EXPERIMENT ONE: SIGNAL TRANSMISSIONS
Abstract. This experiment is designed to learn how signals are encoded and
transmitted by different transmission techniques. By simulating the whole
processes of the signal encoding and decoding, modulating and demodulating,
it is easier for the new comers of communication engineering to understand
the signal transmission techniques in a point-to-point communication system.
1. Details of the experiment
Generally, a point-to-point digital communication system consists of several func-
tional blocks including signal encoder, signal modulator, signal demodulator and
signal decoder. The signal encoder encodes an analog signal into a digitized data
sequence, where the analog signal is sampled at first and quantized. During the
quantizing, the possible signal value is divided into several value periods and the
middle value of the period is usually selected to represent the signal that falls into
the period. Each period is labeled by some digits to identify. The sampled signal
value is analyzed to determine which period it belongs to and the corresponding
label is output as the signal encoder’s output. Notice that the label can be repre-
sented by a codeword of binary digits, we obtain a sequence of binary data after
signal encoder. The decoder receives the transmitted data and decodes it to de-
termine which period the sampled signal value belongs to. Then, the middle value
of the period is output as the approximation of the sampled signal. Therefore, a
distortion is introduced in the signal encoder and decoder because of the quantizing
process.
After the signal encoder, a modulator modulates the data into an analog signal
to transmit them over a channel. In particular, the channel in a wireless commu-
nication system means a frequency bandwidth of the electromagnetic waves. It is
necessary to translate the data into a bandwidth signal by modulating techniques.
In this experiment, the following items need to be done.
(1) Generate a signal s(t) = sin 2πt
k
+ cos πt
k
, t
k
= (0, 1, 2, ...1024)∆t, ∆t =
1/16;
(2) Write a program to encode the generated signal by an A-law PCM encoder
(please refer to books to find how an A-law PCM encoder works);
(3) Modulate the encoded signal by binary PSK with center frequency f
0
=
256Hz;
(4) Demodulate the modulated signal;
(5) Decode the demodulated signal, compare the decoded signal and the gen-
erated signal.
Date: October 4, 2008.
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