An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with M1β antennas receives independent messages from its
N1β serving users, while the second BS with M2β antennas transmits
independent messages to its N2β 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
(N1βN2β+min(M1β,N1β)(N1ββN2β)++min(M2β,N2β)(N2ββN1β)+)/max(N1β,N2β),
M1β+N2β,M2β+N1β, max(M1β,M2β), and max(N1β,N2β), 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.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
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