Antenna Experiments Yield Military Benefits
Academic investigations are establishing the future of transmission technology for troops and civilians.
Improving antennas for defense or commercial purposes has as much to do with mathematics as it does with hardware. Researchers in the Wireless Networking and Communications Group at the University of Texas at Austin are exploring algorithms along with other properties that should improve communications systems on the battlefield.
A key focus of the work has honed in on multiple input, multiple output (MIMO) technology, which features many transmit and receive antennas. A large portion of that effort involves studying limited feedback—an idea that if receivers can send information back to an original transmitter, that transmitter can better configure links. This process should reduce interference and has applications for MIMO and cellular communications. Dr. Robert W. Heath Jr., director of the Wireless Networking and Communications Group (WNCG), says the research has advanced beyond single point-to-point links to examine how base stations can connect to many users and how to coordinate multiple base stations together to reduce interference.
Team members are looking into the fundamental limitations of such systems, and their results demonstrate that complete elimination of interference is not feasible only through the coordination of base stations. “That’s been something that’s surprising,” Heath states. Graduate students under his direction also are studying new analysis techniques in which they try to understand system performance and how antennas would play a role in situations with randomly located base stations. On the cellular side, experiments are underway to see how antennas can improve facets of functionality. Team members are exploring how distributing antennas throughout the cell instead of locating them all at the base station impacts performance.
A big concern for government and commercial communications networks users is how to share more video, so researchers also are looking into specific video applications in terms of configuring multiple antennas to support different needs. Heath and other faculty are leading a project to leverage perceptual optimization and configure a whole wireless link to deliver high-quality video as measured by a person. “Our objective is not necessarily to deliver all of the bits to the handset,” Heath explains. “Rather, it’s to give the user a good experience ... we actually reconfigure how we use the antennas as a function of the video.”
Research by the WNCG focuses on using existing antenna arrays better, not on making new antennas. An application for this is in mobile ad hoc networks (MANETs), which are of particular interest to the military. Heath recently participated in the large Defense Advanced Research Projects Agency (DARPA) Information Theory MANET (ITMANET) effort for which he examined the role of numerous antennas in ad hoc networks with no centralized base station or wired infrastructure. The process of going through multiple hops in these networks is inefficient, and users must deal with challenges of routing, neighbor discovery, overhead and high levels of interference. Heath’s research involved understanding how much networks could improve by exploiting multiple antennas at the nodes, for example for interference cancellation. The ITMANET project involved many researchers from various universities.
Heath says a main insight garnered from the work revealed that antennas are useful for beam steering and interference cancellation. However, there is little gain from using MIMO communications when sending a lot of data because the networks are dominated by interference. Heath believes the results are a good insight for the military, which is focusing on using multiple antennas. “They’re trying to catch up with the wireless LAN [local area network] and the cellular research over the past 10 years, but that research may not be as important for their networks,” he explains. If networks use antennas to cancel interference, more people can communicate at the same time.
The more than a dozen graduate-student members of the WNCG currently have or recently have had funding from U.S. government organizations, including DARPA, the Army Research Laboratory, the Naval Research Laboratory and the National Science Foundation as well as major companies in the defense industry. Omar El Ayach, a graduate student in the group, uses funding from the military to look into interference alignment. He spends more time studying ad hoc networks of vehicles and soldiers than cellular networks. “They might not seem very different, but they are very different,” he explains.
Regardless of funding sources, many of the concepts the researchers are exploring could affect government communications. Intercepting signals that come from base stations with hundreds of antennas is a difficult task, so choosing that set-up can make connections more secure. It also is difficult to intercept signals emanating from such a source, so groups required to listen in on transmissions need to find solutions. Using multiple antennas for interception can help, but it also makes it obvious to enemies where listeners are located.
Heath believes that on the military side, personnel are starting to understand the benefits of multiple antennas, but MIMO and other techniques still lag a decade or more behind commercial industry in terms of their adoption. “They’re missing out right now on the potential benefits in terms of capacity, spectrum efficiency, improved efficiency,” he states. “This is being able to send more data in a battlefield environment, having a more reliable connection. These things are important.” Other benefits include the ability to talk from farther away or from a sheltered environment. “There’s a lot of potential in the military for multiple antennas, and it’s just not being realized yet,” Heath says.
Many in the defense arena have doubts about this type of antenna arrangement because there have been several “revolutions” of it that failed to pan out. “I understand that they’re skeptical, but we’re using antennas now in more holistic fashion than they were used years ago, because right now we’re using them in a lot of different ways,” Heath explains. “We’re developing algorithms that can reconfigure the antenna based on what we might want to do with it. So I think that the military needs to get more into that, [to] look at the different configuration options and into how they might use it.”
The U.S. Army, U.S. Navy and DARPA all have funded research in interference alignment to help nodes in a network communicate simultaneously without creating interference. Group members are demonstrating the idea of using transmit antennas in such a way that at the receiver end, many interferers look like only a few, and thus can be canceled more easily. “We’ve actually done some propagation measurements to show that interference alignment is viable,” Heath explains, though challenges remain in extending it to large networks.
Heath says there are three research directions where multiple antennas are really interesting. The first is massive MIMO in which very large numbers of antennas are used at a base station. Though Heath says the work is promising, it holds several challenges because of the need to create compact arrays and potentially to integrate power amplifiers and other elements. Benefits of employing large numbers of antennas include more users communicating at the same time and computational power savings in terms of expending less power per antenna because each one supports fewer users. They also make it easier to separate out individual users.
The second direction is millimeter wave communication. Because antennas can be so much closer together when using these frequencies, soldiers potentially could have thousands of antennas on their helmets. In addition, developers could achieve high bandwidth rates, but propagation rates to make that reality are not as favorable as in lower frequencies. The research is examining how to fabricate a one-centimeter antenna array as well as how and where a millimeter wave system would work.
Heath’s recent research into millimeter wave technology has evolved an extremely secure protocol using the presence of many antennas. It essentially steers a beam toward one user while randomizing it in other directions so if enemies try to listen in, they receive both less power and a random-looking signal. “We’ve come up with a way of doing it, and it’s actually very different from the way people do communication now,” Heath explains. “It could be potentially big or maybe it won’t really work. I don’t know yet.”
El Ayach also has been looking into millimeter wave communications, but not with military funding. He explains that many challenges exist—including what antennas are available to support the frequencies—that must be overcome, but interest in the technology is growing because it opens up new areas of spectrum.
The third multiple antenna research direction is in femtocell or heterogenous networks. This work involves determining how to coordinate the many transmitters signaling to large numbers of receivers and particularly applies to cellular systems with various sizes of base stations that have wide distribution.
In other efforts, Jackson Massey, a graduate student with the WNCG, had government funding last semester to build on theoretical work done by previous students implementing interference alignment using transceivers with software-defined radio, or SDR, techniques. He helped create a testbed to show how well promising theoretical concepts performed in reality with three transmitters and three receivers that each have two antennas. “So basically we’re getting three two-by-two MIMO systems to work with each other to reduce interference at each one of the receivers,” Massey explains. Doing this will enable all three transceivers to transmit at the same time and in the same frequency spectrum. Researchers hope to see higher network sum data rates for all three systems.
By looking at the many possible features of such a technique, the researchers are building a complete picture of how this technology potentially could impact mobile ad hoc networks and military communications networks. Moving forward, other teammates are separating all of the users from one computer onto several to make the system more realistic and to observe the effects.
Massey believes that at home and on the battlefield, the desire for wireless devices will increase, meaning the number of antennas and amount of interference will increase as well. His research will be applicable to putting more antennas on devices to increase directionality and data rates as well as to reduce system issues when too many people try to communicate at once.