Digital Signal Processing Reference
In-Depth Information
related to power consumption. Unfortunately, the increased efforts are not nec-
essarily reflected in a decent data throughput in an Ultra-Wideband system.
The low spectral density of the received signal forces the designer to use a
limited modulation depth (bpsk/qpsk)intheofdm subbands. But reducing
the modulation depth does not result in more relaxed linearity requirements in
the front-end of the receiver, because accidental in-band interferers are only
filtered later on in the digital back-end.
Based on the aforementioned observations, it seems that ofdm may not be the
best option for wideband radio communication after all. The underlying reason
for these problems is that the real opportunities of wideband radio are being ig-
nored. First, one must realize that no earth-shocking results are to be expected
from a system that has a very large spectral footprint, but is only allowed to use
a very limited amount of transmission power. So what is the actual advantage
of using such a large spectral footprint? The answer being looked for is found
in the characteristics of the propagation channel. A wireless communication
device that is operated in an indoor environment suffers from a very specific
form of multipath fading. The delay spread (
τ
rms
) of the channel is limited to
a few hundreds of nanoseconds, which at first sight may sound very promising
because of the limited amount of intersymbol interference. However, this also
implies that the multipath energy related to a single transmitted symbol arrives
in a very concise time frame at the antenna of the receiver. A narrowband re-
ceiver will see a flat fading channel, because the delay spread of the indoor
channel is below its resolution bandwidth.
Increasing the resolution bandwidth as a means to improve diversity
A wireless system cannot make a distinction between
multipath components that are below its resolution
bandwidth. From a frequency domain point of view,
a channel with a low delay spread corresponds with a
frequency-selective fading channel, while flat fading is
experienced over wide portions of the frequency spec-
trum. A transmission with a spectral footprint smaller than the coherence band-
width of the channel will thus suffer from flat fading, meaning that destructive
interference causes intermittent link outages. The designer of a system can
tackle this issue in a number of ways. For example, a combination of error
coding and using the diversity of time allows the system to bridge short pe-
riods of destructive interference. However, this approach turns out not to be
very reliable in static channels: the long coherence time here causes long in-
terruptions of the radio link. A second option is to use the spatial diversity of a
multi-antenna setup. For a sufficient distance between the individual antennas,
each branch of this hardware rake receiver system sees an independently fad-
ing channel. Switching between or combining the energy from both antennas
TX
RX
