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

The Feasibility of Interference Alignment for Reverse TDD Systems in MIMO Cellular Networks