Degrees of Freedom of Uplink–Downlink Multiantenna Cellular Networks

An uplink-downlink two-cell cellular network is studied in which the first base station (BS) with M₁ antennas receives independent messages from its N₁ serving users, while the second BS with M₂ antennas transmits independent messages to its N₂ 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 (DoFs) is completely characterized as the minimum of (N₁N₂ + min(M₁, N₁)(N₁ - N₂)⁺ + min(M₂, N₂)(N₂ - N₁)⁺)/ max(N₁, N₂), M₁ + N₂, M₂ + N₁, max(M₁, M₂), and max(N₁, N₂), 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 with 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.

I. Introduction

Characterizing the capacity of cellular networks is one of the fundamental problems in network information theory. Unfortunately, even for the simplest setting consisting of two base stations (BSs) having one serving user each, which is referred to as the two-user interference channel (IC), capacity is not completely characterized for general channel parameters [1], [2]. Exact capacity results being notoriously difficult to obtain, many researchers have recently studied approximate capacity characterizations in the shape of so-called “degrees of freedom (DoF)”, which captures the behavior of capacity as the signal-to-noise ratio (SNR) becomes large.