New interference alignment schemes for downlink MIMO heterogeneous networks

We consider a downlink heterogeneous network (HetNet), where several picocells coexist in the same spectrum as a macrocell. For a general framework of the multiple-input multiple-output (MIMO) network consisting of arbitrary number of picocells and macro users within the macrocell, designing a closed-form solution to achieve the degrees of freedom (DoF) of (C + K)d while guaranteeing the minimum overall antenna usage is an open problem, where C, K and d are the number of picocells, marco users within the macro cell and data streams per user, respectively. Therefore, by relaxing the minimum antenna utilization constraint, we present a zero-forcing based transceiver design at first, which outperforms the existing closed-form solution based on interference alignment (IA) in terms of antenna usage in most cases. That is to say, alignment processing is not necessary in most cases for the existing scheme because simple zero-forcing scheme with efficient antenna utilization can also attain the same DoF. Besides, we propose two IA schemes without any iterative computations, which have the same antenna configuration but different CSI feedback overhead. Also, our proposed IA schemes require lower usage of antennas compared to the zero-forcing scheme and existing IA scheme.