With wireless everywhere, designers have incredible options, including high-end HSDPA/WCDMA cellular technology at one end of the spectrum and near-field communications and RFID on the other.
Short-range wireless technologies abound, like the ever popular Wi-Fi, the ubiquitous Bluetooth, the commercial/industrial ZigBee, the emerging 60-GHz WirelessHD, and proprietary industrial, scientific, and medical (ISM) designs. There’s something for everyone. But where does Ultra-Wideband (UWB) fit? After more than five years of development and a year of product availability, UWB is more than ready for prime time.
UWB has always been a bit different. It started out as a pulse technology transmitting binary data as very short wavelets or impulses using a pulse position or pulse phase modulation scheme at very low power. The resulting signal resembled that generated by the original spark gap radio of the late 1800s and early 1900s.
The technology produces a very wide bandwidth, hundreds of megahertz or gigahertz wide, occupying a huge swath of spectrum. Yet its low power level means it offers very little if any interference with other services over the operating bandwidth. This made it a very secure kind of signal that the government and military glommed on to and developed for high-resolution radar and stealth communications.
In the late 1990s and early 2000s, UWB emerged from the “dark side” as a new shortrange, high-speed wireless technology. The FCC authorized the use of UWB in 2002, setting off a commercial bonanza of chip development. Out of that activity, a single international standard has emerged, a form of UWB called multiband orthogonal frequencydivision multiplexing (MB-OFDM).
A group of companies joined to form the WiMedia Alliance, which promotes this form of UWB and provides testing and certification processes to ensure interoperability among different products. In 2005, WiMedia became a formal standard of Ecma International (ECMA 368 and ECMA-369), leading to recognition by the International Standards Organization (ISO) as an international wireless standard.
Virtually all new wireless technologies today are based on OFDM. In this broadband approach, the high-speed data stream is divided into multiple slower streams and used to modulate one of hundreds or even thousands of adjacent carriers spread over a wide bandwidth. Since the narrow-band carriers are orthogonal, they do not affect one another.
However, they do provide a wide bandwidth signal that is ever so tolerant of non-line-of-sight and multipath conditions common in the microwave spectrum. The resulting signal is then reassembled at the receiver. What makes this complex technique possible is digital signal processing (DSP), an inverse fast Fourier transform (FFT) for transmission, and an FFT for reception. Add to that the various coding techniques and transmit at a very low power level, and you have UWB.
WiMedia UWB uses a 128-point FFT where each point defines a channel, tone, or bin in the spectrum. The channels are 4.125 MHz wide. The resulting OFDM signal is 528 MHz wide. Signals that are at least 500 MHz wide qualify as UWB, according to the FCC. The modulation within each channel can be binary phaseshift keying (BPSK), quadrature phase-shift keying (QPSK), or some quadrature amplitude modulation (QAM) variation to ensure high speed.
While a UWB signal may use only one channel, the WiMedia radios use three adjacent 528-MHz bands for transmission, implementing a frequency-hopping approach that can boost the average transmitter power and increase range. The spectrum assigned to UWB in the U.S. ranges from 3.1 to 10.6 GHz. That range is divided into 14 bands of 528 MHz (Fig. 1).
These bands are divided into band groups. Some groups are three bands wide, and others are two bands wide. The UWB radios may use just one band or alternatively two or three bands with a frequency-hopping scheme. Obviously, at least to a wireless engineer, the three lower bands called band group 1 are used the most because their range can be greater—not to mention that ICs in this lower range are easier to make.
The problem is that not all countries use the same UWB spectrum, putting UWB chip and end-product manufacturers in a quandary. Band group 6 (7.3 to 8.8 GHz) is the only band where all countries agree (Fig. 2). Some countries are still considering their options and there may be more common agreement in the future, especially if the radios adopt what is called detect and avoid (DAA). DAA circuits listen to the bands (e.g., 3.1 to 4.2 GHz). If signals are present, the radio reduces the power level in the frequency range where signals were detected. This minimizes or prevents interference.
Continue on Page 2
One of the most stringent FCC regulations is UWB’s low transmit power level. The maximum output transmit power is –41.3 dB/Hz when averaged over a 1-ms period. The instantaneous peak power cannot exceed 0 dBm. The average power level also depends on whether the radio is using a single band or is hopping over a three-band range.
Such a low power level puts the UWB signal down in the noise in some cases. But it is so low, it typically won’t interfere with any other signal in that range, mainly radar, wireless local-area networks (802.11a), and other wireless services. This low power means short range but also an attendant high level of security. Add encryption, and you have one of the most secure wireless technologies around.
The data transmission rates for UWB span 53.3 to 480 Mbits/s. That rate depends primarily upon environmental conditions and range. The maximum range is about 10 m, where the lower rate will prevail. The maximum rate can generally only occur over a 3- to 4-m range assuming few if any obstructions.
A key characteristic of the WiMedia standard is the WiMedia Common Radio Platform (Fig. 3). It consists of the basic UWB physical layer (PHY) and media access controller (MAC) layers, which can readily support higher-level protocol stacks and their protocol adoption layers (PALs).
The WiMedia architecture provides mechanisms for device discovery, wireless personal-area network (WPAN) management and medium access arbitration, and device power management, as well as two independent data transfer mechanisms for the secure exchange of data. Quality of service (QoS) provisions support latency-sensitive applications. Already, several other special interest groups (SIGs) and companies have taken advantage of this feature.
The Wireless USB Implementer’s Forum defines a wireless version of its USB 2.0 interface, which is used by virtually every PC, laptop, and peripheral device. The Bluetooth SIG recently chose WiMedia UWB as an “alternate MAC PHY” or “AMP” for higher speeds in the near future. And, some companies have developed proprietary protocols for special applications. For greater detail on WiMedia, go to www.wimedia.org.
Designers are still discovering many possible applications for UWB’s unique capabilities. The Bluetooth application of UWB will open some new possibilities. The movement to a Bluetooth version also is causing UWB chip vendors to make new versions of their chips to operate in the higher band groups. The Bluetooth SIG requires UWB chips to operate above 6 GHz specifically in band group 6. A faster version will eventually emerge to provide that magical 1-Gbit/s data rate.