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.
Spread
Spectrum Signals
The spread spectrum signals have the signal strength distributed
as shown in the following frequency spectrum figure.
Following are some of its features −
- Band of signals occupy
a wide range of frequencies.
- Power density is
very low.
- Energy is wide
spread.
With these features, the spread spectrum signals are highly
resistant to interference or jamming. Since multiple users can share the same
spread spectrum bandwidth without interfering with one another, these can be
called as multiple access techniques.
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 FHSSFHSS
- Direct
Sequence Spread Spectrum DSSSDSSS
Frequency
Hopped Spread Spectrum FHSSFHSS
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 DSSSDSSS
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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