Students will learn about spread spectrum techniques and PN sequence generator

1. DIGITAL COMMUNICATION
Course Code : PC601EC
Presented
Mr.A.Abhishek Reddy,
Assistant Professor,
Dept. ECE,MECS
Email:abhireddy470@gmail.com
8121444996

2. UNIT – V
Spread Spectrum Communication
 Advantages of Spread Spectrum.
 Generation and characteristics of PN sequences.
 Direct sequence spread spectrum and their
applications.
 Frequency hopping spread spectrum systems and
their applications.
 Acquisition and Tracking of DSSS and FHSS signals.

3. What is Spread Spectrum Communication?
 A major concern in the study of digital communications is to provide
efficient utilization of bandwidth and power.
 However, there are situations where it is required to sacrifice efficient
use of these two resources to meet certain design objective.
 For example the system may be required to provide a form of secure
communication in a hostile environment such that the
transmitted signal is not easily detected by unwanted listeners
 This requirement is catered to by a class of signaling techniques
known as spread-spectrum (SS) modulation.

4. Spread Spectrum
 Spread spectrum – technique whereby an already modulated
signal is modulated a second time, producing a waveform that
interferes in a barely noticeable way with any other signal
operating in the same frequency band
– Interfering signals are transparent to spread spectrum signals
–Spread spectrum signals are transparent to interfering signals
 The main advantage of a SS signal is its ability to reject
unintentional interference by another user simultaneously sharing
the channel, or to reject intentional interference by a hostile
transmitter attempting to jam the transmission.

5. The definition of SS modulation may be stated in two parts :
 SS is a means of transmission in which the data sequence
occupies bandwidth in excess of the minimum bandwidth
necessary to send it.
 The spectrum spreading is accomplished before transmission
through the use of a code that is independent of the data
sequence. The same code is used in the receiver (operating in
synchronism with the transmitter) to despread the received
signal so that the original data sequence may be recovered

8. General Block Diagram Of SS

11. PN SEQUENCE(PSEUDO NOISE/PSEUDO RANDOM)
 A pseudonoise (PN) sequence may be defined as a coded
sequence of 1’s and 0’s with certain autocorrelation properties.
 The PN sequence is a deterministic, periodic signal that is
known to both the transmitter and receiver.
 Even though the signal is deterministic it appears
to have the statistical properties of sampled white noise.
 Hence, it appears to be a truly random signal, to an unauthorised
listener.

15. Properties of PN sequence
Balance Property: In each period of the sequence, the number of 1’s is always one
more than the number of 0’s.
No. of 1’S = 2𝑚−1
No of 0’s =2𝑚−1-1
Run Property: A run is defined as a sequence of a single type of binary digit(s). In
every period, half of the runs of the same sign have length one, one fourth have
length two, one eighth have length three, and so on. Also the number of positive
runs equals the number of negative runs.
Correlation Property: The autocorrelation function of a PN-sequence is
periodic and two-valued.
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2m

25. THANK YOU