Digital Signal Processing Reference
In-Depth Information
to survive in a wide range of hostile channel conditions, such as in presence of
in-band interference. Probably the most important aspect of a (wireless) com-
munication system is the way the information is injected into the channel by
the transmitter. During transport over the channel, noise and other unwanted ar-
tifacts will accumulate on the signal-of-interest, as a result of which a distorted
image of the transmitted signal arrives at the receiver. It is the responsibility of
the receiver to recover the original data, preferably with a low error frequency.
For this purpose, the receiver is commonly assisted by a combination of er-
ror coding and a clever modulation technique at transmission side. This is the
topic of Chapter 2, which provides a high-level overview of the principles
and use of coding and channel modulation in some widespread applications,
such as the v.34 analog telephone modem and the successful wireless lan
networks (802
.
11a/b/g). One of the important lessons learned from these sys-
tems is that coding and modulation should not be considered as two separate
processes in the transmission chain: depending on the mapping of information
on the analog symbols in the modulator or the frequency-dependent transfer
characteristic of the channel, certain bit positions in the transmitted data se-
quence may experience an increased probability of error. In a system ignoring
this fact, the finite resources of the coding subsystem may be dispatched to the
wrong location, resulting in a system performance that deviates from the the-
oretical optimum. Chapter 2 also introduces the notion of
coherence time
.In
fast-varying channels, a short coherence time prevents the transmitter to adapt
the transmitted signal to the current channel conditions. Chapter 2 concludes
with a comparison between wideband single- and multicarrier systems, finding
that single-carrier modulation techniques may not be completely ruled out in
advance as they allow the use of an energy efficient, nonlinear power amplifier
in the transmitter.
Chapter 3 pursues the same train of thought, showing that for a single-carrier
modulated system in a frequency-selective fading channel, it is theoretically
possible to cut away up to 40% of the frequency spectrum without any loss
of information at receiver side. This is because in a single-carrier system, the
energy that belongs to a single bit of information is automatically spread over
the complete spectrum. This implies that, if some portion of the frequency
spectrum goes missing for some reason (either due to fading or interference),
the receiver must be able to recover the original data without intervention from
transmission side.
In the second part of Chapter 3, the principle of
interferer suppression and
signal reconstruction
(issr) is introduced. Running on the back-end signal
processor of the receiver, the issr system is specifically aimed at the recon-
struction of a qpsk-modulated signal with some missing frequency bands. It
is also shown by simulation that issr, when combined with a Turbo-coder, is
able to perform within 0
.
4 dB of the theoretical Shannon limit (Figure 3.7).
