WHAT IS PN SEQUENCE :
"It's a randomly created sequence to secure the information bits"
The PN sequence consists of number of bits depending on the application where it is used . The name i e: "pseudo-noise " sequence is due to the fact that it is created randomly during the transmission of information bits through the channel.
HOW RECEIVER RECEIVES THE SIGNALS:
If you transmit some signals from your device say mobile, then how an receiver it may be any electronic device receives the signal ???? .
Here is the answer , We all know that when we are transmitting the signal. In order to secure that signal, at transmitter we use a signal called as "PN sequence " which will be "mixed with the message signal " and transmitted through the channel. To generate the PN sequence we use a set of shift registers connected in series with each other and also an feedback circuitry in it .
NOW , We have transmitted the signal in order to receive the signal properly "we should know the PN sequence which is used at the transmitter". The received signal is multiplied with the locally generated PN sequence which is exactly same as the PN sequence used at the transmitter part of the system.
Here the multiplication basically is the EX-OR of the two sequences , so that if both the sequence are same then result is "1" otherwise it will be "0". Thus we will be getting the only "message signal" at the receiver end ,And then it is further processed by using various filters mostly the "Band pass filter" and amplified with the help of "Amplifiers" to get good strength signal.
EXAMPLE:
The below figure shows how a transmitted signal is received back at the receiver side with the help of PN sequence which is used at both transmitting as well as receiving ends of the of the system . In the particular example if we observe the message bits or "data bits " are : "10111" and the PN sequence bits are : "1010 ". Here the PN sequence is "post padded" to the data bits then it becomes :"101111010 " which will be our whole signal at transmitting end .
Now we have received the signal at the receiving end and we want to "extract our message signal" from the whole transmitted signal. in order to do so we have to multiply the received signal with the locally generated PN sequence.
101111010
EX-OR 000001010
101110000
Now we are having received signal as "101110000 " out of which the last 4 bits are padded bits which has got no significance so we leave these bits and we will be left with only bits:"10111" which is our transmitted message bits at the transmitter end .
Thus with the help of PN sequence we will be able to transmit the message signal securely and receive the same at receiver side .
The below figure shows how the signal is flowed from transmitter to the receiver .......
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| General communication model |
WHY IT IS NEEDED:
The main application of the PN sequence is to secure the information bits i e : "To encrypt the message bits ". for example assume that you are operating an TV transmission channel named x at a frequency of around 2MHz (Assume). And you have got certain number of subscribers who paid for your service , so when you transmit the data that should be only received by the subscribed users . so the one who knows the "PN sequence" which is used at the transmitter will be able to receive the signal . otherwise if the non-subscribed users tune their receiver to 2MHz then it may possible that they also get access to your channel . This is just an one example where we will be using the PN sequence , there are "n " number of applications where it is used mainly in "defense " applications where the data is very confident .
APPLICATION:
1) It is mainly used in "Defense" applications , where the transmitted signal should not be known to the enemy country soldiers . so it is very important to secure the message data .
2) In TV transmission where it is needed that only the subscribed members should get access to your channel , here we will be using the different length of PN sequences depending on the type of application .
3) In communication of "fighter planes " where the information being sent by our "pilot " will be tried to extract by enemy country also , so in order to avoid this we use PN sequences of larger lengths as the priority for security is more here .
4) Not only these it can be every where there is need for the security of data .
HOW THE SEQUENCE IS GENERATED:
The generation PN sequence is simpler as it only needs some "flip-flops " and some feedback circuitry .
The below figure shows how to generate a PN sequence of length 7 bits .
The circuit consists of 3 flip-flops (note: A flip-flop can store 1 bit at a time ) . so there is formula to find number of output bits created depending on the number of shift registers i e :
Maximum Length Of PN Sequence is
= 2^n - 1 => Formula
=2^3 -1
=8-1
=7 bits
where ,
n= number of "flip-flops "
similarly ,
If we use 4 flip-flops then length of the PN sequence generated will be equal to
= 2^4 - 1
= 16-1
= 15 bits
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| PN Sequence generator circuit |
PROPERTIES OF PN SEQUENCES :
1) Balance Property :
This property states that the "number of 1's in the PN sequence will be always greater than the number of 0's ".
Example :
consider a PN sequence : 1101001
number of 1's = 2^n/2 =>> Formula
= 2^3 / 2
= 8/ 2
= 4 bits >>>>>>>>>>>1
where ,
n= number of flip-flops
number of 0's = 2^n / 2 - 1 =>>Formula
= 2^3 / 2 - 1
= 4 - 1
= 3 bits >>>>>>>>>>>>2
n= number of flip - flops
Thus by comparing equation 1 and 2 we come to know that number of 1's will be more than the number of 0's in any PN sequence .
2) Run Property :
Among the 1's and 0's in each cycle of the maximum length PN sequence . The number of run's is given by
= N+1 / 2
= 7+1 / 2
= 8 / 2
= 4 >> implies number of run's needed for a 7 bit PN sequence .
3) Auto correlation property :
The auto correlation property is basically is used to find the relation between the "present binary symbol and previous binary symbol ".
Let us assume that the present binary symbol as Cn and the previous binary symbol as
Cn-k . Where "k" is the delay of the correlation sequence .
Then the auto correlation function is given by
N Rc(k)=1 / N∑ Cn * Cn-k
n=1
The value of
Rc(k) = ❴ 1 if k=0
0 if k ⧣ 0 ❵
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