Pilot-based product superposition for downlink multiuser MIMO

In the fading MIMO broadcast channel, until recently the results under full CSIR and no CSIR indicated that degrees of freedom (DoF) cannot be improved beyond what is available via TDMA. Recently, however, it was discovered that when one node has full CSIR and the other has none, TDMA is no longer DoF-optimal and a product decomposition with noncoherent (Grassmannian) signaling achieves the optimal DoF under certain receiver antenna configurations. This work extends product superposition to the domain of coherent signaling with pilots, showing that it can also achieve the optimal DoF. In addition, this work demonstrates the advantages of product superposition in low-SNR as well as high-SNR, and extends the set of receiver antenna configurations under which product decomposition achieves optimal DoF.