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ANALOGUE TELECOMMUNICATIONS
1
MAIN TOPICS (Part I)
1)
2)
3)
4)
5)
6)
Introduction to Communication Systems
Filter Circuits
Signal Generation
Amplitude Modulation
AM Receivers
AM Transmitters
2
MAIN TOPICS (Part II)
7)
8)
9)
10)
11)
Single-Sideband Communications Systems
Angle Modulation Transmission
Angle Modulated Receivers & Systems
Introduction To Transmission Lines & Antennas
Mobile Telecommunications
3
Elements of a Communication System
• Communication involves the transfer of information
or intelligence from a source to a recipient via a
channel or medium.
• Basic block diagram of a communication system:
Source
Transmitter
Receiver
Recipient
4
Brief Description
• Source: analogue or digital
• Transmitter: transducer, amplifier, modulator,
oscillator, power amp., antenna
• Channel: e.g. cable, optical fibre, free space
• Receiver: antenna, amplifier, demodulator, oscillator,
power amplifier, transducer
• Recipient: e.g. person, speaker, computer
5
Modulation
• Modulation is the process of impressing information
onto a high-frequency carrier for transmission.
• Reasons for modulation:
– to prevent mutual interference between stations
– to reduce the size of the antenna required
• Types of analogue modulation: AM, FM, and PM
• Types of digital modulation: ASK, FSK, PSK, and
QAM
6
Frequency Bands
BAND
Hz
 ELF 30 - 300
 AF
300 - 3 k
 VLF 3 k - 30 k
 LF
30 k - 300 k
 MF
300 k - 3 M
 HF
3 M - 30 M
BAND
Hz
 VHF 30M-300M
 UHF 300M - 3 G
 SHF 3 G - 30 G
 EHF 30 G - 300G
•Wavelength, l = c/f
7
Information and Bandwidth
 Bandwidth required by a modulated signal depends
on the baseband frequency range (or data rate) and
the modulation scheme.
 Hartley’s Law: I = k t B
where I = amount of information; k = system constant; t =
time available; B = channel bandwidth
 Shannon’s Formula: I = B log2 (1+ S/N) in bps
where S/N = signal-to-noise power ratio
8
Transmission Modes
 Simplex (SX) – one direction only, e.g. TV
 Half Duplex (HDX) – both directions but not at the
same time, e.g. CB radio
 Full Duplex (FDX) – transmit and receive
simultaneously between two stations, e.g. standard
telephone system
 Full/Full Duplex (F/FDX) - transmit and receive
simultaneously but not necessarily just between two
stations, e.g. data communications circuits
9
Time and Frequency Domains
• Time domain: an oscilloscope displays the
amplitude versus time
• Frequency domain: a spectrum analyzer displays the
amplitude or power versus frequency
• Frequency-domain display provides information on
bandwidth and harmonic components of a signal
10
11
Non-sinusoidal Waveform
• Any well-behaved periodic waveform can be represented as a
series of sine and/or cosine waves plus (sometimes) a dc
offset:
e(t)=Co+SAn cos nw t + SBn sin nw t (Fourier series)
12
Effect of Filtering
• Theoretically, a non-sinusoidal signal would require
an infinite bandwidth; but practical considerations
would band-limit the signal.
• Channels with too narrow a bandwidth would
remove a significant number of frequency
components, thus causing distortions in the timedomain.
 A square-wave has only odd harmonics
13
Mixers
• A mixer is a nonlinear circuit that combines two
signals in such a way as to produce the sum and
difference of the two input frequencies at the output.
• A square-law mixer is the simplest type of mixer and
is easily approximated by using a diode, or a
transistor (bipolar, JFET, or MOSFET).
14
Dual-Gate MOSFET Mixer
Good dynamic range and fewer unwanted o/p frequencies.
15
Balanced Mixers
• A balanced mixer is one in which the input
frequencies do not appear at the output. Ideally, the
only frequencies that are produced are the sum and
difference of the input frequencies.
Circuit symbol:
f1
f1+ f2
f2
16
Equations for Balanced Mixer
Let the inputs be v1 = sin w1t and v2 = sin w2t.
A balanced mixer acts like a multiplier. Thus
its output, vo = Av1v2 = A sin w1t sin w2t.
Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]
Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t].
 The last equation shows that the output of the
balanced mixer consists of the sum and difference of
the input frequencies.
17
Balanced Ring Diode Mixer
Balanced mixers are also called balanced modulators.
18
External Noise
• Equipment / Man-made Noise is generated by any
equipment that operates with electricity
• Atmospheric Noise is often caused by lightning
• Space or Extraterrestrial Noise is strongest from the
sun and, at a much lesser degree, from other stars
19
Internal Noise
• Thermal Noise is produced by the random motion of
electrons in a conductor due to heat.
Noise
power, PN = kTB
where T = absolute temperature in oK
k = Boltzmann’s constant, 1.38x10-23 J/oK
B = noise power bandwidth in Hz
Noise voltage,
VN  4kTBR
20
Internal Noise (cont’d)
• Shot Noise is due to random variations in current
flow in active devices.
• Partition Noise occurs only in devices where a single
current separates into two or more paths, e.g.
bipolar transistor.
• Excess Noise is believed to be caused by variations
in carrier density in components.
• Transit-Time Noise occurs only at high f.
21
Noise Spectrum of Electronic Devices
Device
Noise
Transit-Time or
High-Frequency
Effect Noise
Excess or
Flicker Noise
Shot and Thermal Noises
1 kHz
fhc
f
22
Signal-to-Noise Ratio
• An important measure in communications is the
signal-to-noise ratio (SNR or S/N). It is often
expressed in dB:
PS
VS
S
(dB)  10 log
 20 log
N
PN
VN
In FM receivers, SINAD = (S+N+D)/(N+D)
is usually used instead of SNR.
23
Noise Figure
• Noise Factor is a figure of merit that indicates how
much a component, or a stage degrades the SNR of
a system:
F = (S/N)i / (S/N)o
where (S/N)i = input SNR (not in dB)
and (S/N)o = output SNR (not in dB)
• Noise Figure is the Noise Factor in dB:
NF(dB)=10 log F = (S/N)i (dB) - (S/N)o (dB)
24
Equivalent Noise Temperature and Cascaded Stages
• The equivalent noise temperature is very useful in microwave
and satellite receivers.
Teq = (F - 1)To
where To is a ref. temperature (often 290 oK)
• When two or more stages are cascaded, the total noise factor
is:
F2  1 F3  1
FT  F1 +
+
+ ...
A1
A1A 2
25
High-Frequency Effects
• Stray reactances of components (including the
traces on a circuit board) can result in parasitic
oscillations / self resonance and other unexpected
effects in RF circuits.
• Care must be given to the layout of components,
wiring, ground plane, shielding and the use of
bypassing or decoupling circuits.
26
Radio-Frequency Amplifiers
27
Narrow-band RF Amplifiers
• Many RF amplifiers use resonant circuits to limit their
bandwidth. This is to filter off noise and interference and to
increase the amplifier’s gain.
• The resonant frequency (fo) , bandwidth (B), and quality factor
(Q), of a parallel resonant circuit are:
fo
RL
fo 
; B ; Q
Q
XL
2 LC
1
28
Narrowband Amplifier (cont’d)
• In the CE amplifier, both the input and output
sections are transformer-coupled to reduce the
Miller effect. They are tapped for impedance
matching purpose. RC and C2 decouple the RF from
the dc supply.
• The CB amplifier is quite commonly used at RF
because it provides high voltage gain and also
avoids the Miller effect by turning the collector-tobase junction capacitance into a part of the output
tuning capacitance.
29
Wideband RF Amplifiers
• Wideband / broadband amplifiers are frequently used
for amplifying baseband or intermediate frequency
(IF) signals.
• The circuits are similar to those for narrowband
amplifiers except no tuning circuits are employed.
• Another method of designing wideband amplifiers is
by stagger-tuning.
30
Stagger-Tuned IF Amplifiers
31
Amplifier Classes
An amplifier is classified as:
• Class A if it conducts current throughout the full
input cycle (i.e. 360o). It operates linearly but is very
inefficient - about 25%.
• Class B if it conducts for half the input cycle. It is
quite efficient (about 60%) but would create high
distortions unless operated in a push-pull
configuration.
32
Class B Push-Pull RF Amplifier
33
Class C Amplifier
• Class C amplifier operates for less than half of the
input cycle. It’s efficiency is about 75% because the
active device is biased beyond cutoff.
• It is commonly used in RF circuits where a resonant
circuit must be placed at the output in order to keep
the sine wave going during the non-conducting
portion of the input cycle.
34
Class C Amplifier (cont’d)
35
Frequency Multipliers
 One of the applications of class C amplifiers is in “frequency
multiplication”. The basic block diagram of a frequency
multiplier:
Input
fi
High
Distortion
Device +
Amplifier
Tuning
Filter
Circuit
Output
N x fi
36
Principle of Frequency Multipliers
• A class C amplifier is used as the high distortion
device. Its output is very rich in harmonics.
• A filter circuit at the output of the class C amplifier is
tuned to the second or higher harmonic of the
fundamental component.
• Tuning to the 2nd harmonic doubles fi ; tuning to the
3rd harmonic triples fi ; etc.
37
Waveforms for Frequency Multipliers
38
Neutralization
• At very high frequencies, the junction capacitance of
a transistor could introduce sufficient feedback from
output to input to cause unwanted oscillations to
take place in an amplifier.
• Neutralization is used to cancel the oscillations by
feeding back a portion of the output that has the
opposite phase but same amplitude as the unwanted
feedback.
39
Hazeltine Neutralization
40
Review of Filter Types & Responses
•
•
•
•
4 major types of filters: low-pass, high-pass, band pass, and bandreject or band-stop
0 dB attenuation in the passband (usually)
3 dB attenuation at the critical or cutoff frequency, fc (for Butterworth
filter)
Roll-off at 20 dB/dec (or 6 dB/oct) per pole outside the passband (# of
poles = # of reactive elements). Attenuation at any frequency, f, is:
 f 
atten. (dB) at f  log   x atten. (dB) at f dec
 fc 
41
Review of Filters (cont’d)
• Bandwidth of a filter: BW = fcu - fcl
• Phase shift: 45o/pole at fc; 90o/pole at >> fc
• 4 types of filter responses are commonly used:
– Butterworth - maximally flat in passband; highly non-linear phase
response with frequecny
– Bessel - gentle roll-off; linear phase shift with freq.
– Chebyshev - steep initial roll-off with ripples in passband
– Cauer (or elliptic) - steepest roll-off of the four types but has
ripples in the passband and in the stopband
42
Low-Pass Filter Response
Gain (dB)
BW = fc
Vo
0
Ideal
-20
1
-40
0.707
Passband
BW
0
-60
fc
Basic LPF response
f
fc
10fc 100fc 1000fc
f
LPF with different roll-off rates
43
High-Pass Filter Response
Gain (dB)
0
Vo
-20
1
-40
0.707
0
Passband
fc
Basic HPF response
-60
f
0.01fc 0.1fc
fc
f
HPF with different roll-off rates
44
Band-Pass Filter Response
Centre frequency:
Vout
1
0.707
BW
fc1
fo fc2
BW = fc2 - fc1
fo 
f c1 f c 2
Quality factor: Q  f o
BW
Q is an indication of the
selectivity of a BPF.
Narrow BPF: Q > 10.
Wide-band BPF: Q < 10.
f
Damping Factor: DF  1
Q
45
Band-Stop Filter Response
• Also known as band-reject,
or notch filter.
• Frequencies within a certain
BW are rejected.
• Useful for filtering interfering
signals.
Gain (dB)
0
-3
Pass
band
Passband
fc1 fo fc2
f
BW
46
Filter Response Characteristics
Av
Chebyshev
Bessel
Butterworth
f
47
Damping Factor
Vin
Frequency
selective
RC circuit
Vout
+
_
R1
R2
General diagram of active filter
The damping factor (DF)
of an active filter sets
the response characteristic
of the filter.
R1
DF  2 
R2
Its value depends on the
order (# of poles) of the
filter. (See Table on next
slide for DF values.)
48
Values For Butterworth Response
Order
1st Stage
Poles
DF
1
1
optional
2
2
1.414
3
2
4
2
2nd Stage
Poles
DF
1
1
1
1.848
2
0.765
49
Active Filters
• Advantages over passive LC filters:
– Op-amp provides gain
– high Zin and low Zout mean good isolation from source or load
effects
– less bulky and less expensive than inductors when dealing with
low frequency
– easy to adjust over a wide frequency range without altering
desired response
• Disadvantage: requires dc power supply, and could be
limited by frequency response of op-amp.
50
Single-pole Active LPF
R
Vin
C
+
_
Vout
R1
R2
1
fc 
2 RC
R1
Acl  1 +
R2
Roll-off rate for a single-pole
filter is -20 dB/decade.
Acl is selectable since DF is
optional for single-pole LPF
51
Sallen-Key Low-Pass Filter
CA
RA
Selecting RA = RB = R,
and CA = CB = C :
RB
Vin
CB
+
_
Vout
R1
Sallen-Key or VCVS
(voltage-controlled
voltage-source) secondorder low-pass filter
R2
1
fc 
2 RC
The roll-off rate for a
two-pole filter is
-40 dB/decade.
For a Butterworth 2ndorder response, DF = 1.414;
therefore, R1/R2 = 0.586.
52
Cascaded Low-Pass Filter
CA1
RA1
RB1
Vin
CB1
2 poles
+
_
Roll-off rate: -60 dB/dec
RA2
CA2
R1
R2
+
_
Vout
R3
1 pole
R4
Third-order (3-pole) configuration
53
Single-Pole High-Pass Filter
C
Vin
R
+
_
Vout
R1
• Roll-off rate, and formulas
for fc , and Acl are similar to
those for LPF.
• Ideally, a HPF passes all
frequencies above fc.
However, the op-amp has an
upper-frequency limit.
R2
54
Sallen-Key High-Pass Filter
RA
CA
CB
Vin
RB
+
_
Vout
R1
Basic Sallen-Key
second-order HPF
R2
Again, formulas and
roll-off rate are similar
to those for 2nd-order
LPF.
To obtain higher rolloff rates, HPF filters
can be cascaded.
55
BPF Using HPF and LPF
CA1
Vin
RA1
RA2
+
_
R1
Av (dB)
CA2
+
_
Vout
R3
R2
R4
0
-3
HP response
LP response
fc1
fo fc2
f
56
More On Bandpass Filter
If BW and fo are given, then:
f c1 
BW 2
BW
2
+ fo 
; fc2 
4
2
BW 2
BW
2
+ fo +
4
2
A 2nd order BPF obtained by combining a LPF and a HPF:
BiFET op-amp
has FETs at
input stage and
BJTs at output
stage.
57
Notes On Cascading HPF & LPF
• Cascading a HPF and a LPF to yield a band-pass
filter can be done as long as fc1 and fc2 are
sufficiently separated. Hence the resulting
bandwidth is relatively wide.
• Note that fc1 is the critical frequency for the HPF and
fc2 is for the LPF.
• Another BPF configuration is the multiple-feedback
BPF which has a narrower bandwidth and needing
fewer components
58
Multiple-Feedback BPF
C1
R1
C2
Making C1 = C2 = C,
R2
_
Vin
R3
Vout
1
fo 
2 C
R1 + R3
R1 R2 R3
Q = fo/BW
+
Max. gain:
Q
Q
; R2 
2 f oCAo
 f oC
R2
Ao 
Q
2R1
R3 
2
2 f oC (2Q  Ao )
2
R1 
R1, C1 - LP section
R2, C2 - HP section
Ao < 2Q
59
Broadband Band-Reject Filter
A LPF and a HPF can also be combined to give a broadband
BRF:
2-pole band-reject filter
60
Narrow-band Band-Reject Filter
Easily obtained by combining the inverting output of a
narrow-band BPF and the original signal:
The equations for R1, R2, R3, C1, and C2 are the same as for BPF.
RI = RF for unity gain and is often chosen to be >> R1.
61
Multiple-Feedback Band-Stop Filter
C1
R1
Vin
C2
R2
_
Vout
+
R3
R4 When
C1 = C2 =C
1
fo 
2 C R1 R2
The multiple-feedback
BSF is very similar to
its BP counterpart. For
frequencies between fc1
and fc2 the op-amp will
treat Vin as a pair of
common-mode signals
thus rejecting them
accordingly.
62
Filter Response Measurements
• Discrete Point Measurement: Feed a sine wave to the filter
input with a varying frequency but a constant voltage and
measure the output voltage at each frequency point.
• A faster way is to use the swept frequency method:
Sweep
Generator
Filter
Spectrum
analyzer
The sweep generator outputs a sine wave whose frequency
increases linearly between two preset limits.
63
Signal Generation - Oscillators
• Barkhausen criteria for
sustained oscillations:
 The closed-loop gain, |BAV|
= 1.
 The loop phase shift = 0o or
some integer multiple of
360o at the operating
frequency.
Output
AV
AV = open-loop gain
B = feedback factor/fraction
B
64
Basic Wien-Bridge Oscillator
Voltage
Divider
R1
R1
_
R2
R3
+
C1 R4
Vout
Lead-lag
C2 circuit
R2
R3
R4
C1
_
+
Vout
C2
Two forms of the same circuit
65
Notes on Wien-Bridge Oscillator
•
•
•
At the resonant frequency the lead-lag circuit provides a positive
feedback (purely resistive) with an attenuation of 1/3 when
R3=R4=XC1=XC2.
In order to oscillate, the non-inverting amplifier must have a closedloop gain of 3, which can be achieved by making R1 = 2R2
When R3 = R4 = R, and C1 = C2 = C, the resonant frequency is:
1
fr 
2 RC
66
Phase-Shift Oscillator
Rf
_
C1
C2
C3
Vout
+
R1
R2
Each RC section provides 60o of
phase shift. Total attenuation of
the three-section RC feedback,
B = 1/29.
R3
Acl 
Rf
R3
 29
Choosing
R1 = R2 = R3 = R,
C1 = C2 = C3 = C,
the resonant
frequency is:
1
fr 
2 6 RC
67
Hartley Oscillators
L1 + L2
B
L1
1
fo 
; LT  L1 + L2
2 LT C1
L2
B
L1
68
Colpitts Oscillator
C1
1
C1C2
B
; fo 
; CT 
C2
C1 + C2
2 LCT
69
Clapp Oscillator
C2
1
B
; fo 
C2 + C3
2 LCT
1
CT 
1
1
1
+
+
C2 C3 C4
The Clapp oscillator is a variation of the Colpitts circuit. C4 is
added in series with L in the tank circuit. C2 and C3 are chosen
large enough to “swamp” out the transistor’s junction capacitances
for greater stability. C4 is often chosen to be << either C2 or C3,
thus making C4 the frequency determining element, since CT = C4.
70
Voltage-Controlled Oscillator
• VCOs are widely used in electronic circuits for AFC, PLL,
frequency tuning, etc.
• The basic principle is to vary the capacitance of a varactor
diode in a resonant circuit by applying a reverse-biased voltage
across the diode whose capacitance is approximately:
Co
CV 
1+ 2Vb
71
72
Crystals
• For high frequency stability in oscillators, a crystal
(such as quartz) has to be used.
• Quartz is a piezoelectric material: deforming it
mechanically causes the crystal to generate a
voltage, and applying a voltage to the crystal causes
it to deform.
• Externally, the crystal behaves like an electrical
resonant circuit.
73
Packaging, symbol, and characteristic of crystals
74
Crystal-Controlled Oscillators
Pierce
Colpitts
75
IC Waveform Generation
• There are a number of LIC waveform generators
from EXAR:
–
–
–
–
XR2206 monolithic function generator IC
XR2207 monolithic VCO IC
XR2209 monolithic VCO IC
XR8038A precision waveform generator IC
• Most of these ICs have sine, square, or triangle wave
output. They can also provide AM, FM, or FSK
waveforms.
76
Phase-Locked Loop
• The PLL is the basis of practically all modern
frequency synthesizer design.
• The block diagram of a simple PLL:
fr
Phase
Detector
Vp
LPF
Loop
Amplifier
VCO
fo
•Examples of a PLL I.C.: XR215, LM565, and CD4046
77
Operation of PLL
 Initially, the PLL is unlocked, i.e.,the VCO is at the
free-running frequency, fo.
 Since fo is probably not the same as the reference
frequency, fr , the phase detector will generate an
error/control voltage, Vp.
 Vp is filtered, amplified, and applied to the VCO to
change its frequency so that fo = fr. The PLL will
then remain in phase lock.
78
PLL Frequency Specifications
There is a limit on how far apart the free-running
VCO frequency and the reference frequency can be
for lock to be acquired or maintained.
Lock Range
Capture Range
Free-Running
Frequency
fLL
fLC
fo
fHC
fHL f
79
Basic PLL Frequency Synthesizer
fr
Phase
comparator
fc = fout/N
LPF
VCO
fout = Nfr
N
For output frequencies in the VHF range and higher,
a prescaler is required. The prescaler is a fixed divider
placed ahead of the programmable divide by N counter.
80
Frequency Synthesizer Using Prescaling
fr
Phase
comparator
N
LPF
VCO
fout
=(NP+M)fr
Prescaler
P or (P+1)
M
2-modulus prescaler divides by P+1 when M counter is non zero;
it divides by P when M counter reaches zero. N counter counts
down (N-M) times. E.g. of I.C. prescaler: LMX5080 for UHF
operation.
81
AM Waveform
ec = Ec sin wct
em = Em sin wmt
AM signal:
es = (Ec + em) sin wct
82
Modulation Index
• The amount of amplitude modulation in a signal is
given by its modulation index:
Em
Emax  Emin
m
or
Ec
Emax + Emin
where, Emax = Ec + Em; Emin = Ec - Em (all pk values)
When Em = Ec , m =1 or 100% modulation.
Over-modulation, i.e. Em>Ec , should be avoided
because it will create distortions and splatter.
83
Effects of Modulation Index
m=1
m>1
In a practical AM system, it usually contains many
frequency components. When this is the case,
mT  m12 + m22 + ... + mn2
84
AM in Frequency Domain
• The expression for the AM signal:
es = (Ec + em) sin wct
can be expanded to:
es = Ec sin wct + ½ mEc[cos (wc-wm)t-cos (wc+wm)t]
• The expanded expression shows that the AM signal
consists of the original carrier, a lower side
frequency, flsf = fc - fm, and an upper side frequency,
fusf = fc + fm.
85
AM Spectrum
Ec
mEc/2
mEc/2
fm
flsf
fm
fc
fusf
f
fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2
Bandwidth, B = 2fm
86
AM Power
• Total average (i.e. rms) power of the AM signal is: PT
= Pc + 2Psf , where
Pc = carrier power; and Psf = side-frequency power
• If the signal is across a load resistor, R, then: Pc =
Ec2/(2R); and Psf = m2Pc/4. So,
m2
PT  Pc (1 +
)
2
87
AM Current
• The modulation index for an AM station can be
measured by using an RF ammeter and the following
equation:
I  Io
m2
1+
2
where I is the current with modulation and
Io is the current without modulation.
88
Complex AM Waveforms
• For complex AM signals with many frequency
components, all the formulas encountered before
remain the same, except that m is replaced by mT.
For example:
2
mT
mT
PT  PC (1 +
); I  I o 1 +
2
2
2
89
Block Diagram of AM TX
90
Transmitter Stages
• Crystal oscillator generates a very stable sinewave
carrier. Where variable frequency operation is
required, a frequency synthesizer is used.
• Buffer isolates the crystal oscillator from any load
changes in the modulator stage.
• Frequency multiplier is required only if HF or higher
frequencies is required.
91
Transmitter Stages (cont’d)
• RF voltage amplifier boosts the voltage level of the
carrier. It could double as a modulator if low-level
modulation is used.
• RF driver supplies input power to later RF stages.
• RF Power amplifier is where modulation is applied
for most high power AM TX. This is known as highlevel modulation.
92
Transmitter Stages (cont’d)
• High-level modulation is efficient since all previous
RF stages can be operated class C.
• Microphone is where the modulating signal is being
applied.
• AF amplifier boosts the weak input modulating
signal.
• AF driver and power amplifier would not be required
for low-level modulation.
93
AM Modulator Circuits
94
Impedance Matching Networks
• Impedance matching networks at the output of RF
circuits are necessary for efficient transfer of power.
At the same time, they serve as low-pass filters.
Pi network
T network
95
Trapezoidal Pattern
• Instead of using the envelope display to look at AM
signals, an alternative is to use the trapezoidal
pattern display. This is obtained by connecting the
modulating signal to the x input of the ‘scope and
the modulated AM signal to the y input.
• Any distortion, overmodulation, or non-linearity is
easier to observe with this method.
96
Trapezoidal Pattern (cont’d)
m<1
m=1
Vmax  Vmin
m
Vmax + Vmin
m>1
Improper
-Vp>+Vp
phase
97
AM Receivers
• Basic requirements for receivers:
ability to tune to a specific signal
 amplify the signal that is picked up
 extract the information by demodulation
 amplify the demodulated signal
Two important receiver specifications:
sensitivity and selectivity
98
Tuned-Radio-Frequency (TRF) Receiver
• The TRF receiver is the simplest receiver that meets
all the basic requirements.
99
Drawbacks of TRF Receivers
 Difficulty in tuning all the stages to exactly the same
frequency simultaneously.
 Very high Q for the tuning coils are required for good
selectivity  BW=fo/Q.
 Selectivity is not constant for a wide range of
frequencies due to skin effect which causes the BW
to vary with fo.
100
Superheterodyne Receiver
Block diagram of basic superhet receiver:
101
Antenna and Front End
• The antenna consists of an inductor in the form of a
large number of turns of wire around a ferrite rod.
The inductance forms part of the input tuning circuit.
• Low-cost receivers sometimes omit the RF amplifier.
• Main advantages of having RF amplifier: improves
sensitivity and image frequency rejection.
102
Mixer and Local Oscillator
• The mixer and LO frequency convert the input
frequency, fc, to a fixed fIF:
High-side injection: fLO = fc + fIF
103
Autodyne Converter
• Sometimes called a self-excited mixer, the autodyne converter
combines the mixer and LO into a single circuit:
104
IF Amplifier, Detector, & AGC
105
IF Amplifier and AGC
• Most receivers have two or more IF stages to
provide the bulk of their gain (i.e. sensitivity) and
their selectivity.
• Automatic gain control (AGC) is obtained from the
detector stage to adjusts the gain of the IF (and
sometimes the RF) stages inversely to the input
signal level. This enables the receiver to cope with
large variations in input signal.
106
Diode Detector Waveforms
107
Diagonal Clipping Distortion
Diagonal clipping distortion is more pronounced at
high modulation index or high modulation frequency.
108
Sensitivity and Selectivity
• Sensitivity is expressed as the minimum input signal
required to produce a specified output level for a
given (S+N)/N ratio.
• Selectivity is the ability of the receiver to reject
unwanted or interfering signals. It may be defined
by the shape factor of the IF filter or by the amount
of adjacent channel rejection.
109
Shape Factor
B60dB
SF 
B6 dB
110
Image Frequency
• One of the problems with the superhet receiver is
that an image frequency signal could interfere with
the reception of the desired signal. The image
frequency is given by: fimage = fsig + 2fIF
where
fsig = desired signal.
• An image signal must be rejected by tuning circuits
prior to mixing.
111
Image-Frequency Rejection Ratio
• For a tuned circuit with a quality factor of Q, its
image-frequency rejection ratio is:
IFRR  1 + Q x
2
x
f image
f sig

2
where,
f sig
f image
In dB, IFRR(dB) = 20 log IFRR
112
IF Transformers
• The transformers used in the IF stages can be either
single-tuned or double-tuned.
Single-tuned
Double-tuned
113
Loose and Tight Couplings
• For single-tuned transformers, tighter coupling
means more gain but broader bandwidth:
114
Under, Over, & Critical Coupling
• Double-tuned transformers can be over, under,
critically, or optimally coupled:
115
Coupling Factors
• Critical coupling factor kc is given by:
1
kc 
Q p Qs
where Qp, Qs = prim. & sec. Q, respectively.
IF transformers often use the optimum coupling
factor, kopt = 1.5kc , to obtain a steep skirt and
flat passband. The bandwidth for a double-tuned
IF amplifier with k = kopt is given by B = kfo.
Overcoupling means k>kc; undercoupling, k< kc
116
Piezoelectric Filters
• For narrow bandwidth (e.g. several kHz), excellent
shape factor and stability, a crystal lattice is used as
bandpass filter.
• Ceramic filters, because of their lower Q, are useful
for wideband signals (e.g. FM broadcast).
• Surface-acoustic-wave (SAW) filters are ideal for
high frequency usage requiring a carefully shaped
response.
117
Suppressed-Carrier AM Systems
• Full-carrier AM is simple but not efficient in terms of
transmitted power, bandwidth, and SNR.
• Using single-sideband suppressed-carrier (SSBSC
or SSB) signals, since Psf = m2Pc/4, and Pt=Pc(1+m2/2
), then at m=1, Pt= 6 Psf .
• SSB also has a bandwidth reduction of half, which in
turn reduces noise by half.
118
Generating SSB - Filtering Method
• The simplest method of generating an SSB signal is to
generate a double-sideband suppressed-carrier (DSB-SC)
signal first and then removing one of the sidebands.
Balanced
Modulator DSB-SC
USB
BPF
AF
Input
Carrier
Oscillator
or
LSB
119
Waveforms for Balanced Modulator
V2, fm
Vo
V1, fc
fc-fm fc+fm
f
120
Mathematical Analysis of Balanced Modulator
• V1 = A1sin wct; V2 = A2sin wmt
• Vo = V1V2 = A1A2sin wct sin wmt
= ½A1A2{cos(wc- wm)t – cos(wc+ wm)t}
• The equation above shows that the output of the
balanced modulator consists of a lower sidefrequency (wc - wm) and an upper side-frequency (wc+
wm)
121
LIC Balanced Modulator 1496
122
Filter for SSB
• Filters with high Q are needed for suppressing the
unwanted sideband.
fa = f c - f2
fb = fc - f1
fd = fc + f1
fe = f c + f 2
f c anti log( X dB / 20) where X = attenuation of
Q
4f
sideband, and f = fd - fb
123
Typical SSB TX using Filter Method
124
SSB Waveform
125
Generating SSB - Phasing Method
• This method is based on the fact that the lsf and the
usf are given by the equations:
cos {(wc - wm)t} = ½(cos wct cos wmt + sin wct sin wmt)
cos {(wc + wm)t} = ½(cos wct cos wmt - sin wct sin wmt)
• The RHS of the 1st equation is just the sum of two
products: the product of the carrier and the
modulating signal, and the product of the same two
signals that have been phase shifted by 90o.
• The 2nd equation is similar except for the (-) sign.
126
Diagram for Phasing Method
Modulating
signal
Em cos wmt
Balanced Modulator 1
Carrier
oscillator
90o phase
shifter
Ec cos wct
90o phase
shifter
+
SSB
output
Balanced Modulator 2
127
Phasing vs Filtering Method
Advantages of phasing method :
 No high Q filters are required.
 Therefore, lower fm can be used.
 SSB at any carrier frequency can be generated in a
single step.
Disadvantage:
Difficult to achieve accurate 90o phase shift across
the whole audio range.
128
Peak Envelope Power
• SSB transmitters are usually rated by the peak
envelope power (PEP) rather than the carrier power.
With voice modulation, the PEP is about 3 to 4 times
the average or rms power.
PEP 
Vp
2
2 RL
where Vp = peak signal voltage
and RL = load resistance
129
Non-coherent SSB BFO RX
130
Coherent SSB BFO Receiver
RF SSBRC
RF
input
signal
RF amplifier
and
preselector
IF SSBRC
IF amp. &
RF mixer
bandpass
filter
RF LO
Carrier recovery
and frequency
synthesizer
IF
mixer
Demod.
info
BFO
131
Notes On SSB Receivers
• The input SSB signal is first mixed with the LO
signal (low-side injection is used here).
• The filter removes the sum frequency components
and the IF signal is amplified.
• Mixing the IF signal with a reinserted carrier from a
beat frequency oscillator (BFO) and low-pass
filtering recovers the audio information.
132
SSB Receivers (cont’d)
• The product detector is often just a balanced
modulator operated in reverse.
• Frequency accuracy and stability of the BFO is
critical. An error of a little more than 100 Hz could
render the received signal unintelligible.
• In coherent or synchronous detection, a pilot carrier
is transmitted with the SSB signal to synchronize the
RF local oscillator and BFO.
133
Angle Modulation
 Angle modulation includes both frequency and
phase modulation.
 FM is used for: radio broadcasting, sound signal in
TV, two-way fixed and mobile radio systems, cellular
telephone systems, and satellite communications.
 PM is used extensively in data communications and
for indirect FM.
134
Comparison of FM or PM with AM
Advantages over AM:
1)
2)
3)
better SNR, and more resistant to noise
efficient - class C amplifier can be used, and less
power is required to angle modulate
capture effect reduces mutual interference
Disadvantages:
1)
2)
much wider bandwidth is required
slightly more complex circuitry is needed
135
Frequency Shift Keying (FSK)
Carrier
Modulating
signal
FSK
signal
136
FSK (cont’d)
• The frequency of the FSK signal changes abruptly
from one that is higher than that of the carrier to one
that is lower.
• Note that the amplitude of the FSK signal remains
constant.
• FSK can be used for transmission of digital data (1’s
and 0’s) with slow speed modems.
137
Frequency Modulation
Carrier
Modulating
Signal
FM
signal
138
Frequency Modulation (cont’d)
• Note the continuous change in frequency of the FM
wave when the modulating signal is a sine wave.
• In particular, the frequency of the FM wave is
maximum when the modulating signal is at its
positive peak and is minimum when the modulating
signal is at its negative peak.
139
Frequency Deviation
• The amount by which the frequency of the FM signal
varies with respect to its resting value (fc) is known
as frequency deviation: f = kf em, where kf is a
system constant, and em is the instantaneous value
of the modulating signal amplitude.
• Thus the frequency of the FM signal is:
fs (t) = fc + f = fc + kf em(t)
140
Maximum or Peak Frequency Deviation
• If the modulating signal is a sine wave, i.e., em(t) =
Emsin wmt, then fs = fc + kfEmsin wmt.
• The peak or maximum frequency deviation:
d = kf Em
• The modulation index of an FM signal is:
mf = d / fm
• Note that mf can be greater than 1.
141
Relationship between FM and PM
• For PM, phase deviation, f = kpem, and the peak
phase deviation, fmax = mp = mf.
• Since frequency (in rad/s) is given by:
d (t )
w (t ) 
dt
or  (t )   w (t )dt
the above equations suggest that FM can be
obtained by first integrating the modulating
signal, then applying it to a phase modulator.
142
Equation for FM Signal
• If ec = Ec sin wct, and em = Em sin wmt, then the
equation for the FM signal is:
es = Ec sin (wct + mf sin wmt)
• This signal can be expressed as a series of
sinusoids: es = Ec{Jo(mf) sin wct
- J1(mf)[sin (wc - wm)t - sin (wc + wm)t]
+ J2(mf)[sin (wc - 2wm)t + sin (wc + 2wm)t]
- J3(mf)[sin (wc - 3wm)t + sin (wc + 3wm)t]
+ … .}
143
Bessel Functions
• The J’s in the equation are known as Bessel
functions of the first kind:
mf J o
J1
J2
J3
J4
J5
J6 . . .
0
0.5
1
2.4
5.5
1
.94
.77
0.0
0.0
.24
.44
.52
-.34
.03
.11
.43
-.12
.02
.20
.26
.06
.40
.02
.32
.19 . . .
144
Notes on Bessel Functions
• Theoretically, there is an infinite number of side
frequencies for any mf other than 0.
• However, only significant amplitudes, i.e. those
|0.01| are included in the table.
• Bessel-zero or carrier-null points occur when mf =
2.4, 5.5, 8.65, etc. These points are useful for
determining the deviation and the value of kf of an
FM modulator system.
145
Graph of Bessel Functions
146
FM Side-Bands
• Each (J) value in the table
gives rise to a pair of sidefrequencies.
• The higher the value of mf,
the more pairs of significant
side- frequencies will be
generated.
147
Power and Bandwidth of FM Signal
• Regardless of mf , the total power of an FM
signal remains constant because its
amplitude is constant.
• The required BW of an FM signal is:
BW = 2 x n x fm ,where n is the number of pairs of
side-frequencies.
• If mf > 6, a good estimate of the BW is given by
Carson’s rule: BW = 2(d + fm (max) )
148
Narrowband & Wideband FM
• FM systems with a bandwidth < 15 kHz, are
considered to be NBFM. A more restricted definition
is that their mf < 0.5. These systems are used for
voice communication.
• Other FM systems, such as FM broadcasting and
satellite TV, with wider BW and/or higher mf are
called WBFM.
149
Pre-emphasis
• Most common analog signals have high frequency
components that are relatively low in amplitude than
low frequency ones. Ambient electrical noise is
uniformly distributed. Therefore, the SNR for high
frequency components is lower.
• To correct the problem, em is pre-emphasized before
frequency modulating ec.
150
Pre-emphasis circuit
• In FM broadcasting, the high
frequency components are
boosted by passing the
modulating signal through a
HPF with a 75 ms time
constant before modulation.
 t = R1C = 75 ms.
151
De-emphasis Circuit
• At the FM receiver, the
signal after demodulation
must be de-emphasized by a
filter with similar
characteristics as the preemphasis filter to restore the
relative amplitudes of the
modulating signal.
152
FM Stereo Broadcasting: Baseband Spectra
• To maintain compatibility with monaural system, FM
stereo uses a form of FDM or frequency-division
multiplexing to combine the left and right channel
information:
19 kHz Pilot
Carrier
L+R
(mono)
.05
15 23
L-R
L-R
38
SCA
(optional)
53 60 67 74
kHz
153
FM Stereo Broadcasting
• To enable the L and R channels to be reproduced at
the receiver, the L-R and L+R signals are required.
These are sent as a DSBSC AM signal with a
suppressed subcarrier at 38 kHz.
• The purpose of the 19 kHz pilot is for proper
detection of the DSBSC AM signal.
• The optional Subsidiary Carrier Authorization (SCA)
signal is normally used for services such as
background music for stores and offices.
154
Block Diagram of FM Transmitter
FM
Modulator
Frequency
Multiplier(s)
Buffer
Pre-emphasis
Antenna
Driver
Power
Amp
Audio
155
Direct-FM Modulator
• A simple method of generating FM is to use a reactance
modulator where a varactor is put in the frequency determining
circuit.
156
Crosby AFC System
• An LC oscillator operated as a VCO with automatic
frequency control is known as the Crosby system.
157
Phase-Locked Loop FM Generators
• The PLL system is more stable than the Crosby system and can
produce wide-band FM without using frequency multipliers.
158
Indirect-FM Modulators
• Recall earlier that FM and PM were shown to be
closely related. In fact, FM can be produced using a
phase modulator if the modulating signal is passed
through a suitable LPF (i.e. an integrator) before it
reaches the modulator.
• One reason for using indirect FM is that it’s easier to
change the phase than the frequency of a crystal
oscillator. However, the phase shift achievable is
small, and frequency multipliers will be needed.
159
Example of Indirect FM Generator
Armstrong
Modulator
160
Block Diagram of FM Receiver
161
FM Receivers
• FM receivers, like AM receivers, utilize the
superheterodyne principle, but they operate at much
higher frequencies (88 - 108 MHz).
• A limiter is often used to ensure the received signal
is constant in amplitude before it enters the
discriminator or detector. The limiter operates like a
class C amplifier when the input exceeds a threshold
point. In modern receivers, the limiting function is
built into the FM IF integrated circuit.
162
FM Demodulators
• The FM demodulators must convert frequency
variations of the input signal into amplitude
variations at the output.
• The Foster-Seeley discriminator and its variant, the
ratio detector are commonly found in older
receivers. They are based on the principle of slope
detection using resonant circuits.
163
S-curve Characteristics of FM Detectors
vo
Em
d
fi
fIF
d
164
PLL FM Detector
• PLL and quadrature detectors are commonly found
in modern FM receivers.
FM IF
Signal
Phase
Detector
f
LPF
Demodulated
output
VCO
165
Quadrature Detector
• Both the quadrature and the PLL detector are
conveniently found as IC packages.
166
Types of Transmission Lines
• Differential or balanced lines (where neither
conductor is grounded): e.g. twin lead, twisted-cable
pair, and shielded-cable pair.
• Single-ended or unbalanced lines (where one
conductor is grounded): e.g. concentric or coaxial
cable.
• Transmission lines for microwave use: e.g.
striplines, microstrips, and waveguides.
167
Transmission Line Equivalent Circuit
R
Zo
C
L
G
R
C
L
G
“Lossy” Line
R + jwL
Zo 
G + jwC
L
Zo
C
L
C
Lossless Line
L
Zo 
C
168
Notes on Transmission Line
• Characteristics of a line is determined by its primary
electrical constants or distributed parameters: R
(/m), L (H/m), C (F/m), and G (S/m).
• Characteristic impedance, Zo, is defined as the input
impedance of an infinite line or that of a finite line
terminated with a load impedance, ZL = Zo.
169
Formulas for Some Lines
For parallel two-wire line:
m 2D

120 2 D
L  ln
; C
; Zo 
ln
2D

d
d
r
ln
D
d
d
m = momr;  = or; mo = 4x10-7 H/m; o = 8.854 pF/m
D
d
For co-axial cable:
m D
2
60
D
L
ln ; C 
; Zo 
ln
D
2 d
r d
ln
d
170
Transmission-Line Wave Propagation
Electromagnetic waves travel at < c in a transmission
line because of the dielectric separating the conductors.
The velocity of propagation is given by:
v
1
1
c


LC
m
r
m/s
Velocity factor, VF, is defined as: VF  v  1
c
r
171
Propagation Constant
• Propagation constant, , determines the variation of
V or I with distance along the line: V = Vse-x; I = Isex, where V , and I are the voltage and current at the
S
S
source end, and x = distance from source.
•  =  + j, where  = attenuation coefficient (= 0 for
lossless line), and  = phase shift coefficient = 2/l
(rad./m)
172
Incident & Reflected Waves
• For an infinitely long line or a line terminated with a
matched load, no incident power is reflected. The
line is called a flat or nonresonant line.
• For a finite line with no matching termination, part or
all of the incident voltage and current will be
reflected.
173
Reflection Coefficient
The reflection coefficient is defined as:
Er

Ei
or
It can also be shown that:
Ir
Ii
Z L  Zo

  f
Z L + Zo
Note that when ZL = Zo,  = 0; when ZL = 0,  = -1;
and when ZL = open circuit,  = 1.
174
Voltage
Standing Waves
Vmax = Ei + Er
l
2
Vmin = Ei - Er
With a mismatched line, the incident and reflected
waves set up an interference pattern on the line
known as a standing wave.
Vmax 1 + 
The standing wave ratio is : SWR  V  1  
min
175
Other Formulas
When the load is purely resistive:
(whichever gives an SWR > 1)
Zo
ZL
SWR 
or
Zo
ZL
Return Loss, RL = Fraction of power reflected
= ||2, or -20 log || dB
So, Pr = ||2Pi
Mismatched Loss, ML = Fraction of power
transmitted/absorbed = 1 - ||2 or -10 log(1-||2) dB
So, Pt = Pi (1 - ||2) = Pi - Pr
176
Simple Antennas
• An isotropic radiator would radiate all electrical power supplied
to it equally in all directions. It is merely a theoretical concept
but is useful as a reference for other antennas.
• A more practical antenna is the half-wave dipole:
l/2
Balanced Feedline
Symbol
177
Half-Wave Dipole
• Typically, the physical length of a half-wave dipole is 0.95 of l/2
in free space.
• Since power fed to the antenna is radiated into space, there is
an equivalent radiation resistance, Rr. For a real antenna,
losses in the antenna can be represented by a loss resistance,
Rd. Its efficiency is then:
Pr
Rr


PT Rr + Rd
178
3-D Antenna Radiation Pattern
179
Gain and Directivity
• Antennas are designed to focus their radiation into
lobes or beams thus providing gain in selected
directions at the expense of energy reductions in
others.
• The ideal l/2 dipole has a gain of 2.14 dBi (i.e. dB
with respect to an isotropic radiator)
• Directivity is the gain calculated assuming a lossless
antenna
180
EIRP and Effective Area
• When power, PT, is applied to an antenna with a gain
GT (with respect to an isotropic radiator), then the
antenna is said to have an effective isotropic
radiated power, EIRP = PTGT.
• The signal power delivered to a receiving antenna
with a gain GR is PR = PDAeff where PD is the power
density, and Aeff is the effective area.
EIRP
l2GR
PD 
; Aeff 
2
4r
4
181
Impedance and Polarization
• A half-wave dipole in free space and centre-fed has a
radiation resistance of about 70 .
• At resonance, the antenna’s impedance will be
completely resistive and its efficiency maximum. If
its length is < l/2, it becomes capacitive, and
if > l/2, it is inductive.
• The polarization of a half-wave dipole is the
same as the axis of the conductor.
182
Ground Effects
• Ground effects on antenna pattern and resistance
are complex and significant for heights less than one
wavelength. This is particularly true for antennas
operating at HF range and below.
• Generally, a horizontally polarized antenna is
affected more by near ground reflections than a
vertically polarized antenna.
183
Folded Dipole
• Often used - alone or with other elements - for TV and FM
broadcast receiving antennas because it has a wider bandwidth
and four times the feedpoint resistance of a single dipole.
184
Monopole or Marconi Antenna
Main characteristics:
 vertical and l/4
 good ground plane is
required
 omnidirectional in the
horizontal plane
 3 dBd power gain
 impedance: about 36
185
Loop Antennas
Main characteristics:
 very small dimensions
 bidirectional
 greatest sensitivity in the
plane of the loop
 very wide bandwidth
 efficient as RX antenna with
single or multi-turn loop
186
Antenna Matching
• Antennas should be matched to their feedline for
maximum power transfer efficiency by using an LC
matching network.
• A simple but effective technique for matching a short
vertical antenna to a feedline is to increase its
electrical length by adding an inductance at its base.
This inductance, called a loading coil, cancels the
capacitive effect of the antenna.
• Another method is to use capacitive loading.
187
Inductive and Capacitive Loading
Inductive Loading
Capacitive Loading
188
Collinear Array
 all elements lie along a straight line, fed in phase, and often
mounted with main axis vertical
 result in narrow radiation beam omnidirectional in the
horizontal plane
189
2-Way Mobile Communications
• 1) Mobile radio, half-duplex, one-to-many, no dial
tone:
– e.g. CB, amateur (ham) radio, aeronautical, maritime, public safety,
emergency, and industrial radios
• 2) Mobile Telephone, Full-duplex, one-to-one:
– Analogue cellular (AMPS) using FDMA or TDMA
– Digital cellular (PCS) using TDMA, FDMA, and CDMA
– Personal communications satellite service (PCSS) using both
FDMA and TDMA
190
Mobile Telephone Systems
• Mobile telephone began in the early 1980s first as
the MTS (Mobile Telephone Service) at 40 MHz and
later as the IMTS (Improved MTS) at 150 and 450
MHz.
• Narrowband FM and relatively high transmit power
were used.
• Limited channels (total of only 33) and interference
were problems.
191
Advanced Mobile Phone System
• AMPS divide area into cells with low power transmitters in each
cell.
• Max. 4 W ERP for mobile radios; max. 600 mW for portable
phones; to reduce interference min. power needed for
communications is used at all times.
• Base station: 869.040 – 893.970 MHz; mobile unit’s frequency is
45 MHz below.
• Total of 790 duplex voice channels and 42 control channels
available at 30 kHz each.
• Channels are divided in 7- or 12-cell repeated pattern and
frequencies are reused
192
Block Diagram Of Analogue Cell Phone
Antenna
Speaker
RF amp
Duplexer
mixer
Frequency
synthesizer
IF
amp
IF
detector
De-emphasis
Audio
amp
Display
Microprocessor
Keypad
Data
RF power
amp
FM
modulator
Audio preamp
& Pre-emphasis
Mic
6 mW – 3W
193
7-Cell Pattern
6
5
4
1
3
3
7
2
5
6
1
4
• Each cell has a base station.
• All cell sites in a region are
tied to a mobile switching
centre (MSC) or mobile
telephone switching office
(MTSO) which in turn is
connected to other MSCs.
In a real situation, the cells are
more likely to be approximately
circular, with some overlap.
194
Cellular Radio Network
BSC: Base Station Controller
MSC: Mobile Switching Centre
BSC
To other
MSCs
BSC
MSC
To other BSCs
BSC
BSC
BSC
Gateway
MSC
To Public Switched
Telephone Network
BSC
BSC
MSC
BSC
195
Cell-Site Control
• BSC assigns channels and power levels,
transmitting signaling tones, etc.
• MSC routes calls, authorizing calls, billing, initiating
handoffs between cells, holds location and
authentication registers, connects mobile units to
the PSTN, etc.
• Sometimes BSC and MSC are combined.
• Cells can be subdivided into mini and micro cells to
increase subscriber capacity in a region.
196
Digital Cellular Telephone
• The United States Digital Cellular (USDC) system is backward
compatible with the AMPS frequency allocation scheme but
using digitized signals and PSK modulation.
• It uses TDMA (Time-Division Multiple Access) to increase the
number of subscribers threefold with the same 50-MHz
frequency spectrum.
• It provides higher security and better signal quality.
• TDMA Service in the 1900 MHz band is also in use since there
is no room in the 800 MHz band for expansion.
197
Code-Division Multiple-Access System
• CDMA is a totally digital cellular telephone system.
• It is more commonly found in the 1900 MHz PCS band with up
to 11 CDMA RF channels.
• Each CDMA RF channel has a bandwidth of 1.25 MHz, using a
single carrier modulated by a 1.2288 Mb/s bitstream using
QPSK.
• Each RF channel can provide up to 64 traffic channels.
• It uses a spread-spectrum technique so all frequencies can be
used in all cells – soft handoff possible.
• Each mobile is assigned a unique spreading sequence to
reduce RF interference.
198
Global System For Mobile Communications
• GSM uses frequency-division duplexing and a
combination of TDMA and FDMA techniques.
• Base station frequency: 935 MHz to 960 MHz; mobile
frequency: 45 MHz below
• 1800 MHz is allocated for PCS in Europe while North
America utilizes the 1900 MHz band.
• RF channel bandwidth is 200 kHz but each can hold
8 voice/data channels.
199
Personal Communications Satellite System
• PCSS uses either low earth-orbit (LEO) or medium
earth-orbit (MEO) satellites.
• Advantages: can provide telephone services in
remote and inaccessible areas quickly and
economically.
• Disadvantages: high risk due to high costs of
designing, building and launching satellites; also
high cost for terrestrial-based network and
infrastructure. Mobile unit is more bulky and
expensive than conventional cellular telephones.
200