Of all the wavelengths in the spectrum used for wireless data transmission, perhaps the least well known is the millimeter wave band. However, it is precisely this band (and the continuous bandwidth it provides) that enables wireless data transmission at speeds and bandwidth that compare to the high quality of fiber optic communication systems.

Millimeter waves (30-300gHz) are a subset of the microwave band, which is itself part of the larger radio wave spectrum. These waves derive their name from the size of the wavelength, which measures from one to 10 millimeters.

Unlike low frequency radio signals, millimeter waves are not appropriate for long distance transmissions through the atmosphere, due to higher signal loss. Instead, MMW radios typically operate over distances of several kilometers using highly directional, pencil-thin beams that also help prevent interference.

It is this characteristic, along with continuous bandwidth not available at more commonly used lower frequencies, that makes millimeter wave technology the ideal solution for point-to-point, high-speed, high-bandwidth wireless.

The technology, available as commercial transmitter/receiver units that operate at gigabit per second speed, is already being utilized in multi-billion dollar markets such as cellular communications for the next generation of micro and picocell towers, high definition/3D digital video for broadcasting organizations and the motion picture industry, and for high frequency trading on Wall Street.

The Millimeter Wave

In spite of the relative anonymity of MMW radios in the commercial area, the MMW spectrum has been utilized for military satellite-satellite communications for decades.

Due to the dramatically reduced costs recently of MMW integrated circuits (a trend that is expected to continue), the technology is now being increasingly utilized for commercial applications.

The incredible promise of the millimeter wave, however, has as much to do with the Federal Communications Commission as any other factor. The FCC was formed by the Communications Act of 1934.  As part of its mandate, the FCC (News - Alert) allocates specific wavelength frequencies for everything from FM/AM radio stations to television, cell phones, satellites, aeronautics, and the military – to name a few. However, with the explosion of wireless applications, most are jammed into small bands at lower frequencies of the radio spectrum.

Although the millimeter wave band is also regulated by the FCC, if the more crowded bands can be compared to the population per square foot of Manhattan, then the wide open expanses available to millimeter waves are more like Yellowstone National Park. This extra space is critical because it provides the continuous bandwidth required for high-bandwidth, high-speed data transmission. Without it, lower frequency products (despite being capable of such speeds were it not for their neighbors) are hitting a glass ceiling that even refinements and improvements in wireless technology cannot overcome.

Lower frequency allocations, for example, are typically 2-5mHz. In the millimeter wave spectrum the total allocation potential is up to 250gHz, with 5, 7, 10, 15, even 20gHz of continuous bandwidth available. 

With so much room to work with, practical data rates in the millimeter band top out above 40gbps.

Wireless for Next Generation Cellular

The highly directional characteristic of millimeter waves is ideally suited to cellular communications, particularly in crowded urban environments.

In a market that analysts estimate will exceed $5 billion by 2015, the installation of small base stations called micro and picocells is expected to outnumber traditional cell towers by as much as 20 to one. Micro and picocells cover only a limited area, but require less power, cost less and have a much smaller footprint than larger macro cell towers. This makes them ideal for indoor locations such as entertainment venues, malls, airports, train stations, office buildings, and hotels.

But the advent of next generation cellular networks is creating a new backhaul connectivity problem: how to connect the growing number of smaller base stations to the core, either through wired or wireless connections. This is exacerbated by concerns over frequency congestion and interference in dense cell deployments where four or more picocells could be mounted on light poles in a single parking lot or on a rooftop. 

The most 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 to each micro 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 products are being considered by providers. With typical link distances for picocell backhaul estimated at a few hundred meters between sites, and microcells less than two kilometers, millimeter wave products are ideally suite for such applications.

“If you can’t run fiber optic cabling, millimeter wave wireless is the fastest, quickest, smallest and least expensive solution,” says Wayne Pleasant, former chairman of the Wireless Communication Industry Association committee charged with helping the FCC establish guidelines for the 80gHz light licensed millimeter wave band.

“In many key ways millimeter wave devices can be more reliable, and even faster, than fiber optics,” says Pleasant.  “Due to a reduction in latency, transmission speed is improved.”

Millimeter wave radios require only very small antennas, measured in inches rather than feet for Wi-Fi and other wireless options. This addresses the concern over potential visual pollution caused when mounting a large quantity of such products to light poles, billboards, or sides of buildings. 

“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,” says Pleasant.

Jeff Elliott is a Torrance, Calif.-based technical writer. He put together this article for Renaissance Electronics (News - Alert) & Communications LLC.