This paper focuses on interference issues arising in the downlink of a heterogeneous network (HetNet), where small cells are deployed within a macrocell. Interference scenario in a HetNet varies based on the type of small cell access modes, which can be classified as either closed subscriber group (CSG) or open subscriber group (OSG) modes. For these two types of modes, we propose hierarchical interference alignment (HIA) schemes, which successively determine beamforming matrices for small cell and macro base stations (BSs) by considering a HetNet environment in which the macro BS and small cell BSs have different numbers of transmit antennas. Unlike prior work on interference alignment (IA) for homogeneous networks, the proposed HIA schemes compute the beamforming matrices in closed-form and reduce the feedforward overhead through a hierarchical approach. By providing a tight outer bound of the degrees-of-freedom (DoF), we also investigate the optimality of the proposed HIA schemes with respect to the number of antennas without any time expansion. Furthermore, we propose a new optimization process to maximize the sum-rate performance of each cell while satisfying the IA conditions. The simulation results show that the proposed HIA schemes provide an additional DoF compared to the conventional interference coordination schemes using a time domain-based resource partitioning. Under multicell interference environments, the proposed schemes offer an approximately 100% improvement in throughput gain compared to the conventional coordinated beamforming schemes when the interference from coordinated BSs is significantly stronger than the remaining interference from uncoordinated BSs.Index Terms-Interference alignment, interference management, heterogeneous network, sum rate maximization.

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With the development of wireless mobile communication technology, the demand for wireless communication rate and frequency increases year by year. Existing wireless mobile communication frequency tends to be saturated, which demands for new solutions. Terahertz (THz) communication has great potential for the future mobile communications (Beyond 5G), and is also an important technique for the high data rate transmission in spatial information network. THz communication has great application prospects in military-civilian integration and coordinated development. In China, important breakthroughs have been achieved for the key techniques of THz high data rate communications, which is practically keeping up with the most advanced technological level in the world. Therefore, further intensifying efforts on the development of THz communication have the strategic importance for China in leading the development of future wireless communication techniques and the standardization process of Beyond 5G. This paper analyzes the performance of the MIMO channel in the Terahertz (THz) band and a discrete mathematical method is used to propose a novel channel model. Then, a channel capacity model is proposed by the combination of path loss and molecular absorption in the THz band based on the CSI at the receiver. Simulation results show that the integration of MIMO in the THz band gives better data rate and channel capacity as compared with a single channel.

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