MIMO
Multiple-input and multiple-output is a wireless technology that multiplies the capacity of a radio link using multiple transmit and receive antennas. MIMO has become a core technology for broadband wireless communications, including mobile standards—4G WiMAX, and 3GPP 4G LTE and 5G NR, as well as Wi-Fi standards, IEEE 802.11n, ac, and ax.
MIMO uses the spatial dimension to increase link capacity. The technology requires multiple antennas at both the transmitter and receiver, along with associated signal processing, to deliver data rate speedups roughly proportional to the number of antennas at each end.
MIMO starts with a high-rate data stream, which is de-multiplexed into multiple, lower-rate streams. Each of these streams is then modulated and transmitted in parallel with different coding from the transmit antennas, with all streams in the same frequency channel. These co-channel, mutually interfering streams arrive at the receiver's antenna array, each having a different spatial signature—gain phase pattern at the receiver’s antennas. These distinct array signatures allow the receiver to separate these co-channel streams, demodulate them, and re-multiplex them to reconstruct the original high-rate data stream. This process is sometimes referred to as spatial multiplexing.
The key to MIMO is the sufficient differences in the spatial signatures of the different streams to enable their separation. This is achieved through a combination of angle spread of the multipaths and sufficient spacing between antenna elements. In environments with a rich multipath and high angle spread, common in cellular and Wi-Fi deployments, an antenna element spacing at each end of just a few wavelengths can suffice. However, in the absence of significant multipath spread, larger element spacing is required at either the transmit array, the receive array, or at both.
History
Early research in multiple antennas
MIMO is often traced back to 1970s research papers concerning multi-channel digital transmission systems and interference between wire pairs in a cable bundle: AR Kaye and DA George, Branderburg and Wyner, and W. van Etten. Although these are not examples of exploiting multipath propagation to send multiple information streams, some of the mathematical techniques for dealing with mutual interference proved useful to MIMO development. In the mid-1980s Jack Salz at Bell Laboratories took this research a step further, investigating multi-user systems operating over "mutually cross-coupled linear networks with additive noise sources" such as time-division multiplexing and dually-polarized radio systems.Methods were developed to improve the performance of cellular radio networks and enable more aggressive frequency reuse in the early 1990s. Space-division multiple access uses directional or smart antennas to communicate on the same frequency with users in different locations within range of the same base station. An SDMA system was proposed by Richard Roy and Björn Ottersten, researchers at ArrayComm, in 1991. Their US patent describes a method for increasing capacity using "an array of receiving antennas at the base station" with a "plurality of remote users."
MIMO invention
In December 1991, while working on a DARPA project involving signal separation algorithms at Stanford University, Arogyaswami Paulraj discovered that signals from two phones held in one hand could be separated using a three-element receive antenna array in a rich multipath environment. This discovery led to the foundational patent on MIMO, filed in February 1992 with Professor Thomas Kailath as a co-inventor. The patent proposed a method for increasing data rates on MIMO links in proportion to the number of antennas used.While Paulraj’s patent initially emphasized applications in broadcast TV, which he believed would be an early adopter of the technology, it also proposed broader uses for MIMO in cellular communications. Paulraj joined Stanford faculty in 1993, where he built a research group on MIMO. Later in 1998 and 2004, he founded two startups to commercialize MIMO for mobile networks.
Paulraj has received many recognitions for his work. These include the Royal Academy of Engineering Prince Philip Medal, the Institution of Engineering and Technology Faraday Medal, the IEEE Alexander G. Bell Medal, the Marconi Prize, and induction into the U.S. Patent and Trademark Office's National Inventors Hall of Fame.
MIMO advancements
In 1995, G. Foschini and Michael Gans of Bell Labs wrote influential papers on MIMO wireless capacity and proposed the BLAST scheme to layer MIMO data streams and maximize channel capacity. Foschini received the IEEE Alexander Graham Bell Medal.Many other key publications followed, significantly advancing the field: G. Raleigh and V. Jones introduced space-time methods. E. Telatar established the fundamental capacity limits of MIMO channels. S. Alamouti developed a simple but effective transmit diversity scheme that has been widely adopted. R. Calderbank et al. made crucial contributions to the development of space-time codes. H. Sampath et al. described the first MIMO-OFDM cellular system developed by Iospan Wireless. R. Heath advanced the areas of limited feedback and multi-user MIMO systems.
A torrent of research has followed, and as of 2024, there are over 450,000 research publications on MIMO technology and more than 570,000 global patent publications referencing MIMO or its related techniques.
MIMO commercialization
Mobile networks
Iospan Wireless in late 1998 to develop a MIMO-OFDM physical layer based cellular system was Iospan Wireless in late 1998). Iospan’s product consisted of a core network, base stations, and CPE terminals. Airburst did not initially support mobile handovers. The system was trialed in Santa Clara during 2000-2002 and underwent a customer trial in Dubai in 2002. Following the 2001 collapse of the Dot-Com bubble, Iospan could not raise additional venture funding and was acquired by Intel in 2003. Intel integrated Iospan’s MIMO-OFDM technology into the WiMAX broadband mobile standard, IEEE 802.16e standard in 2004.In the early 2000s, several semiconductor companies also entered the MIMO-OFDM-based WiMAX technology market. They included Sequans, Samsung, Intel, Alvarion, and Beceem Communications, who developed modem semiconductors for WiMAX phones. Beceem gained 65% share of the global market, and was acquired by Broadcom Corp.
The 3rd Generation Partnership Project standards body adopted MIMO for HSPA+ in 20XX and MIMO-OFDM based 4G Long Term Evolution in 2008. MIMO-OFDM has since remained the core technology since 2008 for mobile networks, including 5G NR.
5G added native support for MU-MIMO.
WiFi networks
In the early 2000s, several companies—Atheros, Cisco, Broadcom, Intel, and Airgo Networks—entered the MIMO‑OFDM Wi‑Fi semiconductor market. Due to competing proposals within the IEEE 802.11, the first MIMO‑OFDM Wi‑Fi standard was not finalized until 2009. Several pre-standard products were developed, but market grew only after the 802.11n standard was ratified. Airgo Networks was acquired by Qualcomm in December 2006, and Atheros was also acquired by Qualcomm in May 2011. Sequans did an IPO in 2011 and Alviron filed for bankruptcy in 2013.Wi-Fi 6 added native support for MU-MIMO.
MIMO economic impact
Currently, 4G/5G and Wi-Fi powered by MIMO enable approximately 70% of internet-based services, accounting for 10% of global GDP. The GSMA industry alliance estimated the global economic value of mobile networks at $5.7 trillion, and the WiFi alliance estimated the corresponding value for WiFi networks at $3.5 trillion in 2023.Functions
MIMO can be sub-divided into three main categories: precoding, spatial multiplexing, and diversity coding.Precoding is multi-stream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. In beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase and gain weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal gain – by making signals emitted from different antennas add up constructively – and to reduce the multipath fading effect. In line-of-sight propagation, beamforming results in a well-defined directional pattern. However, conventional beams are not a good analogy in cellular networks, which are mainly characterized by multipath propagation. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding with multiple streams is often beneficial. Precoding requires knowledge of channel state information at the transmitter and the receiver.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, 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. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios. The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. Spatial multiplexing can be used without CSI at the transmitter, but can be combined with precoding if CSI is available. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers, known as space-division multiple access or multi-user MIMO, in which case CSI is required at the transmitter. The scheduling of receivers with different spatial signatures allows good separability.
Diversity coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods, a single stream is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding. Diversity coding can be combined with spatial multiplexing when some channel knowledge is available at the receiver.