SCK LAB MANUAL SAMPLE
VERSION 1.2
THIS SAMPLE INCLUDES:
TABLE OF CONTENTS
TWO SELECTED LABS
FULL VERSION IS PROVIDED FREE WITH KITS
Phone: +92 51 8356095, Fax: +92 51 8311056
Email: info@renzym.com, URL:www.renzym.com
REVISION HISTORY
Revision
Description
Date
1.0
1.1
1.2
First draft
Preface added and few labs updated
Digital Communication labs added
March 20, 2013
May 15, 2013
Feb 25, 2014
PREFACE
There has been always existed a gap between how communication systems are ‘taught in theory’
and ‘designed in practice’. Over the last decade or so the practice of communication system design
has gone through a drastic change. The classical approach was focused on the use of hardware
electronic components to build communication circuits. With the rapid increase in the digital
computational power and introduction of Software Defined Radio (SDR) concept, now the focus is on
building communication system in which main tasks are performed by the software. The key
advantage of this approach is that now it is possible for an undergrad student or a professional
engineer who is learning communication theory to directly apply his theoretical concepts and rapidly
build real-time communication system.
While the communication industry has adopted the new SDR technology for quite some time now,
education sector is still lacking behind. Most of books, lab manuals and training equipment are still
not up to date with the current trends of the market. Although the trend has started to change over
the last few years but still a lot of work still needs to be done. This lab manual is an effort to bridge
this gap between theory and practice of communication system design.
If we look at the existing lab equipment, it primarily falls into following two main categories.
1) Conventional Electronic Communication Trainers: These trainers are easy to use but they
don’t provide hands on system design experience. Such trainers come with fixed circuitboards and students are required to just change jumper settings to observe the outputs on
oscilloscope or voltmeter.
2) High-end SDR-based Platforms: These are relatively high end trainers which do provide
system design experience but students require specialized expertise and programming skills
to use them. These skills are not usually available with the undergraduate and even graduate
students. This is the reason why these platforms are not a good option for students’ labs
especially at undergraduate level.
Renzym lab equipment is an attempt to fill this void because not only it provides hands on design
experience but also it doesn’t require any specialized programming expertise. This lab manual
demonstrates the design of different analog communication systems using SDR Communication Kits
(SCK). SCK is USB powered, plug and play device that enables true SDR development directly from
class room Simulink/LabView simulations. It can be used to readily build real-time communication
systems by directly applying the concepts learnt from theory with a minimum of implementation
effort in the hardware. This blending of theory and practice is generally missing from most of the
communication courses. Theoretical performance of various techniques can now be quickly
compared with their performance in a real-world environment. Furthermore it provides a unique
opportunity to the researchers working on receiver design to verify their algorithms in various
practical scenarios with a minimum of implementation effort.
SDR Communication Kit
Complex baseband representation of passband communication system has been adopted in this
manual. This representation simplifies the system design by separating processing of information
bearing baseband signals from the carrier signals. An excellent description of this approach can be
found in a book by Michael P. Fitz titled “Fundamentals of Communications Systems”. The theory of
analog communication systems described in Professor Fitz’s book has been used for designing
related labs in this lab manual. Although we did provide brief theoretical explanation at the start of
every lab, it is still recommended that this book should be consulted by both, instructors and
students, during the labs.
With the help of powerful design tools like Simulink/Labview now it is possible to exactly implement
the theoretical concepts in practice. This is the reason why in this manual theoretical explanation of
the concepts precedes the Simulink implementation in every lab. Complete implementable block
diagrams have been explained with the help of mathematical equations. Students must understand
the theory before they attempt to design a system in Simulink. Lab tasks are provided wherever
necessary for student to take their learning experience to the next level by designing at their own
using what they learned in the previous labs.
It is also important to take a step-by-step approach while designing relatively complex system. A
common mistake that students often make is that they try to build a complete system first before
start testing it. It would make almost impossible to debug for mistakes and identify the source(s) of
error in a big system with tens of functional blocks. Therefore it is of utmost importance for students
to divide a big system into smaller subsystems and verify each system separately before connecting
them together to make complete system. In order to be able to subdivide a system and to be able to
verify the output of each individual system, one must have a clear theoretical understating of the
system functionality. Without such understanding it would be extremely difficult to successfully
design the complete system.
This manual covers a range of analog modulation schemes often used in practice. First two labs
provide introduction to Simulink and how to build and analyze a basic system in Simulink. In the
third lab complex baseband representation of passband system is introduced and key basic building
blocks like ‘Baseband to Passband Converter’ and ‘Passband to Baseband Converter’ are designed.
Fourth lab deals with the basic Double Sideband Amplitude Modulation (DSB-AM) scheme. SDR
Communication Kit (SCK) is introduced in the fifth lab and students will learn to interface SCK with
computer and send/receive basic physical signals using SCK. Furthermore DSB-AM system will also
be built using a pair of SCKs. Lab 6 and 7 will cover two popular schemes of amplitude modulation
namely Large Carrier-AM (commonly known as AM) and Single Sideband Carrier AM. Angle
modulation is treated in the Labs starting from 8 to 11 where Phase Modulation (PM) and Frequency
Modulation (FM) systems are covered. Students will build angle modulators and verify the Carson’s
bandwidth rule. Moreover they will design different modulators for both FM and PM signals
including direct phase detector, discriminator detector and PLL based modulator.
CONTENTS
Preface
Contents
Lab 1: Simulink Fundamentals
Lab 2: Basic Signal Processing
Lab 3: Complex Baseband Representation of Passband Communication Signals
Lab 4: Double-Sideband Suppressed-carrier Amplitude Modulation (DSBSC-AM)
Lab 5: Getting Started with SDR Communication Kit
Lab 6: Large Carrier Amplitude Modulation System (LC-AM)
Lab 7: Single Sideband AM (SSB-AM) using Transmitted Reference Based Demodulation
Lab 8: Angle Modulation
Lab 9: Angle Demodulation using Direct Phase Demodulator
Lab 10: Angle Demodulation Using Discriminator Detector
Lab 11: Digital Communication
Lab 12: Pulse Amplitude Modulation (PAM)
Lab 13: Phase Shift Keying (PSK)
Lab 14: Frequency Shift Keying (FSK)
Lab 15: C/C++ MEX S-Function
Lab 16: Symbol Timing Error and Recovery
Lab 17: Carrier Error and Recovery
Lab 18: Unique Word Phase and Frame Recovery
Lab 19: Bit Error Rate (BER)
Lab 6:
Large Carrier Amplitude Modulation System (LC-
AM)
6.1 Lab Objective:
In this lab we will design Large Carrier Amplitude Modulation (LC-AM) systems. Key lab objectives
will be as follows
1) Simulate the analog amplitude modulation through envelop detection i.e. LC-AM.
2) Test the same model using SDR Communication Kits
3) Single Sideband AM using Transmitted Reference Based Demodulation
Following Simulink blocks will be used in this lab
1) ‘Sine Wave’ Block from Signal Processing Sources.
2) ‘Digital Filter Design’ block from Signal Processing Block set.
3) ‘Bandpass to Baseband’ and ‘Baseband to Passband’ converters (designed in previous labs)
6.2 LC-AM Modulator and Non-coherent Demodulator
The main advantage of LC-AM over DSBSC-AM is that it can detected by simple envelope detector
without any phase estimation. DSBSC-AM message signal modulates the
1) Envelope of the bandpass carrier signal in a continuous manner
2) Phase of the binary bandpass carrier in a binary fashion i.e. the carrier of the phase changes
from 0 to  when message signal amplitude goes from +ve to –ve.
In fact, if the message signal never goes negative the envelope of the bandpass signal and the
message are identical up to a multiplicative constant. This desired characteristic is obtained if a DC
signal is added to the message signal to guarantee that the resulting signal always is positive. This
implies the complex envelope is an affine function of the message signal, i.e.
Where “ ” is a positive number. This modulation has
and
so the imaginary portion of the complex envelope is not used again in LC-AM. The resulting bandpass
signal and spectrum are given as
Block diagram of LC-AM modulator is shown in Figure ‎6.1.
Figure ‎6.1: LC-AM Modulator
The demodulator of LC consists of simple envelope detector. The output of bandpass to be baseband
converter at the receiver, in the absence of noise, can be written as
We can see that envelope of signal can be used to recover message signal,
, by ignoring the
phase, , because it doesn’t change its value. The envelope of the
is given below
Message signal can be recovered from this envelope by applying a highpass filter, DC remover, as
shown in Figure ‎6.2.
Figure ‎6.2: LC-AM Demodulator
LC-AM differs from DSBSC-AM in that a DC term is added to the complex envelope. This DC term is
chosen such that
or equivalently the envelope of the bandpass signal never passes
through zero. This implies that
or equivalently
This constant “ ”, here denoted the modulation coefficient, is important in obtaining good
performance in a LC-AM system. Typically the time average of
is zero the average power
There are two parts to the transmitted/received power:
(1) The power associated with the added carrier transmission
(2) The power associated with the message signal transmission
It is desirable to maximize the power in the message signal transmission and a factor that
characterizes this split in power in LC-AM is denoted the message to carrier power (MCPR).
It shows in order to maximize
the modulation coefficient, , should be maximized. But it is
not possible to increase it beyond a certain level and
for audio transmission practically
remains between 10-15%. This is the penalty which we have to pay for simplified envelope detection
based detection.
6.3 Design in Simulink
A Simulink design of LC-AM modulator and demodulator for sinusoidal message signal is shown in
Figure ‎6.3. Students are required to simulate an LC-AM system with the following parameters.
Input message amplitude,
Sampling frequency,
Carrier Amplitude,
Carrier frequency,
Hz
Hz
Student should calculate appropriate value of modulation co-efficient, , and
and should
make it part of lab report. A careful design of highpass filter, DC remover, is also required at the
demodulator.
Figure ‎6.3: LC-AM System in Simulink
Students should verify their results by comparing the demodulator output signal with the input
message signal as shown in Figure ‎6.4.
Figure ‎6.4: Message signal and LC-AM demodulator output
Furthermore energy spectrums of the baseband signal,
verified by using spectrum scope as shown in Figure ‎6.5.
, and passband signal,
, should be
Figure ‎6.5: Spectrum plots for baseband and passband signals
6.4 Speech Transmission Using SDR Communication Kits
In this section we try to setup an LC-AM link for audio signal using SDR Communication kits. As
described earlier, we need two SCKs connected with two different computers; one acting as
modulator and other as receiver or demodulator. Simulink models for modulator and demodulator
will also split into two separate Simulink models. The setup would look like as shown in figure below
k
ulin
Sim odel
M
Tx-side
USB
SDR Kit
Sim
u
Mo link
de
l
Rx-side
SDR Kit
USB
Figure ‎6.6: Transmission Setup using SCK
When connected, SCK should appear as a default sound card to the computer. It should be checked
from the audio device properties and SCK should be selected as default sound card. At the
transmitter end, modulator output should be sent to the SCK by using “To audio device” block which
can be found in ‘commonly used blocks’. SCK can also be selected as default output audio device
from “To audio device” block properties. LC-AM modulator providing output to SCK is shown in
Figure ‎6.7.
Figure ‎6.7: LC-AM Modulator with SCK
Please also note that block ‘From Multimedia File’ used in Figure ‎6.7 to provide input message signal
to the LC-AM modulator. Using this block any audio file can be provided as input signal. File will keep
on repeating after during the simulation. It is important to calculate audio signal bandwidth before
transmission and also adjust the variables like carrier frequency, modulation coefficient, sampling
frequency (it should be matched with the audio file sampling rate).
Simulink. Output of the demodulator can be sent to audio device by connecting it to “To audio
device” block as shown in Figure ‎6.8.
Figure ‎6.8: LC-AM Demodulator with SCK
Matlab is computationally intensive software so in order to have a smooth non-interrupted
transmission please remover all the plotting blocks like time ‘Scope’ or ‘Spectrum Scope’ etc.
Lab 14:
Frequency Shift Keying (FSK)
14.1: FSK Modulator
FSK modulators can be categorized into two types; discontinuous phase FSK (DPFSK), continuous
phase FSK (CPFSK). DPFSK is also known as noncoherent FSK and CPFSK is also known as coherent
phase FSK. The categorization is shown in Figure ‎14.1.
FSK
Discontinuous
Phase FSK
Continuous Phase
FSK
Figure ‎14.1: FSK Categorization
Discontinuous phase FSK and continuous phase FSK modulated signals are shown in Figure ‎14.2. As
shown in figure, DPFSK modulated signal has discontinuities at symbol boundaries. These
discontinuities increase signal bandwidth. Similarly CPFSK is also shown in the figure; where the
phase is coherent at symbol boundaries.
Discontinuous Phase FSK/Noncoherent FSK
Continuous Phase FSK(CPFSK)/Coherent FSK
Figure ‎14.2: FSK Modulated Signals
14.1.1: DPFSK Modulator
Binary DPFSK modulator is given in Figure ‎14.3. The multiplexer switches between the sinusoids
at
and . In general
and
are not the same, therefore the modulated signal is not
continuous at symbol boundaries. BFSK spectrum is given in Figure ‎14.4, that visualizes the relation
between
and
.
𝑠0 ( )
𝑠1 ( )
M
U
X
( )
Binary Data
Source
Figure ‎14.3: Continuous Time Binary DPFSK Modulator
Consider,
𝑠
𝑠
Therefore,
This produces the sinusoids,
𝑠
𝑠
For better detection the frequencies are chosen to be at maximum separation i.e. achieved by
making then orthogonal. The two frequencies are orthogonal if the following equation is equal to
zero
When,
is equal to zero. This means that frequencies are orthogonal for multiples of,
The required frequency shifts for coherent CPFSK basis signals to be orthogonal can be expressed as.
This means all the frequencies shifts are multiples of half the symbol rate and frequency separation
is
.
1
0
0
𝑐
𝑐
1
Figure ‎14.4: BFSK Spectrum
14.1.2: CPFSK Modulator
For a binary FSK two waveforms are used to transmit a bit as given below
𝑠
(‎14.1)
𝑠
For a CPFSK the phase at the symbol transition boundaries should be continuous. To do so the
modulator must remember the phase during symbol transition. This is achieved by representing the
modulated signal in terms of current and previous symbol. A continuous phase BPSK modulator is
given in Figure ‎14.5.
1,0,1
data
+1,-1+1
LUT
a(k)
DAC
x(t)
VCO
y(t)
Figure ‎14.5: VCO Based FSK
The look up table outputs is
DAC produces a bipolar square wave with amplitude
. DAC output can be expressed as
Where,
𝑠
The VCO output is expressed as
(‎14.2)
Where
to produce a unit energy pulse shape. Consider look up table output during
to be
, then the phase term in (‎14.2) becomes
Where,
Is constant
, resulting in
(‎14.3)
Substituting (‎14.3) in (‎14.2) produces
Suppose
then
For
(‎14.4)
For
(‎14.5)
As seen (‎14.4) and (‎14.5) are of the form given in (‎14.1). Now consider
output during
the DAC output is,
Then the phase term of VCO output becomes
be the look up table
Where
This produces the VCO output to be
The frequency shift is normalized by symbol rate, producing,
This makes the VCO output to be,
Where,
or is chosen so that the two basis signals of coherent CPSK are orthogonal. Correlation function
is used to check orthogonality as shown below.
When,
is equal to zero. This means that frequencies are orthogonal for multiples of,
The required frequency shifts for coherent CPFSK basis signals to be orthogonal can be expressed as.
This means all the frequencies shifts are multiples of quarter the symbol rate and frequency
separation is
. The minimum frequency shift that produces orthogonal signals is.
FSK with
is called Minimum Shift Keying (MSK). In terms of modulation index ( ), when
is equal to zero, This means that frequencies are orthogonal for multiple
. For
MSK
. Using proper frequencies or modulation index the following signals can be orthogonal
basis signals for BSK.
𝑠
𝑠
Or,
𝑠
𝑠
Modulator for orthogonal BSK is given in Figure ‎14.6.
𝜙0 ( )
LUT0
DAC
LUT1
DAC
data
0(
)
1(
)
( )
𝜙1 ( )
Figure ‎14.6: Continuous Time Binary Orthogonal FSK Modulator
Discrete time CPFSK modulated signal is represented as
Where,
Therefore,
For
, modulation index
is,
Therefore,
Where
is sampled version of
This produces a discrete time CPSK modulator given in Figure ‎14.7.
d
data
LUT
N
FIR
Filter
c
z
1
( )
cos()
DAC
( )
VCO
Figure ‎14.7: Discrete Time CPFSK Modulator
14.2: FSK Demodulator
FSK demodulator has two types, coherent demodulators and noncoherent demodulators. In
coherent demodulation theoretically it is assumed that the demodulator know the phase of received
signal, practically the phase of the received signal is estimated using phase recovery algorithm. In
noncoherent demodulation the demodulator does not require the phase of the received signal,
therefore noncoherent demodulation is preferred over coherent demodulation. DPKSF signals have
discontinuous phase at symbol boundaries i.e. phase changes at symbol boundaries therefore,
DPKSF received signal can only be demodulated noncoherently. CPFSK has continuous phase at
symbol boundaries therefore, CPFSK received signal can be demodulated coherently or
noncoherently.
14.2.1: FSK Coherent Demodulator
Discrete time BFSK coherent demodulator is given in Figure ‎14.8. The received signal is sampled
using an ADC. The sampled signal is project onto 𝑠
and 𝑠
producing an estimate of
and . The projections are downsampled by , producing of
and
.
𝜙0 ( )
( )
( )
(. )
1(
)
DAC
Decision
(. )
0(
)
𝜙1 ( )
Figure ‎14.8: Discrete Time BFSK Coherent Demodulator
𝜙1
0
1
Select
Index of
Largest
0
+
> 0?
1
>
0
0
>
𝜙0
1
1
Figure ‎14.9: Decision Block
Figure ‎14.10: BFSK Decision Region
Decisions are taken on
and
, decision regions are defined in Figure ‎14.10. The decision
block can be implemented using different methods; some of them are shown in Figure ‎14.9.
Problem with this demodulator is that it is difficult to produce phase and frequency coherent
replicas of the basis function, therefore alternate methods are used for demodulation, where phase
coherent replicas are not required.
14.2.2: FSK Noncoherent Demodulator
There are many noncoherent demodulators for FSK, such as; Differential detection FSK demodulator,
Foster Sealy FSK demodulator and Square Law FSK demodulator. Foster Sealy FSK demodulator and
Square Law FSK demodulator are discussed in the following sections.
14.2.2.1: Foster Sealy FSK Demodulator
Foster Sealy BFSK demodulator is shown in Figure ‎14.11, it is a noncoherent demodulator, i.e. the
demodulator does not require knowing the phase of the received signal.
( )
( )
Bandpass
Filter
Envelope
Detection
DAC
Decision
Bandpass
Filter
Envelope
Detection
Figure ‎14.11: Foster Sealy BFSK Demodulator
Signals at different stages are shown in Figure ‎14.11. As seen two bandpass filters are used, one
centered at
and the other centered at
. When frequency contents at
are
present during a symbol time the frequency contents at
are absent as shown by signals at
outputs of bandpass filters, similarly when frequency contents at
are present during a
symbol time the frequency contents at
are absent. This produces the envelope detector
outputs as shown by signals in figure. A difference of the envelope detector outputs produces the
desired symbol value.
14.2.2.2: Square Law FSK Demodulator
Continuous Time Square Law demodulator is shown in Figure ‎14.12. Continuous Time Square Law
demodulator uses a noncoherent projection of the received signal on to the two basis functions by
using quadrature mixers at each of the two possible frequencies. The outputs are integrated over a
symbol time and squared. The quadrature results are summed and passed to the decision block
which chooses the symbol associated with the largest output.
𝜙0 ( )
( )
(. )
.
2
(. )
.
2
𝜙1 ( )
Decision
𝜙0 ( )
(. )
.
2
(. )
.
2
𝜙1 ( )
Figure ‎14.12: Continuous Time Square Law BFSK Demodulator
To see how this works, let
𝑠
𝑠
Be the two possible transmitted signals. Now, suppose the received signal is
Now let’s compute the sampled outputs of the four integrators for
:
Thus
And the correct decision is made. The key to proper performance is the orthogonality of the two
possible transmitted signals i.e.
Discrete Time implementation Square Law demodulator is shown in Figure ‎14.13
𝜙0 ( )
( )
( )
(. )
.
2
(. )
.
2
𝜙1 ( )
DAC
Decision
𝜙0 ( )
(. )
.
2
(. )
.
2
𝜙1 ( )
Figure ‎14.13: Discrete Time Square Law Demodulator
14.3: Design in Simulink
Using concepts described in section 14.1 and 14.2 CPFSK modulator and Square Law detector are
given in Figure ‎14.14.
The modulator is a direct implementation of the discrete time CPFSK modulator shown in Figure
‎14.7. For BFSK the look up table values are set to
, where -1 is for bit 0 and 1 is for bit 1. -1
produces a frequency shift
and 1 produces a frequency shift
. Value of
is
selected using the following equation.
Where
for BFSK is loot up table values. The demodulator is a direct implementation of
Square Law FSK demodulator shown in Figure ‎14.12. The quadrature sinusoids for both paths are
generated using the sin wave block. The sin wave block is configured to produce a complex sinusoid,
where real part is the inphase cosine wave and the complex part is the quadrature phase sin wave.
The frequency is set to
. The decision block used here is same as that used in PAM and PSK.
Figure ‎14.14: CPSK Modulator and Square Law Demodulator in Simulink
The output signals at different stags at the demodulator are shown below, where Figure ‎14.15
shows the receiver constellation and Figure ‎14.16 shows the transmitter filter output and the
upconverted transmitted signal.
Figure ‎14.15: Receiver Constellation
1.5
1
Amplitude
0.5
0
-0.5
-1
-1.5
7.81
7.815
7.82
7.825
7.83
7.835
7.84
7.83
7.835
7.84
Time (secs)
1
Amplitude
0.5
0
-0.5
-1
7.81
7.815
7.82
7.825
Time (secs)
Offset=0
Figure ‎14.16: Transmitter Filter Output and Upconverted Transmitted Signal
14.4: Lab Task
1. Design a non coherent BFSK modulator and Square Law BFSK Demodulator.
2. Design a coherent BFSK modulator and Foster Sealy BFSK Demodulator.
3. Design a coherent 4-FSK modulator and Foster Sealy 4-FSK Demodulator.
4. Design a coherent 4-FSK modulator and Square Law 4-FSK Demodulator.