2. 2.26 SPREAD SPECTRUM
MODULATION
It is a wideband modulation Technique.
Spread spectrum transmission offers the following three main
advantages over fixed frequency transmission:
(a) Spread spectrum signals are highly resistant to noise and
interference. The process of re-collecting a spread signal
spreads out noise and interference, causing them to recede
into the background.
(b) Spread spectrum signals are difficult to intercept.
(c) Spread spectrum transmissions can share a frequency band
with many types of conventional transmissions with minimal
interference. These signals add minimal noise to the narrow-
frequency communications, and vice versa. As a result,
bandwidth can be utilized more efficiently.
3. Features of SSM
• Spread spectrum techniques meet the following
objectives: (a) operation with a low-energy spectral
density, (b) multiple access capability without external
control, (c) security (difficult for unauthorized receivers
to observe the message), (d) anti-jamming capability, (e)
multipath protection, and (f) ranging.
• Two types of Spread Spectrum Systems
– 1. Averaging system
– 2. Avoidance system
A Direct Sequence (DS) system is an averaging system, whereas
Frequency Hopping (FH), time hopping (TH), and chirping
systems are avoidance systems.
4. 2.27 Pseudo-
Noise Codes
with Properties
and Code
Generation
Mechanisms
A unique code is used to
spread and despread the
signal using the logic
shown in Figure (a). This
unique code is the DS,
known as the pseudo-
noise (PN) code.
Time and frequency domain appearances
5. PN Code, Bit Rate and Spectrum
• Assume that the clock rate is provided for
generating each bit of a PN code (with one clock
cycle, one bit of PN code comes out).
• The bit rate of a PN code is called the chip rate
(1/ tchip ), which is 10 times or more than the data
bit rate.
• The smallest time increment in the sequences of
certain period or duration is t chip and is known
as a time chip . The total period consists of Nc
time chips.
• The chip rate decides the final transmission
spectrum of the DSSS system.
6. Properties of Pseudo-noise Codes
• Balance property
• Run length property
• Autocorrelation property
In General for PN Code
1. In every period, the number of +1’s differs from that of −1’s
by exactly one (balance property). Hence, Nc is an odd
number.
2. 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 forth. In addition, the number of
positive runs equals that of negative runs (run property).
3. The autocorrelation of a periodic sequence is two valued,
that is, N c for shifts 0, N c , 2 N c , 3 N c , and so on and −1
otherwise (without normalization).
7. Aperiodic and Periodic Sequences
• An aperiodic sequence is one that does not
repeat itself in a periodic fashion. It is usually
assumed that the sequence has a value of zero
outside its stated interval. An ideal aperiodic
sequence of Nc chips has the autocorrelation to
be Nc for no shift and 0 or 1 (Barker sequences)
• A periodic sequence is a sequence of plus or
minus 1’s that repeats itself exactly with a
specified period. Periodic sequences are made
longer by more shift registers, so it appears as
random to the users.
8. Maximum Length Sequences
ML sequence operators (a) Typical PN code generator with three shift register stages and its
application for data spreading (b) Fibonacci implementation of LFSR (c) Galois
implementation of LFSR
Primitive Polynomial-----
9. (3,1) ML Code Generator and Waveforms
of Code Sequence
10. Other Sequences
• Walsh-Hadamard Sequences
• Gold Sequences
Gold sequences are constructed by XOR-ing two
selected m -sequences of the same length with
each other.
11. 2.28 DIRECT SEQUENCE SPREAD SPECTRUM
SYSTEM
Transmitter Process
Simplified diagram for biphase modulation
12. Receiver Process
Carrier demodulation and despreading of SSM signal to get original data
Detection of signal and despreading operations can be
either by
active method
or
passive method
14. Spectral Density, Bandwidth, and
Processing Gain
Spectral density of binary PN sequence
-----Spectral density of message signal
Processing gain
For Bi-phase Modulation
For Quadriphase Modulation
15. DSSS System Performance
• Performance Parameters
– Interference Rejection
– Antijam characteristics
– Energy and Bandwidth Efficiency
22. 2.29 TIME HOPPING SPREAD SPECTRUM
TH (a) Concept (b) Waveforms showing THSS signal formation on bit-by-bit basis (c) TH with
variable time slots (bit by bit)
25. HYBRID SPREAD SPECTRUM SYSTEMS
• The use of hybrid techniques attempt to capitalize upon the
advantages of a particular method while avoiding the
disadvantages.
• DS, suffers heavily from the near–far effect, which makes this
technique hard to apply to systems without the ability of power
control, but its implementation is inexpensive.
• The PN code generators are easy to implement and the spreading
operation itself can be simply performed by XOR ports.
• FH effectively suppresses the near–far effect and reduces the need
for power control. However, implementation of the (fast) hopping
frequency synthesizer required for a reasonable spreading gain is
more problematic in terms of higher silicon cost and increased
power consumption.
• Selection of SFH/FFH also has its own pros and cones.
• Solutions are
PN/FH, PN/TH, FH/TH, and PN/FH/TH.
