In 5G mobile communication systems, massive multiple-input multiple-output (MIMO) and heterogeneous networks (HetNets) play crucial roles to achieve expected coverage and capacity across venues. This paper correspondingly addresses software-defined network (SDN) as the central controller of radio resource management in massive MIMO HetNets. In particular, we identify the huge spatial domain information management and complicated MIMO coordination as the grand challenges in 5G systems. Our work accordingly distinguishes itself by considering more network MIMO aspects, including flexibility and complexity of spatial coordination. In our proposed scheme, SDN controller first collects the user channel state information in an effective way, and then calculates the null-space of victim users and applies linear precoding to that null-space. Simulation results show that our design is highly beneficial and easy to be deployed, due to its high quality of service performance but low computation complexity
