Sunday, February 20, 2005

UWB From Titanic to Today

Ultra Wideband, UWB, is touted as a once secret esoteric military technology now being readied to take over the short range wireless market. It could potentially lay waste to Bluetooth, ZigBee and Wi-Fi as it offers the desirable characteristics of very high bandwidth and low power consumption. It could potentially lay waste to all communications if ever allowed to rule the airwaves like it once did, nearly a century ago.

Ultra Wideband is a bizarre transmission system by conventional radio design practice. First of all, there is no carrier wave like you have with AM and FM. That's not unprecedented. SSB or single sideband radio sets are often used for two-way communications on the shortwave bands. The carrier isn't really mandatory. It simply makes for a simple transmitter design and ease of tuning at the receiver. In most communications schemes, the information (audio, video, data, etc.) rides on a carrier wave that is discarded at the receiving end. Instead of a carrier, UWB transmissions consist of a stream of pulses only picoseconds wide. A nanosecond is 1,000 picoseconds if that gives you an idea of how short these pulses are. The pulses themselves aren't ordinary sine waves, they're just impulses.

Short impulses result in high frequencies spread over a wide band. Thus the term ultrawideband. How ultra? A 4 GHz UWB transmission would have a minimum bandwidth of 1 GHz. If you tuned into a UWB transmission, it would sound like white noise on a regular radio. But isn't background noise on transmission channels made up of white noise? Ah-ha. Now you know why the military likes this technology. It's hard to detect without the right equipment. Anyone stumbling across a UWB signal thinks it's just extra noise and won't suspect there is a huge datastream being transmitted.

The invention of UWB tends to be credited to research on microwave transmission in the 1960s. Actually, what happened was that researchers finally had the mathematical theory to understand electrical impulse response plus the test equipment to check their theories. With that they could build a body of knowledge that would lead to practical UWB systems, at least modern UWB systems.

The very first UWB transmissions were made by Heinrich Hertz in 1887 and refined by Guglielmo Marconi into a transatlantic radio system in 1901. And this is where the Titanic comes in. It was Marconi spark transmitters that sent the CQD and SOS emergency signals from the doomed ocean liner in April of 1912.

Marconi had invented a radio system based on UWB simply because an electrical spark naturally generates impulse signals with very wide bandwidth. Any transmission system with a bandwidth greater than 25% of its center frequency can be considered ultrawideband. Radio pioneers considered that a liability rather than an asset. Each transmission hogged the entire low frequency spectrum, preventing other stations from being heard. They quickly adopted the narrowband system using sine wave carriers that allowed many stations to share a band of frequencies.

Today's circuit miniaturization and solid state components make it easy to use microwave bands in the GHz range where there is lots of spectral space available. Or so it seemed. UWB goes back to the idea of spreading out to occupy the whole available bandwidth. But today's wrinkle is sophisticated receivers that can decode complex signals riding just above the noise floor. No one, least of all the FCC, is going to allow UWB to ride roughshod on very expensive communications spectrum. In fact, UWB is now allowed only if it stays below the Part 15 "unlicensed" power levels and even less on frequencies used for GPS (Global Positioning System) transmissions.

Fortunately, UWB is so efficient that less than a milliwatt is all that is needed to transmit hundreds of Mbps as far as 30 feet. That puts it into the category of PAN or personal-area networking, now dominated by Bluetooth and WiFi. True, UWB as currently approved doesn't approach even Wi-Fi hotspot coverage. But who's to say what might be constructed with mesh networks and further advances in the technology?

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