Spatial multiplexing or space-division multiplexing (often abbreviated SM, SDM or SMX) is a
multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - ...
technique in
MIMO
In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wi ...
wireless communication
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
,
fibre-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pre ...
and other communications technologies used to transmit independent channels separated in space.
( Other multiplexing techniques include FDM (
frequency-division multiplexing
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separat ...
), TDM (
time-division multiplexing
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...
) or PDM (
polarization-division multiplexing). )
Fibre-optic communication
In fibre-optic communication SDM refers to the usage of the transverse dimension of the fibre to separate the channels.
Techniques
Multi-core fibre (MCF)
Multi-core fibres are fibres designed with more than a single core. Amongst different types of MCFs exist, “Uncoupled MCF” is the most common in which each core is treated to be an independent optical path resulting in increasing in channel capacity. However, the main limitation to these systems is the presence of inter core crosstalk and ways to deal it as well as the coupling/de-coupling mechanism. Although, in recent times, different splicing techniques, coupling methods and schemes have been proposed and demonstrated and despite many of the component technologies still being in the development stage, MCF systems already present the capability for huge transmission capacity.
Multi-mode fibres (MMF)
Multi-mode fibers are fibres designed to allow multiple modes to propagate through it where each mode is considered as separate channel enhancing its capacity in contrast to single mode fibre (SMF) that only supports single spatial mode, however MMF has two polarizations.
The MMFs are limited by high dispersion and attenuation rate causing the signal quality to be diminished over long distances. In addition to this, the MMFs also suffer from intermodal crosstalk and requires digital signal processing to deal with it.
Mode-division multiplexing (MDM)
Mode-division multiplexing utilizes the
transverse spatial modes of the fibre to separate the channels.Mode-division multiplexing (MDM) can be achieved by multiplexing several data inputs into different modes efficiently using multiplexers. There are numerous methods of multiplexing and coupling modes into few mode fibres (FMFs), like photonic lanterns, multi-plane light conversion, and others.
Fibre bundles
Bundled fibres are also considered a form of SDM, these are massive number of fibres bundled tightly together.
Wireless communications
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
Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is ut ...
(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.
Encoding
Open-loop approach
In an
open-loop MIMO
In radio, multiple-input and multiple-output, or MIMO (), is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wi ...
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 (eSM)'' scheme.
Closed-loop approach
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
Precoding is a generalization of beamforming to support multi-stream (or multi-layer) transmission in multi-antenna wireless communications. In conventional single-stream beamforming, the same signal is emitted from each of the transmit antennas ...
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.
See also
*
3G MIMO
*
Space–time code
A space–time code (STC) is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas. STCs rely on transmitting multiple, redundant copies of a data stream to the ...
*
Space–time trellis code
*
WiMAX MIMO
*
Fibre-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pre ...
*
multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - ...
References
{{DEFAULTSORT:Spatial Multiplexing
IEEE 802
Information theory
Radio resource management