This document discusses various spread spectrum communication techniques including frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), code division multiple access (CDMA), multiuser detection, and orthogonal frequency division multiplexing (OFDM). It provides examples and explanations of how each technique works, along with their advantages such as interference suppression, low power operation, multiple access capability, and information security provided by codes. Hedy Lamarr is credited with inventing spread spectrum technology during World War II to prevent jamming of radio controlled torpedoes.

1. Spread Spectrum Communications
Unit IV :
Spread Spectrum Communications: Frequency Hopping Spread Spectrum (FHSS) systems, Direct
Sequence Spread Spectrum, Code Division Multiple Access of DSSS, Multiuser Detection, OFDM
Communications
4/3/2018 1
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)

2. Spread Spectrum Communications
4/3/2018 2
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• Spread Spectrum was actually invented by 1940’s Hollywood actress
Hedy Lamarr (1913-2000), who was an Austrian refugee
• In 1940 she along with music composer George Antheil devised a
system to stop enemy detection and jamming of radio controlled
torpedoes by randomly hopping around a set of frequencies.
• Got patent in 1942 but never profited, considering it as war effort
contribution.
• It is used in military communication systems since 1950s.

3. Beneficial attributes of Spread Spectrum Systems
4/3/2018 3
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• Interference suppression capability: reduces effect of interference, jamming, multipath
interference.
• Energy density reduction (Low power): to anyone who does not know the spreading
signal, spread spectrum signal appear as buried in noise, But the intended receiver can
detect the signal
• Fine time resolution: spread spectrum signal can be used for ranging or location
determination. Due to spreading (high frequency), more precise range measurement can
be done.
• Multiple access with information security: To share communication resource among
numerous users, code division multiple access (CDMA) is used. Spreading signal (code)
also provides encryption of information.

4. Spread Spectrum Communication
4/3/2018 4
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
Digital communication focus on efficient utilization
of bandwidth and power with accepted reliability.
Also data security is a concern. Spread spectrum
systems targets all.
Spread spectrum communication
• Spread spectrum is a means of transmission in
which the data of interest occupies a bandwidth
in excess of the minimum necessary bandwidth
• Spectrum spreading is accomplished before
transmission through the code that is
independent of the data sequence. Same code is
used in receiver to de-spread the received signal
in order to recover the original sequence.
Spread
Spectrum
Techniques
Direct
Sequencing
Spectrum
(DS)
Frequency
Hopping
(FH)
Time
Hopping
(TH)
Hybrid
DS/FH FH/THDS/FH DSS/FH/TH
Spread Spectrum Classification

5. Spread Spectrum Communication Model: DSSS
4/3/2018 5
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
Interference rejection capability of spread spectrum system can be explained as
• Multiplication by Spreading signal once spreads bandwidth
• Multiplication by Spreading signal twice followed by filter recovers original signal
• Desired signal is multiplied twice while jamming signal is multiplied only once
Data Signal x(t)
Data rate R
Spreading code
(Pseudo Noise)
sequence
Chip rate RCh
Recovered
Data signal
Spreading code
(Pseudo Noise)
sequence
Chip rate RCh
filter
Bandwidth R
Spread signal of
bandwidth RCh
Jamming/ interference
Basic Spread Spectrum Technique

6. Pseudo Noise Sequence (PN Sequence)
4/3/2018 6
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• PN sequence is a coded binary sequence with certain autocorrelation properties.
• PN sequence is periodic and can be generated using linear feedback shift register.
• A maximum length PN sequence generated using m bit linear feedback shift register is
periodic with 2m-1 period
Shift Register output
1 0 0
1 1 1 0
2 1 1 1
3 0 1 1
4 1 0 1
5 0 1 0
6 0 0 1
7 1 0 0
FFFFFF
x1 x2 x3
Maximum length sequence
generator

7. Direct Sequence Spread Spectrum Example
4/3/2018 7
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
Data input
1 0 1 1 0 1 0
T
TC
TC
T
Spreading PN
Sequence
Transmitted
Signal
Transmitted
Signal
Spreading PN
Sequence
Data output
Transmitter
Receiver

8. Spread Spectrum Communication Model: DSSS
4/3/2018 8
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
f
f
f
1
T
1
T

1
CT
1
CT

1 1
CT T

1 1
CT T
 
  
 
Spectrum of data signal
Spectrum of pseudo noise signal
Spectrum of DSSS signal

9. Frequency Hopping Spread Spectrum (FHSS) systems
4/3/2018 9
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• Data signal modulates a carrier whose frequency is pseudo randomly determined.
• A frequency synthesiser generates a carrier frequency based on dictates of the PN code.
• At each frequency hop time, PN generator feeds the frequency synthesiser a frequency
word (a sequence of l chips which dictates one of the 2l symbol set).
• Frequency hopping bandwidth WSS and frequency step size ∆f determine minimum
number of chips in frequency word.
Data Signal x(t)
Data rate R
PN sequence generator
Chip rate RCh
Recovered
Data signal
filter
Bandwidth R
Spread signal of
bandwidth RCh
Jamming/ interference
Basic FHSS Technique
FH
modulator
FH
modulator
Frequency synthesizer
PN sequence generator
Chip rate RCh
Frequency synthesizer

10. Frequency Hopping Spread Spectrum (FHSS) systems
4/3/2018 10
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• One hop consists of set of frequencies corresponding to chips of frequency word.
• In a hop, occupied bandwidth may be less than WSS but over several hops, it occupies
entire WSS.
Ex. Frequency hopping bandwidth WSS of 400 MHz and frequency step size ∆f of 100 Hz are
specified. What are minimum number of chips required for frequency word.
6SS
SS
6
2
W 400 MHz
Number of frequencies in W = 4 x 10
100 Hz
Minimum number of chips = log (4 10 ) 22
gives smallest integer not less than
f
x
x x
 

   
  

11. FHSS cycles
4/3/2018 11
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
f1
f8
f7
f6
f5
f4
f3
f2
Carrier
frequencies
Hop periods
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Hop1 Hop2

12. FHSS Example: Bluetooth
4/3/2018 12
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• Bluetooth is a wireless technology standard for exchanging data over short distances.
• Invented by Ericsson in 1994 and named after 10-century Norway king Harald Bluetooth
• operates in 2402-2480 MHz, with guard bands of 2 MHz at bottom and 3.5 MHz at top
end. It is part of globally unlicensed industrial, scientific and medical (ISM) 2.4 GHz band.
• Bluetooth uses a radio technology called FHSS and has low power consumption.
• Bluetooth divides data into packets, and transmits each packet on one of 79 designated
Bluetooth channels, each of 1 MHz bandwidth.
• It usually performs 800 hops per second.
Class Maximum Power (mW) Typical range (m)
1 100 100
2 2.5 10
3 1 1
4 0.5 0.5

13. Code Division Multiple Access of DSSS
4/3/2018 13
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• CDMA allows all users to transmit at the same time using all available
bandwidth separable on the basis of code, using which either
frequency hop or direct sequence spread spectrum modulation is
performed
• Design of codes ensures that each code is approximately orthogonal
• CDMA system operates asynchronously.
Advantages:
• Does not require synchronization like in TDMA
• Gradual degradation in performance as the number of users increase
• Offers external interference rejection capability
Time
Freq
FDMA
Time
Freq
TDMA
Freq
Time
PN Code
CDMA

14. Multiuser Detection (MUD)
4/3/2018 14
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
• MUD development started in 1980s. MUD deals with
detection of the mutually interfering digital signals like in
wireless, satellite communication, digital TV etc.
• Multiuser detection aims for both joint detection or single
signal detection against multiuser interference.
• Mutual interference originates from channel distortion and
from out-of-cell interference
• MUD exploits the interfering signals structure, to increase
spectral efficiency, receiver sensitivity, and system capacity.
• MUD can overcome near-far problem suffered by CDMA by
using an optimum receiver that demodulates all users
simultaneously. It consists of a matched filters bank
followed by a Viterbi algorithm.
MF 1
MF 2
MF 3
Viterbi
Algorithm
searches
for ML bit
sequence
S1(t)+S1(t)+S1(t)
b1+I1
b2+I2
b3+I3

15. Orthogonal frequency-division multiplexing (OFDM)
4/3/2018 15
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier
modulation method.
• A large number of closely-spaced orthogonal sub-carriers are used to carry data.
• data is divided into several parallel data streams (channels), one for each sub-carrier.
• Each sub-carrier is modulated with a modulation scheme (ex. QAM/PSK) at a low symbol
rate, maintaining data rates similar to single-carrier modulations over same bandwidth.
• OFDM finds applications in digital broadcasting, wireless networking and WiMAX.
Advantages
• Can easily adapt to severe channel conditions so robust against co-channel interference,
inter-symbol interference (ISI) and fading caused by multipath propagation.
• High spectral efficiency as compared to modulation schemes, spread spectrum, etc.
• Efficient implementation using Fast Fourier Transform (FFT).
• Low sensitivity to time synchronization errors.

16. OFDM Communications
4/3/2018 16
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
Serial to
parallel
converter
IFFT
DAC
DAC
-900
X0(t)
X1(t)
X2(t)
XN-1(t)
Re
Im
fc
S(t)
S(n)
Serial to
parallel
converter
IFFT
DAC
DAC
-900
fc
S(t)
S(n)
X0(t)
X1(t)
X2(t)
XN-1(t)

17. FDM
4/3/2018 17
NEC 602 by Dr Naim R Kidwai, Professor, F/o Engineering,
JETGI, (JIT Jahangirabad)
OFDM vs FDM
OFDM
Bandwidth
saving
Guard
band