Spread Spectrum multiple access (SSMA) uses signals that have a much greater transmission bandwidth than the minimum required. It uses a pseudorandom number (PN) sequence to convert a narrowband signal into a wideband noise-like signal before transmission. The two main types of SSMA are frequency hopped multiple access (FH) and direct sequence multiple access (DS). SSMA provides advantages like being immune to multipath interference and having robust multiple access capability in a multiple user environment.

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