ECE 4371, Fall, 2014
Introduction to Telecommunication
Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 2
Aug. 27th, 2014
Overview

Chapter 4.1-4.3, basics of amplitude modulation

Other
– GPS
– Type of waves
– Satellite communication basics
Baseband and Carrier Communication


Baseband:

Describes signals and systems whose range of frequencies is measured
from 0 to a maximum bandwidth or highest signal frequency

Voice: Telephone 0-3.5KHz; CD 0-22.05KHz

Video: Analog TV 4.5MHz, TV channel is 0-6MHz. Digital, depending
on the size, movement, frames per second, …

Example: wire, coaxial cable, optical fiber, PCM phone
Carrier Communication:

Carrier: a waveform (usually sinusoidal) that is modulated to represent
the information to be transmitted. This carrier wave is usually of much
higher frequency than the modulating (baseband) signal.

Modulation: is the process of varying a carrier signal in order to use
that signal to convey information.

Example on the board.
Modulation

Modulation




A process that causes a shift in the range of frequencies of a signal.
Gain advantages

Antenna size: half of the antenna size. Thousands of miles for baseband

Better usage of limited bandwidth: less side lopes

Trade bandwidth for SNR: CDMA

Robust to inter-symbol-interference (multipath delay)

Robust to errors and distortions
Types

Analog: AM (DSB, SSB, VSB), FM, Delta modulation

Digital: ASK, FSK, PSK, QAM, …

Pulse modulation: PCM, PDM, … Fiber, phone
Advanced: CDMA (3G), OFDM (WLAN, WMAN), ….
Double Sideband






Modulation: m(t)cos(wct) 0.5[M(w-wc)+M(w+wc)]
Low/upper side band (LSB/USB), Double side band (DSB)
DSB-SC: suppressed carrier, no carrier frequency
Wc >= bandwidth of the signal to avoid aliasing.
Demodulation: e(t)=m(t)(cos(wct))^2=0.5(m(t)+m(t)cos(2wct))
E(w)=0.5M(w)+0.25(M(w+2wc)+M(w-2wc))
Low pass filter to remove the higher frequency
Coherent and non-coherent detection
– Receiver can recover the frequency and phase of the transmitter by
PLL. Error of timing can cause the performance error floor
– Non-coherent receiver has 3dB worst performance than coherent.
– Cheaper for Non-coherent receiver, Nextel.
AM-DSB-SC

Example 4.1
t
t

( t )
( t )
t

m( t )
cos( c t )

F{cos( c t )}

M( )

0

0

 ( )
 c
0
Lower sideband (LSB)
c
c

Upper sideband (USB)

Categories of Modulators

Multiplier Modulators
– Multiply m(t) by cos(wct)
– Hard for linearity for high energy. Expensive. e.g. sound system

Nonlinear Modulators
– Example

Switching Modulators
– FFT transform to series of frequencies
– Series-bridge diode modulator, shunt-bridge diode modulator
– Ring Modulators
Frequency Conversion


Move the signals to other
frequency
Multiplying two sinusoids results
in two frequencies which are the
sum and difference of the
frequencies of the sinusoids
multiplied.
(t )  m(t ) cos(C t )
BPF@ I
cos(MIX t )
EXAMPLE : Let m(t)
be as shown.
m(t)
cos(t ) cos(t )  21 [cos((  )t )  cos((  )t )]

To change the carrier frequency c
of a modulated signal to an
intermediate frequency I we use
an oscillator to generate a sinusoid
of frequency MIX such that
e1 (t )  21 m(t ) cos(I t )
(t)
t
t
0

Example 4.2, 4.3

()
 c
c  
0
Then m(t)cos( c t ) cos(MIX t )  21 m( t )[cos(( c  MIX )t )  cos(( c  MIX )t )]
 21 m( t )[cos((2 c  I )t )  cos((I )t )]
t
M()
SPECTRA
I   c  MIX .
e1(t)
E 1 ( )
I
0
I

Amplitude Modulation

Why DSB-SC not working: do not know the carrier frequency in receiver.

The last impulse functions indicate that the carrier is not suppressed in this
case. For some M() shown, the modulated signal spectrum is as shown.
 AM (t )  [ A  m(t )]cos(ct )
( )  12 M (  c )  M (  c )  A  (  c )   (  c )
M()
0

()
 c

0
c  
With this type of AM the demodulation can be performed with/without a
local oscillator synchronized with the transmitter.
AM Example
• m(t) has a minimum value of about -0.4. Adding a dc offset of A=1 results in
A+m(t) being always positive. Therefore the positive envelope of is just
A+m(t). An envelope detector can be used to retrieve this.
A=1
m(t)
A+m(t)
0.7
0.
-0.4
1.
t
t
 AM ( t )  [ A  m( t )] cos(c t )
t
AM Example (cont.)

The choice of dc offset should be such that A+m(t) should always be
positive. Otherwise envelope detector cannot be used, but coherent still ok

For example, the minimum value of m(t) = -0.4 . Therefore A > |min(m(t))|
for successful envelope detection. What if A< |m(t) |.

In the previous example let A=0.3.
A+m(t)
m(t)
0.7
0.
-0.4
0
t
 AM ( t )  [ A  m( t )] cos(c t )
t
t
Modulation Index
• Let mp be the absolute negative peak of m(t).
 EXAMPLE : Single-tone modulation. Let m(t)=2sin(20t)
A  mp
A is the carrier amplitude.
 
MODULATION INDEX :
mp
A
Then we see that f or A  m p , 0    1
When   1 (or A  m p ) the signal is ov ermodulated, and env elopedetectioncan not be used.
(Howev er, we can stilluse sy nchronous demodulation).
mp
2
. i)   0.5
A
A
For dc of f setof 1   2.
mp  2;
 

A  4 ii)   1
A 2
 1
  0 .5
t
m(t)
t
2
t
t
Sideband and Carrier Power
 AM ( t )  A cos(c t )  m( t ) cos(c t )
The f irstterm is the carrier and the second term is sidebands which contain the signal itself .
The total AM signal power is the sum of carrier power and the sideband power.
A2
Carrier power Pc 
2
Sideband power Ps  21 Pm where Pm is the power of m(t).
The sideband power is the usef ulpower.
Ps
Pm
usef ulpower
Ef f iciency:  


.
Total power
Pc  Ps A 2  Pm
For example, let m(t)  Bcos(m t )
mp  B,   B
or B  A.
A
Pm 
B2
2

2 A 2
2
For   1, max 
 
2
2
2
x100%
1
x100%  33%
2 1
Example 4.4, 4.5
AM Generator
+
m(t)
-
BPF
@ c
+
c cos(ct)
-
AM output
Coherent detector for demodulating DSB-SC
modulated wave.
AM Decoder

Rectifier Detector: synchronous

Envelope Detector: asynchronous
t 
t 
t 
+
AM signal
R
C
vc(t)
-
t 
RC Selection
Assume that the capacitoris charged to v oltage E (the env elope v oltageat the instant)at the instant
when the diode turns OFF.
The capacitorbegins to dischrage through the resistor according to
t
v c ( t )  Ee R C
t )
 E(1  RC
f or RC  1 .
c
dv c ( t )
dt
E
  RC
E .
The slope of the capacitor dischargeis - RC
For the capacitor discharge to f ollow the env elope,the magnitude of the
capacitor discharge slope must be greater than the env elpeslope.
dv c ( t )
dt
E  dE .
 RC
dt
E(t)=A(1+cos(wct))
1  1   2
RC 
wc  




GPS Orbits
GPS Position

By knowing how far one is from three satellites one can ideally
find their 3D coordinates

To correct for clock errors one needs to receive four satellites

Differential GPS: local FM
Type of waves
Radio Frequency Bands
Classification Band
Initials
Frequency Range
Characteristics
Extremely low
ELF
< 300 Hz
Infra low
ILF
300 Hz - 3 kHz
Very low
VLF
3 kHz - 30 kHz
Low
LF
30 kHz - 300 kHz
Medium
MF
300 kHz - 3 MHz
Ground/Sky wave
High
HF
3 MHz - 30 MHz
Sky wave
Very high
VHF
30 MHz - 300 MHz
Ultra high
UHF
300 MHz - 3 GHz
Ground wave
Space wave
Super high
SHF
3 GHz - 30 GHz
Extremely high
EHF
30 GHz - 300 GHz
Tremendously high
THF
300 GHz - 3000 GHz
Satellite Communications






Large communication area. Any
two places within the coverage of
radio transmission by satellite can
communicate with each other.
Seldom effected by land disaster (
high reliability)
Circuit can be started upon
establishing earth station (prompt
circuit starting)
Can be received at many places
simultaneously, and realize
broadcast, multi-access
communication economically(
feature of multi-access)
Very flexible circuit installment ,
can disperse over-centralized traffic
at any time.
One channel can be used in
different directions or areas (multiaccess connecting).
Rain Attenuation