Adaptive Rate Technology Goes the Distance

By: By Gregg Levin, Bridgewave Communications Inc.
( 1 Nov 2007 )

As more and mission-critical voice and data traffic traverses high-capacity wireless networks, high-speed millimeter wave RF systems are gaining ground for improved security, reliability, availability and value. The most-recently allocated 60GHz and 80GHz wireless frequency bands are becoming accepted as preferred solutions for extending gigabit LANs between sites, connecting locations to fiber-optic backbones as well as creating "virtual fiber" backbones through a metropolitan area, without the costs and delays of new metro fiber runs.

These upper millimeter-wave frequency bands feature multi-gigahertz spectrum allocations that can be used on an un-channelized basis, providing multi-gigabit data transmission utilizing simple, low-order modulation techniques. The use of "pencil beam" antennas enables large-scale frequency reuse and provides interference immunity, making the need for traditional narrow RF channel assignments unnecessary.

The laws of physics dictate that rain downpours attenuate RF signals by scattering energy off of the desired path. This is especially true for radios with operating frequencies of more than 10 GHz, so it's important to provision links with sufficient RF link margin to prevent outages during severe rainfall. Traditional tools for adding link budget involve larger antennas or decreased link data rates. Larger antennas improve the focus of energy between two end points, enabling provision of links with enough RF link margin to prevent outages during heavy rain. Still, they add installation time and expense to deployment. Reducing data capacity is another way to improve link budget by using narrower frequency channels and/or permitting the use of lower-order modulation, but this is accomplished at the cost of reduced application performance.

ADAPTIVE MODULATION
Rather than permanently reducing link data capacity or link availability, it is preferable to decrease link capacity during brief periods of intense rainfall, thereby maintaining full link performance for the vast majority of the time. The traditional means of temporarily providing additional link budget to overcome rain fades has been "adaptive modulation" rate schemes. These permit radio links to switch dynamically from higher-order modulation to lower-order to provide improved receiver sensitivity when needed. These approaches are effective for radios operating in 11 to 38 GHz frequency ranges because changes in modulation can provide sufficient increases in link budgets to overcome rain fades. In addition, the cost of highly linear transmit power amplifiers is acceptable in these ranges.

THE EMERGENCE OF NEW ADAPTIVE RATE TECHNOLOGY
Adaptive modulation is not attractive, however, in the case of gigabit speed radio links in the newer 60 GHz and 80 GHz ranges. The cost of providing highly linear transmit power amplification is much greater here than in the lower frequencies. Therefore, it is more desirable to use only lower-order modulation, while taking advantage of massive spectrum allocations available in these frequency ranges.

Recently, advancements in adaptive rate technology provide a more compelling solution for these high frequency bands. Temporary increases in link budget can be achieved, when needed, by reducing over-the-air signal bandwidth—instead of changing modulation order. Decreasing air signal bandwidth by a factor of 10 (Figure 2) lowers the amount of background noise the received RF signal must exceed by a factor of 10 (or 10 dB). This reduction provides an additional 10 dB of rain fade link budget, which is enough to improve availability significantly when strong rain presents challenges.

This breakthrough permits links that normally run at full-rate, full-duplex GigE speeds to shift automatically to an over-the-air 100 Mb/s data rate during heavy rainfall, thus gaining 10 dB rain fade budget. The RF bandwidth reduction of adaptive rate technology permits the links to remain connected at the maximum error-free data rate—despite severe rainfall. In Figure 1, the switch from GigE to 100 Mb/s occurs when the radio's RF-receive signal level (RSL) approaches the minimum for the link to operate error-free at full GigE data rates. When the rain subsides and RSL increases beyond the minimum for error-free GigE operation, the links immediately shift back to full GigE.

Not only does adaptive rate make possible remaining in operation during downpours, the technology can permit link operation over distances that previously would only be supported reliably by reduced data rate links.

APPLICATION INTERFACES
When faced with strong rain fade, these GigE adaptive rate links reduce their RF air interface bandwidth and data rate, but do not make corresponding changes to their fiber or copper (wired) interfaces. The wired interfaces continuously operate at full GigE speed—regardless of air interface speed. In addition, there's no immediate impact on wired interface traffic because radios using internal Ethernet switches can buffer inbound and outbound traffic between wired and air interfaces.

TCP/IP traffic entering the radio via wired interfaces will naturally slow, if needed, in response to delays in peer acknowledgements and/or momentary packet losses due to short-term traffic congestion. If the radio's wired port is configured with flow control enabled and traffic inbound to the radio exceeds 100 Mb/s, the radio will send the end-user's network equipment an Ethernet pause request to prevent data overflow within the radio. This lets the network equipment provide back-pressure through the network and prioritize the radio's traffic flow.

When traffic sent through the link contains both high priority and lowerpriority packets, and the user's network equipment is configured to prioritize high priority packets, the result is high-priority packets continue to flow without impact, while lower-priority traffic will throttle back to accommodate reduced bandwidth. This capability makes adaptive rate technology suitable even for the most demanding applications—such as mobile backhaul, where high-value voice traffic typically consumes much less bandwidth than general end-user data traffic.

REAL-WORLD ADAPTIVE RATE BENEFITS
In January 2007, when Bayonne Medical Center (BMC) needed to establish connectivity with recently acquired Richmond University Medical Center (RUMC), the decision was made to link the two locations, which are approximately 2.25 miles apart.

Bayonne's IT team rejected a bandwidth-constrained 45 Mb/s leased line solution that would exceed $5,000 in monthly fees, and ruled out aggregating multiple 52 Mb/s wireless radios as well as optical wireless using free-space optics because neither technology met reliability or distance requirements. However, they learned an 80 GHz millimeter wave product, from BridgeWave Communications, that includes an AdaptRate capability could bridge the 2.25 miles between the hospitals, while meeting stringent criteria for security, uptime and capacity.

Relying on a rain model developed by the International Telecommunication Union (ITU), BMC was able to determine the GigE link could be impaired less than 30 minutes a year. For BMC, the unprecedented availability provided by an adaptive rate solution clinched the deal while delivering outstanding value-for-performance, compared to slower fiber-optic based alternatives.

Medical X-Ray Center, a practice radiology center, also relies on a millimeter wave link with adaptive rate technology to support a secure, HIPAA-compliant MAN. Having reliable, highly available connectivity between the center and a nearby hospital was critical to daily patient care as well as the continuing success of the business. In seeking replacements for an aging, inadequate wireless bridge, Medical X-Ray Center determined at least 700 Mb/s of bandwidth was needed to ensure access to a Picture Archiving Communications System (PACS) and customized patient treatment plans. Fiber-optic services were considered but rejected after discovering leased lines would cost approximately $10,000 a month.

After a thorough evaluation, Medical X-Ray Center deployed a combination of license-free 60 GHz and licensed-band 80 GHz wireless links with adaptive rate technology for increased link availability. The links have performed flawlessly, with adaptive rate ensuring maximum uptime with transparent switch-over during heavy rainfall.

GIGABIT WIRELESS GAINS MOMENTUM
The ability to go farther with high availability will continue to create unprecedented opportunities in the GigE wireless market for an assortment of implementations, including inter-building LAN extensions and IT consolidations; fiber redundancy and business continuity; IP video entertainment, conferencing and video surveillance; low-latency, high QoS VoIP; medical imaging and electronic records; IT outsourcing and 4G/WiMAX mobile backhaul. When the impact of heavy rainfall is a top consideration, adaptive rate technology is providing a "future proof" solution that delivers up to "five nines" link uptime—in all weather conditions.

About the Author
Gregg Levin is Senior Vice President and Chief Marketing Officer at BridgeWave Communications, (408) 567-6900; www.bridgewave.com

Click here for Illustrations:

Figure 1, Figure 2