its about Spread Spectrum and its all types.

2. Presented To :
Sir. Faheem

3.  Nimrah Shahbaz
 Farwa Akhtar
 ( 003 ) BS -IT
 ( 32 ) BS -IT

4. Spread
Spectrum
1) FHSS
Its
TYPES
2) DSSS

5.  Important form of encoding for wireless communication.
 Transmit either analog or digital data
 Analog signal (transmission)
 Developed initially for military and intelligence requirements
 Spread data over wider bandwidth
 Makes jamming and interception harder
 Types of spread spectrum
1) Frequency hoping :
Signal broadcast over seemingly random series of frequencies
2) Direct Sequence :
Each bit is represented by multiple bits in transmitted signal
 Chipping code

6. Spread spectrum is characterized by :
 Wide band width
 Low power
 jamming and interference have less effect on spread
spectrum becuase it is :
1) Resemble noise
2) Hard to detect
3) Hard to intercept

7. PN generator
produces periodic
sequence that
appears to be
random

8.  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 pseudonoise / Pseudorandom number
generator
 Increases bandwidth significantly
 Spreads the spectrum
 Receiver uses same sequence to demodulate signal
 Demodulated signal fed into channel decoder

9.  Immunity from various noise and multipath
distortion
 Including jamming
 Can hide/encrypt signals
 Only receiver who knows spreading code can retrieve
signal
 Several users can share same higher bandwidth
with little interference
o Cellular telephones
 Code division multiplexing (CDM) or
 Code division multiple access (CDMA)

10.  Generated by algorithm using initial seed
(initial value)
 Deterministic algorithm
 Not actually random
 If algorithm good, results pass reasonable tests of
randomness
Pseudorandom numbers
 Need to know algorithm and seed to predict
sequence
 Only receiver sharing info with transmitter able to
decode the signal successfully.

11.  FHSS stands for Frequency Hopping
Spread Spectrum
 Signal broadcast over seemingly random
series of frequencies at fixed interval
 Receiver hops between frequencies in
syncronious with transmitter to pick
message
 Eavesdroppers hear unintelligible blips
 Jamming on one frequency affects only a
few bits

12.  The data signal is modulated with a narrow band
carrier signal that “ hops ” in a random but
predictable sequence from frequency to frequency
as a function of time over a wide band of
frequencies.
It is used as a multiple access method in the
frequency- hopping code division multiple
access (FH-CDMA) scheme.

13.  Typically 2K carriers frequencies forming 2 k
channels
 Channel frequency spacing corresponds with
bandwidth of input signal
 Each channel used for fixed interval
 Eg:300 ms in IEEE 802.11
 Some number of bits transmitted using some
encoding scheme
o May be fractions of bit
 Sequence dictated by spreading code
 Both transmitter & receiver use same code

15.  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)

17.  Typically large number of frequencies used
 Improved resistance to jamming

20.  DSSS stands for Direct Sequence Spread
Spectrum
 Each bit represented by multiple bits using
spreading code
 Spreading code spreads signal across wider
frequency band
 In proportion to number of bits used
 1,0 bit spreading code spreads signal across
10 times bandwidth of 1 bit code
 One method:
 Combine input with spreading code using
XOR
 Performance similar to FHSS