Spatial multiplexing (seen abbreviated SM or SMX) is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, the space dimension is reused, or multiplexed, more than one time.

If the transmitter is equipped with Failed to parse (Missing texvc executable; please see math/README to configure.): N_t

if a linear receiver is used. This means that Failed to parse (Missing texvc executable; please see math/README to configure.): N_s

In an open-loop MIMO system with Failed to parse (Missing texvc executable; please see math/README to configure.): N_t

where Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbf{x} = [x_1, x_2, \ldots, x_{N_t}]^T

A closed-loop MIMO system utilizes Channel State Information (CSI) at the transmitter. In most cases, only partial CSI is available at the transmitter because of the limitations of the feedback channel. In a closed-loop MIMO system the input-output relationship with a closed-loop approach can be described as