Jack Browne
Technical Director

Millimeter-wave frequencies represent the next great frontier for broadband, albeit limited range, communications. Since the United States Federal Communications Commission (FCC) and other global regulatory agencies have designated or freed bandwidth at millimeter-wave frequencies for point-to-point communications links (see Microwaves & RF, August 2007, p. 40), a number of companies at the system level have experienced increased interest in their high-frequency links while some companies at the component level have shared in the enthusiasm. The growing demand for high-speed voice, data, and video interconnections around the world is driving frequency regulators to free up spectrum at millimeter- wave frequencies.

Millimeter-wave frequencies are generally accepted to be in the range from 30 to 300 GHz, with the name coming from the small size of the signal wavelengths at those frequencies. Components for those frequencies are usually based on waveguide interconnections, with the physical size of the waveguide corresponding to the portion of the millimeter-wave spectrum that is passed with minimal signal loss. Of course, many coaxial connector companies now offer connectors with interface dimensions as small as 1 mm for broadband use as wide as DC to 110 GHz (typically for broadband measurement applications), although most millimeter-wave applications fit within a relative small percentage of the total 30-to-300-GHz spectrum.

One of the companies offering complete point-to-point millimeter-wave link solutions is BridgeWave Communications (www.bridgewave.com), with thousands of 60- and 80-GHz wireless gigabit radios deployed worldwide. The firm's millimeter-wave radios support local-area-network (LAN) backbone extensions, mobile-telephone backhaul applications, and high-capacity Internet access. The radio systems are referred to as "wireless fiber" systems for their capability of providing the performance, reliability, and security of a fiber-optic link, but without the installation headaches.

The company's point-to-point, fixed wireless systems employ proprietary AdaptRate^TM technology and forward error- correction (FER) techniques to achieve reliable performance over the longest link distances possible for millimeter- wave frequencies. BridgeWave recently announced the additional availability of AdaptPath^TM technology to their systems, which allows the integration of their AdaptRate millimeter wave links with secondary connections using complementary wired or wireless technologies. The result is a multiple-technology solution with increased availability and range.

The AdaptPath link switching technology creates an all-weather, dual path data connection by teaming one of the company's 60- or 80-GHz wireless bridges with a lower-speed secondary communications path, such as an unlicensed 5-GHz radio bridge or a licensed 6- or 11-GHz link. Should environmental conditions degrade the performance of the millimeter-wave link, the AdaptPath technology automatically switches data traffic to the secondary path before data errors occur. According to Gregg Levin, senior vice president and chief marketing officer for the company, "AdaptPath is the next step in BridgeWave's strategy to offer enterprises, government entities, and network operators greater flexibility in meeting their network capacity, range, and uptime requirements."

Their customers concur. The combination of technologies has helped broadband service provider Roadstar Internet accelerate the rollout of reliable Internet services in the Washington, DC area. According to the firm's founder and CEO, Marty Dougherty, "Our state-of-the-art GigE wireless backbone enables us to be first to deliver next-generation access services in this fast-growing region. The combination of BridgeWave's AdaptRate and AdaptPath features takes us well beyond what's currently available in the industry."

Roadstar and BridgeWave determined rain fading rates for millimeter-wave signals in the service area and applied the combination of AdaptRate and AdaptPath technologies to deliver the highest-performance service possible even during heavy rainfall. The network employs the AdaptRate feature to switch from a full Gigabit Ethernet (GigE) data rate to a lower 100-Mb/s rate during heavy rains but, if needed, will engage the AdaptPath capability to switch data traffic to a secondary, rain-tolerant 40-Mb/s 5-GHz bridge. Once the rainfall has sufficiently eased, the system quickly reverts to the full GigE rate. Dougherty points out some hidden benefits of the BridgeWave technologies: "AdaptPath also reduces networking equipment costs and complexity since we don't have to provision redundant wireless paths using external Ethernet switches and routers."

Earlier this year, Endwave Corp. (www.endwave.com) introduced a transmitter/receiver module pair for use from 71 to 86 GHz (E-band), nominally for broadband point-to-point radios. The two modules combine the company's MLMS^TM (Multilithic Microsystems) and Epsilon^TM Packaging technologies to achieve small size and low cost with outstanding performance at millimeter-wave frequencies. The MLMS technology is an alternative to monolithic microwave integrated circuits (MMICs) using flip-chip devices and electromagnetic (EM) coupling methods to minimize circuit-board space and the number of wire bonds.

The MLMS approach supports a mixture of technologies, such as GaAs, indium-phosphide (InP), and silicon-germanium (SiGe) active devices on the same MLMS chip for a true system-on-chip (SoC) design at frequencies to 100 GHz. The company supports its MLMS designs with a growing library of MLMS models, including frequency mixers, multipliers, voltage- variable attenuators (VVAs), filters, and Lange couplers.

Epsilon Packaging replaces high-cost elements, such as machined housings, with lower-cost injection-molded metallized- plastic housing and metallized FR-4 circuit boards. The end result is a package with no machined metal parts that is mass producible with minimal weight and size. The packaging approach allows a combination of assembly techniques, such as surface-mount technology (SMT), chip-on-board, and bare chip-and-wire approaches, to be integrated into a single low-cost module capable of excellent performance at millimeter-wave frequencies.

The transmit module typically provides +16 dBm output power with 15-dB conversion gain. It features an integrated power detector on the transmit output. The receiver features a noise figure of better than 9 dB and better than 25- dB RF-to-IF conversion gain. It has an input 1-dB compression point of -25 dBm. Separate models are available for the 71-to-71-GHz and 81-to-86-GHz bands.