## What Is MIMO?

Multiple-input multiple-output (MIMO) technology employs various transmission techniques to increase the capacity of a communications channel. A MIMO system has multiple transmission paths between antennas at the transmitters and the receivers. In a MIMO system, each transmit-receive antenna pair constitutes a single-input single-output (SISO) link. To accommodate your design requirements, a MIMO system can have a variety of single or multiple transmit and receive antenna configurations. Additionally, a MIMO system can transmit to a single user or multiple users. For information about the transmission techniques that you can use in a MIMO system, see MIMO Techniques.

### SISO

In a SISO link, the base station (BS) transmits a single data stream,
*S*_{1}, out of a single transmit antenna through a
channel, *H*, to a single mobile station (MS) user, which has a single
antenna to receive *S*_{1}. The reciprocal transmission
from mobile station to base station is also a SISO link.

### MISO and SIMO

Adding multiple transmit antennas to the base station creates a multiple-input
single-output (MISO) link, transmitting data through channel *H*. In this
case, the reciprocal transmission from mobile station to base station is a single-input
multiple-output (SIMO) link.

### SU-MIMO

Reconfiguring the mobile station by adding multiple receive antennas to the single user
(SU) creates an SU-MIMO system. The system transmits multiple streams of data through
channel *H* for the single user to receive. The reciprocal transmission
from mobile station to base station is also an SU-MIMO system.

### MU-MIMO

Expanding the system further, by adding multiple users (MU) on the receive side, creates
the MU-MIMO system. This system also transmits and receives multiple streams of data through
channel, one per user, to multiple users. However, in this case, the system transmits
multiple different streams of data precoded for reception by specific users. Each user sees
a unique channel matrix. Here, channel *H*_{1} for user
*MS*_{1} and channel
*H*_{2} for user
*MS*_{2}.

### MIMO Channel

In a MIMO system, multiple SISO links result in a MIMO channel with signal paths to each receive antenna from each transmit antenna. Each receive antenna sees the signal from each of the transmit antennas.

Although it is not possible to separate the individual multipath components, it is possible to have a well-behaved channel when you space the receive antennas more than one-half of a wavelength apart. A rich multipath environment, where signals constructively and destructively interfere with each other as they reflect and scatter off surfaces between the transmitter and receiver, allows you to accurately simulate your MIMO channel with a well-behaved channel.

A well-behaved channel can produce linearly independent signals at the receiver
antennas, resulting in a full-rank matrix, *H*. You can model this MIMO
system as:

$$y=Hx+n$$

where *y* and *x* are the receive and
transmit vectors, and *H* and *n* are the channel matrix
and the noise vector. The channel, *H*, can be represented with an
*N*_{T}-by-*N*_{R} matrix.

$$H=\left\{\begin{array}{cccc}{h}_{11}& {h}_{12}& \mathrm{...}& {h}_{1{N}_{R}}\\ {h}_{21}& {h}_{22}& \mathrm{...}& {h}_{2{N}_{R}}\\ {h}_{31}& {h}_{32}& \mathrm{...}& {h}_{3{N}_{R}}\\ \begin{array}{l}\mathrm{...}\\ {h}_{{N}_{T}1}\end{array}& \begin{array}{l}\mathrm{...}\\ {h}_{{N}_{T}2}\end{array}& \begin{array}{l}\mathrm{...}\\ \mathrm{...}\end{array}& \begin{array}{l}\mathrm{...}\\ {h}_{{N}_{T}{N}_{R}}\end{array}\end{array}\right\}$$

You can model fading characteristics of MIMO channels with Rayleigh or Rician fading, and the Kronecker model for the spatial correlation between the links as described in [3].