MIMO Techniques

MIMO technology enhances communications systems by simultaneously transmitting and receiving several data signals over the same radio channel. Multiple antennas at both the transmitter and receiver ends significantly improve data throughput and system performance in MIMO systems that serve a single user or multiple users.

For example, an SU-MIMO system or an MU-MIMO system can transmit a single stream or multiple streams of data and precode the different data streams for reception by specific users.

With two transmit antennas at the base station and two receive antennas at the mobile station, a 2-by-2 SU-MIMO system transmits two different data streams to a single user.
With three transmit antennas at the base station and a total of three receive antennas on the mobile stations, the 3-by-3 MU-MIMO system transmits three different data streams to multiple users. Two of the data steams are precoded for reception by the first mobile station and the third data stream is precoded for reception by the second mobile station.

SU-MIMO and MU-MIMO designs implement spatial dimension techniques to realize MIMO gains in multiple wireless systems, including those that follow 5G NR, LTE, and Wi-Fi^® standards. For background material on the subject of MIMO systems, see References.

Wireless system standards and MIMO research, in general, support MIMO techniques in a variety of combinations:

These types of MIMO techniques are fundamental for improving signal reception reliability and throughput in a MIMO channel:

Technique	How It Works	Benefit
Spatial Multiplexing	Spatial multiplexing increases the data rate by transmitting different data streams simultaneously from multiple transmit antennas to multiple receive antennas. When channel conditions are known at the transmitter, MIMO enables simultaneous data transmissions using the same time-frequency resources to transmit the signal using precoding.	Successful reception of the different data streams increases the link capacity.
Beamforming	Beamforming uses multiple antennas at either or both the transmitter and receiver to form directional beams that focus the energy toward the intended receiver, rather than scattering it in all directions. In systems with more transmit antennas than receive antennas, MIMO can improve performance by using beamforming to focus transmit energy towards the receive antennas.	The focused energy increases SNR, which improves link performance.
Spatial Diversity	Spatial diversity transmits the same data stream from multiple antennas but with some form of coding or time delay to ensure that the signals are distinguishable at the receiver.	The diversity increases SNR, which improves link performance.

Technique

How It Works

Benefit

Spatial Multiplexing

Spatial multiplexing increases the data rate by transmitting different data streams simultaneously from multiple transmit antennas to multiple receive antennas.

When channel conditions are known at the transmitter, MIMO enables simultaneous data transmissions using the same time-frequency resources to transmit the signal using precoding.

Successful reception of the different data streams increases the link capacity.

Beamforming

Beamforming uses multiple antennas at either or both the transmitter and receiver to form directional beams that focus the energy toward the intended receiver, rather than scattering it in all directions.

In systems with more transmit antennas than receive antennas, MIMO can improve performance by using beamforming to focus transmit energy towards the receive antennas.

The focused energy increases SNR, which improves link performance.

Spatial Diversity

Spatial diversity transmits the same data stream from multiple antennas but with some form of coding or time delay to ensure that the signals are distinguishable at the receiver.

The diversity increases SNR, which improves link performance.

These terms have different meanings in the various standards and in literature. For example, some standards interchange precoding and beamforming. In Communications Toolbox™, precoding describes spatial separation using a smaller number of antennas, while beamforming describes spatial steering using a large array of antennas. For more information, see MIMO Terminology.

MIMO Functionality

You can model MIMO techniques using these functionalities from Communications Toolbox and other MathWorks^® products:

Functionality	Functionality
Stream Mapping	See 5G Toolbox™ and WLAN Toolbox™
Precoding	`blkdiagbfweights` `ofdmPrecode` `comm.OSTBCEncoder` OSTBC Encoder
OFDM Modulation	`ofdmmod` `ofdmdemod` OFDM Modulator Baseband OFDM Demodulator Baseband
Beamforming	See Phased Array System Toolbox™
Channel	`ofdmChannelResponse` `comm.MIMOChannel` MIMO Fading Channel OFDM Channel Response
Reception	`ofdmEqualize` OFDM Equalizer `comm.OSTBCCombiner` OSTBC Combiner `comm.SphereDecoder` Sphere Decoder

The MIMO category links to examples that demonstrate MIMO system designs using functionality from Communications Toolbox, 5G Toolbox, WLAN Toolbox, Phased Array System Toolbox, and Antenna Toolbox™.

References

[1] Molisch, Andreas F., Wireless Communications: From Fundamentals to beyond 5G. Third edition, Wiley-IEEE Press, 2023.

[2] George Tsoulos, Ed., "MIMO System Technology for Wireless Communications", CRC Press, Boca Raton, FL, 2006.

[3] Oestges, Claude, and Bruno Clerckx., MIMO Wireless Communications: From Real-World Propagation to Space-Time Code Design. 1st ed. Boston, MA: Elsevier, 2007.

[4] Correia, Luis M., and European Cooperation in the Field of Scientific and Technical Research (Organization), eds. Mobile Broadband Multimedia Networks: Techniques, Models and Tools for 4G. 1st ed. Amsterdam; Boston: Elsevier/Academic Press, 2006.

[5] M. Jankiraman, "Space-time codes and MIMO systems," Artech House, Boston, 2004.

[6] G. J. Foschini, M. J. Gans, "On the limits of wireless communications in a fading environment when using multiple antennas," IEEE Wireless Personal Communications, Vol. 6, Mar. 1998, pp. 311–335.

[7] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE Journal on Selected Areas in Communications, vol. 16, no. 8, pp. 1451–1458, Oct. 1998.

[8] V. Tarokh, H. Jafarkhani, and A. R. Calderbank, “Space-time block codes from orthogonal designs,” IEEE Transactions on Information Theory, vol. 45, no. 5, pp. 1456–1467, Jul. 1999.

[9] Hochwald, B.M., and S. Ten Brink., “Achieving Near-Capacity on a Multiple-Antenna Channel.” IEEE Transactions on Communications 51, no. 3 (March 2003): 389–99. https://doi.org/10.1109/TCOMM.2003.809789.

[10] 3GPP TS 38.201. “NR; Physical layer; General description.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[11] IEEE^® P 802.11be™/D5.0. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 8: Enhancements for Extremely High Throughput (EHT).” Draft Standard for Information Technology — Telecommunications and Information Exchange between Systems — Local and Metropolitan Area Networks — Specific Requirements.

MIMO Techniques

MIMO Functionality

References

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