Spatial multiplexing

Spatial multiplexing (often abbreviated SM or SMX) 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. Spatial multiplexing (often abbreviated SM or SMX) 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. If the transmitter is equipped with N t {displaystyle N_{t}} antennas and the receiver has N r {displaystyle N_{r}} antennas, the maximum spatial multiplexing order (the number of streams) is, if a linear receiver is used. This means that N s {displaystyle N_{s}} streams can be transmitted in parallel, ideally leading to an N s {displaystyle N_{s}} increase of the spectral efficiency (the number of bits per second 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. In an open-loop MIMO system with N t {displaystyle N_{t}} transmitter antennas and N r {displaystyle N_{r}} receiver antennas, the input-output relationship can be described as where x = [ x 1 , x 2 , … , x N t ] T {displaystyle mathbf {x} =^{T}} is the N t × 1 {displaystyle N_{t} imes 1} vector of transmitted symbols, y , n {displaystyle mathbf {y,n} } are the N r × 1 {displaystyle N_{r} imes 1} vectors of received symbols and noise respectively and H {displaystyle mathbf {H} } is the N r × N t {displaystyle N_{r} imes N_{t}} 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.