# ANALOG COMMUNICATIONS

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```					ANALOGUE TELECOMMUNICATIONS

1
MAIN TOPICS (Part I)

1)   Introduction to Communication Systems
2)   Filter Circuits
3)   Signal Generation
4)   Amplitude Modulation
6)   AM Transmitters

2
MAIN TOPICS (Part II)

7)    Single-Sideband Communications Systems
8)    Angle Modulation Transmission
9)    Angle Modulated Receivers & Systems
10)   Introduction To Transmission Lines & Antennas
11)   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                    BAND    Hz
 ELF 30 - 300                 VHF 30M-300M
 AF           300 - 3 k       UHF 300M - 3 G
 VLF 3 k - 30 k               SHF 3 G - 30 G
 LF           30 k - 300 k    EHF 30 G - 300G
 MF           300 k - 3 M
 HF           3 M - 30 M

•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 time-
domain.
 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
Excess or                                     Effect Noise
Flicker Noise

Shot and Thermal Noises

f
1 kHz                              fhc
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:

S               PS          VS
(dB)  10 log     20 log
N               PN          VN

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
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     A1 A 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

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:

1  fo    RL
fo        ; B ; Q
2 LC     Q     XL
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-to-
base 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:

High
Input                                  Tuning       Output
Distortion
Filter
fi         Device +                               N x fi
Circuit
Amplifier

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 band-
reject 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
f 
 c

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
0
Vo         Ideal
-20
1
-40
0.707
Passband                -60
BW
0                fc       f              fc    10fc 100fc 1000fc        f

Basic LPF response                 LPF with different roll-off rates

43
High-Pass Filter Response

Gain (dB)
0
Vo
-20
1
-40
0.707
Passband       -60

0           fc              f                   0.01fc 0.1fc   fc         f
Basic HPF response              HPF with different roll-off rates

44
Band-Pass Filter Response

Vout                    Centre frequency:   fo     f c1 f c 2
1
Quality factor: Q  f o
0.707                                            BW
Q is an indication of the
BW                 selectivity of a BPF.
Narrow BPF: Q > 10.
f    Wide-band BPF: Q < 10.
fc1   fo fc2
BW = fc2 - fc1            Damping Factor: DF  1 Q

45
Band-Stop Filter Response

• Also known as band-reject,
Gain (dB)                         or notch filter.
• Frequencies within a certain
0                              BW are rejected.
-3                            • Useful for filtering interfering
signals.
Pass
Passband
band

f
fc1 fo fc2

BW

46
Filter Response Characteristics

Av
Chebyshev

Bessel

Butterworth

f

47
Damping Factor

The damping factor (DF)
Frequency                              of an active filter sets
Vin       selective                      Vout   the response characteristic
+
RC circuit       _                     of the filter.
R1
R1                 DF  2 
R2

R2          Its value depends on the
order (# of poles) of the
filter. (See Table on next
General diagram of active filter          slide for DF values.)

48
Values For Butterworth Response

Order     1st Stage          2nd Stage

Poles     DF       Poles    DF

1       1      optional

2       2      1.414

3       2         1        1        1

4       2      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                                    1
Vin                Vout           fc 
+                            2 RC
C   _
R1
R1              Acl  1 +
R2
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           Selecting RA = RB = R,
and CA = CB = C :
RA   RB
1
Vin                   +      Vout         fc 
CB         _                        2 RC

R1     The roll-off rate for a
two-pole filter is
Sallen-Key or VCVS        R2     -40 dB/decade.
(voltage-controlled             For a Butterworth 2nd-
voltage-source) second-         order response, DF = 1.414;
order low-pass filter           therefore, R1/R2 = 0.586.

52

CA1
Roll-off rate: -60 dB/dec
RA1   RB1
RA2
Vin                     +
CB1         _                     +              Vout
CA2          _
R1
R3
R2
2 poles                     1 pole         R4

Third-order (3-pole) configuration

53
Single-Pole High-Pass Filter

• Roll-off rate, and formulas
C                        for fc , and Acl are similar to
those for LPF.
Vin           +       Vout   • Ideally, a HPF passes all
_                frequencies above fc.
R
However, the op-amp has an
R1        upper-frequency limit.

R2

54
Sallen-Key High-Pass Filter

RA
Again, formulas and
CA    CB                      roll-off rate are similar
Vin                   +      Vout   to those for 2nd-order
_             LPF.
RB
R1      To obtain higher roll-
off rates, HPF filters
R2      can be cascaded.
Basic Sallen-Key
second-order HPF

55
BPF Using HPF and LPF

CA1
Vin                                 RA2
+
_                        +        Vout
RA1
CA2         _
R1
Av (dB)                                                       R3
R2
R4
0
-3
HP response
LP response
f
fc1     fo fc2
56
More On Bandpass Filter

If BW and fo are given, then:
BW 2        BW         BW 2        BW
f c1       + fo 
2
; fc2       + fo +
2

4           2          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                Making C1 = C2 = C,
R2
1     R1 + R3
R1    C2                               fo 
_                         2 C   R1 R2 R3
Vin                                  Vout
R3           +                        Q = fo/BW

Q               Q    Max. gain:
R1            ; R2 
2 f oCAo         f oC       R2
R1, C1 - LP section                      Q              Ao 
R3                               2R1
R2, C2 - HP section             2 f oC (2Q  Ao )
2
2
Ao < 2Q

59

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
The multiple-feedback
R2               BSF is very similar to
R1   C2                       its BP counterpart. For
Vin                _         Vout   frequencies between fc1
and fc2 the op-amp will
+                treat Vin as a pair of
R3
R4 When             common-mode signals
thus rejecting them
C1 = C2 =C      accordingly.
1
fo 
2 C R1R2

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                           Spectrum
Generator          Filter        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:
Output
 The closed-loop gain, |BAV|       AV
= 1.
 The loop phase shift = 0o or
some integer multiple of
360o at the operating
frequency.
AV = open-loop gain
B
B = feedback factor/fraction

64
Basic Wien-Bridge Oscillator

R1                                        R4
Voltage                                  R1
_                                    C1
Divider                                                   _
R2   +              Vout
C1 R4                R2               +   Vout

R3         Lead-lag            R3              C2
C2 circuit

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 closed-
loop 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
Rf
_        C1        C2    C3            Acl          29
Vout             R3
+                                    Choosing
R1        R2    R3     R1 = R2 = R3 = R,
C1 = C2 = C3 = C,
the resonant
frequency is:
Each RC section provides 60o of
phase shift. Total attenuation of                  1
fr 
the three-section RC feedback,                  2 6 RC
B = 1/29.
67
Hartley Oscillators

L1 + L2           1                         L2
B           fo           ; LT  L1 + L2   B
L1          2 LT C1                     L1
68
Colpitts Oscillator

C1           1           C1C2
B    ; fo         ; CT 
C2        2 LCT        C1 + C2
69
Clapp Oscillator

C2              1
B          ; fo 
C 2 + C3        2 LCT
1
CT 
1    1   1
+   +
C 2 C3 C 4

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              Vp                                  fo
Phase                  Loop
LPF                    VCO
Detector               Amplifier

•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         LPF         VCO        fout = Nfr

N
fc = fout/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                                        fout
comparator           LPF          VCO
=(NP+M)fr

Prescaler
N
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

AM signal:
ec = Ec sin wct         es = (Ec + em) sin wct
em = Em sin wmt
82
Modulation Index

• The amount of amplitude modulation in a signal is
given by its modulation index:

Em    Em ax  Em in
m    or
Ec    Em ax + Em in
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 + m2 + ...+ mn
2         2

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              fm
f
flsf            fc            fusf

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:

m2
I  Io   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                    2
mT                mT
PT  PC (1 +    ); I  I o 1 +
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 high-
level 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     m>1
Improper
Vm ax  Vm in                     -Vp>+Vp
m                            phase
Vm ax + Vm in
97

• 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

• 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

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60 dB
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     2
where,
f image        f sig
x             
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 pQs
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           or
AF
Input            Carrier                           LSB
Oscillator                                 119
Waveforms for Balanced Modulator

V2, fm            Vo

V1, fc

f
fc-fm fc+fm
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 side-
frequency (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                        Balanced Modulator 1
signal
Carrier
Em cos wmt
oscillator
Ec cos wct

+    SSB
90o phase               90o phase             output
shifter                 shifter

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.
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.

2
Vp
PEP 
2 RL
where Vp = peak signal voltage
and RL = load resistance
129
Non-coherent SSB BFO RX

130
Coherent SSB BFO Receiver

RF SSBRC                IF SSBRC
RF amplifier             IF amp. &     IF     Demod.
and        RF mixer   bandpass
RF                                             mixer   info
preselector                filter
input
signal                        RF LO

Carrier recovery   BFO
and frequency
synthesizer

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

• 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

1)   better SNR, and more resistant to noise
2)   efficient - class C amplifier can be used, and less
power is required to angle modulate
3)   capture effect reduces mutual interference
1)   much wider bandwidth is required
2)   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 )            or  (t )   w (t )dt
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 Jo      J1    J2    J3    J4  J5  J6 . . .
0     1
0.5   .94   .24    .03
1     .77   .44    .11    .02
2.4   0.0   .52    .43    .20   .06   .02
5.5   0.0   -.34   -.12   .26   .40   .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 side-
frequencies.
• 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 pre-
emphasis 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                       SCA
L+R                               (optional)
(mono)          L-R        L-R
kHz
.05      15 23          38         53 60 67 74
153

• 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                Frequency
Modulator           Multiplier(s)                    Antenna

Buffer                   Driver   Power
Amp
Pre-emphasis

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, 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.

Phase
Detector
FM IF                                  Demodulated
Signal        f               LPF        output

VCO

165

• 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      L        R   L           L        L

Zo                             Zo
C       G   C       G            C        C

“Lossy” Line                Lossless Line

R + jwL                      L
Zo                          Zo 
G + jwC                      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
D      L  ln      ; C         ; Zo      ln
    d        ln
2D          r    d
d
d
m = momr;  = or; mo = 4x10-7 H/m; o = 8.854 pF/m
For co-axial cable:
D
m D             2        60    D
L     ln ; C           ; Zo     ln
2 d            ln
D         r d
d
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:
1    1    c
v                      m/s
LC   m   r

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 = Ise-
x, 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

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        Ir
      or
Ei        Ii

It can also be shown that:       Z L  Zo
            f
Z L + Zo

Note that when ZL = Zo,  = 0; when ZL = 0,  = -1;
and when ZL = open circuit,  = 1.

174
Standing Waves
Voltage

Vmax = Ei + Er
Vmin = Ei - Er
l
2
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:         ZL    Zo
SWR     or
(whichever gives an SWR > 1)               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

Symbol
Balanced Feedline
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 
4r 2
4
181
Impedance and Polarization

• A half-wave dipole in free space and centre-fed has a
• 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

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
mixer          IF        IF                       Audio
RF amp                                     De-emphasis
amp     detector                     amp

Display
Frequency
Duplexer                         Microprocessor
synthesizer
Data

RF power         FM                   Audio preamp
Mic
amp          modulator              & Pre-emphasis
6 mW – 3W

193
7-Cell Pattern

• Each cell has a base station.
• All cell sites in a region are
tied to a mobile switching
6       3                   centre (MSC) or mobile
5       7       6               telephone switching office
1       5                   (MTSO) which in turn is
4       2       1
3       4                   connected to other MSCs.

In a real situation, the cells are
more likely to be approximately
circular, with some overlap.

194

BSC: Base Station Controller
MSC: Mobile Switching Centre

BSC      BSC    BSC      BSC   BSC     BSC    BSC

To other                         Gateway           MSC
MSC
MSCs                             MSC

To other BSCs                                BSC
To Public Switched
Telephone Network
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

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