Simulator of a PPM radio
communication system with a mobile
terminal
Student: Firas Ismael.
I.D.
: 066569260.
E-mail : firas.ismael@mail.huji.ac.il.
Advisor: Dr. Dana Porrat.
E-mail: dporrat@gmail.com
School of Engineering and Computer Science,
The Hebrew University of Jerusalem.
Project Abstract
Software program that will be able to modulate information signal which will be
randomly generated using PPM modulation scheme, the signal generated will use
second derivation of Gaussian function to modulate the transmitted signal, The program
will plot the transmitted signal for the user and print the data to show the modulated
message.
Then the software will demonstrate transmitting this signal throw channel to show the
effect of noise by adding AWG noise to the signal, after calculating the effects of AWGN
on transmitted signal the user will be able to plot the noisy signal and examine the
effects on transmitted signal.
Then receiving and filtering the signal by using matched filter, the signal will be sampled
to retrieve the data that was modulated in our simulator, the user will be able to plot the
filtered signal and examine the results, the data retrieved will be printed to compare with
the sent data.
The user will be able to change the pulse specifications, transmitted message length,
number of messages to be transmitted and to plot graphs in deferent Eb/No values.
In addition the user will be able to plot BER to Eb/No graph for different message length
to compare the differences between simulation and theory.
Introduction
Pulse position modulation (PPM) is a pulse modulation technique that uses pulses that
are of uniform height and width but displaced in time from some base position according
to the amplitude of the signal at the instant of sampling. Pulse position modulation is
also sometimes known as pulse-phase modulation. Pulse position modulation has the
advantage over pulse amplitude modulation (PAM) and pulse duration modulation
(PDM) in that it has a higher noise immunity since all the receiver needs to do is detect
the presence of a pulse at the correct time; the duration and amplitude of the pulse are
not important.
M message bits are encoded by transmitting a single pulse in one of 2M possible timeshifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits
per second.
It is primarily useful for optical communications systems, where there tends to be little
or no multipath interference.
One of the key difficulties of implementing this technique is that the receiver must be
properly synchronized to align the local clock with the beginning of each symbol.
Therefore, it is often implemented differentially as differential pulse-position modulation,
where by each pulse position is encoded relative to the previous, such that the receiver
must only measure the difference in the arrival time of successive pulses. It is possible
to limit the propagation of errors to adjacent symbols, so that an error in measuring the
differential delay of one pulse will affect only two symbols, instead of affecting all
successive measurements.
The software goal was to develop software that can help examining the effects of
AWGS on information signal and give the engineers an instrument to measure the error
probability in transmitting signals using PPM scheme on deferent noise ratios.
Also, in case engineers know the noise range and environment that the device will work
with they can check for BER using the last test in the program and try to figure which
message length is appropriate for the device to use in order to save energy.
References:
•
Wireless Communication, Andreas F. Molisch.
•
Fundamentals of wireless communications, D. Tse and P. Viswanath,
Cambridge University Press, 2005.
•
Wireless Communications, Andrea Goldsmith, Stanford University, California.
Materials and Methods
The software is constructed from three connected parts:
Modulator: in this part the information signal is created and passed to the next part
Noise channel: in this part the information signal is being transmitted through the AWGN
channel.
Demodulator: in this part the noisy signal is being received and filtered in order to be
sampled
Using the Pulse Position Modulation scheme to
Create transmitted signal.
Signal will pass through location Dependent
channel and will add Additive White Gaussian
Noise (AWGN).
Recover the information content, improving the
signal using matched filter.
Then sample the recovered waveform to
retrieve the information sent.
The modulated signal
presents random
messages created by the
simulator, a signal pulse
is transmitted in one of 2M
possibilities.
In our modulator we use
green dots to highlight
and separate different
messages and red dots to
highlight the position of
the pulse that indicates
the data being modulated.
The modulator builds a signal array that contains the modulated signal, after reading all
the data required for initialization.
The signal will pass
through location
dependent channel and
will add Additive White
Gaussian Noise (AWGN).
The simulated noisy
signal at receiver shows
the effects of AWGN on
transmitted signal.
The effects of the channel
are calculated in noisy
channel arrays for each
one of Eb/No values that
the user decided.
The user will be able to plot a specific noise ratio calculation to the graph or plotting
more than one noisy signal and clear the axes for new plots.
The demodulator to
recover the information
content, improving the
signal using matched
filter.
The received signal is
filtered by matched filter
to improve the signal
power to noise power
and reduce error
probability.
Then sample the recovered waveform to retrieve the information sent.
The filtered two definitional array will be calculated from the noise signal two definitional
array for the noise ratios chosen for the simulation.
The user will be able to plot a specific filtered signal from the two definitional array to the
graph or plotting more than one filtered signal and clear the axes for new plots.
Results
The project implemented in Matlab environment.
The interface of the software easy to use and organized in order, first the user will need
to initialize the parameters needed for the simulator to work:
Sampling frequency, Pulse width, Number of messages, Message length, Period, Eb/No
After initializing the parameters the user can start using the software.
First the user may check the pulse used, and plot a graph of the used pulse, the pulse
used can be modified or even changed to another pulse.
Then the user can hit the run simulator button that will calculate all the data and restore
all the results.
After running the simulator the modulated signal wil appear on the plot area and user
will be able to check the simulation with the data printed, that was randomly chosen.
Now the user may start examining the results and plot the noise signals and filtered
signals, depending on the Eb/No value needed for examining.
The user can plot deferent graphs of noisy signal or filtered signal on the same plot area
and examine the differences and then clean the axes and plot again.
And for last test the user can plot BER to Eb/No graph for different message length to
compare the differences between simulation and theory.
Figure toolbar to
control plots
Initialize the
simulator
parameters needed
for running tests.
Examine used
pulse, run the
simulator, choose
an Eb/No value for
plotting noisy signal
and filtered signal.
Run Eb/No test for
specific message
length
Conclusions
The software presents the theory of transmitting information signals clearly using PPM
scheme, the effects of environment on the signal, and shows how the receiver deals
with received signal and retrieves sent data.
The software implemented gives the engineers a strong instrument for examining
transmitting information signal using PPM scheme but needs to be developed and
extended to be more useful for industry.
Future work that can make the program very useful for industry:

Extend the pulse used to be able to modulate using any other pulse the engineer
needs.

The receiver needs to be properly synchronized to align the local clock with the
beginning of each symbol.

Transmitted signal needs a carrier wave.

Improving Statistical and Physical modeling in the simulator by adding more
realistic effects like fading model.

Adding more tests to calculate the energy being used for transmitting data.