ULTRA
WIDEBAND FOR WIRELESS COMMUNICATIONS
1. Ultra
wideband (UWB) communication is based on the transmission of very short pulses
with relatively low energy. This technology may see increased use in the field
of wireless communications and ranging in the near future. UWB technique has a
fine time resolution which makes it a technology appropriate for accurate
ranging. Because of the huge bandwidth, UWB waves have a good material
penetration capability. As will be explained later in more detail in this
chapter, the UWB radio signal occupies a bandwidth of more than 500 MHz or a
fractional bandwidth of larger than 0.20. According to Shannon’s capacity
formula, this large bandwidth provides a very high capacity. Thus, high
processing gains can be achieved that allow the access of a large number of
users to the system. The impulse radio UWB is a carrier-less (i.e., baseband)
radio technology and accordingly, in this radio technique no mixer is needed.
Therefore, the implementation of such a system is simple, which means that low
cost transmitters/receivers can be achieved when compared to the conventional radio
frequency (RF) carrier systems. Through the years (1960s–1990s) the United
States military developed the UWB technology that was first used for ground
penetrating radar. In 1998, the Federal Communication Commissions (FCC)
recognized the significance of UWB technology and initiated the regulatory
review process of the technology. Consequently, in February 2002 the FCC report
appeared, in which UWB technology was authorized for the commercial uses with
different applications, operating frequency bands as well as the transmitted
power spectral densities.
2. UWB
systems operate in a very large bandwidth, they need to share the spectrum with
other users as well as with the existing communication systems and
consequently, interferences may occur. Besides from the interference from other
users, the UWB propagation channel will cause disturbances.
3. Bandwidth
of the UWB technique is huge. This very wide bandwidth means a fine time
resolution. This main feature of the UWB technology provides the capability of
accurate positioning which has already been used in the radar applications and
is now underway in the wireless communications. The capability of
communications and positioning (with precise performance), in a single
technology (i.e., fusion of positioning and data capabilities in a single
technology) is one of the salient features of the UWB technology. Referring to
the spectrum of the UWB signal we realize that the UWB center frequency is
relatively low. This causes the UWB signal to penetrate many materials and
providing a functionality that would not be present in a system of comparable
bandwidth at the significantly higher center frequencies. Besides from the high
performance of the UWB technique at low cost, another major feature of this
technique is the very low transmit power. This low transmit power (in the order
of microwatts) causes a low level of interference to the existing systems.
Moreover, the UWB method is robust against fading. This robustness further
reduces the required transmit power of this technology.
4. Generally
antennas are elements that radiate the electromagnetic energy of a transmission
line to the free space. Antennas are in fact transition devices (transducers)
between guided wave and free space (and vice versa). They can be considered as
impedance transformers, coupling between an input or line impedance and the
impedance of free space. For the case of the UWB this impedance transformation
of antenna is more important. This is due to huge bandwidth of UWB system. As
an initial approach to the UWB antennas we can start from a dipole and
consequently consider multi-narrowband antennas which are optimized to work in
the entire UWB band. This idea is shown in Fig. together with the antenna’s
corresponding dispersive waveform. The large scale components of this
log-periodic antenna radiate the low frequency components and the smaller scale
components of the antenna radiate high frequency components. For the UWB
communications the dispersive behavior of the antenna waveform is not popular.
Another disadvantage of this antenna is at different azimuth angles around the
antenna the waveform varies, which is again unpopular for wireless
communication applications. There are different types of UWB antennas. They are
categorized into the following classes according to form and function:
A log-periodic antenna (left) which
has a dispersive waveform (right)
a)
Frequency dependent
antennas: The log-periodic antenna is an example of
this type of antennas where the smaller scale geometry of antenna contributes
to higher frequencies and the larger scale part contributes to the lower
frequencies.
b)
Small-element antennas: These are small, omni-directional
antennas for commercial applications. Examples of this type of antennas are
bow-tie or diamond dipole antennas.
c)
Horn antennas: Horn antennas are
electromagnetic funnels that concentrate energy in a specific direction. These
antennas have large gains and narrow beams. The Horn antennas are bulkier than
small-element antennas.
d)
Reflector antenna: These antennas are
high gain antennas that radiate energy in a particular direction. They are
relatively large but easy to adjust by manipulating the antenna feed. Hertz’s
parabolic cylinder reflector is an example of this type of antennas.
UWB
Interference
5. Regarding
the interference two important, aspects should be noted,
a)
The interference caused by the narrowband and
wideband systems on the victim UWB system
b)
The interference caused by UWB systems on the
victim narrowband and wideband systems.
Both
interferences are important and should be considered in the design, evaluation
and implementation of the systems. The spectrum of UWB system and other
wireless systems are shown. As seen from this figure, several other services
exist in or in the neighborhood of the UWB band. For example, the IEEE 802.11-a
works at 5.2 GHz is a main source of interference to indoor UWB systems. Other
systems such as 2.4 GHz band WLANs as well as the GPS system (at 1.5 GHz),
mobile cellular system (at 800 MHz and 1800 MHz band) are also source of
interference to UWB systems. Mutually, UWB systems may affect existing wireless
or navigation systems and cause interference to systems such as 802.11a,
wireless systems in ISM band, Mobile cellular, and GPS.
Interference
Reduction
6. The
effect of UWB interference on the wideband system (such as IEEE 802.11-a) can
be mitigated by using a notch filter. The spectrum of the UWB interfere signal
is filtered (around 5.2 GHz) and its spectral contents in this band is
suppressed. Another method of mitigating the effect of UWB interference on the
victim wideband systems is designing time-hopping codes in such a way that the
power spectral density of the UWB signal has less power in the band of wideband
system (e.g., IEEE 802-11-a). Multibanding is another way of mitigation of UWB
interference on narrowband and wideband systems. According to this method, the
UWB band is divided in subbands and the transmit subbands are selected
according to an interference threshold.
7. The
notch filtering of UWB signal is a well known method of UWB interference
mitigation. The time hopping code design is an interesting method but can only
be applied to impulse radio UWB systems. The multiband method is a practical
solution that can be combined with notch filtering method in each subband. It
is scalable which means that the bit rate, power consumption, complexity and
cost can scale with the demand. Multiband UWB can coexist with other spectrum
users as sub-bands can be turned off to avoid interference.
8. The
effect of narrowband interference on the victim UWB system can be simply
reduced by using a rejection filter. The frequency band of the interferer
should fall in the stopband of the filter. The filter can be inserted in the
receiver before correlator (which correlates the received signal with the
transmit template waveform). According to the center frequency of the
interferer, the cutoff frequency, the ripple in the passband and attenuation in
the stopband can be designed. Use of the rejection filter improves the BER
performance of the UWB system when narrowband interferer exists. However, in
the absence of the interferer and when the channel is only additive white
Gaussian noise (AWGN), the rejection filter slightly reduces the BER
performance.
9. In the near future this technology may
see increased use for high-speed short range wireless communications, ranging
and ad hoc networking. There are two competing technologies for the UWB
wireless communications, namely: Impulse Radio (IR) and Multi-band OFDM (MB-OFDM).
IR technique is based on the transmission of very short pulses with relatively
low energy. The MB-OFDM approach divides the UWB frequency spectrum to multiple
non-overlapping bands and for each band transmission is OFDM. Several proposals
based on these two technologies have been submitted to the IEEE 802.15.3a. Both
technologies are valid and credible.
a) Impulse Radio In impulse radio
UWB pulses of very short duration (typically in the order of sub-nanosecond)
are transmitted. Because of very narrow pulses the spectrum of the signal
reaches several GHz of bandwidth. The impulse radio UWB is a carrier-less
transmission. This technology has a low transmit power and because of
narrowness of the transmitted pulses has a fine time resolution. The
implementation of this technique is very simple as no mixer is required which
means low cost transmitters and receivers. Direct Sequence Ultra Wideband
(DS-UWB) and Time Hopping Ultra Wideband (TH-UWB) are two variants of the IR
technique. These IR techniques DS-UWB and TH-UWB are different multiple access
techniques that spread signals over a very wide bandwidth. Because of spreading
signals over a very large bandwidth, the IR technique can combat interference
from other users or sources. It should be mentioned that Direct Sequence Spread
Spectrum (DSSS) and Time Hopping Spread Spectrum (THSS) may be considered
similar to DS-UW Band TH-UWB, respectively. There are, however, differences
between the spread spectrum and IR-UWB systems. Both systems take advantage of
the expanded bandwidth, while different methods are used to obtain such large
bandwidth. In the conventional spread-spectrum techniques, the signals are
continuous-wave sinusoids that are modulated with a fixed carrier frequency,
while in the IR-UWB (i.e., DS-UWB and TH-UWB), signals are basically baseband
and the narrow UWB pulses are directly generated having an extremely wide
bandwidth. Another difference is the bandwidth. For the UWB signals the
bandwidth has to be higher than 500MHz, while for the spread spectrum
techniques bandwidths are much smaller (usually in the order of several MHz).
b)
Multiband OFDM Multi-band Orthogonal
Frequency Division Multiplexing (MB-OFDM) is another UWB technology which uses
the OFDM method. Multi-Band OFDM combines the OFDM technique with the
multi-band approach. The spectrum is divided into several sub-bands with a –10
dB bandwidth of at least
Proposed
MB-OFDM frequency band plan
500MHz. The
information is then interleaved across sub-bands and then transmitted through
multi-carrier (OFDM) technique. One of the proposals for the physical layer
standard of future high speed Wireless Personal Area Networks (WPANs) uses
MB-OFDM technique. In this MB-OFDM WPANs proposal, the spectrum between 3.1 and
10.6 GHz is divided into 14 bands with 528MHz bandwidth that may be added or
dropped depending upon the interference from, or to, other systems. In Fig. a
possible band plan is presented, where only 13 bands are used to avoid
interference between UWB and the existing IEEE 802.11a signals. The three lower
bands are used for standard operation, which is mandatory, and the rest of the
bands are allocated for optional use or future expansions. MB-OFDM technology
promises to deliver data rates of about 110 Mbps at a distance of 10 m. For the
UWB wireless sensor applications data rates are low, but the (hoping) coverage
might be much larger than 10 m. MB-OFDM may require higher power levels when
compared to the IR technology. MB-OFDM technique is robust to multipath which
is present in the wireless channels.
The advantages of the MB-OFDM technique are as follows:
i.
Capturing
multipath energy with a single RF chain.
ii.
Insensitivity to group delay variations.
iii.
Ability
to deal with narrowband interference at receivers.
iv.
Simplified
synthesizer architectures relaxing the band switching timing requirements.
The
disadvantages are as follows:
i.
Transmitter
is more complex because of IFFT
ii.
High
peak-to-average power ratios
iii.
OFDM
synchronization problems
Comparison
of UWB Technologies
10. The
comparison of IR DS-UWB and MB-OFDM UWB techniques in terms of interference
from, or to, other systems, robustness to multipath, performance, system’s
complexity and achievable range-data rate performance for the WPAN
applications, is provided.
Specifications
Comparison of MB-OFDM and IR DS-UWB techniques for WPAN
11. The major characteristics of UWB, i.e., extremely large bandwidth, low power, short-range high data rate communication, robustness against fading, immunity to multipath, multiple access capability, low cost transceivers and precise positioning, motivate several potential applications for this technology.
Thus far the UWB technology has been
mainly applied to military