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Bridging the Last Mile

March 2, 2012

All telecommunication into my house is via fiberoptics. I had a copper wire POTS (Plain Old Telephone Service) with a somewhat fast DSL Internet connection until just a few years ago. I never had a cable television service since there wasn't that much to entertain me on those hundreds of extra channels; and our proximity to New York City gave us all major television networks (ABC, CBS, Fox, NBC, and PBS) and a few independent stations.

All that changed at the digital television changeover, as I reported in a previous article (Public Photons, August 25, 2010). This migration to digital broadcast television involved moving broadcast stations to higher, UHF frequencies where the propagation is not as good as for VHF. Half of my television stations disappeared, although the ones that I was able to receive had very nice video.

As a consequence, I was forced into cable television, but my cable of choice was fiberoptic. At the same time, my landline telephone and Internet were connected through the fiber. This fiberoptic system competes with a coaxial cable system in my area.

The coaxial cable system attempts to compete by offering a lower price, but coaxial cable has a lower bandwidth than an optical fiber. In short, it's a telecommunications medium that's had a healthy half-century run, but it's a relic of technologies past.

Figure caption

It starts as a beam of light in a pencil-thin cable, but it takes a lot of electronics to interface the optical signals with the real world.

In this FiOS® installation, the interface box is at the left, a back-up battery and its charger are at the right, and a MoCA router with its wireless antenna is seen below.

(Via Wikimedia Commons))

I've had cable salespeople come to my door attempting to sway me with their low price. One of their stock speeches, and one that's seen also in print ads and in television commercials, is that their networks are fiberoptic, also. Their problem, however, is what's called the "last mile" in industry parlance.

The "last mile" is the technology that connects from the major feeder lines to the house. My house is fiber all the way. Their last mile is a considerable length of coaxial cable, usually from the nearest big box of electronics hanging from a utility pole. From that point onwards, the bandwidth is limited by the cable bandwidth, which could be shared among several homes. That's alright for channelized television, but it can be a problem for future data services.

Now that wireless technology is capable of high bandwidth at low cost, many groups are investigating the use of wireless to bridge that last mile. One such group from the Institute of Photonics and Quantum Electronics of the Karlsruhe Institute of Technology (Karlsruhe, Germany) will be presenting their work at the forthcoming Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference (OFC/NFOEC) in Los Angeles, March 4–8, 2012.

High bandwidth at radio frequencies requires a proportionately high carrier frequency, and the Karlsruhe research team is using millimeter-wave frequency of 220 gigahertz. This is nearly ten times the frequency of the ISM band of 24.125 gigahertz that's so high in frequency that it's infrequently used, and quite a bit above the Wi-Fi frequency of 5.8 gigahertz. Not surprisingly, frequencies of 30-300 gigahertz are in what's called the extremely high frequency (EHF) band. The high carrier frequency allowed the Karlsruhe research team to achieve a 20 gigabit-per-second data rate.

This technology competes with optical free-space communications that I reviewed in a previous article (Free-Space Optical Communications, August 18, 2011). Optical communications are affected by weather conditions, such as fog, rain and dust, which are less of a problem for radio. Maintenance of optical systems, such as keeping windows clean would be another problem.

The team developed a wireless gateway that converts data from an optical fiber to the millimeter wave signals feeding an antenna. A complementary gateway at the receiver end does the conversion from radio frequency to optical signals. Fraunhofer IAF participated in the development of the conversion modules. At this point, their wireless transmission distance is about twenty meters, still short of a mile, but that range would span the distance from the nearest utility pole to my house.

Project leader, Ingmar Kallfass, summarized the technology as follows:
"The challenge in integrating a wireless link into a fiber optic environment is to ensure that the wireless link supports data rates comparable to those of the optical link—ideally about 100 gigabits per second (Gbit/s)—and that it's transparent to the data... Besides optoelectronic conversion, no further processing must be involved before the signals reach the antenna. This also holds for the receiving part in a reversed sequence."

This experiment was part of a larger MILLILINK project, led by the Fraunhofer IAF and funded by the German Federal Ministry of Research and Education. The Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference is managed by the Optical Society (formerly, the Optical Society of America, OSA). It's sponsored by OSA, the Communications Society of the IEEE, and the IEEE Photonics Society. Telcordia Technologies acts as a non-financial technical co-sponsor.

Passive millimeter wave image

Another application of millimeter wave technology, passive millimeter wave imaging.

In this application, the natural millimeter wave emissions of the human body are used to scan for concealed objects.

(Via Wikimedia Commons))


  1. Angela Stark, "Record-Speed Wireless Data Bridge Demonstrated: Takes High-Speed Communications the 'Last Mile'," Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference Press Release, February 27, 2012.
  2. Swen Koenig, Jochen Antes, Daniel Lopez-Diaz, Ingmar Kallfass, Thomas Zwick, Christian Koos, Wolfgang Freude and Juerg Leuthold, "High-Speed Wireless Bridge at 220 GHz Connecting Two Fiber-Optic Links Each Spanning up to 20 km," Paper OM2B.1, March 5, 2012, 1:30 PM, of Millimeter-Wave Photonics Session.
  3. Web Site for Optical Fiber Communication Conference and Exposition/National Fiber Optic Engineers Conference, March 4-8, 2012.

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