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Spatial multiplexing (often 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 antennas and the receiver has antennas, the maximum spatial multiplexing order (the number of streams) is,
if a linear receiver is used. This means that streams can be transmitted in parallel, ideally leading to an increase of the spectral efficiency (the number of bits per second and per Hz that can be transmitted over the wireless channel). The practical multiplexing gain can be limited by spatial correlation, which means that some of the parallel streams may have very weak channel gains.
where is the vector of transmitted symbols, are the vectors of received symbols and noise respectively and is the matrix of channel coefficients. An often encountered problem in open loop spatial multiplexing is to guard against instance of high channel correlation and strong power imbalances between the multiple streams. One such extension which is being considered for DVB-NGH systems is the so-called enhanced Spatial Multiplexing (eSM) scheme. 
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
where is the vector of transmitted symbols, are the vectors of received symbols and noise respectively, is the matrix of channel coefficients and is the linear precoding matrix.
A precoding matrix is used to precode the symbols in the vector to enhance the performance. The column dimension of can be selected smaller than which is useful if the system requires streams because of several reasons. Examples of the reasons are as follows: either the rank of the MIMO channel or the number of receiver antennas is smaller than the number of transmit antennas.
- Sangchul Moon, Woo-Suk Ko, David Vargas, David Gozalvez Serrano, M. Danish Nisar, and Volker Pauli. "Enhanced Spatial Multiplexing for Rate-2 MIMO of DVB-NGH System", Invited paper in the 19th International Conference on Telecommunications, April 2012.