DS Spread Spectrum Systems 1

[guddoo]

Can some body help me about understanding "DS Spread Spectrum Systems" as I am facing some difficulty about grasping the real concept behind the scenario.

I will be obliged for your guidance and about any useful web site or e-source (if any) .. Thanks

 

[melone]

What is the final application?

 

[nbucska]

Imagine that the tuners of the transmitter and receiver are
connected and sweept through a wide band synchronously:
If there is a strong station somewhere it can interfere
with the communication just for a short time ; it is like
if its energy would be reduced by a large factor by
distribution in time.

Or -- in frequency --:

Imagine if instead of one 100W xmitter you would have
100 1W xmitters at slightly different frequencies and
the outputs of the 100 receivers were added.


<nbucska@pcperipherals.com>

 

[raghuramkarthik]

Here's an explanation:

In DS SSM, we use a randomly generated bit sequence at high frequency.This sequence we call as pseudo noise and the generator as a Pseudo Random Binary Sequence generator(PRBS). It is pseudo random b'coz(only then,we can demodulate of course) we know the initial 'seed' which we apply to the PRBS generator.

The wideband noise-like sequence when multiplied in time with the incoming sequence, is equivalent to convolution in frequency domain b/w a wide band signal and the message(NarrowBand).This achieves the 'Spreading' for SSM.

Hope my explanation helps..any doubts,.. please post.

 

[johnwiss]

and the reason to do this? One--reduce the power in watts/Hz to prevent interference from affecting other non-spread systems.Two--upon multiplying the code as raghuramkarthik says in the receiver the signal will collapse to a narrowband signal to demodulate, and, any inteference is spread to reduce its effect on the desired signal. In fact if users use different PRBS then we can communicate with several user signal on top of each other in frequency--and demodulate the one we want using the PRBS for the transmitter we want to hear.

 

[DrewBrejda]

Take the example of the 100 1W xmitters, each on a different frequency. You can activate each xmitter in any sequence you choose, independant of all them others. In reality, one xmitter is used and the transmit frequency is digitally controlled to change or &quot;hop&quot; in a predetermined sequence. As long as the receiver knows which frequency to listen for at a given time, the receiver can reconstruct the signal and deliver the reconstructed signal to the demodulator. The receiver and xmitter both must know the sequence and be in time sync. By using unique sequences, multiple xmitter/ receiver pairs can operate in the same overall band without interfering with each other. The military uses this technique to provide what are called Low Probability of Intercept (LPI), Low Probability of Detection (LPD), and Low Probability of Jamming (LPJ). The actual transmitted power on any given frequency at any given moment is so low, the signal may actually be buried in the residual band noise thus making it quite difficult for someone to intercept, detect, or jam the signal. If the signal cannot be easily separated from any background noise, it will be very hard to detect or intercept. And with the signal &quot;spread&quot; out over a wide bandwidth, the power and bandwidth needed in a transmitter to jam the signal would be impractical, if not almost impossible to achieve. However, DS SS DOES NOT provide encryption. With the proper equipment, someone could still capture the original signal and reconstruct it so emcryption is still required to ensure security/privacy.