3D-MIMO Dual Communications in SCMA-Based Secure HetNets

In this paper, we consider a communication scenario for heterogeneous networks (HetNets) in which the base stations transmit information over both microwave (μW) and millimeter wave (mmW) bands. To exploit the spatial characteristics of the channel, we consider three-dimensional multiple input multiple output (3D-MIMO) technology. In addition, to increase the spectral efficiency of the network, sparse code multiple access (SCMA) is used as the transmission technology for both bands. A practical scenario in which the channel information is not known perfectly is assumed, and hence a robust scheme is developed. We formulate the resource allocation problem as an optimization problem which is highly nonlinear and non-convex. To solve the problem, we rely on iterative approximation methods, such as successive convex approximation, and we provide a joint transmit power allocation and beam-forming to increase the secrecy rate of the network. Using simulations, we evaluate the performance of the proposed scheme for different network settings. Simulations show a performance increase of about 24.85% with 3D-MIMO over two-dimensional MIMO (2D-MIMO). In addition, the use of both μW and mmW bands results in a 20.25% performance increase over a case where only μW is used. We also show that 3D-MIMO and dual connectivity have better performance than 2D-MIMO and μW in high-rise and dense urban areas.