  The feasibility conditions of interference alignment (IA) are analyzed for
reverse TDD systems, i.e., one cell operates as downlink (DL) but the other
cell operates as uplink (UL). Under general multiple-input and multiple-output
(MIMO) antenna configurations, a necessary condition and a sufficient condition
for one-shot linear IA are established, i.e., linear IA without symbol or time
extension. In several example networks, optimal sum degrees of freedom (DoF) is
characterized by the derived necessary condition and sufficient condition. For
symmetric DoF within each cell, a sufficient condition is established in a more
compact expression, which yields the necessary and sufficient condition for a
class of symmetric DoF. An iterative construction of transmit and received
beamforming vectors is further proposed, which provides a specific beamforming
design satisfying one-shot IA. Simulation results demonstrate that the proposed
IA not only achieve lager DoF but also significantly improve the sum rate in
the practical signal-to-noise ratio (SNR) regime.
