  In this paper, we analyze the Gaussian X channel in the mixed interference
regime. In this regime, multiple access transmission to one of the receivers is
shown to be close to optimal in terms of sum rate. Three upper bounds are
derived for the sum capacity in the mixed interference regime, and the
subregions where each of these bounds dominate the others are identified. The
genie-aided sum capacity upper bounds derived also show that the gap between
sum capacity and the sum rate of the multiple access transmission scheme is
small for a significant part of the mixed interference region. For any \delta >
0, the region where multiple access transmission to one of the receivers is
within \delta from sum capacity is determined.
