ECE 543

Communication III
Dr Mamdouh Gouda
Dr. Ashraf Samy
 Lecture 2
Spread spectrum system

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 Spread spectrum technique
• A major issue of concern in the study of digital communication is
that of providing for the efficient use of bandwidth and power.

• There are situations where it is necessary to provide a form of

secure communication in a hostile environment such that the
transmitted signal is not easily detected or recognized by unwanted
listeners.
user signal
broadband interference
narrowband interference
P P
i) ii)
f f
P sender P P

iii) iv) v)
f f f
receiver
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 Spread spectrum technique
• This requirement can be satisfied by a class of signaling techniques
known as spread spectrum modulation.

• The primary advantages of a spread spectrum communication

system is its ability to reject interference whether it be the
unintentional interference by another user simultaneously
attempting to transmit through the channel, or the intentional
interference by a hostile transmitter attempting to jam the
transmission.

interference
spread power signal
power signal
spread
interference
detection at
receiver
f f
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 Spread spectrum technique
The definition of spread spectrum modulation may be stated in two parts:
1. Spread spectrum is a means of transmission in which the data
sequence occupies a bandwidth in excess of the minimum bandwidth
necessary to send it.
2. 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
may be recovered.

Note:
Although standard modulation techniques such as frequency modulation
and pulse code modulation do satisfy part 1 of this definition, they are
not spread spectrum techniques because they do not satisfy part 2 of
the definition.
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 Spread spectrum technique

Applications :

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 Spread spectrum technique
spread spectrum techniques can be
classified into two main categories

Direct sequence Frequency hopping

spread spectrum spread spectrum
technique technique

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 Direct sequence spread spectrum system
In direct sequence spread spectrum (DSSS), each bit in the original signal
is represented by multiple bits in the transmitted signal, using a spreading
code. The spreading code spreads the signal across a wider frequency
band in direct proportion to the number of bits used. One technique with
direct sequence spread spectrum is to combine the digital information
stream with the spreading code bit stream using an exclusive-OR (XOR).

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 Direct sequence spread spectrum system
To see how DSSS system works, an example of a BPSK modulation
scheme is assumed to be used. The transmitter and receiver block
diagrams are shown in Fig.1, and Fig.2, respectively.
At the transmitter, rather than represent binary data with 1 and 0, it is
more convenient for our purposes to use +1 and -1 to represent the two
binary digits. In that case, a BPSK signal can be represented as:

sd t   A d t  cos2 f c t 
where A is the amplitude of the signal, f c is the carrier
frequency and d t   1,1 . To produce the DSSS signal,
we multiply sd t  by ct  which is the PN sequence taking
on values of +1 and -1. Then the transmitted DSSS
signal is given by
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 Direct sequence spread spectrum system
sd t   A d t  cos2 f c t 
DS spreader
Spread spectrum
Binary data sd t  signal
Modulator
s t 
(BPSK)

ct 

Pseudonoise
bit source

st   A d t  ct  cos2 f c t 

Fig. (1) Transmitter block diagram of direct sequence
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spread spectrum system. Communication 3
 Direct sequence spread spectrum system
r t   s t  ct 
 sd t 

DS despreader
Spread spectrum Binary data
Demodulator
signal
st  sd t 
(BPSK)

ct 

Pseudonoise
bit source

Fig. (2) Receiver block diagram of direct sequence spread

spectrum system.
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 Direct sequence spread spectrum system

Example data and chip sequences for DS/SS with BPSK information and
BPSK spreading
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 Direct sequence spread spectrum system

Illustration of PSD of original and spread signals with DSSSS.

13
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 Pseudo noise sequences
A pseudo noise (PN) sequence is a periodic binary sequence with a noise
like waveform that is usually generated by means of a feedback shift
register, a general block diagram of which is shown in the following
figure.

Feedback shift register

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 Pseudo noise sequences
A feedback shift register consists of an ordinary shift register made up
of m flip-flops (two state memory stages ) and a logic circuit that are
interconnected to form a multiloop feedback circuit.
The PN sequence generation is determined by :
•The length of the shift register
•Its initial state
•The feedback logic.

From the definition of a shift register, we have

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 Pseudo noise sequence

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 Pseudo noise sequence

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 Pseudo noise sequence

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 Maximal length sequence generator
Example:

Maximal length sequence generator of m=3

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 Maximal length sequence generator

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 XOR - XNOR Gates

• In addition to AND, OR, NOT, NAND and NOR gates, exclusive-

OR (XOR) and exclusive-NOR (XNOR) gates are also used in the
design of digital circuits.
• These have special functions and applications.
• These gates are particularly useful in arithmetic operations as well as
error-detection and correction circuits.
• XOR and XNOR gates are usually found as 2-input gates. No
multiple-input XOR/XNOR gates are available since they are
complex to fabricate with hardware.

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 XOR Gate:
The exclusive-OR (XOR), operator uses the symbol ⊕, and it performs
the following logic operation:
X ⊕ Y = X Y’ + X’ Y
The graphic symbol and truth table of XOR gate is shown in the figure.
The result is 1 only when either X is equal to 1 or Y is equal to 1, but
not when both X and Y are equal to 1.

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XNOR Gate:
The exclusive-NOR (XNOR), operator uses the symbol ʘ, and it
performs the following logic operation
XʘY = X Y + X’ Y’ = (X ⊕ Y)’
The graphic symbol and truth table of XNOR (Equivalence) gate is
shown in the figure.
The result is 1 when either both X and Y are 0’s or when both are 1’s.
That is why this gate is often referred to as the Equivalence gate.

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 The truth tables clearly show that the exclusive-NOR operation is the
complement of the exclusive-OR.
This can also be shown by algebraic manipulation as follows:
(X ⊕ Y)’ = (X Y’ + X’ Y)’
= (X Y’)’ (X’ Y)’ = (X’ + Y) (X + Y’)
= (XY + X’Y’)
= XʘY

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 Maximal length sequence generator
Some properties of the maximal length sequences are as follows:

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 Maximal length sequence generator
Let binary symbols 0 and 1 of the sequence be denoted by the levels -1
and +1, respectively. Let c(t) denote the resulting waveform of the
maximal length sequence as illustrated in figure (a) for N =7.

Waveform of maximal length sequence for length m = 3 or period N=7

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 Maximal length sequence generator

Autocorrelation function
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 Maximal length sequence generator

Power spectral density

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 Maximal length sequence generator

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 Problem:

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Solution:

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 Problem:

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Solution:

Note :

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 Any
Question

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