2. 2
Contents:
• DEFINITION OF SPREAD SPECTRUM ( SS )
• CHARACTERISTICS OF SPREAD SPECTRUM
• BASIC PRINCIPLES OF DIRECT SEQUENCE
SPREAD SPECTRUM ( DSSS )
• BASIC PRINCIPLES OF FREQUENCY HOPPING
SPREAD SPECTRUM ( FHSS )
• PERFORMANCE IN THE PRESENCE OF
INTERFERENCE
• PSEUDO-NOISE SEQUENCES ( PN )
3. 3
Definition of Spread Spectrum :
Spread spectrum is a modulation method applied to
digitally modulated signals that increases the transmit
signal bandwidth to a value much larger than is needed
to transmit the underlying information bits.
4. 4
Spread Spectrum Signal Characteristics:
1. They are difficult to intercept for unauthorized
person.
2. They are easily hidden, it is difficult to even
detect their presence in many cases.
3. They are resistant to jamming.
4. They have an asynchronous multiple-access
capability.
5. They provide a measure of immunity to
distortion due to multipath propagation.
5. 5
Spread Spectrum Conditions:
• The signal occupies a bandwidth much larger
than is needed for the information signal.
• The spread spectrum modulation is done
using a spreading code, which is
independent of the data in the signal.
• Dispreading at the receiver is done by
correlating the received signal with a
synchronized copy of the spreading code.
6. 6
Processing Gain :
The spread spectrum increases the bandwidth
of the message signal by a factor N, called
the processing gain where is the message
signal bandwidth, ss is the corresponding SS
signal bandwidth.
,N > 1
7. 7
Spread Spectrum Techniques:
There are several forms of spread
Spectrums :
1. Direct sequence spread spectrum (DS/SS)
2. Frequency hopping spread spectrum
(FH/SS)
9. 9
The channel output given by:
y(t) = x(t) + j(t)
= c(t) s(t)+ j(t)
The Coherent detector input u(t) : u(t) =c(t) y(t)
= s(t)+ c(t) j(t)c
2
t( )
Where: for all t
10. 10
Spreading
Input:
•Binary data dt with symbol rate Rs = 1/Ts ( = Bit rate Rb
for BPSK ).
•Pseudo-noise code pnt with chip rate Rc = 1/Tc
Spreading:
The binary data is multiplied with the PN sequence which
is independent on the binary data to produce the
transmitted signal txb.
txb = dt . pnt
11. 11
The effect of multiplication is to spread the base band
bandwidth Rs of dt to a base band bandwidth of Rc
Bwinfo = Rs << BWss = Rc
Processing gain Gp=BWss/BWinfo = Rc/Rs = Tb/Tc =Nc
13. 13
Dispreading
The spread spectrum signal cannot be detected by a narrow
band receiver. In the receiver, the received base band
signal is multiplied with the PN code Pnr .
• If Pnt = Pnr and synchronized to the PN code in the
received data, then the recovered binary data is produced
on dr. the effect of multiplication of the spread spectrum
signal rxb with the PN sequence pnt used in the transmitter
to dispread the bandwidth of rxb to Rs.
• If then there is no dispreading action.
A receiver not knowing the PN code of transmitter cannot
produce the transmitted data.
Pnr Pnt≠
15. 15
t:
Pnt = Pnr
Autocorrelation Ra 0)= average ( Pnt . Pnt)
+ =1
t: Pnr Pnt≠
Cross correlation Rc 0) = average ( Pnt . Pnt)
<< 1 , for all = 0
16. 16
The operating principle of DS-SS multiple access. Two
users are sending two separate messages m1(t) and m2(t)
through the same channel in the same frequency band at
the same time.
17. 17
Pseudo-Noise Sequence
A pseudo-noise ( PN ) sequence is a periodic binary
sequence with a noise like waveform that is usually
generated by a means of a feed back shift register. It
consists of a shift register made up of m flip-flops and a
logic circuit to form a multiloop feedback circuit.
Feedback shift register.
18. 18
Properties of the PN sequences:
• An m-bit codeword produces a sequence of length
• The peak values are
• The autocorrelation function is equal to –1 other than at
the peaks.
• The O/P sequence contains ones &
Zeros.
• Their power density spectrum is uniform so they may
used as white noise sources.
2
m
1−
2
m
1−
2
m 1−( )
2
m 1−( )
1−
20. 20
We use a correlation receiver to determine whether
a +1 or a –1 was transmitted at time t
PN Decorrelators
PN Matched Filters
21. 21
A typical matched filter implements convolution using FIR
filter whose coefficients are the time inverse of the
expected PN sequence to decode the transmitted data.
If the receiver is not synchronized, then the received signal
will propagate through the matched filter, which outputs
the complete correlation function. The large peak confirms
that the correct code is being received providing accurate
synchronization. The output of the FIR filter is the
decoded data.
The polarity of the large correlation peaks indicates the
data value.
22. 22
Positive
1. Signal hiding (lower power density, noise-like) , non
interference.
2. Secure communications (Privacy).
3. Code division multiple access CDMA.
4. Mitigation of multi path effect.
5. Protection to international interference (jamming)
6. Rejection of unintentional interference (narrow
band)
23. 23
Negative
1. No improve in performance in the presence of
Gaussian noise.
2. Increase bandwidth (frequency usage, wideband
receiver).
3. Increase complexity and computational load.
24. 24
References:
• Simon Haykin “Communication Systems” , John Wily
& Sons 2001.
• Emmanuel C. Ifeachor “Digital Signal Processing”,
Prentice Hall 2002.
• ir. J. Meel “Spread Spectrum introduction” DE NAYER
INSTITUTE. Belgium (www.denayer.be).
• B. P. Lathi “Modern Digital and Analog Communication
Systems”, Oxford University Press 1998.
