The division division

The broadband wireless world has learned from experience that point-to-multipoint systems seem to work better for commercial last-mile service than point-to-point. Now the technology debate has largely shifted to the method by which signals are duplexed.

The new discrepancy is over the difference between time division duplexed (TDD) and frequency division duplexed (FDD) systems. Two start-up vendors-Ensemble Communications and Wavtrace-appear to be emerging as the front-runners in TDD, having devoted their business to developing that type of system from the beginning. Essentially, TDD can give a broadband wireless carrier the ability to dedicate any portion of each customer pipe in either direction-uplink or downlink. In an FDD system, the bandwidth available for the uplink and downlink is fixed from the hub and to each individual customer (Figure 1).

"It's automatically fixed that every customer will get twice as much bandwidth up as down-and that's not every customer," says Carlton O'Neal, vice president of marketing and sales at Ensemble. "This only fits for those customers that have that traffic pattern."

With the adaptive TDD system Ensemble proposes, the whole swath of spectrum can be divided and used to its full potential regardless of direction (Figure 2).

"We adjust the center line between transmit and receive," says O'Neal. "The system automatically adjusts to the user traffic."

Ensemble's product features two levels of software-one that controls the allocation to each user link and one that controls overall bandwidth allocation. TDD will offer a cost savings in the makeup of the radios, O'Neal says. Because there is no guard band to deal with, no duplexer is required to divide the uplink from the downlink.

Both developers agree that TDD proposes more spectrum agility, making it better equipped for carriers with licenses in the B band local multipoint distribution service spectrum, which features a government-imposed guard band. Likewise, both agree that FDD systems are better optimized for non-data applications.

"FDD is really a voice-oriented architecture," says Thomas van Overbeek, president and CEO of Wavtrace. The bursty nature of data makes a time division approach much more appropriate, he says.

The TDD architecture seems poised to allow carriers to provide more original service offerings. As more service providers enter the broadband wireless business by offering static pipes to customers, there could be nothing to distinguish them from one another beyond price. With an adaptive system, carriers can make better use of their bandwidth by loading more customers on and could potentially offer more usage-sensitive pricing and service level agreements (SLAs) that guarantee certain bandwidth availability at certain times.

"The key to generating revenue as a carrier is the ability to oversubscribe the channel," van Overbeek says. "When you're developing SLAs, you provide a certain level of assured capacity and a certain level of peak capacity."

That said, neither Ensemble nor Wavtrace is ready yet to enter the commercial stage. Ensemble's adaptive TDD product will be in alpha trials this summer and beta later this year, O'Neal says.

Wavtrace has an operational test system at its Bellevue, Wash., headquarters and plans to start beta testing with customers this month, van Overbeek says. Wavtrace plans to make first TDD release available in

August, but that version will not be adaptive-it will offer a 50% uplink/50% downlink pipe.

Both vendors also plan to forge relationships with larger suppliers and systems integrators. Ensemble, in fact, has already established a non-exclusive OEM arrangement with ADC Telecommunications.