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MIMO
Multiple-input and multiple-output (MIMO) (/ˈmaɪmoʊ, ˈmiːmoʊ/) 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 (802.16 e, m), 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 (wider angle separation) is required at either the transmit array, the receive array, or at both.
MIMO is often traced back to 1970s research papers concerning multi-channel digital transmission systems and interference (crosstalk) between wire pairs in a cable bundle: AR Kaye and DA George (1970), Branderburg and Wyner (1974), and W. van Etten (1975, 1976). 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 (SDMA) 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 (No. 5515378 issued in 1996) describes a method for increasing capacity using "an array of receiving antennas at the base station" with a "plurality of remote users."
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 (Iospan Wireless, and Beceem Communications) to commercialize MIMO for mobile networks.
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MIMO
Multiple-input and multiple-output (MIMO) (/ˈmaɪmoʊ, ˈmiːmoʊ/) 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 (802.16 e, m), 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 (wider angle separation) is required at either the transmit array, the receive array, or at both.
MIMO is often traced back to 1970s research papers concerning multi-channel digital transmission systems and interference (crosstalk) between wire pairs in a cable bundle: AR Kaye and DA George (1970), Branderburg and Wyner (1974), and W. van Etten (1975, 1976). 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 (SDMA) 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 (No. 5515378 issued in 1996) describes a method for increasing capacity using "an array of receiving antennas at the base station" with a "plurality of remote users."
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 (Iospan Wireless, and Beceem Communications) to commercialize MIMO for mobile networks.