Savings Plans

The bandwidth you save (or squander) today is the bandwidth you'll need tomorrow.

North American fixed-wireless carriers aren't tossing and turning at night worrying about system capacity - at least not yet.

"No one in the industry right now is facing capacity issues on networks," said Russ Wiseman, senior vice president - Internet operations for Nucentrix, the third largest U.S. holder of MMDS spectrum. "We don't see any capacity bottlenecks for the next two to three years."

In the LMDS space, XO (formerly Nextlink) is a company that has "frequency to burn," according to Tom Kuchler, Nortel Networks director of strategic marketing for broadband wireless access.

"We're fortunate that we do have a significant amount of bandwidth to work with in the millimeter-wave range (above 10GHz), and that gives us flexibility to provide high-capacity services," said John Kean, XO director of wireless architecture.

Yet fixed-wireless carriers tend to echo Mike Roberts, Winstar vice president of network engineering, who noted that "we're concerned that we take the best advantage of the bandwidth that we have."

Carriers must think about the future when it comes to capacity issues, said Todd Carothers, Adaptive Broadband vice president of marketing.

"Broadband access won't make money for service providers in the long term," Carothers said. "It's the applications they put over it." He sees traditional applications, but they will be much enhanced. This all folds into the capacity issue, and a sophisticated bandwidth-management process will be needed to handle the capacity efficiently, Carothers said.

"In all the technologies that we're looking at, the ability to scale the network, both geographically and capacity-wide, is a core component," Wiseman said.

Nucentrix has concluded that technologies based on orthogonal frequency division multiplexing (OFDM) represent the best balance of high-capacity, standards-based technology and deployability.

In a frequency-division multiplexing system, the bandwidth is divided into multiple data carriers, with the data divided between the sub-carriers. Because each carrier is independent, a frequency guard band is placed around it, lowering the bandwidth efficiency. With OFDM, multiple carriers (or tones) also are used to divide the data across the spectrum, however each tone is considered to be orthogonal or unrelated to the adjacent tones and does not require a guard band. This makes it more efficient.

Nucentrix prefers a platform that allows it to use either a supercell or multicell design and provides high bits per hertz. It has selected Cisco's system, and Wiseman believes that Cisco is closest to market of all the OFDM platforms.

Two Views Nortel's Kuchler noted there are two ways to look at capacity: the bandwidth delivered through a covered cell area and the end-user capacity.

"We have promoted up to 64-QAM (quadrature amplitude modulation) on the downlinks and 16-QAM on the uplinks," Kuchler said.

QAM has various degrees of complexity with 64-QAM popular in wireless-broadband products. The higher the density in QAM, the higher the S/N ratio must be maintained in order to meet the required bit-error rates.

"We haven't come up with the magic formula to take it beyond that, but the modem technologies, the millimetric technologies and the power-amplifier technologies all continue to get better," Kuchler continued. "I've seen modulations proposed at 512-QAM. That's a little stretch, but 128-QAM, if not here today, will be soon. Again, it's trying to fit more bandwidth into the given regulated spectrum."

XO is using Nortel's Reunion product for point-to-multipoint deployments. However XO uses both point-to-point and point-to-multipoint radios, as each serves a different application.

"Multipoint is suited, because of its capacity-limited basis, to serve a specific customer at a lower cost," Kean said. However, in some buildings, it's more efficient to put in a high-capacity point-to-point link and then use a multiplexer off of that link to serve two or more customers in the same building.

Winstar has much the same philosophy.

"Neither (point-to-point or point-to-multipoint) is totally ubiquitous," Winstar's Roberts said. "At first everyone was saying point-to-multipoint was going to save the world. In different parts of the networks they co-habitate very well when used together."

He said point-to-multipoint works well in highly condensed areas, in a building where he hubs a lot of traffic, and with many buildings that are very close in with high bandwidth requirements. But if you need to go farther, or customers need DS3s or higher, point-to-multipoint doesn't work as well.

DMC Stratex customers are telling the vendor that they want higher capacity, but they want spectral efficiency too, according to Paul Kennard, DMC CTO.

"They're saying, `Although I want high capacity, I don't want to use up all of my valuable bandwidth just lighting up one building,'" Kennard said.

Consecutive-Point Daisy Chains Point-to-consecutive-point technology also is a technique XO is testing with Triton in Boston and Dallas.

"A consecutive-point system is a good solution for providing high availability through a ring architecture," Kean said. Sonet features are extended into a radio platform that provides the ring architecture, so if a segment is broken, there is an alternate route for the data.

"Switching time is on the order of tens of milliseconds, so we can continue to operate uninterrupted," Kean said. "Using the right architecture, you can still provide the high capacity to the various nodes along the architecture."

XO is using OC3 or 155Mb radios now, and some vendors have announced OC12 radios for the future. Because XO has so much spectrum in its trading areas, it can offer the higher-rate service.

A set of consecutive-point links can be daisy chained almost to an unlimited degree, Kean said, at least far enough to serve a metro area.

"A requirement is to have enough spectrum to lay out those links and provide an interference-free operation throughout that geographic area," he said, adding that you need a frequency plan to avoid interference.

Planning intelligently is key. Because XO has ample spectrum, it can use radios with higher spectrum efficiency, or higher coding in the modulator. Other techniques to get more efficiency include adding highly directive antennas, going from 1-foot dishes to 2-foot dishes.

"Sometimes it's a trade-off," Kean said. "Higher modulation and coding schemes reduce the link distance. As we increase the density of our system we will wind up putting in shorter and shorter links anyway, so it'll fit well with using those higher-capacity modulation schemes."

This can result in a fine balancing act, Winstar's Roberts said.

"I want to cram as much stuff as I can within the same license areas, not pay a lot more, and it's got to go just as far," he said.

The first time Roberts has that conversation with vendors, he admits he gets funny looks.

"They say, `Don't you know physics? Don't you know Einstein? You're trying to defy that.'

"Yes, I am," he admitted.

Yet Roberts has found in his three to four years with Winstar that vendors have come up with ways to double and triple capacity without giving up a lot of distance.

Sectorization & Cellularization Two techniques used in mobile wireless networks, sectorization and cellularization, are finding a place in fixed-wireless networks as well, especially in the lower bands.

In Sprint's Phoenix MMDS system, Hybrid used narrowbeam antennas of 30 degrees to get a 5-times gain and then cellularized by installing two transmitter sites, each with a radius of 35 miles, to cover the city and overcome topography limitations.

John Frederick, ADC vice president & general manager of the broadband wireless-access division, said the first multicell deployment was done by ADC with WorldCom in its Boston MMDS trial. A 5-cell system currently covers part of Boston, and other cells can be added. ADC's platform can be used in both supercell and multicell deployments, with frequency reuse on the order of that used in PCS sectorization, he said.

LMDS sectorization in North America typically involves 90-degree sectors, Nortel's Kuchler said. That appears to be the efficient boundary between cell range and frequency reuse, and there haven't been the pressures to go to the smaller sectors.

However, XO does not sectorize in its LMDS networks. Although LMDS signals theoretically can go as far as MMDS signals, they are more subject to attenuation from rain, Kean explained.

"To provide high availability on the order of outages lasting only minutes per year, we deliberately limit the distance of our links to between one and three miles, depending on the rain," Kean said.

As MMDS moves toward true non-line-of-sight systems, cellularization will play a bigger part.

"Two things will enable (non-line-of-sight): smaller cell sites with 2- to 5-mile radius cells and advanced modulation schemes," Frederick said.

In November NextNet Wireless announced an MMDS system in alpha testing that is said to be completely non line-of-sight. Based on OFDM, the system uses 6-sector cells with 5-mile, 3-mile and 1-mile radii for rural, suburban or urban deployment.

Fuzion Wireless, a fixed-wireless carrier in the unlicensed U-NII bands (5.725GHz to 5.825GHz), has the benefit of being the only operator using these bands in many of the areas where it is offering service, according to Robert Campbell, vice president of operations.

"We don't expect that to remain the case," Campbell said, adding that Fuzion's ability to get up and rolling quickly has been a tremendous benefit.

If another carrier comes into its space and interferes, that carrier has to solve the problem. However, Fuzion's approach is to work out the differences so both carriers will be able to operate.

Fuzion provides Internet services at speeds up to 155Mb/s to customers that include small- and medium-size enterprises, ASPs and Web-hosting services. It operates in the United States, Canada and Latin America.

Fuzion specifically selected Adaptive Broadband's product for the U-NII bands.

"When we met with Adaptive, we found they had a very high-capacity wireless radio on the unlicensed band, and when we began exploring the product in more detail, we realized that they understand the capacity issue," Campbell said.

"We provide a packet-on-demand technology (AB-Access) that is proprietary to us, and we are working with the standards bodies to get it adopted as a standard," said Todd Carothers, Adaptive Broadband vice president of marketing. The product interleaves many different users within a single channel and provides bandwidth on a packet-by-packet basis, only as needed.

Campbell said Fuzion can deploy a 25Mb radio in certain sectors, and if capacity is consumed, it can overlay additional traffic.

Fuzion also is pursuing additional spectrum. The U-NII band consists of 200MHz allocated to outdoor use and 100MHz allocated to indoor use.

"The sad thing about 5GHz is it won't penetrate drywall," Campbell said.

Fuzion, Adaptive Broadband and others are trying to convince the FCC to add the 100MHz indoor allocation to the outdoor allocation so the full 300MHz will be available.

Today, Fuzion is not running short of bandwidth, said Campbell. But if capacity gets tight, that's not all bad.

"If I'm 100% subscribed, that's a wonderful problem to have," he said.