What Does Multiple-In/Multiple-Out (MIMO) Mean?
Multiple-In/Multiple-Out (MIMO) references multiple transmission and reception antennas for enhanced wireless communications performance, such as data throughput. MIMO uses multiplexing techniques to increase wireless bandwidth and range. Input and output refer to the radio channel, which carries the signal.
MIMO is a key component of wireless technology and communication standards, such as IEEE 802.11n (Wi-Fi), Fourth Generation Wireless (4G), Third Generation Partnership Project (3GPP), Long Term Evolution (LTE), and Worldwide Interoperability for Microwave Access (WiMAX).
MIMO is also known as Multiple-Input/Multiple-Output.
Techopedia Explains Multiple-In/Multiple-Out (MIMO)
MIMO technologies were first explored in the early-mid 1970s. In the mid-1980s, scientists published papers on beamforming, a related precursor technology. Spatial multiplexing, a MIMO technique for multiple signal transmission, was proposed by Arogyaswami Paulraj and Thomas Kailath in 1993, and their 1994 patent emphasized wireless broadcast application. The multiple antenna concept was explored in 1996. In 1998, Bell Laboratories was the first to prove that MIMO technology performance is improved by spatial multiplexing.
MIMO utilizes reflective signals from one or multiple objects after transmission and prior to receipt. Antennas and antenna system designs encourage signals to follow multiple paths. Although these signals are the last to arrive at the receiving antennas and experience the most attenuation from absorption by objects, diffusion, and other factors, they combine with and complement the receiver's strongest straight line signals. At the receiver, special algorithms receive, correlate, and recombine the signals, which increases signal strength significantly, while reducing signal fading. Known as higher spectral efficiency, this process results in a higher number of data bits transferred per second at a bandwidth rate per Hz or cycle per second (CPC).
IEEE 802.11n uses MIMO for the Wi-Fi technology, which creates a theoretical 108 Mbps throughput. The earlier IEEE 802.11g technology only produced 54 Mbps without the benefit of MIMO. Two transmitters double the data rate and two or more receivers allow greater distances between transmitters and receivers.
MIMO has three main categories as follows:
- Precoding: Adjusts all available signal phases and gains for stronger signal strength at the receiver.
- Spatial multiplexing: Requires highly complex signal receivers, employing either Orthogonal Frequency-Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) modulation.
- Diversity coding: Used when there is no way to determine signal propagation through the air. A single data stream uses space-time coding to enhance transmitted signal reliability, due to data redundancy at the receiver.