  We study a two-tier macrocell/femtocell system where the macrocell base
station is equipped with multiple antennas and makes use of multiuser MIMO
(spatial multiplexing), and the femtocells are "cognitive". In particular, we
assume that the femtocells are aware of the locations of scheduled macrocell
users on every time-frequency slot, so that they can make decisions on their
transmission opportunities accordingly. Femtocell base stations are also
equipped with multiple antennas. We propose a scheme where the macrocell
downlink (macro- DL) is aligned with the femtocells uplink (femto-UL) and, Vice
Versa, the macrocell uplink (macro-UL) is aligned with the femtocells downlink
femto-DL). Using a simple "interference temperature" power control in the
macro-DL/femto-UL direction, and exploiting uplink/downlink duality and the
Yates, Foschini and Miljanic distributed power control algorithm in the macro-
UL/femto-DL direction, we can achieve an extremely attractive macro/femto
throughput tradeoff region in both directions. We investigate the impact of
multiuser MIMO spatial multiplexing in the macrocell under the proposed scheme,
and find that large gains are achievable by letting the macrocell schedule
groups of co-located users, such that the number of femtocells affected by the
interference temperature power constraint is small.
