Recursive Spatial Multiplexing: Improving Unitary Precoding with Outdated Channel State Information

We propose a closed-loop multiple-input multiple-output (MIMO) architecture which incorporates unitary precoding and recursive spatial multiplexing. Both techniques take advantage of a feedback channel in a similar way for the transmission of data at high rates with limited signal processing complexity. Unitary precoding is effective in situations with accurate channel state information (CSI) at the transmitter end. In situations with outdated CSI due to time-variant channels, a linear signal reconstruction at the receiver end may result in severe noise amplification. Recursive multiplexing mitigates the noise enhancement by means of retransmissions of signal parts in critical subspaces. This significantly increases the ergodic capacity achieved by precoding based on outdated CSI, as shown for the cases of time-variant (4x4) and (8x8) MIMO channels.