the number of transmit streams. The optimal degrees of freedom for the two-cell MIMO MAC are examined by using transmit zero forcing and null space interference alignment and subsequently, simple receive zero forcing is shown to provide the optimal degrees of freedom for L>1. By the uplink and downlink duality, the degrees of freedom results in this paper are also applicable to the downlink. In the downlink scenario, we study the degrees of freedom of L-cell MIMO interference channel exploring multiuser diversity. Strong convergence modes of the instantaneous degrees of freedom as the number of users increases are characterized.