A Comparison between Ultra-Wideband and
Narrowband Transceivers
David Barras1, Frank Ellinger and Heinz Jäckel
Laboratory for Electronics, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland
1
ETA S.A./Swatch Group, Grenchen, Switzerland
Abstract – The Ultra-Wideband (UWB) technology has
been authorized for commercial applications since the
beginning of this year. This new technology will bring a
new way of thinking wireless communications: timedomain instead of frequency domain. This paper will first
introduce UWB communications and then will make a
short comparison with a standard narrowband
transceiver that can typically be found in WLAN and
WPAN systems.
Index Terms — Ultra-Wideband (UWB), wireless
communication, transceiver, FCC Part 15 amendment.
II. UWB FOR WIRELESS COMMUNICATIONS
The FCC has defined the -10 dB bandwidth of an UWB
signal to be greater than 25% of the center frequency or
greater than 1.5 GHz. UWB for radar applications have
been around for several decades, but with the very
recent adopted proposal of a new regulation with regard
to the use of UWB without license for handheld
wireless communications (between 3.1 GHz and 10.6
GHz, see Figure 1), new applications can be foreseen.
I. INTRODUCTION
On February the 14th 2002, the Federal
Communication Commission in US (FCC) has
approved the use of the very controversial UltraWideband (UWB) technology for commercial
applications [1]. A “First Report and Order” that
describes a proposal for the Part 15 Rule amendment
has been released in April 2002 [2]. At the heart of the
debate was the possibility of interference with other
wireless systems caused by the wideband nature of the
UWB emissions. The most threatened technology
among other security systems was the Global
Positioning System (GPS) with its "sub-noise floor"
power density signal. The U.S. Department of Defense
(DoD), after having voiced strong concerns over this
interference problem, finally obtained from the FCC a
low frequency bound for UWB commercial
applications at 3.1 GHz. However, other entities like
mobile phone providers or airline industries had urged
the FCC to prohibit the use of UWB technology below
6 GHz. The standard adopted represents a cautious first
step with UWB technology.
This decision has been obviously welcomed by all the
companies involved in this technology (Æther Wire &
Location, Intel, Pulse-LINK, Time Domain,
MultiSpectral Solutions, XtremeSpectrum,...). The
targeted applications for UWB technology are those
that traditionally suffer from the multipath fading effect
like indoor high-speed communications and positioning
[3] [4], ground penetrating radars (GPR), through-wall
and medical imaging systems or other security system
(anti-collision).
1
Figure 1: the FCC Part 15.109 average limit of 500
µV/m (measured at 3m distance in a 1 MHz
bandwidth i.e. -41.3 dBm) has been kept for UWB
communications between 3.1 and 10.6 GHz.
For these applications (typically WLAN like HiperLAN
or IEEE 802.11a), the bandwidth is clearly much
greater (about two order of magnitude) than the one
used for today's narrowband communication systems
(see Table 1). But the maximum permissible average
power per MHz is about three order of magnitude
smaller than those in ISM bands, where typically 200
mW EIRP (Effective Isotropically Radiated Power) in
the lower 20 MHz width bands are allowed for indoor
communications. This results in a decrease of about
five orders of magnitude for signal-to-noise ratio for
UWB.
Narrowband
Wideband
Ultra-Wideband
-10 dB Bandwidth
Center frequency
0.01
0.01 … 0.25
0.25 or bandwidth 1.5 GHz
Table 1:Bandwidth definition of wireless systems
However, the maximum theoretical channel capacity
with additional Gaussian noise (see Eq. 1) near the
power emission limit is far greater than today's systems.
This is due to the fact that the capacity C (in bits/sec) is
linearly dependent with the channel bandwidth B (in
Hz), but only logarithmically dependent with the
signal-to-noise ratio S/N.
Eq. 1:
C = B·log2(1+S/N)
[bits/s]
A simple comparison has been done Figure 2. In this
example, the maximum theoretical channel capacity of
a narrowband system is compared to an UWB system.
The narrowband system has a bandwidth of 20 MHz
(like HiperLAN or IEEE 802.11a) and power spectral
density (PSD) limit of 10 mW/MHz for indoor use
(dotted line). The UWB system has a 2 GHz bandwidth
and -41.3 dBm/MHz of PSD limit. A typical indoor
fading with a propagation exponent of 3.5 [5] has been
considered to give an approximation of the channel
capacity vs. distance.
Figure 2: Maximum theoretical channel capacity in
a typical indoor environment
This Figure 2 shows clearly the potentials of UWB for
short wireless range communications. The channel
capacity remains greater or equal than one order of
magnitude than today's narrowband systems for
distances under about 70 meters. Beyond this limit, the
argument of the logarithmic term of Eq. 1 becomes
smaller than 2 and the capacity decrease rapidly. This
makes UWB to be a strong candidate for future
physical layer in high data rate WPANs, like IEEE
802.15.3 [6].
This very low emitted power (typically 100 µW for a 2
GHz bandwidth UWB system) translates into a
2
significant benefit for all other type of portable or
wearable devices with lower data rate but with higher
spatial device density like mobile phones, PDA or
wristwatches. Actually, the battery life in a UWB
system will no longer be limited by the necessary
emitted output power at the antenna, but by the backend consumption (modem and processing), which is
prone to further enhancement with silicon downscaling
[7] and pulse detection techniques.
III. UWB TRANSCEIVER ARCHITECTURE
A. Towards “Software-Defined Radios”
Today's radio technology is mainly based on the Edwin
Armstrong's super-heterodyne architecture. The early
success of this technology has lead to the frequency
discrimination in the spectrum we have today, each
application having its frequency band. UWB will use
the concept of time and/or code discrimination instead
of frequency discrimination. The complexity of an
UWB device will be very close to the one of a today's
GPS device where front-end hardware carry weight for
about 20% of the development effort vs. the remaining
80% for the software development. UWB will push this
sharing out even further and will make future radio
device implementations to finally leave the concept of
"dedicated radio" to reach the concept of "softwaredefined radios" (SDR). SDR will bring the flexibility in
radio device design, allowing by software changes to
develop short-range/high data rate or long-range/low
data rate applications with decimeter location abilities,
or all at the same time. This ability of an UWB device
to know its location towards other devices could bring
interesting solutions in managing mobile ad-hoc
wireless area networks (WAN) as described in project
such as Terminodes [8] or as studied by the MANET
(Mobile Ad-hoc NETwork) working group in IETF [9].
B. Narrowband vs. UWB
Figure 3 shows a comparison between a typical
narrowband and a UWB transceiver. Furthermore,
antenna, front-end, analog and digital baseband are
compared qualitatively in Table 2.
Front-end design challenges for receiving UWB signals
consist mainly in realizing efficient wideband devices.
Losses coming from the difficulty in having powerefficient antennas (especially electrically small ones
[10] [11]), pulse shape distortions and ringing due to
filter characteristics of the antenna and the
communication channel, losses due to wideband
matching and consumption of wideband low-noise
amplifier are major challenges in implementing UWB
front-end in portable devices. For the emitting part, the
antenna can be driven directly from CMOS devices, but
need to have a low impedance characteristic in order to
have sufficient high current from a low voltage source
flowing on the antenna structure. These antennas are
better known as "large-current radiators" [12].
Antenna
Radio
Frequency
Intermediate
Frequency
Analog
Baseband
Front-end
LNA
PA
HF Filters
Up/Down
conversion
AGC, Filter
RF mixers
Analog
Baseband
I/Q channel
LF Filters
AD/DA
Local
oscillator
PLL
RF Osc
Digital
Baseband
System &
Peripherals
Digital
Baseband
Modulation
Demodulation
Synch
Encoding
Decoding
µC
Memory
Core
uP
Memory
Human
Interface
Power
Battery
Narrowband transceiver architecture
Front-end
LNA, AGC
PA or Driver
(Filter)
Analog
Baseband
AGC
Pulse det.
Pulse gen.
AD
Digital
Baseband
Encoding
Decoding
Synch
µC
Memory
Modulation
Demodulation
Core
uP
Memory
Human
Interface
Power
Battery
UWB transceiver architecture
Figure 3: Transceiver architectures comparison
Antennas
RF front-end
Intermediate
frequency
Analog baseband
Digital baseband
Other aspects
Ultra-Wideband
Narrowband
Partial filtering is achieved by the antenna
Non constant envelope modulation (e.g. OFDM) need
very high linearity
Tough filtering is needed to satisfy out-of-band
emission
AGC, Mixers, RF oscillator, PLL
Small antenna design with gain and wide bandwidth
Low impedance antenna and good wideband matching
with few components is difficult
Antenna and front-end co-design is necessary
Wideband LNA are power consuming and hard to
match, the AGC is part of the front-end
Relaxed requirement on linearity
No need
Very high bandwidth A/D converters
Extended-time sampling techniques
Digital sampling oscilloscope techniques (DSO)
Coherent detection with very fine time resolution
Precise time references
On-board noise
External CW jammer
UWB channel characteristics is not completely known
but studies are underway.
Small high Q antenna design with good gain is easily
achievable
50Ω impedance, easy to match
Antenna and front-end can be designed independently
Narrowband LNA are easy to match
Small bandwidth A/D converter (typically twice the data
rate)
Non-coherent detection
Load-pull
LO leakage
LO pulling by PA
Narrow channels are well characterized (fading models)
Table 2 : Comparison of narrowband and UWB transceiver design (boldface means “challenging”)
3
The other big challenge comes from baseband design.
UWB radios communicate with pulses of very short
duration, typically a nanosecond or below. Ultra-fine
time resolutions are needed and will increase
acquisition time and may require additional correlators
to capture signal energy and accurate timing source like
TCXO (Temperature Compensated Crystal Oscillator)
to keep synchronization. Multiple mobile users in an
area will increase the performance requirement of the
baseband part. A purely digital implementation of the
baseband will require extremely fast A/D converters
(sub-nanosecond samples) with a huge number of
correlators.
Other works are currently underway to find solutions
on the analog side to avoid high-speed digital decoding
and thus power-consuming baseband implementations
[13].
I. CONCLUSIONS
In the years to come, many more small form factor
wireless consumer applications will appear on the
market. On the other side, new concepts like pervasive
computing or networking has appeared in the last
decade. The UWB is seen as the most serious candidate
to achieve this vision of wireless interconnection
between computing devices. Today, UWB is only at the
beginning of its development for the consumer market,
which appears to be one of the main segments for
UWB, with applications like high data rate multimedia
home networks or child locator. There is still a lot to be
done to make this technology attractive. Up to now, the
wireless technology has always been focused on
narrowband implementations. Technology requirements
on UWB systems are drastically different. Silicon and
hardware improvements, challenges in software and
algorithms for efficient data transmission and network
management are the technical challenges that should be
taken up by this new technology. Other works around
the standardization and the regulation are to be done in
order to make UWB widespread and popular.
CURRICULUM VITAE OF THE MAIN AUTHOR:
David Barras was born in Sierre, Switzerland, in 1972. He
received the Degree in Electrical Engineering (EE) from the
Swiss Institute of Technology of Lausanne (EPFL),
Lausanne, Switzerland in 1997. From 1997 to 2001, he
worked as RF engineer in ETA S.A./Swatch Group Ltd. Since
2001, he is working as scientific assistant for ETA
S.A./Swatch Group Ltd. at the Swiss Federal Institute of
Technology of Zürich (ETH), Switzerland. His main interests
are WLAN and WPAN, radio-frequency transceivers and
design of low-power silicon based RF circuits for wireless
applications.
4
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