1. Spread Spectrum
Technologies
By-
Rohit Dhongde (B-41)
Swapnil Dondal(B-49)
Vaibhav Deshmukh(B-52)
Sushant Burde(B-48)
Pratik Morey(B-39)

2. Spread Spectrum multiple access (SSMA)
2
Spread Spectrum multiple access (SSMA) uses signals which have
a transmission bandwidth that is several orders of magnitude
greater than the minimum required RF bandwidth. A PN sequence
converts a narrowband signal to a wideband noise-like signal
before transmission.
Two main types of SSMA:
1. Frequency hopped multiple access (FH)
2. Direct sequence multiple access (DS)
Code division multiple access(CDMA)
Advantage:
1. Immune to multipath interference and robust multiple access capability.
2. Efficient in a multiple user environment

3. General Model of Spread
Spectrum System

4. Frequency Hopping Spread Spectrum
l Carrier changes frequency (HOPS) according
to a pseudorandom Sequence.
4 Pseudorandom sequence is a list of frequencies. The
carrier hops through this lists of frequencies.
4 The carrier then repeats this pattern.
4 During Dwell Time the carrier remains at a certain
frequency.
4 During Hop Time the carrier hops to the next frequency.
4 The data is spread over 83 MHz in the 2.4 GHz ISM band.
4 This signal is resistant but not immune to narrow band
interference.

5. 6.5
Frequency hopping spread spectrum (FHSS)

6. 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

7. 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

8. Frequency Hopping Example

9. Direct Sequence Spread Spectrum
February 2005Copyright 2005 All Rights Reserved 9
l Spread spectrum increases the bandwidth of the signal
compared to narrow band by spreading the signal.
l There are two major types of spread spectrum techniques:
FHSS and DSSS.
4 FHSS spreads the signal by hopping from one frequency to
another across a bandwidth of 83 Mhz.
4 DSSS spreads the signal by adding redundant bits to the
signal prior to transmission which spreads the signal across 22
Mhz.
* The process of adding redundant information to the signal
is called Processing Gain .
* The redundant information bits are called Pseudorandom
Numbers (PN).

10. Direct Sequence Spread Spectrum
Using BPSK Example