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In this paper, we study an interference alignment (IA) scheme with finite time extension and beamformer selection method with low computational complexity for X channel. An IA scheme with a chain structure by the Latin square is proposed for K × 3 multiple-input multiple-output X channel. Since the proposed scheme can have a larger set of possible beamformers than the conventional schemes, its performance is improved by efficient beamformer selection for a given channel realization. Also, we propose a condition number-based beamformer selection method with low computational complexity and its performance improvement is numerically verified.INDEX TERMS Beamforming, degrees of freedom (DoF), interference alignment (IA), latin square, multiple-input multiple-output (MIMO), X channel.

Topological interference management (TIM) can obtain degrees of freedom (DoF) gains with no channel state information at the transmitters (CSIT) except topological information of network in the interference channel. It was shown that TIM achieves the optimal symmetric DoF when internal conflict does not exist among messages [6]. However, it is difficult to assure whether a specific topology can achieve the optimal DoF without scrutinizing internal conflict, which requires lots of works. Also, it is hard to design a specific optimal topology directly from the conventional condition for the optimal DoF. With these problems in mind, we propose a method to derive maximal topology directly in TIM, named as alliance construction in K-user interference channel. That is, it is proved that a topology is maximal if and only if it is derived from alliance construction. We translate a topology design by alliance construction in message graph into topology matrix and propose conditions for maximal topology matrix (MTM). Moreover, we propose a generalized alliance construction that derives a topology achieving DoF 1/n for n ≥ 3 by generalizing sub-alliances. A topology matrix can also be used to analyze maximality of topology with DoF 1/n. Index TermsAlliance, alliance construction, degrees-of-freedom (DoF), internal conflict, maximal topology matrix (MTM), topological interference management (TIM).