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If you aren’t a researcher within wireless networking, you might have heard the acronym MIMO but you likely do not know much about this technology and its applications. This technology is now a fundamental component of wireless communications. In this article we examine its origins, present use, and potential evolution in service of a digitally inclusive future.

What is MIMO?

MIMO stands for “multiple input – multiple output,” meaning that there are several inputs to the communication system and several outputs. When we are dealing with wireless communications, these inputs and outputs are the antennas at the transmitter and receiver. MIMO is often referred to as “multi-antenna techniques.”

In cellular communications, transmitting and receiving operations are traditionally performed at the base station and mobile phone. The traditional resources that enable wireless transmission are frequency and time. With the adoption of MIMO, we are able to transmit and receive information using a new resource: the different antennas. 

By incorporating MIMO technology, systems are able to obtain a more robust transmission system through diversity or to increase the data rate through multiplexing.

Diversity means having several choices to choose or combine: To exploit multi-antenna diversity, we transmit redundant signals through different paths (different transmit and receive antennas in this case) in such a way that when they are combined or one of them is selected, it is more likely that the joint or chosen signal will bear a better quality. 

Multiplexing means transmitting via several information channels at the same time. Thanks to the availability of several antennas, we can transmit and receive several data streams in parallel, without spending additional resources such as time or frequency. They can be sent to the same user, increasing the data rate, or addressed to different users.

As multiple signals get combined at the receiving antennas, their power is added and the system is able to optimize the energy use even if the propagation causes a serious decrease of the received power. 

However, just placing several antennas is not enough. As we see in Figure 1, when information is exchanged at the same time and using the same frequency among several antennas, it becomes combined, producing what we call spatial interference and impacting the data’s integrity. Therefore, signal processing techniques must be performed in the transmitter and/or receiver in order to compensate for this interference. That is what we call MIMO processing.

Since its invention [1, 2], MIMO was quickly embraced by the research community as a very promising technique. However, it took years until it made its way through to WiFi and 4G cellular standards thanks to its successful combination with Orthogonal Frequency Division Multiplexing (OFDM).

The present and future of MIMO

The advantages of MIMO increase as the number of antennas increases, thus leading to the concept of massive MIMO[3]. While 4G standards were initially contemplating a maximum of eight antennas at either side of the communication link, massive MIMO means increasing this amount towards the hundreds. Even though there have been some proposals such as using textile antennas [4], it is much easier to implement this volume of antennas in the base station, in particular if higher frequencies are used, which entail smaller size of the antennas. The current massive MIMO paradigm being deployed in 4G and 5G cellular systems means using a large number of antennas at the base stations to simultaneously serve a multiplicity of users whose mobile phones are equipped with a more conventional (reduced) number of antennas.

Even at the base stations, it is not easy to place many antennas at the same location. Besides the problem of finding enough space, it entails some added hardware complexity since each antenna is supported by additional circuitry (such as amplifier, digital-to-analog and analog-to-digital converter, or frequency conversion). 

Another avenue to solving the added hardware complexity is distributed massive MIMO, where the massive number of antennas required are distributed across the system rather than concentrated in one location. One option that may enable such an easy and flexible deployment is the new concept of radio stripes, or small antennas connected over a stripe [5]. This is a concept currently being pushed by Ericsson, aiming to create antennas so small and flexible that they can be integrated into adhesive tape and distributed over large areas to provide ubiquitous coverage. 

Another option is cell-free massive MIMO [6], wherein a large number of distributed, low-cost and low-power antennas cooperate through a centralized controller to serve a smaller number of users. Each of them may be served by all or by a subset of the antennas denoted as “user centric.” In any case, the concept of cell disappears, since a particular user is no longer attached to a single cell tower but rather connected to many (probably smaller) access points. As a consequence, the link quality is more evenly distributed among all users because they will always have some access point that is close enough to deliver a good signal, as opposed to some users being too far away from the cell tower in a classical system (often denoted as cell-edge users). Figure 2 illustrates the classical cellular and new approach.

Today there is an increasing interest in using high frequencies for mobile communications, as high as billions of Hertz (GigaHertz) and even higher (TeraHertz). It turns out that the higher the carrier frequency, the larger the bandwidth that can be used for communications, which translates to higher data rates and/or higher number of users benefiting from these communications. The available bandwidth at the conventionally used lower frequencies is smaller and they get easily overloaded. However, it is not as simple as evolving equipment to support higher frequencies. These high frequencies are characterized by very high propagation loss that causes the received power to decrease as we move away from the antenna. Here massive MIMO could be the solution to the attenuation problem at high frequencies.

As MIMO technology continues to develop, future applications have the potential to improve the efficiency, capacity, and flexibility of wireless communications. An interesting new concept is reconfigurable intelligent surfaces (RIS) [7] made of small elements that can be put together on a wall or a glass window to reflect the signal to where the user is, avoiding obstacles and improving the coverage.

Until now MIMO techniques have provided solutions to many challenges, improving our ability to communicate. The Marconi Society is hosting the Discussion of the Decade, the first-ever decadal survey of all fields within Information and Communications Technologies (ICT) to identify the problems whose solutions will create the greatest impact at microscopic and global scales. Surely MIMO techniques will have an important role in solving some of these challenges.

MIMO and Digital Inclusion

45% of the world’s households are unconnected and even more are under-connected. What role can MIMO play in improving access to connectivity? The current focus has been to use MIMO technologies to increase the capacity in high-traffic networks and developed markets. However, there are several interesting aspects in this technology that we would like to highlight.

MIMO has the potential to uniformize and enlarge coverage. It is a key component of the 5G fixed-wireless-access deployments. To improve access to remote areas, MIMO can also be implemented to increase the capacity and range of the wireless backhaul links, replacing fiber rollouts where it is not geographically possible or affordable.

The efficiency brought by MIMO may allow spectrum in the lower bands, with better propagation characteristics, to be spared for rural and underserved areas.

In combination with the open radio access network (ORAN) paradigm, distributed versions of MIMO and massive MIMO are becoming very affordable in terms of deployment and operation costs. The question is, will these CAPEX savings be leveraged to connect the unconnected?

We have mainly focused on the evolution of cellular communications, but many researchers predict that MIMO techniques will play an important role in the evolution of satellite communications and their possibilities to connect the next billion. An interesting discussion will take place during the 6G Summit on Connecting the Unconnected, a series of virtual sessions that are part of the Marconi Society’s upcoming master class and symposium, The Decade of Digital Inclusion.

REFERENCES

[1] A. Paulraj and C. B. Papadias, “Space-time processing for wireless communications,” IEEE Signal Processing Magazine, vol. 14, no. 6, pp. 49–83, November 1997.

[2] Gerard. J. Foschini, “Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multi-Element Antennas,” Bell Labs Technical Journal, vol. 1, no. 2, pp. 41–59, October 1996.

[3] Erik G Larsson, Ove Edfors, Fredrik Tufvesson, and Thomas L Marzetta, “Massive MIMO for next generation wireless systems,” IEEE Communications Magazine, vol. 52, no. 2, pp. 186–195, February 2014.

[4] Matilde Sanchez-Fernandez, Antonia Tulino, Eva Rajo-Iglesias, Jaime Llorca, and Ana Garcia Armada, “Blended Antenna Wearables for an Unconstrained Mobile Experience,” IEEE Communications Magazine, vol. 55, no. 4, pp. 160–168, April 2017.

[5] “Radio Stripes: re-thinking mobile networks”, available online: https://www.ericsson.com/en/blog/2019/2/radio-stripes 

[6] Hien Quoc Ngo, Alexei Ashikhmin, Hong Yang, Erik G. Larsson, Thomas L. Marzetta, “Cell-Free Massive MIMO Versus Small Cells,” IEEE Transactions on Wireless Communications, vol. 16, no. 3, pp. 1834-1850, March 2017.

[7] C. Liaskos, S. Nie, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. Akyildiz, “A New Wireless Communication Paradigm through Software-Controlled Metasurfaces,” IEEE Communications Magazine, vol. 56, no. 9, pp. 162–169, September 2018.