In the 1960s, the roots of ultrawideband wireless began with work that was generally referred to as impulse radio, baseband or carrier-free communications. Through the 1980s, development was focused primarily on U.S. Department of Defense (DoD) applications, such as ground-penetrating radar. As UWB's potential benefits for public safety, enterprise and consumer communications became more apparent, the FCC began allocating unlicensed radio frequency spectrum.

Prior to final ruling by the FCC regarding the UWB spectrum, some developers moved ahead with implementations that were basically derivative of earlier impulse radio technologies. Pulse-based single-carrier approaches, such as DS-UWB, essentially used one large channel encompassing the entire available spectrum. This had the advantage of leveraging a relatively simple transmitter design but had major inherent disadvantages. These include difficulty in collecting enough signal energy, stringent transmit/receive switching time requirements, analog signal processing challenges and potentially wasted spectral resources to avoid narrowband interference.

Fortunately, the FCC's ultimate ruling provided much more implementation flexibility by not requiring a single channel to use the entire 7.5 GHz spectrum. The FCC has now defined UWB as any radio technology with a spectrum that occupies greater than 20% of the center frequency or a minimum of 500 MHz and has allocated unlicensed spectrum from 3.1 GHz to 10.6 GHz for short-range UWB communications. In effect, the FCC ended up allocating spectrum for UWB but did not dictate the specific radio technology to be used for implementation. This significantly expanded the options avail-able to designers of UWB communications, enabling them to use a combination of sub-bands within the spectrum to optimize performance, minimize power consumption, improve reliability and enhance overall usage of available spectrum resources.

MB-OFDM transmits data simultaneously over multiple carriers spaced apart at precise frequencies. Inherent advantages of MB-OFDM include high spectral flexibility and resiliency to RF interference, which allows MB-OFDM to provide superior resiliency because interference within specific channels can be overcome by dynamically reallocating traffic flows. Given the unlicensed nature of the UWB spectrum, in which many wireless devices must coexist, enhanced spectral flexibility is an important factor that enables MB-OFDM to comply with local requirements by dynamically turning off certain tones or channels in software. With 128 separate OFDM tones carrying traffic in each 500 MHz channel, narrowband interference at virtually any frequency can be overcome simply by nulling out effected tones. This provides an additional level of granularity for enhancing RF resilience within each channel, without compromising FCC compliance because the 500 MHz channel structure remains in place and coexistence requirements are met.

By looking at the design opportunity from a goal of effectively using the available allocated spectrum for optimal results, rather than approaching it from a particular technology mindset, MD-OFDM clearly offered the greatest opportunity for success. (It is also useful to note that OFDM technology has been proven in other high-performance, widely deployed communications systems including, Wi-Fi 802.11a/g, WiMAX 802.16a, HomePlug and global ASDL standards.)

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