Fourier Analysis - Redes de Computadores - UPV

With –o option we specify the name of the resulting binary file. The –lm option is
necessary to link with math library (sin and cos trigonometric functions)
Impact of physical medium on the transmitted signal.
Here we are going to use our program to analyze how the transmitted signal is distorted
when crossing an specific transmission channel. For that purpose, we are going to
produce four different scenarios corresponding with the transmission of one signal with
the same transmission speed under different channel bandwidths. In particular, we will
use channels with bandwidths of 400, 1000, 2000 and 9000 Hz.
Run the program with the following parameters:
./MiFourier –c a –v 1200 –b 400 –a 10000 > bw400.txt
Option –c specifies the character we are going to transmit (also we can specify the
ASCII code: –c 97). Option –v fixes the transmission speed (in bits per second). With
option --b we can define the desired channel bandwidth. Finally, option –a is aesthetic
option used to determine width of the frequency domain plot (in Hz).
As the character and transmission speed are just the same than the default ones, we do
not need to specify them. Run the program to get results for the rest of proposed
bandwidths.
./MiFourier –b 1000 –a 10000 > bw1000.txt
./MiFourier –b 2000 –a 10000 > bw2000.txt
./MiFourier –b 9000 –a 10000 > bw9000.txt
Compare the four graphics, using the GNUPlot program. You can observe that as
bandwidth increases, more harmonics cross the channel and as a consequence the
reconstructed signal is better (closer to the original one). Do you think that a bandwidth
of 400 Hz is enough for transmitting this character with a speed of 1.200 bps. ? Should
it be necessary a channel bandwidth of 9 KHz or it would be enough with 2 KHz?
Impact of transmission speed on the transmitted signal
En general, todos tenemos la idea de que al transmitir por un medio físico una señal
digital, la velocidad máxima de transmisión está acotada. Hay muchos factores que
introducen esta limitación, pero uno de los principales es que el ancho de banda del
medio es limitado. Como ya hemos visto, esto provoca que a partir de una determinada
frecuencia, el resto de armónicos se atenúan.
We can notice that transmission speed is very related with signal period T. So that, as
transmission speed increases the signal period becomes shorter. The fundamental
frequency that determines the distance between harmonics also increases, so a lower
number of harmonics will cross the channel and, as a consequence, the reconstructed
signal will be poorer .
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