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QAM using Matlab/Octave


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Implement, in software, quadrature amplitude modulation and
demodulation for sampled message signals, mI(t) and mQ(t).
Use the following system parameters: sampling rate of 8000
sps; 200 time samples; and, carrier frequency of 2.1 kHz. For
the low-pass filter, use length 35, passband edge 1 kHz, and
stopband edge 2 kHz. For input signal ˜m(t) = mI (t)+jmQ(t),
use the following values for ˜m(t):
• first 50 samples, 1/√2 + j/√2
• second 50 samples, −1/√2 + j/√2
• third 50 samples, −1/√2 − j/√2
• last 50 samples, 1/√2 − j/√2.

Hint: a constant list of 50 complex-valued samples with modulus
1 and angle 45 degrees can be generated as
(1/sqrt(2) + 1j/sqrt(2))*ones(1,50).

From the output of your quadrature amplitude demodulator,
make three plots.
• time plot of received “I” signal
• time plot of received “Q” signal
• time plot of the phase of the received complex-baseband

Note that the phase of a complex number v, in degrees, can
be obtained as angle(v)*180/pi. It is recommended that the
I and Q plots be combined as subpanels within a single figure
using subplot, for easy side-by-side comparison.

figure;%create new figure window
subplot(2,1,1) %2 rows, 1 column, first plot
plot(real(v));title(’In-phase signal’)
plot(imag(v));title(’Quadrature signal’);
xlabel(’sample number’)

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