As anyone familiar with wireless communications can attest, erecting a tower is not a simple task. While almost all constituents want wireless coverage in their area, many object vehemently to the idea of a tower being located near their homes.

In a large-scale domestic emergency, outside aid often includes mobile communications vehicles that can be used to provide coverage where existing infrastructure has failed, or to supply additional capacity in a location where there are a lot of users. However, such a deployment has to be coordinated — ideally at an expedited pace, so response is not interrupted — or the mobile tower may introduce interference that proves to be a hindrance more than a help.

In a battleground scenario, the military faces additional challenges. Often, the military radio signals must be transmitted in a hostile environment, where they are susceptible to jamming efforts or attacks from military weapons.

With this in mind, the Defense Advanced Research Projects Agency (DARPA) has been developing a series of technologies designed to work in difficult battleground arenas. One such effort is DARPA Interference Multiple Access (DIMA) technology — a set of multi-user detection algorithms that enables greater throughput by allowing multiple users to occupy the same time slot and frequency slot, which would constitute interference in traditional systems (see graphic). DIMA promises to greatly reduce the time currently needed to coordinate spectral assets and deploy fixed antennas.

“A very important appeal of this technology is that you do not need the infrastructure,” said Brian Pierce, DIMA program manager. “And, now, you have this aspect that I'm not concerned with stepping on my colleagues and the various nodes [so spectrum can be used more efficiently].”

So far, DIMA has been tested with radios that are hundreds of meters apart, but not kilometers apart, Pierce said. Tim Settle, vice president of advanced system concepts for Ivysis Technologies — a company working with DARPA on DIMA — noted that the range of the radio nodes depends on the radio hardware, not the DIMA software.

“Obviously, a radio that can transmit at 5 watts will have a much longer range than one that can only transmit at 1 watt or 2 watts,” Settle said, noting that factors such as the frequency used and foliage also can impact range.

With DIMA, each radio leverages the multi-user detection capabilities of the technology to serve as a node in an ad-hoc network that is capable of sending, receiving and routing information between other nodes that are within range. In a demonstration last year, DIMA proved capable of allowing a single fixed node to receive information from as many as five other fixed nodes simultaneously, resulting in overall data throughput that was 3.6 times greater than 802.11g, with a packet-error rate of less than 1%.

This year, DARPA has focused on proving the ability of DIMA to work in a mobile environment. A preliminary test conducted during the spring demonstrated that mobility is possible, as a receiver in a vehicle moving at 15 mph was able to receive information from five transmitters simultaneously while maintaining the desired data throughput, although the packet-error rate crept to 3%, Pierce said.

In March 2010, a key DIMA test will be conducted to determine the readiness of the technology. The test will include all radio nodes moving at speeds of as much as 30 mph, Pierce said.

“On the battlefield, I think 30 mph would be a pretty good clip,” Pierce said. “Of course, the faster you can go, the better.”

In addition to the greater rate of mobility, next year's DIMA test is designed to demonstrate the ad-hoc routing function of the system and to lower the packet-error rate below 1%, Pierce said. Such capabilities hopefully will be deemed as desirable for an arm of the military and — eventually — be used in the public-safety arena, where DIMA's capabilities would appear to be ideal for first responders, particularly during a large-scale incident.

DIMA's capability also fits into the software-defined radio model that is being used by the military and, increasingly, within public safety.

“A very key accomplishment of the DIMA program was to show that you could implement this technology and approach on a single [field programmable gate array (FPGA) chip], and hence the power consumption associated with that,” Pierce said.

But the future of the DIMA program is not clear after the test is conducted next March — something that is “not unusual” for a DARPA program at this stage of development, Pierce said. For development of DIMA to continue within DARPA, one of the military services needs to express interest in the technology and provide funding to transition it from a research technology into a product that can be purchased by the military.

One potential military option would be for DIMA to be included in the U.S. Army's Wireless Network After Next (WNAN) program. This software-based communications program already includes several components already developed by DARPA, including mobile MIMO, disruption tolerant networking (DTN) — which is designed to ensure that packets are not lost — and the dynamic spectrum access capabilities provided by the XG program.

These characteristics are expected to be included in a cognitive WNAN portable radio that is being designed by DARPA with the goal of costing less than $500 per unit — a price range that many in the industry believe will be attractive to the public-safety community, as well as the military. The latest design of that radio is scheduled to be tested next month, according to DARPA.

Whether DIMA is included in WNAN or another military communications project is uncertain, but Pierce said he believes the technology is worthy of consideration, if it is demonstrated that it can perform robustly in an unfriendly spectral environment.

While the outcome of DIMA's March 2010 test is critical, Pierce said he believes DIMA has plenty of room to grow, noting that greater capacity and greater mobility — eventually, the technology is expected to allow video from unmanned aerial vehicles to be shared with ground troops — both are within reach. Pierce noted that aerial communications should be simpler than ground communications after the speed portion of the mobility challenge is met, because there is greater line of sight and fewer multipath issues with aircraft.

“Is there more beyond this [March] test? The answer, of course, is ‘Yes,’” Pierce said. “A lot of DIMA is tied to the advances in the FPGA chips. As they become more computationally capable and stay within the power budgets of the given platform, naturally you could foresee doing a greater number of nodes in a simultaneous fashion.” n

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