Millimeter-wave frequencies offer wide usable wireless bandwidths in parts of the spectrum not normally used by electronic devices. Frequencies from 30 to 300 GHz represent a subset of the microwave frequency band, named for the sizes of these wavelengths, which range from one to ten millimeters. Because these millimeter-wave signals suffer high atmospheric losses, they are not suitable for long-distance transmissions. But they can support effective communications over distances to several kilometers using highly directional, “pencil” thin beams that also help prevent interference. Since these frequencies offer continuous bandwidth not available at more commonly used lower frequencies, millimeter-wave technology is an ideal solution for shorter-range, point-to-point, high-speed, high-bandwidth wireless applications.

Millimeter-wave technology, which is available in commercial transmitter/receiver units that can handle high-speed data rates at gigabit/s (Gb/s) transmission speeds, is used in various multi-billion-dollar markets, including cellular communications for next-generation microcell and picocell towers, for wireless trading on Wall Street, and for high-definition/three-dimensional (HD/3D) digital video for broadcasting and motion-picture industries. In addition, millimeter-wave technology has been reliably used for military communications applications for decades.

As the costs of millimeter-wave (MMW) integrated circuits (ICs) continue to decrease, the technology should see increasing use in commercial applications. Also, the promise of millimeter-wave technology has as much to do with the United States Federal Communications Commission (FCC) as any other factor. The FCC was formed by the Communications Act of 1934. As part of its mandate, the FCC allocates specific wavelength frequencies for everything from AM/FM radio stations to television, cellular telephones, satellite communications, aeronautics, and military applications—to name a few. 

Millimeter Waves Extend Wireless Services, Fig. 1

So far, many wireless applications have been crammed into narrow bandwidths at lower frequencies of the radio spectrum. The MMW bands offer available frequencies with wide bandwidths for analog and digital communications. These higher MMW frequencies provide bandwidths in support of applications that are hitting a “glass ceiling” at lower frequencies, that even refinements and improvements in wireless technology cannot overcome because of the limited bandwidths. Lower-frequency wireless allocations, for example, are typically 2 to 5 MHz in bandwidth. In the millimeter-wave spectrum, the total allocation potential is as much as 250 GHz, with 5, 7, 10, 15, and even 20 GHz of continuous bandwidth available.  These millimeter-wave bands readily support practical data rates to 40 Gb/s and more. 

Next-Generation Cellular

Millimeter-wave signals are highly directional in their propagation characteristics, making them well suited for cellular communications applications in crowded urban environments. In a market that analysts estimate will exceed $5 billion by 2015, the building of small base stations called microcells and picocells is expected to outnumber the installation of traditional cellular towers by a factor of as much as 20 to 1. Although these microcells and picocells cover smaller areas than traditional cellular stations, they require less power, with less cost, and occupy much smaller footprints than traditional “macro” cellular towers. This makes them ideal for installation in indoor locations such as entertainment venues, malls, airports, train stations, office buildings, and hotels.

Millimeter Waves Extend Wireless Services, Fig. 2

This growing number of smaller cellular base stations is also creating a new backhaul connectivity problem: how to link these greater numbers of smaller cell sites, either through wired or wireless connections. In addition are concerns over frequency congestion and interference in dense cellular deployments where four or more picocells could be mounted on light poles in a single parking lot or on a rooftop. 

An obvious solution for high-speed transmission of data-intensive content would be to establish a physical connection using fiber-optic cabling. However, the cost and challenge of implementing fiber-optic links to each microcell or picocell site is prohibitive, particularly in urban areas where streets and sidewalks cannot easily be trenched. As a result, outdoor “fiber-optic quality” wireless millimeter-wave products are currently being considered by providers.  With typical link distances for picocell backhaul connections estimated at a few hundred meters between sites, and microcells at less than two kilometers, millimeter-wave transmission and reception products are ideally suited for such applications.

Wayne Pleasant is the former Chairman of the Wireless Communication Industry Association (WCIA) committee charged with helping the FCC establishes guidelines for the 80-GHz light licensed millimeter-wave band. He notes that “if you can’t run fiber optic cabling, millimeter wave wireless is the fastest, quickest, smallest and least expensive solution.” Pleasant notes that, “in many key ways, millimeter wave devices can be more reliable, and even faster, than fiber optics. Due to a reduction in latency, transmission speed is improved.” Millimeter-wave radios operate with small antennas in proportion to their small wavelengths, so that concerns for potential “visual pollution” caused when mounting a large quantity of such products to light poles, billboards, or sides of buildings are minimized.

Pleasant observes that “narrow beam antennas allow systems in these bands to be engineered in close proximity to one another without causing interference.  Since a greater number of highly directive antennas can be placed in a given area, the net result is higher reuse of the spectrum, and higher density of potential users.”

In preparation for these expanding microcell and picocell requirements for millimeter-wave links, Renaissance Electronics and Communications (REC) and its wholly owned subsidiary, HXI, have developed a number of millimeter-wave radio products capable of high data rates. The firm has supported high-frequency component and subsystem military and commercial applications since 1991, with its GigaLink Light Speed radios the first millimeter-wave radios to achieve FCC certification for unlicensed and light licensed commercial applications in the 60- and 70-GHz millimeter-wave bands. With their generous available bandwidths at these frequencies, these radios support full-duplex communications throughputs of 1.25 Gb/s and higher. They are designed to minimize latency, or lags, in data transmission, which is critical to the next generation of data centric devices that must accommodate Voice over IP (VoIP), live digital streaming, large file downloads, and video conferencing through mobile handsets.