1. Spread-Spectrum Techniques
CHAPTER 6
Spread-Spectrum Techniques
School of Computer and Communication Engineering, Amir Razif B. Jamil Abdullah
EKT 431: Digital Communications

2. Spread-Spectrum Techniques
Chapter 12 text.
Spread-Spectrum Overview
Pseudonoise Sequence
Direct-Sequence Spread-Spectrum System
Frequency Hopping Systems
Synchronization
Jamming Considerations
Commercial Application
Cellular System

3. Spread-Spectrum Overview
Initial application of spread-spectrum (SS) was in military guidance and communication system; jamming resistance ~ radar.
The transmission bandwidth employed in the SS is mush greater than the minimum bandwidth required to transmit the information.
The system is defined as spread spectrum if it fulfill the following requirement;
(1) The signal occupies a bandwidth much in excess of the minimum bandwidth necessary to send the information.
(2) Spreading is accomplished by means of spreading signal, often called code signal, which is independent of the data.
(3) At the receiver, despreading (recovering the original data) is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading signal used to spread the information

4. Benefit Attributes of Spread-Spectrum
Interference Suppression Benefits;
Presence of intentional jammer
Signal BW is spread, the jammer can make one of the two choices
(1) Jam all the signal coordinates of the system with an equal amount of power in each one, with the result that little power is available for each coordinate.
- Results in a reduction in jammer noise spectral density by a factor (W/Wss) across the spread spectrum.
Noise spectral density - Broadband jammer noise spectral density.
(2) Jam a few signal coordinates, with increased power in each of the jammed coordinates.
- Results in a reduction in the number of signal coordinates that the jammer occupies.

5. Spread spectrum advantages
Prevent jamming (destroying the signal by another party)
Covert communications (prevents eavesdropping or unauthorized listening), as signal is below noise floor!
 Military
Less multi-path fading (fading is frequency dependent)
Multiple access in wireless applications
Cordless, mobile phones sharing a small geographical area
Code Division Multiple Access (CDMA) : modern mobile telephony

6. Spread spectrum Issues
Receive much wider range of frequencies now  Low-noise design imperative
How to generate the same random string at two different locations?
Agree on a protocol for exchange of the “key” (or seed)
Ex: MATLAB: RAND('state',sum(100*clock)) resets RAND to a different state each time
Ex: Bluetooth
Algorithms: Barker, M-Sequence, Gold, Hadamard-Walsh
Complex sequence  more robust SS link
Higher “wasted” BW offset by the fact that more than one user can transmit in the same BW  CDMA

7. Cont’d…
Figure 6.1; effect of spectrum spreading in the presence of white noise with spreading in the presence of an international jammer.
Figure 6.1: Effect of spectrum spreading.
(a) Spectrum spreading in the presence of white noise.
(b) Spectrum spreading in the presence of international jammer.

8. Energy Density Reduction
Cont’d…
The larger the dimensionality of the signal set or the more signal coordinates the communicator can choose from the greater is the jammer’s uncertainty regarding the effectiveness of the jamming technique; better protection against jamming.
Energy Density Reduction
In spread spectrum system, the signal are spread over many signaling coordinate, the resulting signal power in average spread thinly and uniformly in the spread domain.
Without synchronous replica of the spreading signal, the spread spectrum seems to be buried in the noise (difficult to perceived).
SS systems exhibit LPI may also exhibit low probability of position fix (LPPF); the signal is perceived but the direction of transmitter cannot be identified.

9. Fine Time Resolution Multiple Access Cont’d…
SS signal can be used to determine the position location, through time delay of pulses it traverse the channel.
Uncertainty in delay measurement is inversely proportional to the bandwidth of the pulse.
The larger the bandwidth, the more precisely can measure the range.
Multiple Access
SS can be used as a multiple excess techniques, share communication resource among numerous users in coordinate manner.
Code-division multiple access (CDMA)~ provide communication privacy between users with different spreading signal.

10. Catalog of Spreading Techniques
For a signal of bandwidth W and duration T, the dimensionality of signal space is approximately 2WT.
To increase the dimensionality;
(1) increase W by spectrum spreading or,
~ the signal is spread in frequency domain.
(2) increase T by time spreading or time hopping (TH)
~ a message with data rate R is allocated a longer transmission- time duration compare to the conventional.
~ signal is spread in time domain.
Direct sequencing (DS) and frequency hopping (FH) are the most commonly used techniques for SS

11. Model for Direct Sequence SS Interference Rejection
Figure 12.5; model for Direct Sequence spread-spectrum (DS/SS) interference rejection.
At the modulator signal x(t); data rate of R bits/s, is multiplied by spreading code signal g(t) having a chip rate Rch chip/s.
Multiplication in time domain is equivalent to convolution in frequency domain.
At the demodulator the received signal is multiplied by a synchronized replica of spreading code signal g(t) results in despreading of signal.

12. Model for Direct Sequence SS Interference Rejection
Filter with bandwidth R will attenuate higher frequency components.
Interference rejection of a spread–spectrum system:
(1) multiplication by spreading signal once spreads the signal bandwidth.
(2) multiplication by the spreading signal twice, follow by filtering, recover the original signal.

13. Frequency Hopping System
Modulation used is M-ary frequency shift keying (MFSK).
K=log2M information bits are used to determine which one of M frequencies is to be transmitted.
At the signal is first FH demodulated (dehopped) by mixing with the same sequence of pseudo-randomly selected frequency tones that was used for hoping.
The dehopped signal is applied to a conventional bank of M noncoherent energy detector to select the most likely symbol.

14. Frequency Hopping
Frequency Hopping as a spread spectrum technique used by M-ary Frequency Shift Keying (MFSK).
the position of the M-ary signal set is shifted pseudo-randomly by the frequency synthesizer over a hopping bandwidth.
Information bits used to determine which one of M frequencies is to be transmitted

15. Example: Frequency Hopping
A hopping bandwidth Wss of 4 MHz and a frequency step size Δf of 100 Hz are specified. What is the minimum number of PN chips that are required for each frequency word?
Solution:

16. Frequency Hopping System
Frequency hopping using 8-ary FSK modulation

17. FHSS: change carrier frequency at “random”
Time

18. Frequency Hopping Spread Spectrum
Transmitter

19. FHSS
Receiver

20. Frequency Hopping Robustness
Characterize a signal’s ability to withstand impairments from the channel.
A signal configured with multiple replicate copies, each transmitted on a different frequency – greater likelihood of survival than does a single such signal with equal total power.
Greater diversity – the more robust the signal against random interference

21. Fast Frequency Hopping versus Slow Frequency Hopping
Fast Frequency Hopping (FFH)
Fast-frequency hopping.
~ Several frequency hops per modulation symbol.
~ The shortest uninterrupted waveform in the system is that of the hop..
Slow Frequency Hopping (SFH)
Slow-frequency hopping.
~ Several modulation symbols per hop.
~ The shortest uninterrupted waveform in the system is that of data symbol.

22. Frequency Hopping In general SFH
~ in which the symbol rate, Rs of the MFSK signal is an integer multiple of the hop rate, Rh. Several symbols are transmitted on each frequency hop.
FFH
~ in which the hope rate, Rh is an integer multiple of the MFSK symbol rate, Rs. The carrier frequency will change or hop several times during the transmission of one symbol.

23. Frequency Hopping DS versus FH Direct-Sequence Frequency Hopping
For mobile applications – DS represents a reliable mitigation method, signaling renders all multipath signal copies that delayed by more than one chip time.
DS radios encounter more randomly distributed errors that are continuous & lower level.
Frequency Hopping
For mobile applications – FH provide the same mitigation if the hopping rate faster than the symbol rate, hopping BW is large.
SFH radios typically suffer occasional strong burst errors.
Used for providing diversity in fixed wireless access applications