About Multiple Input Multiple Output (MIMO) Technology

MIMO is abbreviated as Multiple-input multiple-output. This is a wireless radio communication and multi-path technology that is being mentioned and used in many new technologies these days. This technique has been developed for enhancing the wireless communication system by using multiple antennas at transmitter, receiver or both. Vo-LTE, LTE (Long Term Evolution), Wi-Max, Wi-Fi, and much other radio, wireless and RF technologies are using the new MIMO wireless technology to provide enlarged link capacity and spectral efficiency combined with enhanced link reliability.

MIMO -Multiple Input Multiple Output Basics

Multiple-In, Multiple-Out (MIMO) communication sends the same information as several signals simultaneously through multiple antennas, using a single radio channel.

MIMO System
MIMO System

It uses multiple antennas to improve signal quality and strength of an RF link channel in a form of antenna diversity. The data are divided into multiple data streams at the transmission point and rearranged on the receive side by another MIMO radio configuration with the same number of antennas.

Basically, a communication medium may be affected by signal fading and this will affect the signal to noise ratio. If these can be made to be affected in different ways by the signal path, the probability that they will all be affected at the same time is considerably reduced. Accordingly, diversity helps to stabilize a link and improves performance, reducing error rates.

Spatial multiplexing and spatial diversity two methodologies are used to provide improvements in the signal to noise ratio (SNR) and they are characterized by improving the reliability of the system with respect to the various forms of fading.

The Concept of Spatial Diversity

The principle of diversity is to provide the receiver with multiple versions of the same signal. In most environments where wireless communication systems operate, the strength of the received signal varies with the time, which is called as Fading.

Fading significantly degrades communications performance by causing the probability of bit error to increase compared to what it would be if only white noise were present.

The below figure shows the probability of bit error as a function of bit energy to noise power spectral density, Eb/N0. The second observation is that for Rayleigh fading, which is the type of fading assumed in this figure and that often occurs in practice, the error probability decreases linearly when plotted on a logarithmic scale against Eb/N0 plotted in dB.

A Logarithmic Scale Against Eb/N0 plotted in dB
A Logarithmic Scale Against Eb/N0 plotted in dB

The Concept of Spatial Multiplexing

Spatial multiplexing refers to transmitting multiple data streams over a multipath channel by exploiting multipath. By so doing, multiple data channels are able to be transmitted simultaneously over the same frequency band, enabling a potentially large number of bits per second to be transmitted per hertz of spectrum.

Spatial multiplexing is analogous to other more common types of multiplexing schemes such as frequency division multiplexing (FDM), Time division multiplexing (TDM).

Single user and Multi-user MIMO

Single user MIMO refers to a conventional MIMO where only one transmitting node and one receiving node, and the transmitter node has multiple antennas. In multiuser MIMO, mobile cellular users, each with a single antenna, transmit to a base station, and the base station processes the signals from each of the individual mobiles as if they were coming from multiple transmit antennas on a single node.

In this case, the base station performs the same operation as the receiver. So multiple mobile users can transmit the data over the same bandwidth, and the base station is able to decouple the individual data streams using spatial coding techniques.

In multiuser MIMO allows more cellular users to transmit simultaneously on the uplink path over the same bandwidth than would otherwise be possible.

Basic Block Diagram of MIMO System

Below figure shows the basic block diagram of MIMO systems. The information bits to be transmitted are encoded by using a conventional encoder. And that’s to be interleaved. The interleaved codeword is mapped to data symbols (Quadrate amplitude modulation symbols) by using a symbol mapper.

Basic Block Diagram of MIMO System
Basic Block Diagram of MIMO System

These data symbols are input to a space-time encoder that outputs, one or more spatial data streams. The spatial data streams are mapped to the transmit antennas by space-time pre-coding block.

The signals launched from the transmit antennas propagate through the channel and arrive at the receive antenna array. The receiver collects the signals at the output of each receive antenna element and reverses the transmitter operations in order to decode the data: receive space-time processing, followed by space-time decoding, symbol be mapping, deinterleaving, and decoding.

Advantages of MIMO

  • Multiple-in Multiple-out takes advantage of spatial multiplexing to increase wireless bandwidth and range.
  • MIMO algorithms send information out over two or more antennas and the information is received via multiple antennas as well.
  • MIMO systems provide a precise capacity gain over conventional single antenna RF systems, along with more reliable communication.

Disadvantages

The main disadvantage is only its complexity. Apart from this, it will provide precise output.

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