3. I. History of Spread Spectrum
II. Spread Spectrum System Model
III. Spread Spectrum Classification
IV. Spread Spectrum Techniques
Outline
4. History of Spread Spectrum
Spread Spectrum was actually invented
by 1940s Hollywood actress Hedy
Lamarr(1913-2000).
An Austrian refugee, in 1940 at the age of
26, she devised together with music
composer George Antheil a system to stop
enemy detection and jamming of radio
controlled torpedoes by hopping around a
set of frequencies in a random fashion.
She was granted a patent in 1942 (US pat.
2292387) but considered it her
contribution to the war effort and never
profited.
Techniques known since 1940s and used in
military communication systems since
1950s.
5. “Spread” radio signal over a wide frequency range
Several magnitudes higher than minimum requirement
Gained popularity by the needs of military communication
Proved resistant against hostile jammers
Ratio of information bandwidth and spreading bandwidth is
identified as spreading gain or processing gain
Processing gain does not combat white Noise
Introduction to Spread Spectrum
6. Offers the following applications:
able to deal with multi-path
multiple access due to different spreading sequences
spreading sequence design is very important for
performance
low probability of interception
privacy
anti-jam capabilities
7. Spread Spectrum Applications
Interference
̶ Prevents interference at specific frequencies
̶ E.g. other radio users, electrical systems
Military
̶ Prevents signal jamming
̶ Scrambling of ‘secret’ messages
Wireless LAN security
̶ Prevents ‘eavesdropping’ of wireless links
̶ Prevents ‘hacking’ into wireless LANs
CDMA (Code Division Multiple Access)
̶ Multiple separate channels in same medium using different
spreading codes
9. Spread Spectrum Criteria
A communication system is considered a spread spectrum system if it
satisfies the following two criteria:
Bandwidth of the spread spectrum signal has to be greater than the
information bandwidth. (This is also true for frequency and pulse
code modulation!)
The spreading sequence has to be independent from the
information. Thus, no possibility to calculate the information if the
sequence is known and vice versa.
11. Direct Sequence Spread Spectrum
Information signal is directly modulated (multiplicated) by
a spreading sequences (see next slide)
Spreading sequence consists of chips each with a
duration of tchip
A set of chips represent a bit; the exact number of chips
per bit equals the spreading gain
Near far effect
Require continuous bandwidth
16. The information signal is transmitted on different frequencies
Time is divided in slots
Each slot the frequency is changed
The change of the frequency is referred to as slow if more than
one bit is transmitted on one frequency, and as fast if one bit is
transmitted over multiple frequencies
The frequencies are chosen based on the spreading sequences
Frequency Hopping Spread Spectrum
19. Time Hopping Spread Spectrum
Time divided into frames; each TF long
Each frame is divided in slots
Each wireless terminal send in exactly one of these slots per
frame regarding the spreading sequence
No near far effect
20. Comparison of different Spread Spectrum
Techniques
SS Technique advantage disadvantage
Direct Sequence �best behavior in multi
path rejection
�no synchronization
�simple implementation
�difficult to detect
�near far effect
�coherent bandwidth
Frequency
Hopper
�no need for coherent
bandwidth
�less affected by the near
far effect
�complex hardware
�error correction
needed
Time Hopper �high bandwidth
efficiency
�less complex hardware
�less affected by the
near far effect
�error correction
needed
