Flexible Future

Software-defined-radio technology supports multiple radio bands, multiple modes, and multiple applications within the same device.

One of the problems with traditional wireless technology is that it ties carriers and consumers to a single frequency or protocol for the life of the base station or terminal hardware. A relatively new concept in the commercial cellular-phone industry, software-defined radio (SDR) or software radio, promises to improve the flexibility of every aspect of wireless networks.

There are three key elements of SDR systems. The first is the capability to support multiple radio bands and protocols such as TDMA, CDMA, and GSM. The second element is a new way to access the flexible programming capabilities of the devices, so the programs can reach down into the radios to change their functionality. The third element is the ability to have a secure and reliable software download, so that you can add services after deployment.

Lee Hamilton, AirNet Communications CEO, said, "We like to compare SDR to computers. The early computers were task specific. Now we take for granted that you can change what a computer does by changing the software."

Although the vision of SDR has been around for some time, it has gathered attention only in the last couple of years as a technology that can be practically implicated. The military already has started to deploy the technology to enable radios that operate over a range of 2MHz to 2GHz. In the commercial realm it has been limited to base stations, owing to the size and power requirements of SDR equipment.

Eventually SDR technology promises to play a role in creating the universal handset that could be used on a variety of networks, frequency bands and applications. A consumer could buy one handset to communicate over a commercial network in the field, a LAN in the home, and via a short-range Bluetooth network to various peripherals.

About 120 companies have joined the SDR Forum (www.sdrforum.org) to help focus discussions on the various technologies and methodologies that need to be coordinated to make SDR a success. John Ralston, SDR Steering Committee chairman and MorphICs Technology vice president of intellectual property standards, said, "One of the challenges is that SDR still means lots of different things to lots of different people. The forum is really an attempt to streamline this whole line of discussion."

Base-Station Deployment Deploying SDR technology in the base station will help operators reduce the cost of upgrading their equipment and allow them to deploy multiple services over their spectrum at the same time.

"When we decided to deploy a GPRS system, I realized how important the idea of SDR was," said Dave Miller, president of Cross Communications, which has deployed a network of AirNet AdaptaCell SDR base stations. "We could do just a couple of minor hardware changes and use the existing network that we already have."

According to Ralston, Ericsson, Lucent and Nortel have been deploying SDR technologies in their equipment for the last five to seven years.

"However they don't use SDR for the glamorous new stuff," Ralston pointed out. "The manufacturers have essentially exploited a common platform of SDR so they could build a smaller number of boxes to serve all of the different customer segments. It improves your time to market and the reliability of the product. The engineering and manufacturing efficiencies go way up."

The real power of SDR comes when it is used across a broad band of frequencies. For example, AirNet first deployed the AdaptaCell broadband SDR base station in 1997 that allows up to 12 200kHz GSM channels to be carried across a single piece of equipment operating across 5MHz of bandwidth.

This approach has enabled AirNet to shrink its equipment by combining many components within the base station. A traditional narrowband base station requires a separate power amplifier, digital signal processing (DSP) card and air interface for each channel. But by performing all of these functions in software, AirNet's broadband SDR base station is able to share all of these components across 12 separate channels. Hamilton claimed, "We have the smallest, highest-capacity base station that is out there. Our solution of adaptive software gives us about 10% of the hardware of a traditional base station."

Broadband SDR also has enabled AirNet to deploy adaptive-array (sometimes called phased array) capabilities into its base station. This technology allows the base station to break the area into smaller cells so that it can handle far more capacity by reusing the spectrum in each of these smaller cells. It allows the antenna to steer the radio signals like a beam onto a particular phone.

Adaptive-array technology requires multiple antenna elements to help steer the beam. Hamilton said that with traditional hardware, an operator has to deploy one base station for each antenna element. However, by doing all of the processing in software, it is possible to drive all of the antenna elements with a single base station.

Hamilton said that none of the major base-station manufacturers is working on broadband SDR base stations at the moment because it is not perceived as a big enough market.

"There is a tremendous investment around narrowband hardware base stations, and if the incumbents have a choice, they will continue selling more hardware like that," he explained. "But in a few years, everyone will be building base stations with a broadband SDR approach, because it is just such a compelling approach."

SDR in the Terminal In the long run, many believe that SDR technology could play a valuable role in wireless terminals such as handsets, PDAs and car-based systems. SDR technology will allow these devices to be dynamically upgraded in the field giving them a longer life span and providing consumers a wealth of different applications.

Stephen Blust, SDR Forum chairman and Cingular Wireless director of technology, strategy and standards, noted, "If the FCC opens up a new frequency band, it would be nice to incorporate those in a new manner. If a cell-phone customer buys a phone that does TDMA, they might go somewhere where they need GSM. The vision becomes one of enabling their phone, either temporarily or permanently, to support a new service. Now you can extend the lifetime of the consumer device and increase the customer choice. You are no longer constrained the way you were previously by the technology."

There are still many concerns and limitations that need to be worked out before SDR becomes common in portable applications. The technology required to do SDR is currently too power intensive for many portable applications. Furthermore, there are valid commercial reasons for handset vendors and commercial operators to discourage the deployment of highly flexible devices. There also are security issues associated with wireless devices that could have the potential to disrupt communications for a variety of other users and applications.

SDR has not taken off in handsets because the flexible processors required to implement it consume too much power. Single-function application-specific integrated circuits (ASIC), which are used in traditional fixed-function phones are able to achieve an efficiency of 1,000 million instructions per second (MIPS) per milliwatt (mW) of power. However, the more programmable DSPs and field-programmable gate arrays (FPGA) needed for SDR are significantly less efficient, achieving efficiencies of only 5MIPS/mW to 10MIPS/mW.

Ralston said that we need to achieve a 10- to 100-fold improvement in the programmable chips for them to become practical in mobile devices.

The current voice-only wireless devices only have to process a few 10s of MIPS; 3G systems are going to require hundreds to thousands of MIPS of processing power. Phased-array antennas, which will eventually allow mobile devices to beam their energy in a spot, thereby increasing range and reducing the required transmission energy, will require from 5,000MIPS to 7,000MIPS.

Although few consumers would be happy with a phone that died after a few minutes of use, phone vendors need to figure out how to make the programmable processors required for SDR far more efficient.

"What has emerged is an energy versus flexibility gap," Ralston said. "The hardwired ASIC gives you the efficiency you need, but removes the flexibility. The DSP and FPGA give you flexibility, but they don't have the energy efficiency. One of the biggest challenges is to plug that gap."

Another concern is security. These phones could turn into an ideal target for various hackers. Although most in the industry seem to brush off such fears with the contention that there already is sufficient security built into the communications links with phones, any security system is only as strong as the weakest link. All it takes is for one person to inadvertently (or deliberately) broadcast a self-propagating virus to everyone on his mailing list before the virus has spread around the world.

Although viruses traditionally only have been able to affect computer applications, viruses that affect terminals with SDR capabilities could potentially create radio interference over any frequencies to which the phone has access.

Another concern with SDR is from a business perspective. Handset vendors make a lot of money selling new terminals to people as the standards change. The widespread adoption of SDR could potentially reduce their revenues. SDR also could make it easier for new competitors to get a foothold in the terminal arena.

"SDR brings down the barriers to entry to becoming a handset player," said Bryan Prohm, Dataquest senior analyst. "You have to protect your own turf in a sense."

New competitors could use SDR to create a single terminal to address a variety of markets. Established terminal vendors may not begin offering SDR equipment until they are dragged kicking and screaming into the arena by upstart competitors.

"SDR is a disruptive technology," Blust said. "It is disruptive in the sense that it is a new way of doing things. It takes time to displace some of the traditional ways. But at the same time, it is an opportunity technology because it lets new entrants come into an industry with well entrenched ideas and competitors."

The carriers also may be leery of providing their customers with SDR technology that enables the subscriber to change carriers on a whim. A fixed-function terminal provides a disincentive to churn.

Prohm does not expect to see SDR handsets anytime soon.

"It is fine there is a forum to talk about the issues, but until you start to hear the big three and the bundle behind them starting to talk about time frames and what kind of impact this will have on the marketplace, I am hesitant to suggest that the market will suddenly change overnight," he said.

Although there is lots of talk about the technology, Prohm does not expect to see any of the major terminal vendors give SDR technology anything more than lip service for the next few years.

"It is one of those things that is yet to climb the hype cycle to where everyone in the industry is talking about it," he said. "It is still in the nascent stage, and it has not started to generate interest and notice from the key players other than lip service. Everyone recognizes that it is the ultimate destination, but in the short term, by having a hodgepodge of multiple access standards, it behooves all of these guys to exploit the revenue stream, before consolidation tightens the marketplace."

In March 2000, the FCC first began an inquiry into how to make rules more efficient in order to simplify the deployment and licensing of software-defined radio (SDR) technologies. In December, it released a Notice of Proposed Rule Making with its proposal.

1 The FCC wants to create an official definition of SDR, such as a transmitter that can have its operating parameters changed in software.

2 The FCC wants to create a permission-to-change-parameters procedure. Since the current FCC approval process to change parameters is slow, it wants to develop a methodology to speed the process along.

3 The FCC wants to create an option of electronically labeling equipment. Currently, labels must be physically placed on the transmitter. Electronic labeling would allow the transmitter to be updated and labeled remotely.