Vendors serving the public-safety sector have long desired to leverage wireless technology developed in the commercial sector to drive down costs and spur innovative applications. That desire could be sated to some degree by the massive development efforts from commercial vendors in smart antenna technology known as multiple input/multiple output (MIMO), which holds the promise of technological improvements that would be a boon for public-safety wireless communications.

MIMO promises to increase data speed, range and reliability of wireless networks by using multiple antennas on both sides of a communications link to increase spectral efficiency. While a conventional radio system uses one antenna to transmit a data stream, MIMO uses multiple antennas to combat distortion and interference by simultaneously transmitting multiple data streams through the same channel.

The technology also creates multiple parallel independent channels from a base station to the mobile device and recognizes the unique signatures of these independent paths. Knowing the individual path signature allows them to be used as independent parallel highways, carrying much more data than a single link without increasing network power.

“Increasing power is extremely inefficient,” said Renaldo Valenzuela, director of wireless communications research with Lucent Technologies' Bell Laboratories. “With multiple antennas, I can combine the signals to make them stronger and pull them above other messages without using additional spectrum. Spectrum is like beachfront property and no one is getting any more of it.”

Virtually every major wireless infrastructure vendor is developing MIMO capabilities. MIMO, which is interface agnostic, is expected to be widely deployed in future iterations of commercial third-generation networks to significantly increase data capacity, deliver high-speed data services more efficiently and improve coverage.

“There is a huge interest in MIMO,” said Robert Heath, assistant professor in the Department of Electrical and Computer Engineering at the University of Texas. “Every major company has people researching it and sitting around thinking about the issue.”

Nortel Networks conducted a MIMO demonstration on CDMA 1xRTT radio equipment in 2000, and completed an extensive propagation trial in central London in 2001, proving the feasibility of MIMO to provide substantial performance gains in an urban deployment, said John Hoadley, the company's vice president of advanced technology, wireless networks.

Bell Labs also has conducted several demonstrations of the technology and has accomplished lab experiments that have enabled wireless networks to increase capacity by 20 times, said Valenzuela. Meanwhile, wireless LAN semiconductor company Agere Systems, formerly part of Bell Labs, demonstrated the technology using a three transmitter/three receiver MIMO system supporting 162 Mb/s wireless networking speeds.

“The academic community has been turned upside down,” said Valenzuela. “MIMO has created a fascinating field.”

Nevertheless, the first MIMO deployments likely won't be seen for a while. MIMO is expected to be incorporated in the next generation of 802.11 technology, known as 802.11n, which proposes to boost data rates to 100 Mb/s. However, that standard probably won't be ratified for another two years.

Consequently, some 802.11 chip vendors are poised to ship pre-802.11n chips using MIMO right now. For example, chipset provider Airgo has announced a handful of original equipment manufacturer customers — including SOHOware, Planex, Askey and Taiyo Yuden — plan to manufacture and distribute the world's first MIMO 802.11 products based on its True MIMO technology, which offers the same throughput as 100 Mb/s Ethernet, according to Airgo.

The conviction among 802.11 advocates is that MIMO can dramatically boost performance and range while still operating with existing 802.11a/ b/g radios, with a minimal cost increase over legacy products.

“The general idea of using 802.11 technology is using wider channels but more efficiently,” said Sheung Li, product line manager with Atheros Communications, a developer of WLAN chipsets. “When MIMO comes in, this is another step along this line of thinking. It's the same idea of, how do I pack more people into the same space? Not only am I using spectral efficiency inherent in 802.11, but I'm taking advantage of the physics opportunities of using multiple antennas efficiently.”

What does all this mean for public safety? Marcello Pagnozzi, project officer with Project MESA (Mobility for Emergency and Safety Applications), an international partnership designed to produce global standards for next-generation public-safety applications, said MIMO's applicability to public-safety communications systems should be explored for several critical reasons.

Project MESA desires applications requiring significant bandwidth that would usually leverage high-level modulation techniques or higher frequencies with their propagation shortcomings, said Pagnozzi. While data throughput speeds decrease the closer a device moves to the edge of a cell, MIMO boosts signal strength at the edge of the cell to replicate the strength achieved in the middle of the cell to increase data speeds.

“MIMO gives you some diversity that translates into better reliability at the edge of a cell,” said Heath. “It scales with the number of transmitters. For high data rates, you're talking about factors of two, three or four, and 10 is possible.”

In addition, Project MESA systems are unlikely to enjoy the same link budgets — the signal strength necessary for reliable communication — that cellular systems use because public-safety agencies typically deploy a smaller number of base stations than do commercial carriers. MIMO solves that problem by improving the air interface link budget, which translates into improved coverage.

Public safety also is looking to deploy 802.11-compliant equipment in the 4.9 GHz band, which was set aside by the Federal Communications Commission for exclusive use by first responders. While the bandwidth offered by 802.11 networks is enticing, Wi-Fi networks are designed to serve only a localized environment because of the propagation limitations of the technology, said Pagnozzi. However, MIMO could make Wi-Fi technologies more cost effective for wide-area deployment, according to Li.

“I get a stronger signal through more conditions, and I can get a signal where I couldn't before, deep within a building or city canyon,” he said.

MIMO also can combat other problems associated with 802.11 technology. While the 4.9 GHz band is allocated to public safety, it can be used by all licensed public-safety entities within a region, which threatens to consume all of the available frequency channels and cause interference.

MIMO has the ability to suppress interference and adapt to local network conditions in real time, said Valenzuela.

For instance, in an emergency situation, first responders can commandeer all transmitters in a particular area to transmit high volumes of data such as real-time video feeds and then switch to normal operations when the incident is over.

MIMO's advantages for public-safety networks may soon be demonstrated, perhaps as early as this fall, according to Valenzuela.

In February, Lucent received a one-year, $11.5 million contract with the U.S. Defense Advanced Research Projects Agency to demonstrate an ultra-high capacity, highly secure communications system for DARPA's MIMO program.

The overall program goal is to deploy the first MIMO-based mobile ad hoc network that provides a 20-fold increase in spectral efficiency.