Unit IV_SS_MMS.ppt

 INTRODUCTION
 PN SEQUENCE & MAXIMAL LENGTH SEQUENCE
 DEFINITION OF SPREAD SPECTRUM ( SS )
 CHARACTERISTICS OF SPREAD SPECTRUM & TYPES
 BASIC PRINCIPLES OF DIRECT SEQUENCE SPREAD
SPECTRUM ( DSSS )
 BASIC PRINCIPLES OF FREQUENCY HOPPING SPREAD
SPECTRUM ( FHSS )
 PERFORMANCE IN THE PRESENCE OF INTERFERENCE
2

Text Books
 1. Simon Haykin, “Digital Communication Systems”, John
Wiley&Sons, Fourth Edition.
 2. A.B. Carlson, P B Crully, J C Rutledge, “Communication Systems”,
Fourth Edition, McGraw Hill Publication.
Reference Books
 1. P Ramkrishna Rao, Digital Communication, McGrawHill Publication
 2. Ha Nguyen, Ed Shwedyk, “A First Course in Digital Communication”,
Cambridge University Press.
 3. B P Lathi, Zhi Ding “Modern Analog and Digital Communication
System”, Oxford University Press, Fourth Edition.
 4. Bernard Sklar,Prabitra Kumar Ray, “Digital Communications
Fundamentals and Applications” Second Edition,Pearson Education
 5. Taub, Schilling, “Principles of Communication System”, Fourth Edition,
McGraw Hill.

Topic Books lectures Teaching Method
Introduction T1, R1
1
PPT
Pseudo noise sequences T1, T2
2
Conventional
A notion of spread spectrum T1
1
PPT
Direct sequence spread
spectrum with coherent BPSK
T1, R1 Conventional
Signal space dimensionality &
processing gain
T1
1
Conventional
Probability of error T1 Conventional &
simulation
Concept of Jamming T1
http://nptel.ac.in/co
urses/117105136/4
1
Conventional & PPT
Frequency hop spread spectrum R3
2
Conventional & PPT

Subjects
1 Signals and Systems
2 Analog Communication
3 Digital Electronics
4 Engineering Mathematics
Concepts
1 Shift Register
2 Power Spectral Density & Auto-correlation
3 Random Variables and Central Limit Theorem, Mean and Variance
4 BPSK/MFSK
5 Coherent & non-coherent Detection

In communication systems we focused on:
1)How to utilize bandwidth? &
2)How to minimize transmitted power?
 Problems in communication systems
1) Unauthorized user can uses data.
2) Interference of another user
3)Hostile transmitter can Jam the transmitter
4) Military application problems.
6

 In combating the intentional interference
(Jamming)
 In rejecting the unintentional interference from
users
 To avoid interference due to Multipath
propagation.
 In low probability of Intercept (LPI) signals
 In obtaining the message privacy.
7

 Spread Spectrum
a) Averaging type systems
eg. DS-SS
b) Avoidance type Systems
eg. FH-SS
Time Hopping
Chirp
Hybrid
8

1
■ Feedback shift register becomes “linear” if the
feedback logic consists entirely of modulo-2 adders.
■ From implementation standpoint, the most convenient
way to generate a pseudo-noise sequence is to employ
several shift-registers and a feedback through
combinational logic.
Pseudo-Noise (PN) sequence generator
Fig. Block diagram of Pseudo-Noise (PN) sequence generator

 A Pseudo Noise (PN) Sequence is defined as a coded
sequence of 1’s and 0’s with certain autocorrelation
properties.
 It consists of a shift register made up of m flip-flops and a
logic circuit to form a multiloop feedback circuit as a
Combinational Logic.
 It generally uses D type of FF & Logic circuit as a Modulo 2
Adder.
 A feedback resister having ‘m’ flip-flops can generate a
maximum-length sequence with a period of
N = 2m – 1
 PN is a Periodic binary sequence with noise like waveform.
 Becomes periodic after 2m – 1 states
 zero state is not permitted. 1-Feb-23 11

□ The Maximum Length sequence is a type of cyclic code which
represents a commonly used Periodic PN sequence.
□ A PN sequence generated by a linear feedback shift register must
eventually become periodic with period at most 2m  1, where m
is the number of shift registers.
□ A PN sequence whose period reaches its maximum value is
named the maximum-length sequence or simply m-sequence
12

13
 A shift register of length ‘m’ consists of ‘m’ flip flops and all of
them operated on the same clock. At each clock pulse, the state of
each Flip Flop is shifted to the next one.
 To prevent the shift register from getting empted, we have to apply
an input continuously to the first FF. This input is called as a
Feedback, is computed by using a logical function of the states of
all the flip flops.
 For a linear type of Feedback Shift Register, a feedback function
is obtained by using Modulo 2 addition (i.e. EX-OR gate) of the
outputs of various flip flops.
 For m= 3, the Maximum Length sequence at the generator output
will always be periodic with a period of,
N = 2m  1 , N = 7
 With increase in value of m, the sequence length increases and the
sequence becomes random in true sense.

14
1. Balance property
2. Correlation property
3. Run property
1. Balance property:
In each period of maximal-length sequence, the number
of “ 1’s “ is always one more than the number of “ 0’s ".
2. Correlation property:
The Autocorrelation function of maximal-length
sequence, is periodic & binary valued. This property
is called the Correlation Property.

15
3. Run property:
 The period of a Maximal-length sequence is given by,
 By Runs we mean subsequence of identical symbols ( 1’s & 0’s)
within one period of Sequence.
 Among the runs of 1’s and 0’s in each period of a maximum
length sequence, one half the runs of each kind are of length 1,
one fourth are of length two, one eighth are of length three and
so on.
 For a maximum length sequence generated with on shift registers
has (N+1)/2 runs
where, N =2m – 1.
 e.g. 000 1111 0 1 0 11 00 1

16
A PN sequence is an NRZ type signal with logic ‘1’
represented by +1 and binary ‘0’ is represented by -1.
4. Chip Duration (Tc):
The duration of every bit is known as the Chip
Duration. It denoted as Tc.

17
5. Chip rate ( Rc ):
The chip rate is defined as the number of bits (Chips)
per second.
We know, Tc = 1/Rc
Rc = 1/Tc
6. Period of PN ( Pseudo-noise) Sequence:
The period of PN sequence is given by,
Where Tc is bit duration.

18
Shift Register Stages M Feedback Taps
2 (2,1)
3 (3,2)
4 (4,1)
5 (5,2) (5,4,3,2) (5,4,2,1)
6 (6,1) (6,5,2,1) (6,5,3,2)
7 (7,1) (7,3) (7,3,2,1)(7,4,3,2) (7,6,4,2)
(7,6,3,1) (7,6,5,2) (7,,6,5,4,2,1)
(7,5,4,3,2,1)
8 (8,4,3,2) (8,6,5,3) (8,6,5,2) (8,5,3,1)
(8,6,5,1) (8,7,6,5,2,1) (8,6,4,3,2,1)

19
S1 S2 S3
CLK
O/P
sequence
+
PN with m= 3 (N=7)
Example1:
For the PN sequence generator shown in figure obtain the
output PN sequence. If chip rate is 107 chips/sec, calculate:
i) Chip, (ii) PN duration, (iii) Period of output Sequence.

20
Assuming initial states S3 S2 S1 = 0 0 1.
The output sequence is 0010111
which repeats with period 23 – 1 = 7
CLK Shift Register Output S3 S2 O/P
S3
S3 S2 S1
0 0 0 1 0 0
1 0 1 0 1 0
2 1 0 1 1 1
3 0 1 1 1 0
4 1 1 1 0 1
5 1 1 0 0 1
6 1 0 0 1 1
7 0 0 1 0 0
8 0 1 0 1 0
9 1 0 1 1 1
10 0 1 1 1 0
+

21
Rc = 107 chips/sec
 i) Chip (Duration):
Chip duration Tc = 1/ Rc
= 1 / 107 chips/sec
= 0.1 µsec
 ii) Duration of PN Sequence (N):
N = 2m -1
N= 7 bits
 Iii) Period of Output Sequence (Tb):
Tb = N x Tc
= 7 x 0.1 µsec
= 0.7 µsec

22
PN Sequence:
PN Sequence Examples 2:
Answer: Output Sequence

23
PN Sequence Examples 2 …..

24
PN Sequence Examples 3:
Answer: Output Sequence

Spread spectrum is a modulation method applied to
digitally modulated signals that increases the
transmit signal bandwidth to a value much larger
than is needed to transmit the underlying
information bits.
26

27
1. They are difficult to intercept for unauthorized
person.
2. They are easily hidden, it is difficult to even detect
their presence in many cases.
3. They are resistant to jamming.
4. They have an asynchronous multiple-access
capability.
5. They provide a measure of immunity to distortion due
to multipath propagation.

28
• The signal occupies a bandwidth much larger
than is needed for the information signal.
• The spread spectrum modulation is done
using a spreading code, which is
independent of the data in the signal.
• Dispreading at the receiver is done by
correlating the received signal with a
synchronized copy of the spreading code.

31
Information signal b(t) is narrowband signal.PN signal
c(t) is a wideband signal. The baseband transmission,
the product signal m(t) represented the transmitted
signal.
Received signal r(t) is a m(t) & additive interference i(t)
To recover the original signal b(t), received signal r(t) is
applied to a demodulator that applied to the multiplier
followed by a integrator.

32
There are several forms of spread Spectrums
1. Direct sequence spread spectrum (DSSS)
2. Frequency hopping spread spectrum (FHSS)
1. Direct sequence spread spectrum (DSSS):-

33
Block diagram of DSSS Transmitter system.

34
Block diagram of DSSS Transmitter first converts the
incoming binary data sequence bk into a polar NRZ
waveform b(t), which is followed by two stages of
modulation.
- First stage consists of a product modulator or
multiplier with data signal b(t) & the PN signal c(t)
i.e. PN sequence as input.
-The second stage consists of a binary PSK modulator.
The transmitted signal x(t) is thus Direct Sequence
Spread binary phase- shift keyed (DS/BPSK) signal.
-The phase modulation Ө(t) of x(t) has one of two
values, 0 & π depending on the polarities of the message
signal b(t) and PN signal c(t) at time t, as in Table.

37
Block diagram of DSSS Receiver system.

38
Block diagram of DSSS Receiver system consist of two
stages of demodulation.
1. In First stage received signal y(t) and a locally
generated carrier are applied to a product modulator
followed by a low pass filter whose bandwidth is equal
to that of the original message signal m(t).
This stage of the demodulation process reverse the
phase-shift keying applied to the transmitted signal.
2. In the second stage of demodulation performs
spectrum dispreading by multiplying the low pass filter
output by locally generated replica of the PN signal c(t),
followed by integration over a bit interval 0 ≤ t ≤ T Tb
and finally decision make as received signal is 0 or 1 bit

39
The channel output given by:
y(t) = x(t) + j(t)
= c(t) s(t)+ j(t)
The Coherent detector input u(t) : u(t) =c(t) y(t)
= s(t)+ c(t) j(t)
= 1
Where : for all t

40
1. Processing Gain
2. Probability of Error
3. Jamming margin
1. Processing Gain(PG):
The processing Gain of a DS-SS system presents the
gain achieved by processing a spread spectrum signal over
an Unspread signal.
processing Gain can also be defined as that the ratio
of the Bandwidth of the spread signal to the Bandwidth of
the Unspread signal.
Spread code signal is m(t)= b(t) c(t) ……(1)

41
-One bit period of the spread signal m(t) is given by Tc.
The bandwidth of the NRZ signal is equal to the
reciprocal of its one bit period.
Bandwidth of the Signal= 1/ Tc ……(2)
- The Unspread signal b(t) is an NRZ signal & BW of
NRZ is reciprocal of the bit period Tb. BW of
Unspread Signal= 1/ Tb ……(3)
- Processing Gain PG= (1/Tc) / (1/Tb)
- PG= Tb / Tc ……(4)

44
-The ability of spread-spectrum system to combat the effect
of Jammers is determined by the processing gain of the
system, which is a function of the PN sequence period.
-The processing gain can be made larger by employing
PN sequence with narrow chip duration, which, in turn,
Permits a greater transmission bandwidth & more chips
per bit.
-However, the capabilities of physical devices used to
generate the PN spread- spectrum signals impose a practical
limit on the attainable processing gain. Also processing gain
so attained is still not large enough to overcome the effects
of some jammers of concern.

45
- One such alternative method is to force the Jammer to
cover a wider spectrum by randomly hopping the data
modulated carrier from one frequency to the next.
- Then the spectrum of the transmitted signal is spread
sequentially rather than instantaneously.
- The type of spread spectrum in which the carrier hops
randomly from one frequency to another is called
Frequency-Hop (FH) spread spectrum.
-The common modulation format for FH systems is that of
M-ary Frequency-shift keying (MFSK). The combination
of these two techniques is referred to simply as FH/MFSK.

46
- Since frequency hopping does not cover the entire spread
spectrum instantaneously, we are considering the rate at
which the Hops occur. We have two basic technology
characterizations of frequency hopping.
A) Slow-Frequency Hopping:
In which is the symbol rate Rs of the MFSK signal is an
Integer multiple of the Hop rate Rb. That is, several
symbols are transmitted on each frequency hop.
B) Fast-Frequency Hopping:
In which the hop rate Rb is an integer multiple of the
MFSK symbol rate Rs. That is, the carrier frequency will
Change or hop several times during the transmission of
one symbol.

47
Slow Frequency hopping spread spectrum TX:

48
- Block diagram of an FH/MFSK transmitter, which involves
frequency modulation followed by Mixing. First , the
incoming signal binary data are applied to an M-ary FSK
modulator.
- The resulting modulated wave and the output from a digital
Frequency Synthesizer are then applied to a mixer that
consist of a multiplier followed by a Band-Pass Filter.
- The filter is designed to select the sum frequency
component resulting from the multiplication process as the
transmitted signal.
- In particular, successive k-bit segment of a PN sequence
drive the frequency synthesizer, which enables the carrier
frequency to hop over d distinct values.

49
- On a single hop, the bandwidth of the transmitted signal is
the same as that resulting from the use of conventional
MFSK with an alphabet of orthogonal signals.
- For a complete range of frequency hops, the transmitted
FH/MFSK signal occupies a much larger bandwidth (GHz)
which is larger than that achievable with DSSS.
- An implication of these large FH bandwidths is that
coherent detection is possible only within each hop,
because frequency synthesizers are unable to maintain
phase coherence over successive hops.
- Accordingly, most frequency hop spread-spectrum
communication systems use noncoherent M-ary
modulation schemes.

51
- In the receiver shown in above figure the frequency hopping is first
removed by mixing (Down- Converting) the signsl with the output of
a local frequency synthesizer that is synchronously controlled in the
same manner as that in the transmitter.
- At the output of the multiplier we get the input signals, their sum &
difference. Out of these difference frequency components, is selected
by the bandpass filter that follows the multiplier. Difference signal is
the MFSK signal.
- The resulting output is then band-pass filtered and subsequently
processed by a noncoherent M-ary FSK detector. To implement this
M-ary detector, we may use a bank of M noncoherent matched
filters.
- An estimate of the original symbol transmitted is obtained by
selecting the largest filter output.

52
- In an individual FH/MFSK tone of shortest duration is referred to as a
chip. The chip rate Rc for an FH/MFSK system is defined as,
Rc = max(Rb, Rs)
where Rb is the hop rate & Rs is the symbol rate.
A slow FH/MFSK signal is characterized by having multiple symbols
transmitted per hop. Hence each symbol of a slow FH/MFSK signal
is a Chip.
Rc = Rs =(Rb/K) ≥ Rb
where K = log2 (M)
Processing Gain PG= (Wc / Rs )
=
-

58
Wireless Telephone Systems
-In last few years Wireless telephone systems
haveundergone a tremendous transformation and
have becomes a necessity now.
-Now Wireless telephone systems over comes the
landline telephone system.
-Two primary Wireless systems are:
i) Cellular Telephones System
ii) Personal Communication Systems OR Services
(PCS )

59
I Cellular Telephones System
-Cellular telephone system is a wireless telephone
system.
-It is a multiuser system.
Basic Concept:
-It is wireless communication just like cordless.
-Distance is not restricted to within home but one
can travel in the city or even outside the city
without interruption in communication.
-In this city is divided in to small areas called
“CELLS” of near about 10 square KM.

62
Cell:
-The basic geographic unit of a Cellular
Communication system is called as Cell.
-Its shape is Hexagonal as shown.
-The size of cell is not fixed. Practically the
shape of the cell is not be perfect Hexagon.
A
B
C
A
C
A
C

63
Cluster:
- Agroup of a cells is called as a Cluster.
-Its size is not fixed, it is according to
requirements of perticular area.
A
B
C
A F
E
G
D
E
F
D E

64
Frequency Reuse:
-Frequency reuse refers to the use of radio channels
operating on the frequency to cover different areas,
that are physically separate from each other
-must manage reuse of frequencies
-power of base transceiver controlled allow
communications within cell on given frequency
limit escaping power to adjacent cells
allow re-use of frequencies in nearby cells typically 10 – 50
frequencies per cell
K

65
Frequency Reuse:
Frequency Reuse Pattern :

67
Hand off:
Concept :
When you move to another cell within the same
system, you get a handof
You are transferred automatically to that cell’s cellsite

69
PCS:
The term Personal Communication Service
(enabling communication with a person at
anytime, at any place, and in any form) include
Various Wireless Access
Personal Mobility Services
Two of the most popular cellular systems
High Tier Digital Cellular Systems
Lower Tier Cordless Telecommunication
Systems

73
Positive
1. Signal hiding (lower power density, noise-like) , non
interference.
2. Secure communications (Privacy).
3. Code division multiple access CDMA.
4. Mitigation of multi path effect.
5. Protection to international interference (jamming)
6. Rejection of unintentional interference (narrow
band)

74
Negative
1. No improve in performance in the presence of
Gaussian noise.
2. Increase bandwidth (frequency usage, wideband
receiver).
3. Increase complexity and computational load.

75
References:
• Simon Haykin “Communication Systems” , John Wily
& Sons 2001.
• Emmanuel C. Ifeachor “Digital Signal Processing”,
Prentice Hall 2002.
• ir. J. Meel “Spread Spectrum introduction” DE NAYER
INSTITUTE. Belgium (www.denayer.be).
• B. P. Lathi “Modern Digital and Analog Communication
Systems”, Oxford University Press 1998.

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