Pseudorandom Noise Sequence(PN sequence) Basics

PN sequence is widely used in CDMA systems for the following reasons.
• spread the bandwidth of baseband modulated signal to the much larger bandwidth before transmission
• to distiguish between different users by allocating unique PN sequences to them.

PN sequence stands for Pseudorandom Noise Sequence. Though the name suggests they are random sequences; but they are not random. PN sequences are deterministic and periodic in nature. The same pattern repeats after some duration. Following are important properties of a PN sequence.
•  Relative frequencies of one and zero are each equal to one half.
•  For 1s and 0s; half of all run lengths are of length 1; 1/4 are of length 2;1/8 are of length 3 and so on.
•  If new sequence is generated by shifting original sequence by nonzero elements than equal number of aggrements and also disaggrements exist between these two sequences.

PN sequences are generated by combining outputs of feedback shift registers. One such circuit is depicted in the figure.

PN sequence MATLAB code

%%PN Sequence Generator in matlab of length 1023
h = commsrc.pn('GenPoly', [[8 2 0]], 'Mask', [1 0 0 0 0 0 1 0]);
set(h, 'NumBitsOut', 1023);
pnseq = generate(h);
for k=1:length(pnseq)
if(pnseq(k) == 0)
pnseq(k) = -1;
end
end
maxpeak = xcorr(pnseq,pnseq);
plot(maxpeak);

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Pseudorandom Noise Sequence(PN sequence) Basics

PN sequence MATLAB code

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