This document discusses spread spectrum communication techniques. It explains that spread spectrum spreads a signal's bandwidth much wider than the minimum required to transmit the information being sent. This is done by modulating the signal with a pseudorandom code that is known to both the transmitter and receiver. The document covers direct sequence spread spectrum and frequency hopping spread spectrum, and discusses their advantages like interference rejection, multipath protection, and ability for multiple users to access the same spectrum.

1. Spread Spectrum Communication
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2.  Spread spectrum communication systems are widely
used today in a variety of applications for different
purposes such as access of same radio spectrum by
multiple users (multiple access), anti-jamming
capability (so that signal transmission can not be
interrupted or blocked by spurious transmission from
enemy), interference rejection, secure
communications, multi-path protection, etc.
However, irrespective of the application, all spread
spectrum communication systems satisfy the
following criteria-
 (i) As the name suggests, bandwidth of the
transmitted signal is much greater than that of the
message that modulates a carrier.
 (ii) The transmission bandwidth is determined by a
factor independent of the message bandwidth.
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3.  The power spectral density of the modulated signal is
very low and usually comparable to background noise
and interference at the receiver.
 SS is widely used to solve a problem of security that
means SS is a technique for secure communication.
 SS is pseudorandom in nature. This makes it appear like
random noise.
 The normal receiver can not demodulate it to recover the
original signal.
 A pseudorandom process is a process that appears to
be random but is not. Pseudorandom sequences typically
exhibit statistical randomness while being generated by
an entirely deterministic causal process.
 Such a process is easier to produce than a genuinely
random one, and has the benefit that it can be used
again and again to produce exactly the same numbers.
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4. Advantages of Spread Spectrum
(SS) Techniques
 a) Reduced interference: In SS systems, interference from
undesired sources is considerably reduced due to the processing
gain of the system.
 b) Low susceptibility to multi-path fading: Because of its
inherent frequency diversity properties, a spread spectrum
system offers resistance to degradation in signal quality due to
multi-path fading. This is particularly beneficial for designing
mobile communication systems.
 c) Co-existence of multiple systems: With proper design of
pseudo-random sequences, multiple spread spectrum systems
can co-exist.
 d) Immunity to jamming: An important feature of spread
spectrum is its ability to withstand strong interference,
sometimes generated by an enemy to block the communication
link. This is one reason for extensive use of the concepts of
spectrum spreading in military communications. 5/5/20204 nidhi jain

5. Types of SS
 Based on the kind of spreading modulation,
spread spectrum systems are broadly classified
as-
 (a) Averaging Type
(i) Direct sequence spread spectrum (DS-SS)
 (b) Avoidance type system
(i) Frequency hopping spread spectrum (FH-SS)
systems
(ii) Time hopping spread spectrum (TH-SS)
systems.
(iii) Hybrid systems
(iv) Chirp
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6. Spread Spectrum Concept
 Input fed into channel encoder
 Produces narrow bandwidth analog signal around central
frequency
 Signal modulated using sequence of digits
 Spreading code/sequence
 Typically generated by pseudo noise/pseudorandom number
generator
 Increases bandwidth significantly
 Spreads spectrum
 Receiver uses same sequence to demodulate signal
 Demodulated signal fed into channel decoder
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7. General Model of Spread Spectrum
System
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8. Pseudorandom Sequences
generator
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10. Pseudorandom Sequences
 The spread of the “random” sequence of
frequencies is determined by a pseudo noise
(PN) sequence generator.
 A PN generator outputs a stream of bits (1s and
0s) that appears random (has no apparent
pattern).
 PN sequence generators are easy to construct
using simple logic components:
 XOR gates, and
 a shift register, made up of flip flops
 A PN sequence is not truly random (hence,
“pseudo”), but is periodic and repeats at a fixed
interval.
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11. Pseudorandom Numbers
 Generated by a deterministic algorithm
not actually random
but if algorithm good, results pass reasonable tests
of randomness
 Starting from an initial seed
 Need to know algorithm and seed to predict
sequence
 Hence only receiver can decode signal
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12.  The PN sequence length is the number of iterations
(the number of 1s and 0s) before the sequence
repeats.
 The sequence length is determined by the:
number of flip flops, n, in the shift register
selection of feedback taps that are applied to one or
more XOR gates.
 The sequence length can have a maximum value of:
 Maximum PN Sequence length=(2^n)-1 ; n is no. of
FF.
 A PN sequence that has this length is said to be a
maximal length sequence.
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13.  Assuming the circuit below is a maximal length
PN sequence generator, how many outputs (1s
and 0s) would it produce before the sequence
repeats?
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14. Direct Sequence Spread Spectrum
(DSSS)
 Each bit represented by multiple bits using spreading
code
 Spreading code spreads signal across wider
frequency band
 In proportion to number of bits used
 10 bit spreading code spreads signal across 10 times
bandwidth of 1 bit code
 One method:
 Combine input with spreading code using XOR
 Input bit 1 inverts spreading code bit
 Input zero bit doesn’t alter spreading code bit
 Data rate equal to original spreading code
 Performance similar to FHSS
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16. Direct Sequence Spread Spectrum
Transmitter
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17. Direct Sequence Spread Spectrum
Receiver
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18. Frequency Hopping Spread
Spectrum (FHSS)
 Signal broadcast over seemingly random series
of frequencies
 Receiver hops between frequencies in sync with
transmitter
 Eavesdroppers hear unintelligible blips
 Jamming on one frequency affects only a few bits
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19. Basic Operation
 Typically 2k carriers frequencies forming 2k
channels
 Channel spacing corresponds with bandwidth of
input
 Each channel used for fixed interval
 300 ms in IEEE 802.11
 Some number of bits transmitted using some
encoding scheme
 May be fractions of bit (see later)
 Sequence dictated by spreading code
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20. Frequency Hopping Example
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21. Frequency Hopping Spread
Spectrum System (Transmitter)
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22. Frequency Hopping Spread
Spectrum System (Receiver)
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23. 3D view of FHSS
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24. Slow and Fast FHSS
 Frequency shifted every Tc seconds
 Duration of signal element is Ts seconds
 Slow FHSS has Tc  Ts
 Fast FHSS has Tc < Ts
 Generally fast FHSS gives improved performance
in noise (or jamming)
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25. Slow Frequency Hop Spread Spectrum
Using MFSK (M=4, k=2)
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26. Fast Frequency Hop Spread Spectrum
Using MFSK (M=4, k=2)
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27. CDMA( Code Division Multiple
Access)
A method for multiple access {single channel ; multiple users}
• The conventional method was FDMA. Then , after developing
synchronization methods, TDMA was introduced
In early 1980s CDMA was introduced
• First for military usage.
• In early 1990s used in Cellular Communication
Systems
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28. General concept of CDMA
Transmitted signal occupies a bandwidth much larger than the original
signal !
• The bandwidth is spread by means of a PN code independent of the
data
• The receiver synchronizes to the code to recover the data
• Many signals share the same times and frequencies but independent
codes
Spreading a Bit by means of many Chips
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29. CDMA in a DSSS Environment
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30. Comparison
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OFDM Basic Concept
 Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier
modulation scheme
 First break the data into small portions
 Then use a number of parallel orthogonal sub-carriers to transmit the data
 Conventional transmission uses a single carrier, which is modulated
with all the data to be sent
Single Carrier Company
Multi Carrier Company

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OFDM Basic Concept
 OFDM is a special case of
Frequency Division Multiplexing
(FDM)
 For FDM
 No special relationship between the
carrier frequencies
 Guard bands have to be inserted to
avoid Adjacent Channel Interference
(ACI)
 For OFDM
 Strict relation between carriers: fk = k·Df
where Df = 1/TU
(TU - symbol period)
 Carriers are orthogonal to each other
and can be packed tight

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OFDM – Signal properties
Time domain
Frequency domain
Power Spectrum for OFDM symbol
frequency

34. Multiuser Detection
 Multiuser detection considers all users as signals
for each other -> joint detection
 Reduced interference leads to capacity increase
 Alleviates the near/far problem
 MUD can be implemented in the BS or mobile, or
both
 In a cellular system, base station (BS) has
knowledge of all the chip sequences
 Size and weight requirement for BS is not
stringent
 Therefore MUD is currently being envisioned for
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35. MUD Algorithms
Optimal
MLSE
Decorrelator MMSE
Linear
Multistage Decision
-feedback
Successive
interference
cancellation
Non-linear
Suboptimal
Multiuser
Receivers
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