explain for me about dsss modulation

[hamid.abbaszadeh]

HI ALL
please explain for me about dsss modulation on the shape.
expecting your coments

 

[ted]

Hi,

DSSS = Direct-sequence spread spectrum. The name points out, that the modulation spreads the transmitted spectrum. This method of spreading can be explained (very much simplified!):

- Spreading modulation is done by pseudo-random sequence, which operates much higher speed, than the information bit rate.
- Thus pieces of each information bit are transmitted on multiple frequencies
- The receiver knows the sequence, and is thus able to put those pieces (called chips) back together

As a result, some of advantages are achieved, such as:
- In case of disturbing signals on same of generated frequencies, there is high likelihood, that many other chips, falling on different frequencies, but belonging to a certain bit, are passing undisturbed. Thus the data stream can still be received correctly. (That protects against jamming and from disturbance by other devices using the same frequency)
- Certain degree of protection for eavesdropping can be achieved, because without knowledge of the right sequence it is difficult to reconstruct the data stream
- Increased immunity against the background noise (so-called process gain)
- Use of one and same frequency band for many independent users is easy: There is no need to synchronize those data streams with each other, but they can still co-exist without significant mutual interference.

The signal "shape" in this kind of modulation is pretty complex, and has similar properties to white noise. The frequency spectrum is, however, centered on the carrier frequency with amplitude reducing when moving outwards from the center frequency, leading to a wide bell-shaped frequency/amplitude plot.

I hope this helps you forward. I eliminated all math by purpose, as those equations you get from books. Some slightly deeper description than mine you can find even in "encyclopedia-level" writings such as the one in Wikipedia: https://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum


- ted

 

[hamid.abbaszadeh]

tanks ted
i know "The signal "shape" in this kind of modulation is pretty complex"
but explain on the shape