Spread Spectrum Modulation - Digital Communication

What is Spread Spectrum Modulation?

Before transmitting a signal, there are classes of signaling techniques that are employed to deliver a secure communication known as Spread Spectrum Modulation. The advantage of spread spectrum technique is it prevents interference if it is intentional or unintentional.

The signals modulated using these techniques are very hard to interfere and cannot jam. Anyone with no official access is not allowed to crack them and used for military purposes. These signals are transmitted at very low power density.

Pseudo-Noise Sequence

It is the sequence of 0s and 1s with auto-correlation properties used in spread spectrum techniques. It is also a sequence of maximum-length type of cyclic code.

Narrow-band and Spread-spectrum Signals

Both the signals are easily understood by looking their frequency spectrum as shown below.

Narrow-band Signals

These signals have the concentrated signal strength as shown below.

Narrow-band Signals

Following are some of its features −

  • A narrow range of frequencies is occupied with band of signals
  • Very high power density.
  • Low and concentrated energy is spread.
  • Even though they are good features, they are gullible to interference.

Spread Spectrum Signals

These signals have the concentrated signal strength distributed as shown below.

Spread Spectrum Signals

Below are few features −

  • Band of signals occupy a wide range of frequencies.
  • Very low power density.
  • Wide spread energy.

The above features make the signals to be highly resistant to jamming or interference. This is also called as multiple access techniques as the same spread spectrum bandwidth is shared without interfering with each other.

FHSS and DSSS / CDMA

The techniques discussed use signals which has a transmission bandwidth of magnitude greater than then required RF bandwidth.

These are of two types.

  • Frequency Hopped Spread Spectrum (FHSS)
  • Direct Sequence Spread Spectrum (DSSS)

Frequency Hopped Spread Spectrum (FHSS)

In this technique, the users change the frequencies of usage in a specified time interval, hence it is called as frequency hopping.

The frequencies of the data are switched from one to other to provide a secure transmission. Dwell time is the time spent on each frequency hop.

Direct Sequence Spread Spectrum (DSSS)

Using this technique, user sends the data by multiplying each and every bit with a secret code known as chipping code. In general, it is the spreading code the original message is multiplied with for transmission. At receiving end, same code is used to retrieve the original message.

Comparison between FHSS and DSSS/CDMA

As the two spread spectrum techniques are popular for their characteristics, to have a better idea let us have 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

Below are the spread spectrum advantages −

  • Cross-talk elimination
  • Better output provided with data integrity
  • Multipath fading effect is reduced.
  • High security
  • Noise Reduction
  • Co-existence with other systems
  • Longer operative distances
  • Hard to detect
  • Demodulating/decoding is not easy.
  • Signal jamming is difficult.

Even though these techniques are designed originally for military use, they are now widely used for commercial purpose.

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