Massive MIMO
Beyond providing speedy cell phone service, 5G technology promises unprecedented new applications. Machine-to-machine communication and the Internet of Things continue to expand, competing with cell phone users for internet throughput. In fact, the Ericsson Mobility Report forecasts an increase in mobile network traffic by 77 percent by 2026, to a global level of 226 exabytes every month.
How can wireless communications technologies evolve to meet this ever-increasing demand for connectivity? The answer may reside with a technology known as massive multiple input, multiple output (MIMO).
Wireless communication depends upon electromagnetic waves carrying information from a transmitter located in a base station to one or more receivers. A base station contains three parts that work together to send and receive a wireless signal: A baseband unit determines which frequency to generate. A radio unit then generates the signal. And the base station antenna radiates the signal.
In current wireless technology, the signal propagates in all directions from the transmitter—often a cell phone tower—in a “spray.” The energy from the signal dissipates as distance increases. So the fixed location of the base station determines its coverage area, or cell.
Consumer demand for greater throughput and data speed necessitates technology that can boost network capacity. Increasing the number of antennas in the base station can address this demand. But the signals that each antenna emits can interfere with each other, causing noise. Noise means disruption in the performance of the network.
A technology that improves this ratio—minimizing noise while boosting signal strength—will improve the quality of connectivity for all users. Enter massive MIMO.
Movement from MIMO to Massive MIMO
MIMO is an antenna technology that focuses the energy of a signal specifically at a receiver. The technology involves complex mathematical algorithms that combine the strength of the radiation from multiple antennas.
Typically, a user is not standing directly in the path of a base station. The antenna emits a signal that must reflect off of various objects until it reaches the receiver, such as a person holding a 5G-equipped mobile phone. Traditional wireless technology involves trying to minimize the degradation of the signal as it bounces off of those various objects.
By contrast, MIMO technology features multiple antennas that use even more reflection paths. These paths combine to create a communication channel that carries even more information. However, increasing the number of antennas alone does not improve performance. Engineers must configure the antennas properly.
Spatial Streaming
With MIMO, multiple antennas carry multiple data streams over the same frequency. The transmission band subdivides into several different channels, or spaces, to carry the data.
The data sets, or spatial streams, remain separated despite travelling together. This technique, also called spatial multiplexing, maintains the integrity of the information. The receiver reconstructs the information mathematically and presents it to the end user in a way that makes sense.
As an analogy, a garden hose can deliver a certain amount of water to a spot of land. Two garden hoses double the amount of water. The water finishes at the same spot but takes two different, parallel channels to get there. Increasing the number of garden hoses has a direct influence on how much water the spot receives.
Beamforming
Spatial multiplexing works in conjunction with another technology particular to 5G: 3D beamforming. Rather than spreading a signal in all directions from an antenna, beamforming organizes the signals from various antennas and directs information straight to an end user. The more antennas that radiate a signal, the narrower and more powerful the beam.
Multiple Users
Paths do not have to originate from the same base station. Multiple antennas at multiple base stations can aggregate signals to meet the same user. Furthermore, multiuser MIMO communicates with multiple devices simultaneously instead of users having to wait their turn to access the network.
Move Toward Massive MIMO
Essentially, massive MIMO extends the capabilities of MIMO and has to do with the number of antennas involved with signal transmission and reception. There is no distinct classification of the number of antennas that delineate where MIMO leaves off and massive MIMO begins.
Antennas can organize in arrays, meaning multiple antennas connect and work together as a single antenna. Arrays greatly increase the amount of throughput—how much data transfers from source to destination in the system.
A 4G network takes advantage of MIMO in smaller multiple antenna arrays, say up to 8 × 8. But massive MIMO promises to live up to its moniker in the 5G ecosystem, with a massive MIMO antenna array as large as 256 × 256.
Signal Processing That Enables Massive MIMO
Conveniently, 5G technology can make use of a wide range of frequencies in wireless communication. Scientists measure these frequencies in hertz (Hz), referring to the number of wave cycles per second.
Sharing Frequencies with 4G
A 5G network can share infrastructure with a 4G LTE network in certain frequency bands. For example, T-Mobile claimed to deploy the first 5G network in the United States, based on its 600 MHz spectrum. These lower-frequency waves can penetrate objects and travel long distances. So low-band 600 MHz spectrum works effectively to provide coverage indoors and bandwidth over a wide area.
Low band, encompassing all frequencies less than 1 GHz, is one range of frequencies that a 4G and 5G network can share. The other range encompasses midband frequencies, generally between 1 GHz and 2.6 GHz. Engineers built much of the 4G LTE infrastructure with an eye toward converting to 5G networking.
Massive MIMO and Millimeter Wave Frequencies
But because existing low-band and midband spectrum already is reaching its physical limitations, the Federal Communications Commission has made available high-band frequencies. These frequencies, ranging upward from 24 GHz, are called millimeter wave (mmWave) bands. These extremely high frequencies deliver download speeds of multiple gigabits per second and can handle large numbers of users simultaneously.
But mmWave bands attenuate, meaning their power dissipates quickly over distance, a phenomenon in physics known as the inverse square law. The inverse square law states that as the receiver moves away from the transmitter, energy drops as a square of distance or as the square of frequency.
Thus, a 5G network operating on mmWave frequencies can benefit greatly from implementing massive MIMO. Without an antenna array to focus the energy, a base station would require massive amounts of power to transmit a viable signal. Massive MIMO provides a way to focus energy, analogous to a laser beam, right at a user.
Pros and Cons of Massive MIMO
The implementation of 5G massive MIMO involves an engineering trade-off. Multiple antennas do provide spectral efficiency, providing end users with greater bandwidth and speed. However, multiple antennas also require an increase in energy to power them.
Disadvantages of Massive MIMO
One disadvantage of massive MIMO involves this energy consumption. Calculating the trajectory of multiple signals both stationary and on the move requires highly complex computing, which carries an energy penalty.
Also, unless 5G massive MIMO is operating with mmWave frequencies, an antenna array can rapidly become large and unwieldy. The smaller wavelengths associated with mmWave bands allow a device to pack multiple antennas into a smaller area.
But longer wavelengths currently associated with 4G LTE would necessitate unwieldy, large devices to host antenna arrays. A typical vertical panel operating as an antenna in a contemporary cellular network stands around five feet tall and eight inches wide. A four-MIMO setup using low-band or midband frequencies would require four of such panels.
Another concern is cost. 5G Massive MIMO antennas are relatively expensive, which can drive up the cost of 5G deployment. The development of new materials and antenna designs using graphene and metamaterials may help reduce the size and cost of Massive MIMO in the future.
Advantages of Massive MIMO
Massive MIMO contributes to the capacity of the entire 5G network. Multiple antennas enhance throughput and improve signal strength. When parallel streams of information combine, the user experiences improved performance.
Furthermore, massive MIMO’s multiple antennas minimize the signal loss associated with attenuation. Massive MIMO solves the issue of poor coverage near a cell’s edge by transmitting a more powerful signal. Thus the end user’s experience becomes more uniform and stable when connecting to a massive MIMO 5G network.
Similarly, beamforming technology in 5G reduces noise and allows the signal to follow users on the move. The complex mathematics involved with integrating and focusing the signal provide excellent coverage, even if the 5G receiver is not stationery.
Massive MIMO and the Future of 5G Technology
A Global Industry Analysts report indicates that the global market for massive MIMO technologies could reach nearly $22 billion by 2027. The lightning speeds, capacity for greater throughput, and spectrum efficiency of massive MIMO will help power burgeoning digital technologies.
Ease of 5G Rollout
The Internet of Things, machine-to-machine communication, virtual reality applications, smart cities, and autonomous vehicles all rely on instantaneous, powerful wireless communication.
Rollouts in 5G mmWave technology benefit from massive MIMO. Massive antenna arrays intensify the signal, minimizing attenuation that a signal encounters at frequencies above 27 GHz.
Reducing this signal strength loss means that communications companies can deploy fewer sites. Rather than having a radio and antenna at every light pole, for example, massive MIMO can reduce deployments, depending on the strength of the antenna array.
Massive MIMO and Safety
And because of the nature of spatial streaming and beamforming, only a person with a 5G device receives the signal. Unlike current wireless systems, whose signals spray throughout the cell, massive MIMO technology focuses energy directly at 5G users. Little collateral radiation affects other people nearby.
A group of experts within IEEE known as the Committee on Man and Radiation (COMAR) studies health and safety issues related to electromagnetic fields. COMAR does not establish safety standards, but it monitors issues within a safety standard set by IEEE in 1991.
This standard, known as IEEE C95.1-2019, establishes safety levels for human exposure to radio frequency electromagnetic fields. COMAR has determined there is nothing inherently dangerous about 5G or massive MIMO technology. Rather, any potential danger lies in intensity and length of time of exposure, much like standing in front of a heat lamp or frequency modulation transmitter.
Environmental Impact
Because massive MIMO relies on beamforming, its use minimizes stray radiation in the environment. Beamforming focuses the signal in a specific direction: think laser pointer instead of flashlight. Thus, massive MIMO actually reduces the amount of radiated energy that dissipates into the environment.
Massive MIMO arrays also improve connectivity while simultaneously requiring less power consumption than other technologies, as studies have shown. For example, a 2018 Ghent University study explored the characteristics of a 256-antenna array researchers built. The study found that their massive MIMO base station provided two hundred times more capacity than a 4G reference network for the same coverage, while consuming eight times less power.
Other researchers have found similar energy efficiency models using massive MIMO. During low-traffic loads, parts of the antenna array can switch off, saving power and increasing efficiency.
Massive MIMO and the Future of Connectivity
Find out more about the future of 5G, massive MIMO deployment, and other burgeoning 5G technologies by exploring IEEE’s 5G initiative.
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