February 06, 2020
Amitraj
Spread Spectrum Techniques
-> Spread-Spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.
-> These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming, to prevent detection, and to limit power flux density.
-> Spread spectrum is designed to be used in wireless applications (LANs and WANs). In wireless applications, all stations use air (or a vacuum) as the medium for communication.
OR
Spread Spectrum Modulation
Spread spectrum is a technique used for transmitting radio or telecommunications signals. The term refers to the practice of spreading the transmitted signal to occupy the frequency spectrum available for transmission.
The advantages of spectrum spreading include noise reduction, security and resistance to jamming and interception.
Pseudo-Noise Sequence
A coded sequence of 1s and 0s with certain auto-correlation properties, called as Pseudo-Noise coding sequence is used in spread spectrum techniques. It is a maximum-length sequence, which is a type of cyclic code.
Narrow-band and Spread-spectrum Signals
Narrow-band Signals
The Narrow-band signals have the signal strength concentrated as shown in the following frequency spectrum figure.
Features:-
Band of signals occupy a narrow range of frequencies.
Power density is high.
Spread Spectrum Signals
The spread spectrum signals have the signal strength distributed as shown in the following frequency spectrum figure.
Features:-
Band of signals occupy a wide range of frequencies.
Power density is very low.
Energy is wide spread.
-> 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.
1. Frequency Hopped Spread Spectrum (FHSS)
2. 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.
Advantages of Spread Spectrum
Better output with data integrity
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
Difference between FSSS and DSSS / CDMA-
-> Spread-Spectrum techniques are methods by which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth.
-> These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming, to prevent detection, and to limit power flux density.
-> Spread spectrum is designed to be used in wireless applications (LANs and WANs). In wireless applications, all stations use air (or a vacuum) as the medium for communication.
OR
Spread Spectrum Modulation
Spread spectrum is a technique used for transmitting radio or telecommunications signals. The term refers to the practice of spreading the transmitted signal to occupy the frequency spectrum available for transmission.
The advantages of spectrum spreading include noise reduction, security and resistance to jamming and interception.
Pseudo-Noise Sequence
A coded sequence of 1s and 0s with certain auto-correlation properties, called as Pseudo-Noise coding sequence is used in spread spectrum techniques. It is a maximum-length sequence, which is a type of cyclic code.
Narrow-band and Spread-spectrum Signals
Narrow-band Signals
The Narrow-band signals have the signal strength concentrated as shown in the following frequency spectrum figure.
Features:-
Band of signals occupy a narrow range of frequencies.
Power density is high.
Spread Spectrum Signals
The spread spectrum signals have the signal strength distributed as shown in the following frequency spectrum figure.
Features:-
Band of signals occupy a wide range of frequencies.
Power density is very low.
Energy is wide spread.
-> 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.
1. Frequency Hopped Spread Spectrum (FHSS)
2. 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.
Advantages of Spread Spectrum
Better output with data integrity
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
Difference between FSSS and DSSS / CDMA-
|
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
|
