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Massive MIMO, millimeter waves, and small cell infrastructure will pay off for future tech generations
A worker rebuilds a cellular tower with 5G equipment in Orem, Utah.
This is a guest post. The views expressed here are solely those of the author and do not represent positions of IEEE Spectrum or the IEEE.
Much of 5G's buzz centers on the potential it offers society: Lightning-fast downloads, barely-there latencies, and new and improved technologies like virtual reality and self-driving cars. Behind the buzz, however, one amazing fact is all but disregarded—the fundamental technological breakthroughs of 5G. Millimeter wave technology, small cell densification, and massive multiple-input multiple-output (massive-MIMO) antenna systems are paving the way for the next several decades of the wireless industry. Together, these technologies will futureproof wireless networks as we enter an era of wireless cognition and human-style computing. In fewer than 20 years, wireless networks will carry information at the speed of the human brain.
To fully appreciate just how revolutionary the 5G era will be, and the impact it will have on the wireless industry going forward, let's first consider the past 10 years. Over that period of time, the growth in data capacity and data consumption over the global cellular network has far outstripped Cooper's law. Coined by engineer Martin Cooper, the 'law' originally suggested that cellular telephone links would double in capacity about every 30 months (a factor of 16 each decade).
It's true that average download speeds have increased in obedience with Cooper's law from a few megabits per second in 2010 to about 50 Mbps in 2020. However, peak throughput data rates over the same time frame have scaled by more than a factor of 1000, from megabits per second to gigabits per second. In fact, the wireless industry trade association CTIA has shown that the total capacity carried over the US cellular network increased a staggering 96 times from 2010 to 2019, with the average smartphone user consuming 9.2 gigabytes of data per month in 2019.
What is astounding is that this nearly hundredfold increase in carried capacity happened, in the United States at least, with only 40 percent more available spectrum in the past 10 years as consumers embraced smart phones. What's more, it happened before the industry's adoption of the key 5G enablers: small cells, millimeter wave spectrum, and M-MIMO antenna technology. The vast spectrum resources available at frequencies above 100 GHz promise many orders of magnitude of additional spectrum in the U.S. alone in the decades to come.
Just like how Moore's law brought millions of times more processing power over four decades, the three technological pillars of 5G will unleash an exponential expansion that will bring vast new capacity and use cases in the coming decades. 5G will support average and peak data rate increases for each user, as well as carried traffic across the network, such that in ten years from now, industry metrics will surely be one hundred times today's levels—and truthfully, most likely closer to two or three hundred times. This means that the average smart phone user in 2031 is likely to consume more than a terabyte per month, and the typical 2 to 3 Gbps peak wireless download speeds in today's nascent 5G networks will in 2031 approach peak data rates of 1 terabit per second.
The Federal Communications Commission (FCC) saw the clear need for legislation that would enable wireless carriers to rapidly densify the network throughout the United States. The agency's 2018 Small Cells Order made small cells one of the three key technical pillars that will eventually support future terabit per second wireless communications.
The FCC also saw the need and potential of spectrum above 95 GHz when it opened that spectrum up through its Spectrum Horizons order in 2019. In England, the Office of Communications (Ofcom) followed the FCC's lead in opening up spectrum above 100 GHz for the first time in 2020, as well as pursuing more available spectrum in this sub-terahertz region.
Lastly, the third technology pillar for 5G, massive-MIMO, will shift base station antennas from having 2-by-2 antenna elements to 16-by-16 arrays and eventually 64-by-64 arrays and beyond, greatly multiplying a single base station's capacity. Carriers around the world are already deploying massive-MIMO in mid-band spectrum using time-division duplexing (which simply means that the signals between a base station and user share the exact same frequency but are spaced out in time so as to not collide with one another). The technology will find its way into millimeter wave and sub-terahertz wireless systems in the coming years.
Graduate students (from left) Shihao Ju, Ojas Kanhere, and Yunchou Xing (seated) take rooftop measurements of a 142 GHz frequency signal at NYU Wireless' Brooklyn campus.NYU Wireless
And here's the reason all of this really matters: Contrary to popular belief, wideband data transfers perform better at millimeter wave and terahertz frequencies than at the lower frequencies used in the first four generations of cellular technology. At NYU Wireless, we showed that, between sub-6 GHz and 140 GHz frequencies, the propagation path loss for an urban radio channel doesn't really differ at different frequencies, after accounting for the radiated signal's first meter of travel. This means that once a radio signal reaches what's called the "far field" (in other words, beyond the first meter or so), the frequency has surprising little impact on a signal's attenuation as it travels through urban and indoor channels—barring inclement atmospheric conditions like rain, or frequencies that are liable to be absorbed by molecular resonances, like for oxygen. This also means that the extensive work to densify cell sites today will pay a huge dividend in future networks that operate above 100 GHz. Networks won't likely require further densification, and today's new tower sites for 5G will be usable for decades to come without the need to build many more.
The wireless industry groupthink has traditionally considered omnidirectional antennas to be the norm in wireless communications. But beginning with 5G, wireless systems are using directional antennas that have high antenna gains and narrow beamwidths on both the mobile and the base station ends of each link. This offers more, not less, signal strength to each user as we move to millimeter wave, sub-terahertz, and ultimately terahertz frequencies.
Additionally, it's a myth that for a given distance traveled, radio energy becomes more lossy in free space as we increase above millimeter frequencies. We've shown that up to around 400 GHz, typical air only has a 10 decibels of loss per kilometer. This is merely a loss of only 1 decibel per 100 meters, which is about the range for a typical 5G small cell today. Even up to 900 GHz, we've shown that much of the spectrum suffers a 100 dB/km loss, or, for a small cell, only 10 decibels over 100 meters. 10 dB will be relatively easy to make up for with directional antennas at such high frequencies. While rain and foliage are problematic, once 5G cells are engineered to alleviate the effects of rain for transmissions up to 70 GHz, there is no further degradation beyond those frequencies up to 1 THz. This again shows that the efforts to deploy 5G will hold for decades to come. Snow is a different issue, but engineers must realize how the directional nature of future antenna technologies overcome the preconceived notions of greater loss at higher frequencies, as such thinking stems from a bygone era when omnidirectional antennas were prevalent.
The upshot of all this is that the additional link gain with these 5G technologies more than offsets any radio channel loss when site-specific deployment is used to avoid massive obstructions. Most things become reflective at and above millimeter wave frequencies, allowing more opportunities for directional antennas to find and combine signal paths. This improves the radio frequency power budget, which can then be used for providing much wider bandwidth channels with no worsening of a channel's signal-to-noise ratio compared to today's wireless systems. In turn, that means that wider bandwidth channels may be supported using the existing 5G tower infrastructure as we move up to wider bandwidth channels and higher carrier frequencies. Remarkably, the advantages offered by the three technical pillars of cell densification, wider bandwidth channels, and massive-MIMO will allow the engineering and deployment of 5G systems to carry forward for decades into the future as more spectrum is opened up on the sub-terahertz and terahertz bands. In fact, we predicted this nearly a decade ago.
Future 6G and 7G cellular networks will be able to use the same cellular infrastructure—and the same fundamental breakthroughs in radio circuits and antenna technologies—needed for 5G. Yet those cellular generations will do so with much greater data rates that will create massive data capacities and new use cases in the coming decades. This alone should motivate governments, funding agencies, wireless providers, and citizens to make the rollout of 5G a priority. On top of that, the vast capabilities of 5G and beyond should also motivate both the industry and governments to formulate new architectures, like Open RAN, as well as a new emphasis on security. There is a tremendous amount of opportunity at stake for building today's 5G network, with benefits to inure for decades to come as we increasingly come to depend on these invisible waves.
Theodore (Ted) S. Rappaport is the David Lee/Ernst Weber Professor of Electrical Engineering at the NYU Tandon School of Engineering (NYU-Tandon), and founding director of NYU WIRELESS, a multidisciplinary research center focused on the future of wireless communications and applications. He is a professor of computer science at New York University's Courant Institute of Mathematical Sciences and of radiology at the NYU School of Medicine. Earlier in his career, he founded the Wireless Networking and Communications Group (WNCG) at the University of Texas at Austin in 2002 and the mobile communications research center now called Wireless@ at Virginia Tech in 1990.
A very nice article by an expert on Wireless Communications. Thank you, Prof. Rappaport!
The prosthetics industry is too focused on high-tech limbs that are complicated, costly, and often impractical
The author, Britt Young, holding her Ottobock iLimb bionic arm.
In Jules Verne’s 1865 novel From the Earth to the Moon, members of the fictitious Baltimore Gun Club, all disabled Civil War veterans, restlessly search for a new enemy to conquer. They had spent the war innovating new, deadlier weaponry. By the war’s end, with “not quite one arm between four persons, and exactly two legs between six,” these self-taught amputee-weaponsmiths decide to repurpose their skills toward a new projectile: a rocket ship.
The story of the Baltimore Gun Club propelling themselves to the moon is about the extraordinary masculine power of the veteran, who doesn’t simply “overcome” his disability; he derives power and ambition from it. Their “crutches, wooden legs, artificial arms, steel hooks, caoutchouc [rubber] jaws, silver craniums [and] platinum noses” don’t play leading roles in their personalities—they are merely tools on their bodies. These piecemeal men are unlikely crusaders of invention with an even more unlikely mission. And yet who better to design the next great leap in technology than men remade by technology themselves?
As Verne understood, the U.S. Civil War (during which 60,000 amputations were performed) inaugurated the modern prosthetics era in the United States, thanks to federal funding and a wave of design patents filed by entrepreneurial prosthetists. The two World Wars solidified the for-profit prosthetics industry in both the United States and Western Europe, and the ongoing War on Terror helped catapult it into a US $6 billion dollar industry across the globe. This recent investment is not, however, a result of a disproportionately large number of amputations in military conflict: Around 1,500 U.S. soldiers and 300 British soldiers lost limbs in Iraq and Afghanistan. Limb loss in the general population dwarfs those figures. In the United States alone, more than 2 million people live with limb loss, with 185,000 people receiving amputations every year. A much smaller subset—between 1,500 to 4,500 children each year—are born with limb differences or absences, myself included.
Today, the people who design prostheses tend to be well-intentioned engineers rather than amputees themselves. The fleshy stumps of the world act as repositories for these designers’ dreams of a high-tech, superhuman future. I know this because throughout my life I have been fitted with some of the most cutting-edge prosthetic devices on the market. After being born missing my left forearm, I was one of the first cohorts of infants in the United States to be fitted with a myoelectric prosthetic hand, an electronic device controlled by the wearer’s muscles tensing against sensors inside the prosthetic socket. Since then, I have donned a variety of prosthetic hands, each of them striving toward perfect fidelity of the human hand—sometimes at a cost of aesthetics, sometimes a cost of functionality, but always designed to mimic and replace what was missing.
In my lifetime, myoelectric hands have evolved from clawlike constructs to multigrip, programmable, anatomically accurate facsimiles of the human hand, most costing tens of thousands of dollars. Reporters can’t get enough of these sophisticated, multigrasping “bionic” hands with lifelike silicone skins and organic movements, the unspoken promise being that disability will soon vanish and any lost limb or organ will be replaced with an equally capable replica. Prosthetic-hand innovation is treated like a high-stakes competition to see what is technologically possible. Tyler Hayes, CEO of the prosthetics startup Atom Limbs, put it this way in a WeFunder video that helped raise $7.2 million from investors: “Every moonshot in history has started with a fair amount of crazy in it, from electricity to space travel, and Atom Limbs is no different.”
We are caught in a bionic-hand arms race. But are we making real progress? It’s time to ask who prostheses are really for, and what we hope they will actually accomplish. Each new multigrasping bionic hand tends to be more sophisticated but also more expensive than the last and less likely to be covered (even in part) by insurance. And as recent research concludes, much simpler and far less expensive prosthetic devices can perform many tasks equally well, and the fancy bionic hands, despite all of their electronic options, are rarely used for grasping.
Activity arms, such as this one manufactured by prosthetics firm Arm Dynamics, are less expensive and more durable than bionic prostheses. The attachment from prosthetic-device company Texas Assistive Devices rated for very heavy weights, allowing the author to perform exercises that would be risky or impossible with her much more expensive iLimb bionic arm.Gabriela Hasbun; Makeup: Maria Nguyen for MAC cosmetics; Hair: Joan Laqui for Living Proof
In recent decades, the overwhelming focus of research into and development of new artificial hands has been on perfecting different types of grasps. Many of the most expensive hands on the market differentiate themselves by the number and variety of selectable prehensile grips. My own media darling of a hand, the iLimb from Ottobock, which I received in 2018, has a fist-shaped power grip, pinching grips, and one very specific mode with thumb on top of index finger for politely handing over a credit card. My 21st-century myoelectric hand seemed remarkable—until I tried using it for some routine tasks, where it proved to be more cumbersome and time consuming than if I had simply left it on the couch. I couldn’t use it to pull a door shut, for example, a task I can do with my stump. And without the extremely expensive addition of a powered wrist, I couldn’t pour oatmeal from a pot into a bowl. Performing tasks the cool bionic way, even though it mimicked having two hands, wasn’t obviously better than doing things my way, sometimes with the help of my legs and feet.
When I first spoke with Ad Spiers, lecturer in robotics and machine learning at Imperial College London, it was late at night in his office, but he was still animated about robotic hands—the current focus of his research. Spiers says the anthropomorphic robotic hand is inescapable, from the reality of today’s prosthetics to the fantasy of sci-fi and anime. “In one of my first lectures here, I showed clips of movies and cartoons and how cool filmmakers make robot hands look,” Spiers says. “In the anime Gundam, there are so many close-ups of gigantic robot hands grabbing things like massive guns. But why does it need to be a human hand? Why doesn’t the robot just have a gun for a hand?”
It’s time to ask who prostheses are really for, and what we hope they will actually accomplish.
Spiers believes that prosthetic developers are too caught up in form over function. But he has talked to enough of them to know they don’t share his point of view: “I get the feeling that people love the idea of humans being great, and that hands are what make humans quite unique.” Nearly every university robotics department Spiers visits has an anthropomorphic robot hand in development. “This is what the future looks like,” he says, and he sounds a little exasperated. “But there are often better ways.”
The vast majority of people who use a prosthetic limb are unilateral amputees—people with amputations that affect only one side of the body—and they virtually always use their dominant “fleshy” hand for delicate tasks such as picking up a cup. Both unilateral and bilateral amputees also get help from their torsos, their feet, and other objects in their environment; rarely are tasks performed by a prosthesis alone. And yet, the common clinical evaluations to determine the success of a prosthetic are based on using only the prosthetic, without the help of other body parts. Such evaluations seem designed to demonstrate what the prosthetic hand can do rather than to determine how useful it actually is in the daily life of its user. Disabled people are still not the arbiters of prosthetic standards; we are still not at the heart of design.
The Hosmer Hook [left], originally designed in 1920, is the terminal device on a body-powered design that is still used today. A hammer attachment [right] may be more effective than a gripping attachment when hammering nails into wood.Left: John Prieto/The Denver Post/Getty Images; Right: Hulton-Deutsch Collection/Corbis/Getty Images
To find out how prosthetic users live with their devices, Spiers led a study that used cameras worn on participants’ heads to record the daily actions of eight people with unilateral amputations or congenital limb differences. The study, published last year in IEEE Transactions on Medical Robotics and Bionics, included several varieties of myoelectric hands as well as body-powered systems, which use movements of the shoulder, chest, and upper arm transferred through a cable to mechanically operate a gripper at the end of a prosthesis. The research was conducted while Spiers was a research scientist at Yale University’s GRAB Lab, headed by Aaron Dollar. In addition to Dollar, he worked closely with grad student Jillian Cochran, who coauthored the study.
Watching raw footage from the study, I felt both sadness and camaraderie with the anonymous prosthesis users. The clips show the clumsiness, miscalculations, and accidental drops that are familiar to even very experienced prosthetic-hand users. Often, the prosthesis simply helps brace an object against the body to be handled by the other hand. Also apparent was how much time people spent preparing their myoelectric prostheses to carry out a task—it frequently took several extra seconds to manually or electronically rotate the wrists of their devices, line up the object to grab it just right, and work out the grip approach.The participant who hung a bottle of disinfectant spray on their “hook” hand while wiping down a kitchen counter seemed to be the one who had it all figured out.
In the study, prosthetic devices were used on average for only 19 percent of all recorded manipulations. In general, prostheses were employed in mostly nonprehensile actions, with the other, “intact” hand doing most of the grasping. The study highlighted big differences in usage between those with nonelectric, body-powered prosthetics and those with myoelectric prosthetics. For body-powered prosthetic users whose amputation was below the elbow, nearly 80 percent of prosthesis usage was nongrasping movement—pushing, pressing, pulling, hanging, and stabilizing. For myoelectric users, the device was used for grasping just 40 percent of the time.
In the United States alone, more than 2 million people live with limb loss, and 185,000 people receive amputations every year.
More tellingly, body-powered users with nonelectric grippers or split hooks spent significantly less time performing tasks than did users with more complex prosthetic devices. Spiers and his team noted the fluidity and speed with which the former went about doing tasks in their homes. They were able to use their artificial hands almost instantaneously and even experience direct haptic feedback through the cable that drives such systems. The research also revealed little difference in use between myoelectric single-grasp devices and fancier myoelectric multiarticulated, multigrasp hands—except that users tended to avoid hanging objects from their multigrasp hands, seemingly out of fear of breaking them.
“We got the feeling that people with multigrasp myoelectric hands were quite tentative about their use,” says Spiers. It’s no wonder, since most myoelectric hands are priced over $20,000, are rarely approved by insurance, require frequent professional support to change grip patterns and other settings, and have costly and protracted repair processes. As prosthetic technologies become more complex and proprietary, the long-term serviceability is an increasing concern. Ideally, the device should be easily fixable by the user. And yet some prosthetic startups are pitching a subscription model, in which users continue to pay for access to repairs and support.
Despite the conclusions of his study, Spiers says the vast majority of prosthetics R&D remains focused on refining the grasping modes of expensive, high-tech bionic hands. Even beyond prosthetics, he says, manipulation studies in nonhuman primate research and robotics are overwhelmingly concerned with grasping: “Anything that isn’t grasping is just thrown away.”
TRS makes a wide variety of body-powered prosthetic attachments for different hobbies and sports. Each attachment is specialized for a particular task, and they can be easily swapped for a variety of activities. Fillauer TRS
If we’ve decided that what makes us human is our hands, and what makes the hand unique is its ability to grasp, then the only prosthetic blueprint we have is the one attached to most people’s wrists. Yet the pursuit of the ultimate five-digit grasp isn’t necessarily the logical next step. In fact, history suggests that people haven’t always been fixated on perfectly re-creating the human hand.
As recounted in the 2001 essay collection Writing on Hands: Memory and Knowledge in Early Modern Europe, ideas about the hand evolved over the centuries. “The soul is like the hand; for the hand is the instrument of instruments,” Aristotle wrote in De Anima. He reasoned that humanity was deliberately endowed with the agile and prehensile hand because only our uniquely intelligent brains could make use of it—not as a mere utensil but a tool for apprehensio, or “grasping,” the world, literally and figuratively.
More than 1,000 years later, Aristotle’s ideas resonated with artists and thinkers of the Renaissance. For Leonardo da Vinci, the hand was the brain’s mediator with the world, and he went to exceptional lengths in his dissections and illustrations of the human hand to understand its principal components. His meticulous studies of the tendons and muscles of the forearm and hand led him to conclude that “although human ingenuity makes various inventions…it will never discover inventions more beautiful, more fitting or more direct than nature, because in her inventions nothing is lacking and nothing is superfluous.”
Da Vinci’s illustrations precipitated a wave of interest in human anatomy. Yet for all of the studious rendering of the human hand by European masters, the hand was regarded more as an inspiration than as an object to be replicated by mere mortals. In fact, it was widely accepted that the intricacies of the human hand evidenced divine design. No machine, declared the Christian philosopher William Paley, is “more artificial, or more evidently so” than the flexors of the hand, suggesting deliberate design by God.
Performing tasks the cool bionic way, even though it mimicked having two hands, wasn’t obviously better than doing things my way, sometimes with the help of my legs and feet.
By the mid-1700s, with the Industrial Revolution in the global north, a more mechanistic view of the world began to emerge, and the line between living things and machines began to blur. In her 2003 article “ Eighteenth-Century Wetware,” Jessica Riskin, professor of history at Stanford University, writes, “The period between the 1730s and the 1790s was one of simulation, in which mechanicians tried earnestly to collapse the gap between animate and artificial machinery.” This period saw significant changes in the design of prosthetic limbs. While mechanical prostheses of the 16th century were weighed down with iron and springs, a 1732 body-powered prosthesis used a pulley system to flex a hand made of lightweight copper. By the late 18th century, metal was being replaced with leather, parchment, and cork—softer materials that mimicked the stuff of life.
The techno-optimism of the early 20th century brought about another change in prosthetic design, says Wolf Schweitzer, a forensic pathologist at the Zurich Institute of Forensic Medicine and an amputee. He owns a wide variety of contemporary prosthetic arms and has the necessary experience to test them. He notes that anatomically correct prosthetic hands have been carved and forged for the better part of 2,000 years. And yet, he says, the 20th century’s body-powered split hook is “more modern,” its design more willing to break the mold of the human hand.
“The body powered arm—in terms of its symbolism—(still) expresses the man-machine symbolism of an industrial society of the 1920s,” writes Schweitzer in his prosthetic arm blog, “when man was to function as clockwork cogwheel on production lines or in agriculture.” In the original 1920s design of the Hosmer Hook, a loop inside the hook was placed just for tying shoes and another just for holding cigarettes. Those designs, Ad Spiers told me, were “incredibly functional, function over form. All pieces served a specific purpose.”
Schweitzer believes that as the need for manual labor decreased over the 20th century, prostheses that were high-functioning but not naturalistic were eclipsed by a new high-tech vision of the future: “bionic” hands. In 2006, the U.S. Defense Advanced Research Projects Agency launched Revolutionizing Prosthetics, a research initiative to develop the next generation of prosthetic arms with “near-natural” control. The $100 million program produced two multi-articulating prosthetic arms (one for research and another that costs over $50,000). More importantly, it influenced the creation of other similar prosthetics, establishing the bionic hand—as the military imagined it—as the holy grail in prosthetics. Today, the multigrasp bionic hand is hegemonic, a symbol of cyborg wholeness.
And yet some prosthetic developers are pursuing a different vision. TRS, based in Boulder, Colo., is one of the few manufacturers of activity-specific prosthetic attachments, which are often more durable and more financially accessible than robotic prosthetics. These plastic and silicone attachments, which include a squishy mushroom-shaped device for push-ups, a ratcheting clamp for lifting heavy weights, and a concave fin for swimming, have helped me experience the greatest functionality I have ever gotten out of a prosthetic arm.
Such low-tech activity prostheses and body-powered prostheses perform astonishingly well, for a tiny fraction of the cost of bionic hands. They don’t look or act like human hands, and they function all the better for it. According to Schweitzer, body-powered prostheses are regularly dismissed by engineers as “arcane” or derisively called “Captain Hook.” Future bionic shoulders and elbows may make a huge difference in the lives of people missing a limb up to their shoulder, assuming those devices can be made robust and affordable. But for Schweitzer and a large percentage of users dissatisfied with their myoelectric prosthesis, the prosthetic industry has yet to provide anything fundamentally better or cheaper than body-powered prostheses.
Bionic hands seek to make disabled people “whole,” to have us participate in a world that is culturally two-handed. But it’s more important that we get to live the lives we want, with access to the tools we need, than it is to make us look like everyone else. While many limb-different people have used bionic hands to interact with the world and express themselves, the centuries-long effort to perfect the bionic hand rarely centers on our lived experiences and what we want to do in our lives.
We’ve been promised a breakthrough in prosthetic technology for the better part of 100 years now. I’m reminded of the scientific excitement around lab-grown meat, which seems simultaneously like an explosive shift and a sign of intellectual capitulation, in which political and cultural change is passed over in favor of a technological fix. With the cast of characters in the world of prosthetics—doctors, insurance companies, engineers, prosthetists, and the military—playing the same roles they have for decades, it’s nearly impossible to produce something truly revolutionary.
In the meantime, this metaphorical race to the moon is a mission that has forgotten its original concern: helping disabled people acquire and use the tools they want. There are inexpensive, accessible, low-tech prosthetics that are available right now and that need investments in innovation to further bring down costs and improve functionality. And in the United States at least, there is a broken insurance system that needs fixing. Releasing ourselves from the bionic-hand arms race can open up the possibilities of more functional designs that are more useful and affordable, and might help us bring our prosthetic aspirations back down to earth.