  We develop an interference alignment (IA) technique for a downlink cellular
system. In the uplink, IA schemes need channel-state-information exchange
across base-stations of different cells, but our downlink IA technique requires
feedback only within a cell. As a result, the proposed scheme can be
implemented with a few changes to an existing cellular system where the
feedback mechanism (within a cell) is already being considered for supporting
multi-user MIMO. Not only is our proposed scheme implementable with little
effort, it can in fact provide substantial gain especially when interference
from a dominant interferer is significantly stronger than the remaining
interference: it is shown that in the two-isolated cell layout, our scheme
provides four-fold gain in throughput performance over a standard multi-user
MIMO technique. We show through simulations that our technique provides
respectable gain under a more realistic scenario: it gives approximately 20%
gain for a 19 hexagonal wrap-around-cell layout. Furthermore, we show that our
scheme has the potential to provide substantial gain for macro-pico cellular
networks where pico-users can be significantly interfered with by the nearby
macro-BS.
