Feedback-aided selective subspace retransmission for outage-free spatial multiplexing

We study a novel spatial multiplexing scheme for a multiple-input/multiple-output (MIMO) architecture with vertical coding and a feedback channel. Rather than for sending back channel state information (CSI), the feedback channel is used to request the retransmission of signal parts in critical signal subspaces. This enables the receiver to perform linear reconstruction of the layered signals without the destructive noise enhancement in ill-conditioned channel matrices that is experienced with conventional linear processors. Moreover, a given target signal-to-noise ratio at the decoder input can be ensured. A comparison against standard MIMO architectures in terms of achievable throughput lets us conclude that the proposed multiplexing facilitates an advantageous closed-loop MIMO system which does not rely on accurate transmitter-side CSI.