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Spread Spectrum Modulation

Spread spectrum modulation is a class of signaling techniques used to provide secure communication by preventing interference, whether intentional or unintentional. There are two main types: frequency hopped spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). FHSS works by rapidly switching a signal between multiple frequencies, while DSSS multiplies the signal by a secret spreading code. Both techniques have advantages like better security, reduced multipath fading and coexistence with other systems.

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166 views2 pages

Spread Spectrum Modulation

Spread spectrum modulation is a class of signaling techniques used to provide secure communication by preventing interference, whether intentional or unintentional. There are two main types: frequency hopped spread spectrum (FHSS) and direct sequence spread spectrum (DSSS). FHSS works by rapidly switching a signal between multiple frequencies, while DSSS multiplies the signal by a secret spreading code. Both techniques have advantages like better security, reduced multipath fading and coexistence with other systems.

Uploaded by

saknisurka
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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 Modulation

A collective class of signaling techniques are employed before transmitting a signal to


provide a secure communication, known as the Spread Spectrum Modulation. The main
advantage of spread spectrum communication technique is to prevent “interference” whether
it is intentional or unintentional.
The signals modulated with these techniques are hard to interfere and cannot be jammed. An
intruder with no official access is never allowed to crack them. Hence, these techniques are
used for military purposes. These spread spectrum signals transmit at low power density and
has a wide spread of signals.

FHSS and DSSS / CDMA


Spread spectrum multiple access techniques uses signals which have a transmission
bandwidth of a magnitude greater than the minimum required RF bandwidth.
These are of two types.

 Frequency Hopped Spread Spectrum (FHSS)


 Direct Sequence Spread Spectrum (DSSS)

Frequency Hopped Spread Spectrum (FHSS)


This is frequency hopping technique, where the users are made to change the frequencies of
usage, from one to another in a specified time interval, hence called as frequency hopping.
For example, a frequency was allotted to sender 1 for a particular period of time. Now, after
a while, sender 1 hops to the other frequency and sender 2 uses the first frequency, which
was previously used by sender 1. This is called as frequency reuse.
The frequencies of the data are hopped from one to another in order to provide a secure
transmission. The amount of time spent on each frequency hop is called as Dwell time.

Direct Sequence Spread Spectrum (DSSS)


Whenever a user wants to send data using this DSSS technique, each and every bit of the
user data is multiplied by a secret code, called as chipping code. This chipping code is
nothing but the spreading code which is multiplied with the original message and
transmitted. The receiver uses the same code to retrieve the original message.

Comparison between FHSS and DSSS/CDMA


Both the spread spectrum techniques are popular for their characteristics. To have a clear
understanding, let us take a look at their comparisons.

FHSS DSSS / CDMA


Multiple frequencies are used Single frequency is used

Hard to find the user’s frequency at any instant of time User frequency, once allotted is always the
same

Frequency reuse is allowed Frequency reuse is not allowed

Sender need not wait Sender has to wait if the spectrum is busy

Power strength of the signal is high Power strength of the signal is low

Stronger and penetrates through the obstacles It is weaker compared to FHSS

It is never affected by interference It can be affected by interference

It is cheaper It is expensive

This is the commonly used technique This technique is not frequently used

Advantages of Spread Spectrum


Following are the advantages of spread spectrum −

 Cross-talk elimination
 Better output with data integrity
 Reduced effect of multipath fading
 Better security
 Reduction in noise
 Co-existence with other systems
 Longer operative distances
 Hard to detect
 Not easy to demodulate/decode
 Difficult to jam the signals
Although spread spectrum techniques were originally designed for military uses, they are
now being used widely for commercial purpose.

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