multiple input multiple output technology

in radio communications, multiple input multiple output is the use of multiple antennnas at both the transmitter and receiver to improve communication performance.

       This technology is one of the several forms of smart antenna technology. This technology is used in wireless communications because it offers significant increases in data throughput without additional bandwidth or power requirements.

Working: it acheives this goal by spreading the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency and to achieve diversity gain that improves fading.

Because of these factors this is used in latest technologies like Wi-Fi , 4G, 3GPP long term evolution, Wi Max and HSPA+.

MIMO can be sub divided into three main categories:

Precoding

Spatial multiplexing

Diversity coding

PRECODING : it is a multistream beamforming. It is considered as all spatial processing that occurs at the transmitter. In single stream beamforming , the same signal is emitted from each of the transmit antenna with appropiate phase and gain such that signal power is maximed at the receiver input. The benefit of this is to increase the received signal gain by making ignals emitted from different antennas , adding them constructively and to reduce multipath fading effect. In line of sight propagation, it results in well defined directional patterns.  When the receiver has multiple antennas , the transmit beamforming cannot simultaneously maximise the signal levelat all of the receive antennas. There is a requirement of channel state information at the transmitter and the receiver.

SPATIAL MULTIPLEXING : this requires MIMO antenna configuration.  In this, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures and the receiver has accurate CSI  , it can separate these streams into parallel channels. This technique is very effective for increasing channel capacity and higher signal to noise ratios. This can be used without CSI at the transmitter but can be combined with precoding if CSI is available.

DIVERSITY CODING  :  these techniques are used when there is no channel knowledge at the transmitter. In this, a single stream is transmitted, but the signal is coded using techiques called spac time coding. The signal is transmitted from each of transmit antenas with full or near orthogonal coding. Diversity coding can be combined with spatial multiplexing when some channel knowledge is available at the transmitter.