A simplified channel and beamforming model for ns-3

Currently, ns-3 provides the TR 38.901 channel model as the default option for sampling MIMO wireless channels which take into account the presence of (possibly directional) multiple antenna elements at both transmitter and receiver. However, this model represents a major performance bottleneck for most end-to-end simulations, especially whenever they involve upwards of hundreds of nodes. Accordingly, the goal of my proposal is twofold: (i) to provide ns-3 with a simplified statistical channel model, to be used for analyses which are focused on the upper layers of the protocol stack only ; and (ii) to improve the performance of the TR 38.901-based framework which is currently available in ns-3, for the remainder of the use cases.