![]() Bit and work clock used in digital communication code#This is the principle of Code Division Multiple Access (CDMA) cellular systems, in other words, share the same frequency and time with multiple users with different codes. Other users will have very little amplitude of the original signal. If we take them all and plot them so that there are 15 points before 0 and 15 after:įigure 5: Correlation of a) example sequence and b) other sequence with polynomial created with LabVIEW and MathScriptĪs seen, only if the end user having the exact sequence is able to demodulate the message when the sequence is synchronized (peak at correlation = 1). ![]() This is the autocorrelation for each shift point. = 15 which is multiply each value by itself and add them all ( ). The formal definition of discrete autocorrelation is: Using this scheme, the initial state is only needed to generate exactly the same sequence of length (the only forbidden state is all zeros since the register will lock in this state).Īfter the fifteenth shift, the values on the registers will be again the starting seed.Īfter this number of ‘1’ and ‘-1’ the sequence will start to repeat since the starting symbols will be the same. This sequence appears to have random pattern but in fact can be recreated by using the shift register structure in Figure 4 with M=4, polynomial and initial state ‘1 1 0 0’.įigure 4: Shift register structure for m-sequence These codes (DSSS codes) will all be treated as pseudonoise (PN) sequences because resembles random sequences of bits with a flat noiselike spectrum. These sequences are generated by m-sequences. This “key” is in fact a pseudo random sequence (rapid phase transition) also known as pseudo noise (PN). This is like a key, only the demodulator that “knows” such a key will be able to demodulate and get the message back. ![]() To get the signal back, the exact same high bandwidth signal is needed. In fact, the power density amplitude of the spread spectrum output signal is similar to the noise floor. The rapid phase transition (chip rate ) signal has a larger bandwidth given that the rate is greater (without changing the power of the original signal) and behaves similar to noise in such a way that their spectrums are similar for bandwidth in scope. We encourage the reader to seek the references for advance knowledge of spread spectrum systems. The last section will give the reader some insight of more advance topics but will not deeply explore them. ![]() This document will explore basics concepts of spread spectrum for the remaining of the introduction and then it will explore the supporting concepts of the most used technique in spread spectrum systems. This makes the signal more difficult to distinguish from noise and therefore more difficult to jam or intercept. ![]() A spread spectrum signal does not have a clearly distinguishable peak in the spectrum. The idea behind spread spectrum is to use more bandwidth than the original message while maintaining the same signal power. easy to detect).įigure 1: Narrow band signal, relatively easy to jam or intercepted. Likewise, the signal can also be intercepted since the frequency band is fixed and narrow (i.e. These narrowband signals are easily jammed by any other signal in the same band. Spread Spectrum refers to a system originally developed for military applications, to provide secure communications by spreading the signal over a large frequency band.įigure 1 represents a narrow band signal in the frequency domain. ![]()
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