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PN SEQUENCE GENERATOR
USING MAT LAB
BY
S.Dileep
07891A0426
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WHAT IS PN SEQUENCE GENERATOR?
Pseudo random Noise sequence generator is a
circuit which generates pseudo random noise
sequences.
A pseudo- noise sequence is a binary sequence
with a noise like wave form that usually
generated by means of a Linear feedback shift
registers.
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BLOCK DIAGRAM OF LINEAR FEEDBACK SHIFT
REGISTER
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WHAT IS LINEAR FEEDBACK SHIFT
REGISTER?
A Feedback shift register consists of an ordinary shift
register made up of m Flip-Flops .
The flip-flops in the shift register are regulated by a single
timing clock.
At each pulse of the clock, the state of each flip-flop isshifted to the next one i.e. the contents of the register is
shifted right by one position at each clock cycle.
The feedback from the left most register are XOR- ed
together, to give a random sequence.
But this binary sequence is not totally random, because itdoes repeat after many cycles, thats why it is so called as
pseudo random noise sequence generator.
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SHIFT REGISTER
LENGTH:
This is often referred to as the degree, and in general, the
longer the shift register, the longer the duration of the pn
sequence before it repeats.
The combination of taps and their location is often referred
to as a polynomial, and expressed as:
P(x)=1+x^2+x^3
Where the leading 1 represents x^0, which is the output of
the last register, x^2 is the output of register stage 3 and
x^3 the output of XOR.
A maximal sequence for a shift register of length L is
referred as m-sequence, and is defined as:
L=2^m-1.
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PN SEQUENCE
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APPLICATIONS :
These pn sequence generators are widely usedin Cmos testing process as to generate test
patterns.
And also used to generate spreading codes in
CDMA,FDMA and PCM.
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PROPERTIES:
In each period of a maximal length sequence, the number
of 1s is always one more than the number of 0s.This
property is called as the Balance property.
Example:
101000111101001 Among the runs of 1s and of 0s in each period of a
maximal-length sequence, one half the runs of each kind
are of length one, one fourth are of a length two, one-
eighth are of length three, and so on as long as these
fractions represent meaningful numbers of runs. This
property is called the Run property.
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The autocorrelation function of a maximal-length sequence
is periodic and binary value. This property is called ascorrelation property.
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PN sequences have single autocorrelation peak, thisfeature permits the transmission to be
asynchronous.
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ADVANTAGES ANDDISADVANTAGES :
The sequences generated by
an LFSR can be nearly
random with long periods,which aids in making the
spread signal appear noise
like.
Easy of implementation.
Output of lfsr is periodic.
For any given size of LFSR, a
maximum of different unique
m-sequences can be generated
.
Cross correlation properties
are good for more than two
users. Thats why we go forgold sequences.
Advantages: Disadvantages:
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