Digital Signal Processing Reference
In-Depth Information
1.5
Future Research for Adaptation
Most of the techniques and issues that were described in the previous sections still need
further research for the development of more efficient adaptation and parameter esti-
mation algorithms. At the same time, there is a significant amount of effort in evolving
current wireless communications systems to provide higher data rates, higher capacity,
and better performance. New technologies are being introduced to accommodate these
goals, like multicarrier wireless communications, MIMO, and ultrawideband (UWB).
Adaptation techniques will be a significant factor in efficient and successful deployment
of these technologies.
UWB is a promising technology for future data communications systems, high-
accuracy (indoor) geolocation devices, sensor applications, etc. Any signal that occupies
more than 500 MHz of bandwidth and meets the spectrum mask requirements enforced
by spectrum regulation agencies is considered a UWB signal [70]. For example, in the
United States, the Federal Communications Commission (FCC) has allocated 7.5 GHz
of spectrum (between 3.1 and 10.6 GHz) for unlicensed use of UWB devices. One of
the most popular UWB systems, which is based on impulse radio (IR), utilizes car-
rierless transmission with very low-power spectral density. IR-based UWB techniques
are based on the transmission of nanosecond-level short pulses that generate extremely
wide spectrums. This results in a covert noise-like signal in a radio channel. Note that
within the transmission band of UWB, other technologies also coexist. For example, the
OFDM-based WLAN technology at the 5 GHz U-NII band is a big concern for UWB
signals, as it might create significant interference for UWB signals. In order to provide
robustness against narrowband interference, adaptive implementation of UWB systems
is very important. For this purpose, several strategies have been developed recently.
Multiband UWB is one of the techniques proposed to reduce the effect of narrowband
interference. In such techniques, the whole 7.5 GHz bandwidth is divided into several
narrower bands that are still wider than 500 MHz. The information is transmitted in
these bands depending on the narrowband interference situation. Several versions of
multiband schemes are available, some of which can be found in [70]. Estimations of the
existence and level of narrowband interference are interesting research topics that fall
under the parameter estimation algorithms described before.
The wide bandwidth of UWB offers a capacity much higher than the current narrow-
band systems. Short-range data transmission rates of over 500 Mbps have been theoreti-
cally shown [71, 72]. However, these high data rates are only possible with excellent signal
quality values and for short-range communications. High data rates can be traded off
with longer ranges and for lower link quality values. Depending on the link quality and
distance between transmitter and receiver, the rate can be changed through adaptation
of the processing gain. UWB achieves processing gain due to pulse repetition, i.e., trans-
mitting more than one pulse within a bit. For example, by transmitting 100 pulses per
bit, a processing gain of 20 dB is obtained. Additional processing gain is obtained due
to the low duty cycle, which is the ratio of the pulse repetition interval and the pulse
width. Adaptation of processing gain is a research topic that needs to be explored for
UWB systems. Similarly, multiple-access capability of UWB systems, which is primarily
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