Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks

TL;DR

This paper characterizes the sum degrees of freedom in a two-cell uplink-downlink cellular network, showing that mixed operation modes can outperform traditional single-mode operation, with implications for heterogeneous networks.

Contribution

It provides a complete characterization of the sum DoF for uplink-downlink cellular networks and demonstrates the potential gains of mixed operation modes over conventional methods.

Findings

Uplink-downlink operation can improve sum DoF compared to traditional modes.

The sum DoF is explicitly characterized by a formula involving network parameters.

The results are applicable to heterogeneous networks with hotspots and delayed CSI.

Abstract

An uplink-downlink two-cell cellular network is studied in which the first base station (BS) with $M_1$ antennas receives independent messages from its $N_1$ serving users, while the second BS with $M_2$ antennas transmits independent messages to its $N_2$ serving users. That is, the first and second cells operate as uplink and downlink, respectively. Each user is assumed to have a single antenna. Under this uplink-downlink setting, the sum degrees of freedom (DoF) is completely characterized as the minimum of $(N_1N_2+\min(M_1,N_1)(N_1-N_2)^++\min(M_2,N_2)(N_2-N_1)^+)/\max(N_1,N_2)$, $M_1+N_2,M_2+N_1$, $\max(M_1,M_2)$, and $\max(N_1,N_2)$, where $a^+$ denotes $\max(0,a)$. The result demonstrates that, for a broad class of network configurations, operating one of the two cells as uplink and the other cell as downlink can strictly improve the sum DoF compared to the conventional uplink or downlink operation, in which both cells operate as either uplink or downlink. The DoF gain from such uplink-downlink operation is further shown to be achievable for heterogeneous cellular networks having hotspots and with delayed channel state information.