Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks

An uplink-downlink 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. 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 (N1N2 + 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, N-1 + M-2, max(M-1, M-2), and max(N-1, N-2), where a(+) denotes max(0, a). The result demonstrates that, depending on the network configuration, operating one of the cells as uplink and the other cell as downlink can improve DoF compared to the conventional uplink or downlink operation, in which both cells operate as either uplink or downlink.