Digital Signal Processing Reference
In-Depth Information
In Chapter 4, the foundations are laid for the main body of this work
(Chapter 5 - Pulse-based wideband radio). A general overview is provided
of the characteristics of a wireless transmission channel, including
power de-
lay spread
,
frequency-selective
and
flat fading
,the
coherence bandwidth
and
the
coherence time
of the wideband indoor channel. It is discussed here that
a wideband transmission in a multipath environment suffers from flat fading,
on top of the frequency-selective component. This flat fading component is
a major issue for the reliability of the system, as it may cause periodic drop-
outs of the communication link. The designer of a communications system
may employ different techniques to prevent this from happening, which all
rely on some form of diversity. In a spatial diversity scheme, for example,
the incoming energy from multiple antennas is combined. If the signals that
arrive at each of the antennas have independent fading characteristics, the
probability that the combined signal experiences a deep fade is considerably
reduced. A major drawback of the latter approach is that multiple antennas are
an inefficient way to increase diversity, as it requires the duplication of costly
(external) hardware.
With these caveats in mind, Chapter 5 introduces the concept of pulse-based
wideband radio. The advantage is that, for exactly the same symbol rate, short
pulses provide a better multipath resolvability than a continuous-time mod-
ulated carrier. As a result, the receiver will be able to distinguish between a
larger number of independently fading multipath components, which can be
combined to form a more reliable link. What is repeatedly stressed through-
out this chapter is that it is not a good idea to focus on the generation and
detection of
individual pulses
. After all, the latter way of doing would require
a perfect knowledge of the channel characteristics, which is a problem in a
fast-varying environment (i.e. a channel with a short coherence time). Dealing
with individual pulses does not allow for a straightforward solution to cope
with intersymbol interference (isi) or in-band interferers. Also, tracking indi-
vidual pulses poses a serious synchronization issue to the receiver. For this
reason, the proposed pulse-based radio system approaches the problem from a
slightly different angle: instead of using pulses as a starting point, the pulse-
based radio system that is introduced in Chapter 5 is founded on a basic qpsk
radio subsystem. On top of this continuous-time qpsk sublayer comes a pulse-
based extension layer. Although the transmitted signal has the appearance of
a stream of individual pulses, the underlying system is still based on a regular
qpsk transmission. As will be explained in more detail in Chapter 5, the ad-
vantage of this system is that it allows the qpsk subsystem in the receiver to
employ a regular coherent detection scheme, without the need to be aware of
the pulse-based extension shell.
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Remember that the main function of the pulse-based extension layer is to increase multipath resolvability.
