1. Subject Name: Digital Communication Subject Code: 10EC61
Prepared By: Lakshmi C R, Pallavi
Department: ECE
Date:
Date :

2. UNIT 8 SPREAD SPECTRUM MODULATION

3. pseudo noise sequences Notion of spread spectrum
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Contents
pseudo noise sequences
Notion of spread spectrum
Direct sequence spread spectrum
Coherent binary PSK
Frequency hop spread spectrum
applications

4. Spread spectrum modulation
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Spread spectrum modulation
Spread spectrum modulation is one in which the data sequence occupies a band width in excess of the minimum bandwidth necessary to transmit it.
b(t)= data sequence to be transmitted ,c(t )= wide band code , c(t)*b(t)-wide band

5. Is obtained by special code which is independent of the data sequence.
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The spectrum spreading at the transmitter and despreading at the receiver
Is obtained by special code which is independent of the data sequence.

6. Pseudo Noise Sequence
Generation of pseudo noise sequence
Maximum length sequence generator for n=3
The total number of states of m-state feed back register will be 2^m-1
The output pseudo noise sequence will have a period of 2^m-1

7. Example Consider the linear feed back register involve 3 flip flops
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Example
Consider the linear feed back register involve 3 flip flops
The input So is equal to mod sum of S1 and S3

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Length is 2^m-1= 2^3-1=7 The output Sequence is m is number of stages of flip flops in shift register
State of Flip-Flop
Output PN sequence equal to S3 S1 S2 S3 1

9. Notion of Spread Spectrum
Spread spectrum modulation can provide protection against interfacing signals(jamming).
It purposefully makes the information bearing signals to occupy a Bandwidth far in excess of the minimum Bandwidth necessary to transmit it.
bk= binary data sequence
ck= PN sequence

10. For base band transmission m(t)= transmitted signals
n(t)= Additive interference noise
r(t)= received Signal

11. To recover the original message signal b(t) ,received signal r(t) is applied to demodulator that consists of multiplier followed by an integrator and decision device.

12. Hence the equation becomes
By applying the multiplier output to Low pass filter the Spurious component c(t).n(t) is filtered out.

13. Direct – Sequence Spread Spectrum with coherent binary Phase shift Keying

14. model of direct – sequence spread binary PSK system(alternative form)

15. Frequency Hop-Spread Spectrum
Spread Spectrum in which Carrier hops randomly from one frequency to another its called frequency hop spread spectrum.
Depending on the rate at which hops occur there are two types of frequency hops
(1) Slow frequency hopping
(2) Fast frequency hopping .

16. Slow frequency hopping: In which the symbol rate is Rs is of MFSK is integer multiple of hop rate Rh. Here several symbols are transmitted on each frequency hop.

17. An individual FH/MSK tone of shortest duration is referred as chip
An individual FH/MSK tone of shortest duration is referred as chip. Chip rate Rc for an FH /MSK system is define by
Rc=Max(Rh,Rs) Rh=hop rate
Rs= symbol rate.
Rb= Bit rate
Bit rate, hop rate, symbol rate are related by
Rc= Rs= Rb/ K>= Rh
Where

19. Fig : the above fig illustrate the variation of the frequency of slow FH/MSK signal with time for one complete period of PN sequence is
2^4-1=15
The FH/MSK signal has following parameters
Number of bits per MFSK symbol k=2
Number of MFSK tones M=2^k=4
length of PN segment per hop k=3
total number of frequency hops= 2^k=8

20. Fast Frequency hopping:In which the hop rate Rh is the integral multiple of MFSK symbol rate Rs. Here the carrier frequency will hop several times during the transmission of one symbol.

21. The above figure illustrate the variation of the transmitted frequency of a fast FH/MFSK signal with time. The signal has the following parameters
Numbers of bits per MFSK symbol k=2
Number of MFSK tones M=2^k=4
length of PN segment per hop k=3
total number of frequency per hop=2^k=8

22. Application
Spread spectrum modulation is used in some applications which involve
Anti jammimg capability for narrow band jamming
Interference rejection
Multiple access capability
Multipath protection
Low probability of intercept(LPI)
Secure communications.
Improved spectral efficiency

23. 1. Explain Pseudo noise sequence generator and properties with example
1. Explain Pseudo noise sequence generator and properties with example. (jun12)
Logic Circuit

24. A feedback shift register is said to be Linear when the feed back logic consists of entirely mod-2-address ( Ex-or gates). In such a case, the zero state is not permitted. The period of a PN sequence produced by a linear feedback shift register with ‘n’ flip flops cannot exceed 2^n-1. When the period is exactly 2^n-1, the PN sequence is called a ‘maximum length sequence’
or ‘m-sequence’.
Let initial status of shift register be 1000
We can see for shift Register of length m=4. At each clock the change in state of flip-flop is shown.
Feed back function is modulo two of X3 and X4.
After 15 clock pulses the sequence repeats.
Output sequence is 1 1

25. Properties of PN Sequence
randomness of PN sequence is tested by following properties
1. Balance property
2. Run length property
3. Autocorrelation property
2. Explain DSSS with neat diagram (jun 12)
3. Explain FHSS with a neat diagram (July 11)

26. 3. A Spread spectrum communication system has the following parameters
3. A Spread spectrum communication system has the following parameters. Information bit duration Tb = msecs and PN chip duration of 1µsecs. The average probability of error of system is not to exceed 10-5.
calculate a) Length of shift register b) Processing gain c) jamming margin
Solution:
Processing gain PG =N= Tb/Tc =1024 corresponding length of shift register m = 10
In case of coherent BPSK For Probability of error 10-5.
[Referring to error function table] Eb/N0 =10.8

28. Assignment questions
1. Explain PN sequence and its properties.
2. What is spread spectrum? Explain its advantages and principle of DSSS system.
3. Explain slow FHSS with neat block diagram and waveform.
4. Explain with neat block diagram Fast FHSS/MFSK system.
5. Define processing gain and jamming margin.