Grassmannian-Euclidean superposition for MIMO broadcast channels

In multiantenna broadcast channels without channel state information at transmitter (CSIT), orthogonal transmission (e.g., TDMA) is known to achieve maximal degrees of freedom either with perfect channel state information at receiver (CSIR), or when no receiver has CSIR. Recently, it has been shown [1] that TDMA is no longer optimal in cases of unequal CSIR, e.g., when one receiver has CSIR and another does not. This paper proposes a multiplicative superposition that attains degrees of freedom uniformly higher than [1] and TDMA. The proposed scheme uses coherent signaling for the receiver with CSIR, and Grassmannian signaling for the receiver without CSIR. This signaling structure is shown to attain the optimal degrees of freedom under a wide set of antenna configurations.