DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 393

Verify that the original input signal to the modulator is recovered as the output
from the demodulator. Experiment with different sampling rates, filter characteris-
tics, and carrier frequencies to reduce the occasional output noise.

10.13 MODULATION SCHEMES—PAM AND PSK

This project implements both pulse amplitude modulation and phase shift keying
schemes. See also the projects in Sections 10.11 and 10.12. The files for this project
are included in the folder modulation_schemes .

10.13.1 Pulse Amplitude Modulation

In pulse amplitude modulation (PAM), the amplitude of the pulse conveys the infor-
mation. The information symbols are transmitted at discrete and uniformly spaced
time intervals. They are mapped to a train of pulses in the form of a carrier signal.
The amplitude of these pulses represents a one-to-one mapping of the information
symbols to the respective levels. For example, in binary PAM, bit 1 is represented
by a pulse with amplitude A and bit 0 by A.

At the receiver, the information is recovered by obtaining the amplitude of each
pulse. The pulse amplitudes are then mapped back to the information symbol. Figure
10.35 shows the block diagram of a typical PAM system. This is a simplified version
without the introduction of adaptive equalizers or symbol clock recovery, which
takes into account the effects of the channel. The incoming bit stream (output of
the DSK) is parsed into J -bit words, with different lengths of parsing, resulting in
different numbers of levels. For example, there are eight levels when J

-

3. These
levels are equidistant from each other on a constellation diagram and symmetric
around the zero level, as shown in Figure 10.36. The eight constellation points rep-
resent the levels, with each level coded by a sequence of 3 bits. Tables 10.5-10.7 show
the mapping levels.

=

Serial to
Parallel
Converter

Map J-bit
to 2 J Levels

Impulse
Modulator

Transmit
Filter
G T (w)

Bit Stream

Channel
C(w)

Receive
Filter
G R (w)

Map from
2 J Levels to
J-bit Words

Parallel to
Serial
Converter

Quantizer

Bit Stream

FIGURE 10.35. PAM system.