Spatial multiplexing - AbsoluteAstronomy.com

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 multiplex

Multiplexing

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

ed, 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

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...

(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

Spatial Correlation

Theoretically, the performance of wireless communication systems can be improved by having multiple antennas at the transmitter and the receiver. The idea is that if the propagation channels between each pair of transmit and receive antennas are statistically independent and identically...

, which means that some of the parallel streams may have very weak channel gains.

Open-loop approach

In an open-loop MIMO

MIMO

In radio, multiple-input and multiple-output, or MIMO , is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology...

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.

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.(taken from http://www.cst.uwaterloo.ca/~reza/research_feedback.php)
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

Precoding is a generalization of beamforming to support multi-layer transmission in multi-antenna wireless communications. In conventional single-layer beamforming, the same signal is emitted from each of the transmit antennas with appropriate weighting such that the signal power is maximized at...

matrix.

A precoding matrix

is used to precode the symbols in the vector to enhance the performance. The column dimension

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.

History

Single-user MIMO
- Bell Laboratories Layered Space-Time (BLAST)
  Bell Laboratories Layered Space-Time
  Bell Laboratories Layered Space-Time is a transceiver architecture for offering spatial multiplexing over multiple-antenna wireless communication systems. Such systems have multiple antennas at both the transmitter and the receiver in an effort to exploit the many different paths between the two...
  
  , Gerard. J. Foschini (1996)
- Per Antenna Rate Control (PARC), Varanasi, Guess (1998), Chung, Huang, Lozano (2001)
- Selective Per Antenna Rate Control (SPARC), Ericsson (2004)
Multi-user MIMO: Samsung, Qualcomm, Ericsson, TI, Huawei, Philipse, Alcatel-Lucent, Freescale, et al.
- PU²RC
  PU2RC
  Per-User Unitary Rate Control is the advanced multi-user MIMO technique which utilizes the concept of both pre-coding matrices and scheduling to enhance the system performance of multiple antenna wireless networks....
  
  allows the network to allocate each antenna to the different user which is not considered in single-user MIMO scheduling. Instead of a physical antenna, the network can transmit a user date through a codebook based spatial beam, i.e., a virtual antenna. The efficient user scheduling such as pairing spatially distinguishable users with codebook based spatial beams are additionally used for the simplification of wireless networks in terms of additionally required wireless resource and complex protocol modification.
Enhanced multiuser MIMO
- Employ advanced decoding techniques
- Employ advanced precoding techniques

Open-loop approach

Closed-loop approach

History

See also