MM-Wave Signals Serve Cell Sites

May 18, 2011
The large bandwidths available in certain millimeter-wave frequency bands makes them attractive for use in communications networks that must securely transfer large amounts of data over moderate distances.

Millimeter-wave components and technology have long been associated with radiometry and secure point-to-point communications. But as the means to generate and detect signals at frequencies above 30 GHz become more practical, the uses for millimeter-wave components and subsystems become more widespread. Credit can be given to improved measurement capabilities as well as to the enhanced capabilities of electromagnetic (EM) simulation software tools. As design and measurement methods become more efficient, millimeter-wave designs become more cost effective, enabling their consideration as solutions for a wide range of applications, from automotive cruise control systems and airport threat detection imaging systems to high-data-rate personal area network (PAN) communications equipment.

Millimeter-wave frequencies are generally considered from 30 to 300 GHz, with wavelengths of about 1 to 10 mm. Because of the small wavelengths, circuit dimensions and structures are proportionally fine and typically difficult to machine. Although the production of coaxial cables and connectors has improved to the point where coaxial lines can support frequencies well into the millimeter-wave range typically to about 70 GHz the transmission line of choice for most higher millimeter-wave frequencies is the waveguide, often referred to as "plumbing" due to its pipelike appearance. Waveguide sections are available in various formats, including rectangular and circular types, with dimensions also tied to frequency and wavelength, and becoming minute as frequencies increase beyond 100 GHz.

In spite of traditional difficulties in machining (and testing) millimeter-wave components, including antennas and waveguide, the available bandwidth is attractive for many communications applications. For example, as service providers for the latest wireless communications networks including fourth-generation (4G) cellular systems using Long Term Evolution (LTE) and WiMAX technologies strive to keep pace with customers' growing demands to move large amounts of data, they are increasingly faced with "data jams" at the backhaul connections from one cell site to another. Some of these connections have traditionally been made by long runs of cable or fiber-optic lines.

But microwave radios, specifically in the unlicensed 60-GHz band, represent an attractive alternative to metal cables or glass fibers. In the United States, that unlicensed spectrum runs from 57 to 64 GHz, while in Japan, it extends from 59 to 66 GHz, or 5 GHz of overlapping spectrum. this bandwidth supports data rates to 1 Gb/s. In addition, numerous millimeter-wave frequency bands, such as 60 GHz, are characterized by atmospheric attenuation, rendering these bands almost immune to interception and thus fairly secure for communication of sensitive data.

The use of such frequencies for line-of-sight, short-haul communications was not economically feasible just a few years ago. But advances in millimeter-wave-frequency building-block integrated circuits (ICs), specifically based on silicon IC technologies such as silicon-germanium (SiGe) or silicon CMOS devices, have enabled the development of affordable millimeter-wave components for 60-GHz communications links as well as in 77-GHz adaptive cruise control systems for automotive applications, in licensed E-band point-to-point communications at 71 to 76 GHz and 81 to 86 GHz, and in 94-GHz imaging and data communications systems.

To make radios more affordable, for example, Vubiq, Inc. has worked with silicon-germanium (SiGe) millimeter-wave ICs from IBM to develop low-cost, high-data-rate backhaul radios for use at 60 GHz. The firm created a unique patent-pending WR-15 waveguide package that houses the radio, as well as an embedded clock, intermediate-frequency (IF) electronics, and supporting components.

With its millimeter-wave division located in St. Petersburg, Russia, ELVA-1 has been in business since 1993 designing and manufacturing millimeter-wave components, subsystems, and test equipment, notably for use in select atmospheric absorption bands such as 42 GHz, 70 and 80 GHz, and 94 GHz that enable secure communications. The largest supplier of millimeter--wave hardware in Russia, ELVA-1 offers certified radios in bands of 71 to 76 GHz, 81 to 86 GHz, and 92 to 95 GHz. The radios support full-duplex communications with capacity to 1.25 Gb/s using complex modulation, including quadrature-phase shift-keying (QPSK) and quadrature-amplitude-modulation (QAM).

Backhaul radio vendors, including Exalt Communications and Bridgewave Communications offer backhaul radios operating on licensed and unlicensed millimeter-wave bands at 42 and 60 GHz, respectively. Exalt's ExtremeAir outdoor radios operate in licensed bands to 42 GHz at distances to 24 miles. Bridgewave's products, at 60 and 80 GHz, are suitable for shorter-distance connections.

For those designing millimeter-wave systems from the component level up, a large number of reliable component suppliers offer the passive and active components needed to assemble a radio transceiver or detection system, such as Aerowave, Ducommun, Endwave, Herley Defense Electronics, Microsemi, Millitech, QuinStar Technology, and Spacek Labs. Spacek, for example, offers both active and passive millimeter components, including amplifiers, filters, mixers, and Gunn oscillators through 110 GHz. To aid designers, Aerowave's Paul Chorney offers several calculators (Excel spreadsheets) that are useful for working with waveguide at mm-wave frequencies. Similarly, Ducommun features several references on its website, including rectangular and circular waveguide and flange designations, as well as conversion charts.

One of the keys in developing affordable millimeter-wave device solutions is also designing packages that are affordable at those frequencies, and Endwave has been successful at adapting conventional surface-mount packages to millimeter-wave applications. The firm, a leading provider of high-frequency monolithic microwave integrated circuits (MMIC) and integrated transmit/receive modules, had an internal need for an enhanced high-frequency package. By re-engineering quad-flat-no-lead (QFN) packages, Endwave's engineers were able to double the effective upper-frequency limit from 25 GHz to 50 GHz.

While communications links offer the most promising opportunities for millimeter-wave technology, it has been applied in other key market areas, including in the war on terrorism. Millivision, for example, has developed passive threat detection systems based on millimeter-wave technology. These systems do not generate EM, but use passive millimeter-wave sensors to detect the radiation emitted by organisms. The sensors work with associated video cameras to detect objects on a subject. The firm's Automatic Threat Detection (ATD) Tool merges detected signals into a real-time video image, so that millimeter-wave signals themselves need not be viewed directly. The systems can resolve objects smaller than 2 in. on a side.

For research applications, SynView has developed a three-dimensional (3D) imaging system based on terahertz-frequency (100 GHz to 10 THz) imaging. Many materials, such as paper, plastic, and composites, are transparent to terahertz radiation, making it possible to look inside closed containers (much like x-ray radiation).

Finally, in order to adapt commercial test equipment (such as signal generators, spectrum analyzers, and VNAs) to the frequency ranges of millimeter-wave components for measurement purposes, a number of companies provide measurement accessories (like multipliers and VNA extenders), such as Millitech, OML, and Virginia Diodes. OML is planning to extend the range of its VNA modules through 500 GHz, while Virginia Diodes already offers VNA extenders operating from 750 through 1050 GHz.

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