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.
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