2. INTRODUCTION
WHY SPREAD SPECTRUM MODULATION?
HOW IT WORKS
SPREAD SPECTRUM TECHNIQUES
APPLICATIONS
CONCLUSION
3. Wireless Networks Spring 2005
Spread-spectrum radio communications is a favorite technology of the military
because it resists jamming and is hard for an enemy to intercept, Just as they
are unlikely to be intercepted by a military opponent
4. Spread Spectrum : This is a technique in which
a telecommunication signal is transmitted on a bandwidth considerably
larger than the frequency content of the original information.
Frequency hopping is a basic modulation technique used in spread
spectrum signal transmission.
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.
5. Problem of radio transmission: frequency dependent fading can wipe
out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal
using a special code
detection
at
receiver
interference
spread
signal
signal
spread
interference
f f
power
power
7. Spread spectrum signals are distributed over a wide range
of frequencies and then collected back at the receiver
These wideband signals are noise-like and hence difficult to
detect or interfere with
Initially adopted in military applications, for its resistance
to jamming and difficulty of interception
More recently, adopted in commercial wireless
communications
Wireless Environment and Wireless LANs
7
8. Safeguards for physical
security must be even greater
in wireless communications
Encryption: intercepted
communications must not be
easily interpreted
Authentication: is the node
who it claims to be?
Wireless Environment and Wireless LANs
8
9. A noise-like and random signal has to be generated at the
transmitter.
The same signal must be generated at the receiver in
synchronization.
We limit the complexity by specifying only one bit per
sample, i.e., a binary sequence.
These special "Spreading" codes are called "Pseudo
Random" or "Pseudo Noise" codes. They are called
"Pseudo" because they are not real Gaussian noise
9
10. PN generator produces periodic sequence that appears
to be random
PN Sequences
Generated by an algorithm using initial seed
Sequence isn’t statistically random but will pass many test of
randomness
Sequences referred to as pseudorandom numbers or
pseudonoise sequences
Unless algorithm and seed are known, the sequence is
impractical to predict
Wireless Networks Spring 2005
11. THE MAJOR SPREAD SPECTRUM TECHNIQUES
ARE
DSSS
FHSS
12. Wireless Environment and Wireless LANs
12
1101010010011
11010111010100100001101010010011111010100100111
17. 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
complex hardware
error correction
needed
no need for coherent
bandwidth
less affected by the near far
effect
18. 1. Reduced crosstalk interference:
2. Better voice quality/data integrity and less static
noise
3. Lowered susceptibility to multipath fading
4. Inherent security:
5. Co-existence:
6. Longer operating distances
7. Hard to detect:
8. Hard to intercept or demodulate:
9. Harder to jam
19. • Radio-jamming is ever-present threat to radio channels
• This is an attack on the availability of signals
– Denial-of-Service (DoS) attack
• Traditional anti-jamming techniques rely on pre-shared
secret codes (keys) to increase channel availability
19
RCVR
XMTR JMR
20. • Spread-Spectrum Techniques
– FHSS (Frequency Hopping
Spread Spectrum)
– DSSS (Direct-Sequence
Spread Spectrum)
20
frequency
Hopping sequence (PRNG seed) must be known
to the sender and receiver but not the jammer.
Spreading code (PRNG seed) must be known
to the sender and receiver but not the jammer.
frequency
energyenergy
PRNGPRNG
PRNGPRNG
21. Universal framework to
integrate a diverse set of
devices in a seamless, user-
friendly, efficient manner
Devices must be able to
establish ad hoc
connections
Support for data and voice
Similar security as cables
Simple, small, power-
efficient implementation
Inexpensive!
Wireless Environment and Wireless LANs
21
Bluetooth
printer
module
Bluetooth
phone and
headset
22. Multiple Transmitters 1 Receiver
Choice of unique sequences to spread the
data bits
Ability of receiver to distinguish between
data streams
23. This method of communication
i.e in CDMA cannot offer much of the international
roaming and many other advanced user services
Increased complexity
Large Bandwidth
