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 M1antennas receives independent messages from its N1serving users, while the second BS with M2antennas transmits independent messages to its N2serving 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.