The MIMO advantage

By Russell J. Hoppenstein, applications manager, wireless infrastructure, Texas Instruments

Multiple input, multiple output (MIMO) refers to systems using multiple antennas (and correspondingly multiple channels) to transmit and receive information. MIMO is a form of spatial diversity, which in itself is nothing new. Spatial diversity schemes to improve channel performance have been used for decades, specifically to mitigate troubling issues of fading in urban environments. Modern MIMO techniques take advantage of increased signal-processing power and integrated transceivers to not only improve channel reliability but increase data rates as well.

MIMO basics
Multipath fading plagues many modern communication systems operating in indoor locations and urban outdoor environments. Multiple propagation paths from one or more reflections in the environment cause fading to occur when radiated signals reach the receiver at different points in time. Multipath signals may interfere in a destructive way, contributing to the loss of channel information.

A MIMO system uses multiple transmitters and receivers to achieve system improvements through spatial diversity and spatial multiplexing. Fig. 1  depicts a 3 × 3-channel MIMO system with multiple transmit and receive antennas. Each transmit antenna broadcasts the signal, and each receive antenna picks up the signal from each transmitter antenna.

Fig. 1: Basic MIMO setup

The parameter h _ij   denotes the channel characteristics between an individual transmit antenna and receive antenna; it includes effects for propagation loss, delay, and multipath interference.

MIMO’s ancestors
Telecommunication systems previously used diversity techniques, but their definition takes a different moniker when using MIMO jargon. A system with a single transmitter and multiple receivers (usually limited to two), traditionally called a diversity receiver, is labeled as multiple input, single output (MISO). Older-generation base stations and other communication systems used a receiver diversity scheme to improve channel performance in which spatially separated antennas received signals from a single source impacted by fading and/or interference, and the receiver demodulated the signal and chose the best one.

The probability of the signal destructively interfering at two locations simultaneously is very low. In other words, if a signal destructively interferes at one antenna’s location due to multipath fading, it is unlikely that it will also destructively interfere at another antenna’s location.

The diversity receiver application is useful for base stations that can support multiple receivers and separated antennas. It is not practical for mobile users, wherein there is not sufficient real estate for separated antennas nor space and power allocation for multiple receivers.

To allow mobile receivers to leverage the benefits of diversity, the onus is put back on the base station transmitter. A transmitter diversity scheme employs two or more transmitters broadcasting from antennas that are spatially separated. This is called single input, multiple output (SIMO). Table 1  summarizes the antenna combinations.¹

Table 1: Antenna diversity schemes

The traditional diversity scheme simply chooses the best signal at its disposal. This is a viable technique but requires twice as many transmitters and receivers for the benefit of only one. A more sophisticated system combines the demodulated signal to take advantage of multiple received signals. Ultimately, this extends the reach of the cell by effectively increasing the signal strength while mitigating the detrimental effects of fading.

The MIMO advantage
MIMO does not simply offer additional signal paths to get around detrimental fading; it also provides spatial multiplexing to increase transfer data rates. To increase a channel’s data rate, there are a few options at your disposal. One simple option is to increase the bandwidth.

Modern telecommunications applications in a 4G wireless infrastructure use this option with wider bandwidth signals or multi-carrier signals. Emerging 5G systems seek out open spectrum to use bandwidths as wide as 2 GHz. Widening the bandwidth is always an option, but spectrum is precious, and there is a limit to the space available for any given carrier.

Spatial multiplexing breaks up information into parts and transmits the different signals across different transmitters. The receivers capture information from multiple transmitters and piece the information back together again. The system uses the same frequency to transmit information and relies on different propagation paths to separate information. This technique, in theory, increases the data rate by the (minimum) number of transmit and receive antennas.² Fig. 2  illustrates how the capacity improves.

Fig. 2: Spatial multiplexing

A MIMO system requires some knowledge of the channel characteristics; it uses this information to push more data through favorable channels (those with the best signal-to-noise ratios) and consequently less through sub-par channels (those with the worst noise and/or interference). This technique is colloquially called water-filling.^3,4 Picture multiple water pails of varying capacity representing the quality of the channel. For the best data rates, the largest pails are filled to their maximum before the overflow is routed to the smaller and smaller pails.

Fig. 3 represents how this technique maximizes the capacity of the system by using every ounce of a channel’s capability at any given time. The power per channel must meet the power constraints, meaning that the total overall power of each element cannot exceed the total power available. If the state of the channel is unknown, then the water-filling technique is not applicable. The system defaults back to a uniform distribution of power where each transmitter gets the same amount of information.

Fig. 3: Water-filling technique

Conclusion

A MIMO system is a viable solution for increasing capacity through spatial multiplexing. It is a system that uses the existing spectrum without resorting to increasing the bandwidth to improve performance. Spatial duplexing breaks up information in time and broadcasts it simultaneously across the available transmit channels. With proper channel characterization, a MIMO system can adapt to environmental changes to maximize data throughput by using the cleanest channel at any given time. In the next installment of this series, I’ll explore the mechanism of increasing capacity and mitigating fading with MIMO.

Watch for Parts 2 (The MIMO advantage, Part 2: expandable capacity in MIMO systems) and three (The MIMO advantage, Part 3: advanced MIMO applications) in the upcoming weeks.

References
¹ Akhilesh Kumar, Abhay Mukhetjee, Kamta Nath Mishra and Anil Kumar Chaudhary, “Signal processing and channel capacity enhancement using MIMO Technology,” 2012 International Conference on Devices, Circuits and Systems, March 2012.

² Jaimal Soni (August 2013), “Capacity Characterization of a MIMO-OFDM Wireless Channel with BLAST Implementation.” Retrieved from https://uwspace.uwaterloo.ca/handle/10012/5426.

³ Yang Wen Liang (January 2005), “Ergodic and Outage Capacity of Narrowband MIMO Gaussian Channels.” Retrieved from www.ece.ubc.ca/~yangl/mimo_cap/mimo.pdf.

⁴ Chun-Hung Liu (March 2017), “The Capacity of Wireless Channels.” Retrieved from https://www.coursehero.com/file/21858073/Lecture2.