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Spread Spectrum

Spread spectrum is a mobile communication technique that spreads a signal across a wider frequency band to enhance security, reduce interference, and improve robustness. The two main types are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), both of which offer benefits such as interference reduction, security enhancement, and efficient bandwidth utilization. FHSS rapidly switches frequencies to prevent interference, while DSSS spreads the signal using a pseudo-random noise code to enhance signal integrity and coexistence in the same frequency band.

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13 views15 pages

Spread Spectrum

Spread spectrum is a mobile communication technique that spreads a signal across a wider frequency band to enhance security, reduce interference, and improve robustness. The two main types are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), both of which offer benefits such as interference reduction, security enhancement, and efficient bandwidth utilization. FHSS rapidly switches frequencies to prevent interference, while DSSS spreads the signal using a pseudo-random noise code to enhance signal integrity and coexistence in the same frequency band.

Uploaded by

Christabel Johannah
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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SPREAD SPECTRUM

In mobile communication, spread spectrum is a technique used to


spread a signal across a wider frequency band than the minimum
bandwidth required to transmit the signal.

The primary goal of spread spectrum is to improve signal security,


reduce interference, and enhance the overall robustness of
wireless communication.

TYPES:
▪ Frequency Hopping Spread Spectrum (FHSS)
▪ Direct Sequence Spread Spectrum (DSSS)

6.1
6.2
6.3
Figure 6.27 Spread spectrum

6.4
Spread Spectrum Background:

Hedy Lamarr and George Antheil invented


spread spectrum technology, also known as
frequency hopping spread spectrum
(FHSS). Lamarr was an actress and inventor,
while Antheil was a composer.

6.5
6.6
In FHSS, the transmitter and receiver rapidly switch
between different frequency channels in a
pseudo-random order.
This prevents interference and jamming on any
particular frequency because the signal hops to
different frequencies over time.
It's commonly used in Bluetooth and older cordless
phones.
Bluetooth devices hop between these 79 channels in a
pseudo-random order at a very fast rate (up to 1,600
hops per second). This helps minimize the impact of
interference from other devices operating in the
same frequency range.
6.7
FHSS Process
1.A pseudorandom code generator, called pseudorandom noise (PN),
creates a k-bit pattern for every hopping period Th •
2.The frequency table uses the pattern to find the frequency to be used for
this hopping period and passes it to the frequency synthesizer.
3.The frequency synthesizer creates a carrier signal of that frequency, and
the source signal modulates the carrier signal.

6.8
6.9
Figure 6.30 FHSS cycles

6.10
Figure 6.31 Bandwidth sharing

6.11
Benefits of FHSS
•Interference reduction
FHSS helps reduce interference because the signal hops from
frequency to frequency. This hopping makes it difficult for
interference to affect the overall signal, ensuring smooth and
uninterrupted communication.
•Security enhancement
The pseudorandom nature of the frequency hopping provides an
inherent level of security. It is challenging to intercept or eavesdrop
on the transmission without knowledge of the frequency hopping
pattern.
•Bandwidth utilization
FHSS allows for efficient use of bandwidth. By spreading the signal
over a wide range of frequencies, it ensures optimal utilization of the
available spectrum.
6.12
Direct Sequence Spread Spectrum (DSSS):

•DSSS works by multiplying the original data


signal with a high-rate pseudo-random noise
(PN) code, often referred to as a "chip number,"
which spreads the signal across a wider
frequency band.
•This method helps to resist interference and
jamming by distributing the signal over a larger
portion of the spectrum.
•DSSS is commonly used in technologies like
Wi-Fi (IEEE 802.11b) and CDMA (Code Division
Multiple Access) mobile networks.

6.13
DSSS example

6.14
Benefits of using DSSS
DSSS offers several advantages in wireless communication:
1.Increased Signal Integrity: DSSS enhances signal integrity by
spreading the signal over a broad spectrum, reducing the impact
of interference and noise.
2.Improved Security: Since the signal is spread across a wide
frequency band, it’s harder for unauthorized users to intercept or
eavesdrop.
3.Co-existence: Multiple DSSS systems can coexist in the same
frequency band without interfering with each other due to unique
chipping codes.
4.High Speed: Despite the increased bandwidth, DSSS can
accommodate high-speed data transmission.

6.15

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