Spatial multiplexing


Spatial multiplexing is a transmission technique in MIMO wireless communication, fibre-optic communication and other communications technologies to transmit independent and separately encoded data signals, known as "streams". Therefore, the space dimension is reused, or multiplexed, more than one time.
=Wireless communications=
If the transmitter is equipped with antennas and the receiver has antennas, the maximum spatial multiplexing order 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 practical multiplexing gain can be limited by spatial correlation, which means that some of the parallel streams may have very weak channel gains.

Encoding

Open-loop approach

In an open-loop MIMO system with transmitter antennas and receiver antennas, the input-output relationship can be described as
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 scheme.

Closed-loop approach

A closed-loop MIMO system utilizes Channel State Information 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.