1. Lecture 8: Spread Spectrum
Principle of spread spectrum
Frequency hopping spread spectrum
Direct sequence spread spectrum
Direct sequence CDMA systems
Ben Slimane

2. What is Spread Spectrum?
Spread spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth
Spread spectrum methods:
Frequency hopping spread spectrum
Direct sequence spread spectrum
Time hopping spread spectrum

3. Spread Spectrum At the transmitter side:
Input is fed into a channel encoder
Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits
Spreading code or spreading sequence
Generated by pseudonoise, or pseudo-random number generator
Effect of modulation is to increase bandwidth of signal to be transmitted

4. Spread Spectrum At the receiving end:
digit sequence is used to demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data

5. Spread Spectrum

6. Spread Spectrum What can be gained from apparent waste of spectrum?
Immunity from various kinds of noise and multipath distortion
Can be used for hiding and encrypting signals
Multiple access capability
Several users can independently use the same wider bandwidth with very little interference

7. Frequency Hopping Spread Spectrum (FHSS)
Signal is broadcast over seemingly random series of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals
Transmitter operates in one channel at a time
Bits are transmitted using some encoding scheme
At each successive interval, a new carrier frequency is selected

8. Frequency Hoping Spread Spectrum
The frequency sequence is dictated by the spreading code
Receiver should hop synchronously with the transmitter to be able to recover the message
Advantages
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at knocking out a few bits

9. Frequency Hoping Spread Spectrum

10. Frequency Hopping Spread Spectrum

11. Frequency Hopping Spread Spectrum
Slow-frequency-hop spread spectrum
The hopping duration is larger or equal to the symbol duration of the modulated signal
Tc >= Ts
Fast-frequency-hop spread spectrum
The hopping duration is smaller than the symbol duration of the modulated signal
Tc < Ts

12. Slow Frequency-Hop SS

13. Fast Frequency-Hop SS

14. FHSS Performance Considerations
Large number of frequencies used
Results in a system that is quite resistant to jamming
Jammer must jam all frequencies
With fixed power, this reduces the jamming power in any one frequency band

15. Direct Sequence Spread Spectrum (DSSS)
The modulated signal is spread by a spreading waveform (spreading code)
The spreading code spreads the signal over a wider frequency band
Spread is in direct proportion to number of bits per symbol used
The spreading code is usually periodic with a period larger or equal to the symbol duration of the modulated signal

16. DSSS Using BPSK

19. Direct Sequence Spread Spectrum (DSSS)

20. Spectrum of DS Spread Spectrum Signal

21. Code-Division Multiple Access (CDMA)
CDMA is multiple access scheme that allows many users to share the same bandwidth
3G (WCDMA), IS-95
Basic Principles of CDMA
Each user is assigned a unique spreading code
The processing gain protects the useful signal and reduces interference between the different users
PG = (Bandwidth after spreading)/(Bandwidth before spreading)

22. CDMA for Direct Sequence Spread Spectrum

23. CDMA Example

26. Spreading Sequences
Spreading sequences are very important in the design of spread spectrum communication
Two categories of Spreading Sequences
PN sequences
Orthogonal codes
FHSS systems
PN sequences most common
DSSS CDMA systems

27. PN Sequences
PN sequences are periodic but appear random within one period
PN sequences are very easy to generate
Generated using LFSR
PN sequences are easy to re-generate and synchronize at the receiver
PN sequences have good random properties
PN sequences converge to a Gaussian process when the period tends to infinity

28. Spreading in Cellular CDMA Systems
Cellular CDMA systems use two layers of spreading
Channelization codes (orthogonal codes)
Provides orthogonality among users within the same cell
Long PN sequences (scrambling code)
Provides good randomness properties (low cross correlation)
Reduces interference from other cells