TAMPERE UNIVERSITY OF
TECHNOLOGY
83950 Telecommunications Laboratory Course
Digital Modulations 1
email:
Huone:
tuomok@cs.tut.fi
HC 419 / Tuomo Kuusisto
The purpose of this exercise is to guide a student to the
digital modulations and to the measurement equipment
needed in digital modulations
1. PRE-LAB ASSIGNMENT
1.1
Why
there
are
different
digital
modulations
(advantages / disadvantages) like N-PSK, M-QAM, MSK,
GMSK? Calculate the bandwidth needed for QPSK and
16QAM when the bit rate is assumed to be same for
both modulations.
1.2
Let us send a bit stream 10101110100101010101010101.
Draw the phase diagram, the I- and Q-branch signals
for the GMSK-modulation. I=cos(), Q=sin()
1.3
Tell briefly, how an eye diagram is interpreted and
what kind of things can be read from that diagram
and how?
1.4 Draw the constellations for
types:
QPSK, MSK, GMSK, 32QAM, 64QAM
the
next
modulation
1.5
How noise can be seen in an eye diagram and in a
constellation?
1.6
What is an error vector?
1.7
Draw the normal GSM burst frame. Mark the different
parts and bits of that frame.
2. MEASUREMENTS
When you come to the lab, bring a diskette with you.
The measurements are done using HP vector analyzer
HB89441A. All the needed digitally modulated signals are
created using Rohde&Schwarz’s signal generator SMIQ03.
Also another signal generator SMY02 and the spectrum
analyzer ESA-L1500A are needed in this assignment.
The exercises to be done in the lab are composed of
exploring the characteristics of the most common digital
modulations and also from the little expedition to the
GSM world.
2.1
Study QPSK, /4 DQPSK, 8PSK, 16QAM and 64QAM by
turns, f = 900MHz, level –10 dBm, symbol rate =
70ksym/s.
-
How noise affects to the constellation of each
modulation type?
-
How noise affects to the eye diagram?
-
What is the ‘leakage’ value when the constellation
breaks?
-
How Imbalance affects to the constellation?
-
The affect of Quadrature Offset?
2.2
Use QPSK as a modulation type. SNR is +20 dB
(Bn=200kHz). Calculate a percentual opening from the
eye diagram. What do you get? (A tip: firstly take a
print from the eye diagram without noise to your
diskette. Secondly take a print with noise)
2.3
Add an interfering signal to the QPSK modulated data
signal (without noise). What happens? How you
calculate SNR and what is the result? It is better
to adjust the result length as large as possible.
The frequency of the interfering signal is 900.2 MHz
and the level is –20 dBm. (Take a print to your
diskette).
2.4
Add one multipath-fading signal to the actual data
signal without noise. Explain what happens? Try with
different speeds.
2.5
Set standard GSM as a modulation type. Change the
data produced by the signal generator to the same as
in pre-lab assignment 1.3. The change goes like
this:
2.5.1
Digital
modulation
->
source
->
servdataData -> DLSTO0 .-> edit view
2.5.2
Remove the data in DLSTO0 and insert the
sequence
2.5.3
Fill up the data list with zeros so that
the burst will be full. Check, how many zeros
you need from the structure of the burst.
2.5.4Return to Digital std modulation where you
choose this altered data list
2.5.5
Examine
the
bit
stream
using
vector
analyzer. Is the reception of your own sequence
successful? If it is, draw the phase of that
sequence from the trellis diagram. It is good
to arrange the analyzer so that the trellis
diagram shows only the wanted bit sequence’s
phase diagram. Does this phase match to the one
in pre-lab assignment 1.2?
2.6
Turn off the digital standard modulation. Choose GSM
standard like signal from the digital modulation
part.
2.6.1
Turn source -> pattern -> 0: transmission
using only zeros.
2.6.2
Coding OFF
2.6.3
Filter -> 0.3 : The symbol rate as in GSM
specifications
2.6.4
Examine the spectrum using ESA-L1500A. How
this spectrum change, if we transmit only ones
instead? (pattern -> 1)
What about if the pattern is 01?
2.6.5
Examine how coding will affect. How the
results change if coding is GSM ?
3. POST-LAB ASSIGNMENT
3.1
Answer to the questions.
Some measurement instructions to 2.5
Settings for the signal generator:
Set generator to transmit standard GSM-signal. Use
select, return buttons and round big white wheel .
Set the frequency to 900 MHz and power level to –30 dBm.
Check the values of the BURST / SLOT to be transmitted
Burst type -> Norm
Slot level -> full
Hop trigger
-> off
S
-> 0 bin
Tail 000
Data e.g. PN9
Settings for the vector analyzer:
Instrument Mode:
Demodulation
Demod. Setup
-> demod type -> [digital]
-> MSK1
-> stand. Setups -> GSM
Receiver -> RF section (2-2650MHz)
Measure from
-> INPUT
Set correct frequency, span = 1MHz.
Range -> -30 dBm. Range is used to set the receiver’s
receiving level. The more lower the value, the more
accurately receiver works. However, level should be set
so that no overloading occurs (you can see OV on the
display, if there is overloading). In practice, the level
cannot be lower than the transmission power level is.
Result length should be 148 sym
Pulse search
-> on
Sync search
-> off
Search length
-> 1.434 ksym
Points / symbol
-> 5
From the more setups, set training sequence to the same
as in the burst transmitted by the signal generator.
Calculate the delay of the sequence from the beginning of
the burst. Set sync search on.
LITERATURE
1. Work folder
2. Edward A. Lee, David G. Messerschmitt: Digital
Communication
Other interesting sources...
3. Hewlett & Packard: BACK TO BASICS
4. Ulrich Rohde: Communications Receivers